Deciphering the Medicinal Chemistry Aspects of Akt Inhibitors for the Management of Cancer: Structure–Activity Relationship Frameworks, Landscapes, and Optimization Approaches

ABSTRACT

The Akt pathway is dysregulated in cancer, leading to proliferation, decreased apoptosis, and metastasis, and hence is a major therapeutic target in cancer treatment. Over the past two decades, substantial advances have been made in the design of potent, selective, and pharmacokinetically optimized Akt inhibitors. This review analyzes recent Akt inhibitors, with particular focus on the structure–activity relationship (SAR) of their heterocyclic core scaffolds and the resulting biological activity. Additional strategies identified in this review that have yielded promising preclinical and clinical candidates include scaffold optimization, conformational restriction, and hybrid design. Special emphasis is given to advanced inhibitors such as capivasertib, ipatasertib, and NTQ1062, which illustrate the influence of pharmacophoric refinement on therapeutic success. In this review, we integrate knowledge of medicinal chemistry and structure to direct the rational design of next generation Akt inhibitors that are safer and more effective for targeted cancer therapy.

This review majorly describes the systematic development of Akt inhibitors involving numerous heterocyclic scaffolds along with their structure activity relationships to explore anticancer therapeutic strategies.

Abbreviations

786‐O

renal cell carcinoma cell line

ADME

absorption, distribution, metabolism, and excretion

AKT

protein kinase B

APN

aminopeptidase N

ATP

adenosine triphosphate

BAD

Bcl‐2‐associated death promoter

BT474

breast tumor cell line 474

ClogP

calculated logarithm of the octanol–water partition coefficient

CYP450/CYP3A4

cytochrome P450 (isoform 3A4)

EC₅₀

half‐maximal effective concentration

EDG

electron‐donating group

FBLD

fragment‐based lead discovery

FDA

U.S. Food and Drug Administration

GPCR

G‐protein‐coupled receptor

GSK‐3β/GSK3β

glycogen synthase kinase 3 beta

H460/NCI‐H460

human non‐small‐cell lung carcinoma cell line

HCT116

human colorectal carcinoma cell line 116

HepG2

human hepatocellular carcinoma cell line G2

HER2

human epidermal growth factor receptor 2

hERG

human ether‐à‐go‐go‐related gene (cardiac potassium channel)

HFF

human foreskin fibroblast

HGC‐27

human gastric carcinoma cell line 27

HL‐60

human promyelocytic leukemia cell line 60

HR

hormone receptor

HTS

high‐throughput screening

IC₅₀

half‐maximal inhibitory concentration

KHOS

human osteosarcoma cell line

K_i

inhibition constant

LNCaP

lymph node carcinoma of the prostate (cell line)

MD‐SVR

molecular docking‐support vector regression

MSK1

mitogen‐ and stress‐activated kinase 1

mTOR

mechanistic target of rapamycin

mTORC1/mTORC2

mTOR complex 1/complex 2

MTT

3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (cell viability assay)

OVCAR‐8

ovarian carcinoma cell line 8

p70S6K

ribosomal protein S6 kinase beta‐1 (p70 S6 kinase)

PAM pathway

PI3K/Akt/mTOR pathway

PC‐3

prostate cancer cell line 3

PDK1

phosphoinositide‐dependent kinase 1

pGSK3β

phosphorylated GSK3β

PH domain

pleckstrin homology domain

PI3K

phosphatidylinositol 3‐kinase

PIK3CA

phosphatidylinositol‐4,5‐bisphosphate 3‐kinase catalytic subunit alpha

PIP2

phosphatidylinositol‐4,5‐bisphosphate

PIP3

phosphatidylinositol‐3,4,5‐trisphosphate

PKA

protein kinase A

PKB

protein kinase B

p‐PRAS40/PRAS40

(phosphorylated) proline‐rich Akt substrate of 40 kDa

PTEN

phosphatase and tensin homolog

ROCK1

Rho‐associated coiled‐coil containing protein kinase 1

RTK

receptor tyrosine kinase

SAR

structure–activity relationship

SRB

sulfo‐rhodamine B (cell viability/cytotoxicity assay)

TNBC

triple‐negative breast cancer

TSC2

tuberous sclerosis complex 2

1. Introduction

Cancer is a complicated disease where the genetic and epigenetic alterations interfere with the normal cell regulation mechanisms resulting in the chronic proliferative signaling, evasion of apoptosis, and reprogramming metabolism [1, 2, 3, 4, 5, 6, 7]. Despite major progress in diagnosis and treatment, cancer is still one of the leading causes of morbidity and mortality worldwide and a major challenge to healthcare [8, 9]. Dysregulation of multiple intracellular signaling pathways regulating cell proliferation, survival, metabolism, angiogenesis, migration and apoptosis drives tumor initiation and progression [10, 11]. Among these, the phosphoinositide 3‐kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway has been recognized as one of the most commonly altered oncogenic cascades and plays a central role in the pathogenesis of many solid and hematological malignancies [12, 13]. The PIK3CA mutations, loss of PTEN, and overexpression of receptor tyrosine kinases or activated Akt mutations lead to hyperactivation of this pathway, which promotes uncontrolled cell growth, metastatic progression, therapeutic resistance and poor clinical outcomes, making it an attractive target for anticancer drug discovery [14, 15, 16, 17].

Akt is a serine/threonine protein kinase, a member of the AGC kinase family, and is the principal signaling node in the PI3K/Akt/mTOR pathway [18, 19]. The three Akt isoforms (Akt1, Akt2, and Akt3) are structurally very homologous but differ in tissue distribution and physiological function [20, 21]. Akt activation requires membrane recruitment through its pleckstrin homology (PH) domain, followed by phosphorylation at Thr308 and Ser473, leading to downstream phosphorylation of substrates involved in protein synthesis, glucose metabolism, cell‐cycle progression, inhibition of apoptosis and DNA repair [18, 22]. Persistent Akt activation has been associated with breast, prostate, ovarian, colorectal, lung, pancreatic and endometrial cancer development and progression, emphasizing its importance as a validated therapeutic target [23, 24].

The Akt aberrant activation is typically observed in breast cancer, particularly in hormone receptor‐positive and triple‐negative cancer. The mutation of the upstream regulators: PIK3CA, the loss of tumor suppressor PTEN, or the amplification of Akt itself can cause the dysregulation [25, 26, 27]. The activated Akt predisposes cancer by preventing apoptosis, accelerating cell division, and making cells resistant to endocrine and chemotherapy treatments. Therefore, Akt is a possible therapeutic agent in breast cancer, resulting in selective inhibitors capable of reversing this cancerous effect [25, 26, 28]. On November 16, 2023, the U.S. Food and Drug Administration (FDA) approved capivasertib under the trade name Truqap as the first Akt inhibitor in the treatment of hormone receptor (HR)‐positive, human epidermal growth factor receptor 2 (HER2)‐negative locally advanced or metastatic breast cancer with PIK3CA, Akt1, or PTEN mutations [29, 30]. Capivasertib (AZD5363) is a potent, selective, oral pan‐Akt inhibitor that targets all three Akt isoforms. It has shown a potent clinical response in hormone receptor‐positive (HR+), HER2‐negative advanced or metastatic breast cancer, particularly in patients who have already developed resistance to prior endocrine therapy [31]. Other Akt inhibitors that target various stages of clinical development against breast cancer, hormone receptor‐positive (HR+), HER2‐negative, and triple‐negative disease with aberrant PI3K/Akt pathway activation [32].

The Table 1 represents the key Akt inhibitors currently in clinical development, detailing their developing companies, clinical trial phases, and the types of cancer they target. Additionally, it emphasizes the heterocyclic core and relevant publications to effectively communicate each compound's clinical stage and significance in therapy (see Figure 1).

TABLE 1.

An overview of Akt‐targeting clinical candidates, including their developers and current clinical status, is presented in the table.

Drugs name	Developed by	Status (August 2026)	Binding modes	Isoform selectivity	Heterocyclic core	Notes	References
Capivasertib (AZD5363)	AstraZeneca	FDA approved	ATP‐competitive pan‐Akt inhibitor	Akt1,2 and 3	Pyrrolo[2,3‐d]pyrimidine	First‐in‐class approved Akt inhibitor; based on CAPItello‐291 capivasertib received its first approval, in the USA, in November 2023 for use in combination with fulvestrant.	[37]
Ipatasertib (GDC‐0068)	Genentech/Roche	Phase III completed; not approved	ATP‐competitive pan‐Akt inhibitor	Akt1,2 and 3	Cyclopenta[d]pyrimidine	Development continues mainly in academic/investigator‐initiated combination trials.	[38]
Miransertib (ARQ 092)	ArQule (acquired by Merck 2020)	Phase I/II	Allosteric Akt inhibitor	Akt1,2 and 3	Imidazo[4,5‐b]pyridine	Rare pediatric disease and fast track designations for proteus syndrome.	[39]
M2698	EMD Serono/Merck KGaA	Phase I	ATP‐competitive inhibitor	Akt1 and Akt3 (+p70S6K)	Quinazoline	Dual p70S6K/Akt inhibitor; crosses blood–brain barrier.	[40, 41]
GSK690693	GlaxoSmithKline	Phase I (terminated)	ATP‐competitive pan‐Akt inhibitor	Akt1,2 and 3	Imidazo[4,5‐c]pyridine	Discontinued due to toxicity and insufficient therapeutic window.	[42]
XL‐418	Exelixis	Preclinical/Discontinued	ATP‐competitive inhibitor	Dual Akt/p70S6K	Piperazine	First disclosed dual Akt/S6K inhibitor to enter the clinic	[43]
NTQ‐1062	Not publicly disclosed	Phase I (CTR20211999)	ATP‐competitive inhibitor	Akt1,2 and 3	Piperazine	Preclinical‐stage compound with insufficient clinical data reported.	[44, 45]
Hu‐7691	Huasun Bio	Phase I (China)	ATP‐competitive inhibitor	Akt1 and Akt3	Piperidine–pyrazole–benzamide	Developed to reduce cutaneous toxicity seen with other Akt inhibitors.	[46]
BAY1125976	Bayer Pharma AG	Phase I (terminated)	Allosteric Akt inhibitor	Akt1 and Akt2	Imidazo‐pyridazine	Well tolerated but no radiologic/clinical responses.	[47]
Uprosertib (GSK2141795)	GlaxoSmithKline (GSK)	Phase II (discontinued)	ATP‐competitive pan‐Akt inhibitor	Akt1,2 and 3	Furan carboxamide scaffold	Reached phase II but discontinued due to limited clinical benefit.	[48]
Afuresertib (GSK2110183)	GlaxoSmithKline (GSK)	Phase III (AFFIRM‐205) met its primary endpoint (April 2026)	ATP‐competitive pan‐Akt inhibitor	Akt1,2 and 3	Thiophene carboxamide scaffold	Active in multiple myeloma (v I/II) LAE002 (afuresertib) plus fulvestrant has demonstrated strong positive topline results in the phase III clinical trial (AFFIRM‐205).	[49]
Vevorisertib	Boehringer Ingelheim	Phase I	Allosteric; pan‐Akt inhibitors	Akt1,2 and 3	Imidazo[4,5‐b]pyridine	Early clinical‐stage Akt inhibitor with ongoing evaluation in oncology indications.	[50]
MK‐2206	Merck Sharp and Dohme Corp.	Development discontinued after Phase II because of limited efficacy	Allosteric Akt inhibitor	Akt1,2 and 3	Dihydroisoquinoline‐containing allosteric scaffold	Limited clinical activity as a single agent across multiple phase II trials.	[51]

FIGURE 1.

The progression of a small molecule of Akt inhibitors from a hit to a clinical candidate.

Over the past two decades, extensive medicinal chemistry efforts have resulted in the discovery of diverse ATP‐competitive and allosteric Akt inhibitors [33, 34]. ATP‐competitive inhibitors target the highly conserved catalytic kinase domain and generally exhibit potent enzymatic inhibition, whereas allosteric inhibitors bind to a regulatory pocket formed between the PH and kinase domains, stabilizing Akt in its inactive conformation and often providing improved selectivity. The recent clinical success of capivasertib [35], together with the continued development of several investigational Akt inhibitors, has validated Akt inhibition as a promising therapeutic strategy [36]. However, the clinical translation of many compounds remains limited by challenges including inadequate isoform selectivity, acquired resistance, suboptimal pharmacokinetic properties, dose‐limiting toxicities, and compensatory activation of parallel signaling pathways.

Figure 2 highlights the development of therapeutics targeting Akt has advanced considerably since its cloning and identification as a part of the PI3K/Akt axis in 1991. This evolution includes early staurosporine‐derived pan‐kinase inhibitors, followed by structure‐guided ATP‐competitive and allosteric agents, and most recently, PROTAC‐based degraders. This pathway culminated in the 2019 approval of alpelisib, the first PI3Kα inhibitor for PIK3CA‐mutated breast cancer, and the 2023 approval of capivasertib, recognized as the first Akt inhibitor approved for HR‐positive, HER2‐negative breast cancer. These milestones demonstrate the maturity of the field and the ongoing need for next‐generation agents with better selectivity and pharmacokinetic properties.

FIGURE 2.

Timeline of key discovery and clinical milestones in Akt‐pathway drug development.

2. Literature Search Strategy

A systematic literature search was conducted to identify relevant publications reporting on the biology, structure–activity relationships (SARs), medicinal chemistry, pharmacology, resistance mechanisms and clinical development of Akt inhibitors. The search was conducted in PubMed, Scopus, Web of Science, and Google Scholar and was supplemented by a manual search of the reference lists of key publications. The search strategy employed combinations of the following keywords and including “Akt inhibitor”, “protein kinase B inhibitor”, “Akt1”, “Akt2”, “Akt3”, “PI3K/Akt/mTOR”, “allosteric Akt inhibitor”, “ATP competitive Akt inhibitor”, “structure–activity relationship (SAR)”, “clinical trial”, and “medicinal chemistry”, “drug discovery”, “PI3K/Akt/mTOR”, “resistance,” and “clinical development”. Scaffold‐specific and compound‐specific searches were also carried out where appropriate to provide thorough coverage of the medicinal chemistry literature (see Figure 3).

FIGURE 3.

Literature search strategy and study selection criteria for Akt inhibitor research.

Publications were eligible if they contained relevant information on Akt‐targeted compounds, molecular mechanisms of Akt inhibition, structural features and SAR, isoform selectivity, cellular or biochemical activity, pharmacological properties, resistance mechanisms or clinical development. Original research articles, structural biology studies, pharmacological investigations and clinical studies were prioritized. Review articles and perspectives were used mainly for contextual information and citation tracking. Articles with insufficient relevance to Akt biology or inhibitor development were excluded. The search was performed for publications from (2000) to (2026) and literature search was completed on (31 July 2026). Reference‐list screening and citation tracking were then performed to identify additional relevant studies fitting the predefined scope.

3. PI3K/Akt/mTOR Signaling Pathway

The PI3K/Akt/mTOR signaling pathway, or Akt, is a major regulator of various cell functions, including growth, proliferation, survival, glucose metabolism, and angiogenesis [37, 52, 53, 54]. The binding of growth factors to cell surface receptor tyrosine kinases (RTKs) or G protein‐coupled receptors (GPCRs) activates phosphatidylinositol 3‐kinase (PI3K). When PI3K activated, it converts phosphatidylinositol‐4,5‐bisphosphate (PIP 2) into phosphatidylinositol‐3,4,5‐trisphosphate (PIP 3) [55]. This secondary messenger attracts proteins with a pleckstrin homology (PH) domain, like Akt (protein kinase B) and PDK1 (phosphoinositide‐dependent kinase‐1), to the plasma membrane [52, 56].

As depicted in Figure 4, PDK1 phosphorylates Akt at threonine 308 (Thr308) at the membrane, and mTOR complex 2 (mTORC2) phosphorylates it at serine 473 (Ser473). It fully activates Akt. When fully activated Akt, phosphorylates various downstream targets [57, 58, 59] involved in the regulation of cell‐cycle progression, protein synthesis, and cell survival. These effects are mediated through the modulation of Cyclin D1, p21, and p27, activation of the TSC2–mTORC1 pathway, and inhibition of pro‐apoptotic factors such as BAD and caspase‐9 [52, 60, 61]. Under normal physiology, the pathway is strictly controlled by PTEN (phosphatase and tensin homolog), a tumor suppressor that converts PIP3 to PIP2, thereby suppressing Akt activation [40, 62, 63].

FIGURE 4.

PI3K/Akt/mTOR signaling pathway inhibitors in human cancer.

Nevertheless, PTEN loss or mutation, amplification of Akt isoforms, or PI3K upregulation leads to sustained Akt signaling, which facilitates oncogenic transformation, tumor growth, and resistance to therapy [64]. This Akt aberrant activation has been detected in various cancers such as breast, lung, prostate, and ovarian tumors. The PI3K/Akt/mTOR axis has thus emerged as a major therapeutic target, and several ATP‐competitive and allosteric Akt inhibitors have been developed to inhibit tumor cell growth and reestablish apoptotic signaling. Molecular insights into this pathway are thus crucial for the rational design and optimization of Akt‐targeted anticancer therapies [65, 66, 67, 68, 69].

4. ATP‐Competitive Inhibitors

4.1. Isoquinoline‐5‐sulfonamide Core

Hadas Reuveni et al. [70] reported a systematic design strategy employing combinatorial and parallel chemistry techniques to produce Novel inhibitors of protein kinase B (PKB/Akt) as highlighted in Figure 5. They chose the known kinase inhibitor H‐89 as the starting scaffold. They split it into three modular regions: Core (A Ring) systematically replaced the 5‐isoquinoline part with different bicyclic or aromatic residues and modified the sulfonamide linkage to carbonyl and similar functional groups. Bridge (B Ring): The ethylenediamine linker has been enhanced by modifying the chain length and the strength of the hydrogen bonds. Cinnamoyl moiety (C Ring): This part has been modified by adding various types of aromatics and heteroaromatic substituents to investigate the necessary structural modifications for potency. This structural optimization led to the discovery of compound NL‐71–101 as a promising analogue that inhibited PKB with an IC₅₀ of 2000 nM and PKA with an IC₅₀ of 120 nM, exhibiting a more selective profile than the parent compound H‐89 (IC₅₀ = 2500 nM for PKB and 35 nM for PKA). H‐89 inhibits protein kinase A (PKA) more potently than protein kinase B (PKB) the new compound NL‐71–101, demonstrates a reversed and more desirable selectivity by inhibiting PKB more strongly than PKA.

FIGURE 5.

Lead compound and H‐89 inhibit PKB and PKA at 10 µM ATP.

Ian Collins et al. aimed to replace the metabolically labile alkenyl substituents of H‐89 and NL‐71–101 while reducing their relatively high lipophilicity. A further objective was to enhance selectivity for PKB (Akt) over other AGC kinases, a challenging task given the high homology with PKA (68% sequence identity in the kinase domain, with only three differing residues in the adenine‐binding site) [71]. To accomplish this, the researchers substituted the ethylene linker with an ether bridge and made several modifications to the C‐ring phenyl group. Among these, para‐chloro and 3,4‐dichloro derivatives (1 and 2) showed significant PKB inhibition, with IC₅₀ values of 260 (±20) and 210 nM, respectively, and demonstrated SRB cell growth inhibition with IC₅₀ values of 25 000 and 13 000 nM [72]. Subsequently, optimization involved removing the C‐ring and modifying the B‐ring with cyclic diamines. In this series, 3 includes an (S)‐pyrrolidin‐2‐ylmethanamine group, blocked PKB with an IC₅₀ of 3900 nM. As represented in Figure 6, various heterocyclic substitutions resulted in notable improvements in both potency and selectivity. A pyrrolidine‐linked analogue (4) demonstrated strong activity, with a PKB IC₅₀ of 520 nM and an SRB IC₅₀ of 40 000 nM. In contrast, 5 inhibited PKB with an IC₅₀ of 480 nM and had an SRB IC₅₀ of 28 000 nM.

FIGURE 6.

Lead compounds from the isoquinoline‐5‐sulfonamide series at 30 µM ATP.

Neil Vasdev et al. [72] have reported the discovery of the first imaging agent for monitoring PKA levels using positron emission tomography (PET). Through systematic derivatization of 5‐isoquinoline sulfonamides, researchers discovered that introducing a methyl group to the sulfonamidic nitrogen of the identified PKA inhibitors H‐9 and H‐89 enhances their properties. Within this series, 6 exhibits significant activity against PKB, which shows an IC₅₀ of 1280 nM at a higher ATP concentration of 125 µM. Research was conducted to bring the ligands closer to physiological levels of ATP, as they are ATP‐competitive. Table 2 presents a comparative analysis of the reported isoquinoline‐sulphonamide compounds, highlighting their structural features, biological activities, and development stage.

TABLE 2.

Comparative SAR summary of H‐89 and isoquinoline‐5‐sulfonamide‐based Akt inhibitors.

Lead compounds	Key structural modifications	PKB/Akt IC50	PKA IC50	Cellular activities	Major SAR findings	Remarks
H‐89	Parent isoquinoline‐5‐sulfonamide scaffold with ethylenediamine linker.	2500 nM	35 nM	Not reported	Potent PKA inhibitor; weak PKB inhibition. Isoquinoline sulfonamide is essential for kinase binding.	Starting scaffold for SAR optimization.
NL‐71–101	Modification of isoquinoline core, ethylenediamine linker, and cinnamoyl region.	2000 nM	120 nM	Not reported	Improved PKB selectivity relative to H‐89 by reducing PKA affinity.	Demonstrated reversal of kinase selectivity.
1	Ether linker with p‐chlorophenyl substitution.	260 ± 20 nM	NR	SRB IC50 = 25 µM	Ether bridge and para‐chloro substitution markedly improved PKB potency.	Enhanced biochemical potency.
2	Ether linker with 3,4‐dichlorophenyl substitution.	210 nM	NR	SRB IC50 = 13 µM	Dichloro substitution further increased PKB inhibition compared with mono‐chloro analogue.	Most potent compound in this series.
3	Removal of C‐ring; cyclic diamine ((S)‐pyrrolidin‐2‐ylmethanamine).	3900 nM	NR	NR	Removal of aromatic C‐ring substantially reduced potency.	Requires numerical verification against original publication.
4	Pyrrolidine‐linked analogue.	520 nM	NR	SRB IC50 = 40 µM	Pyrrolidine substitution restored potency compared with 3.	Improved biochemical activity.
5	Alternative heterocyclic analogue.	480 nM	NR	SRB IC50 = 28 µM	Heterocyclic optimization slightly improved potency and cellular activity.	Comparable to 4.
6	N‐Methylated isoquinoline sulfonamide.	1280 nM (125 µM ATP)	NR	PET imaging probe	Activity evaluated under near‐physiological ATP concentration; developed primarily as a PET imaging agent rather than an optimized Akt inhibitor.	ATP concentration should be considered when comparing IC50 values.

4.1.1. Structure–Activity Relationship of H‐89

In this review article, we examined the SAR of H‐89 as a protein kinase A inhibitor and its selectivity over PKB as represented in Figure 7.

FIGURE 7.

Structure–activity relationship of H‐89.

4.1.1.1. A Ring

The isoquinoline sulfonamide ring is essential for activity, and the sulfonyl group is replaced with a carbonyl, resulting in reduced activity. The substitution of the hydroxyl group maintained activity but reduced selectivity over PKB.

4.1.1.2. Side Chain

The methyl substitution on the ethylenediamine side decreased the activity.

4.1.1.3. Linker

Replacing the ethylene linker with a methyl group significantly enhanced the activity, and switching to an ether bridge made it even more promising.

4.1.1.4. B Ring

Replacing the phenyl‐substituted bromo group with chloro and dichloro further increased the compound potency.

4.2. Thiophene and Furan Carboxamide

Xiaodong Lin et al. [73] reported the new class of 2‐pyrimidyl‐5‐amidothiophenes that act as Akt inhibitors. The initial objective was to identify a lead compound that selectively binds to Akt3 and incorporates a 4‐fluorophenyl substituent. However, modifications to this group by incorporating various substitutions led to it being significantly less effective at reducing activity, suggests that the 4‐fluorophenyl group was crucial for maintaining potency. Additional modifications included a 2,4‐dichlorophenyl group and a methylamine chain. Among them 7 demonstrated strong pan‐Akt inhibition with IC₅₀ values of 6 nM for Akt1, 23 nM for Akt2, and 3 nM for Akt3. The data shown in Figure 8 indicates that the compound has a balanced and highly effective Akt inhibitory profile.

FIGURE 8.

Lead compound of thiophene and furan carboxamide as Akt inhibitors.

Wenhu Zhan et al. [74] reported a novel series of pyrazol‐furan carboxamide analogues for potent Akt inhibition, where they modified the core ring by leveraging the possible binding mode between them and the ATP‐bound pocket of Akt1, consisting of three essential interactions (hinge region, acidic hole, and hydrophobic pocket) with various heterocycles, among them, furan‐substituted 8 shows potent activity against Akt1 with an IC₅₀ of 61 nM. The thiophene‐substituted 9 exhibits potent activity against Akt1 with an IC₅₀ of 71 nM (see Figure 8).

Mark A. Seefeld et al. [75] discovered a novel series of pyrrolo‐pyridinyl thiophene carboxamides, which act highly effectively as Akt kinase inhibitors. In this scaffold, the pyrrolo‐pyridinyl ring served as the hinge‐binding region, while the thiophene core acted as the central framework, as described in Figure 9. Initial studies examined the effects of pyrrolo‐pyridine and thiophene regiochemistry on Akt1, including 10, showed strong Akt1 inhibition with an IC₅₀ of 6 nM and moderate activity against pGSK3β with an IC₅₀ of 190 nM. Later on, modify the third position of the thiophene ring and exploring the amide regiochemistry, 11 and 12 emerged as significant Akt1 inhibitors, with IC₅₀ values of 3 and 6 nM. However, they showed less effectiveness against pGSK3β, with IC₅₀ values of 560 and 800 nM. After substituting alkylamine and phenyl groups, 13 and 14 were further optimized. They demonstrated sub‐nanomolar Akt1 activity (IC₅₀ values of 1 and 2 nM) and improved selectivity over pGSK3β (IC₅₀ values of 80 and 260 nM). Finally, the elimination of the methyl chain from the phenyl ring and the introduction of fluorine led to the creation of highly potent analogues (see Figure 9). The m‐fluoro and p‐fluoro‐substituted 15 and 16 exhibited potent Akt1 inhibition (IC₅₀ = 0.03 and 0.06 nM), significant activity across Akt isoforms (Akt2: IC₅₀ = 8 and 6 nM; Akt3: IC₅₀ = 2 and 2 nM), and notable cellular efficacy (pGSK3β: IC₅₀ = 90 and 180 nM; LNCaP: IC₅₀ = 18 and 66 nM). This series demonstrates a systematic design approach in medicinal chemistry, wherein gradual systematic modification improved both potency and selectivity, ultimately leading to a highly effective Akt inhibitor.

FIGURE 9.

Lead compounds from the thiophene carboxamide series.

Melissa Dumble et al. [76] reported novel ATP‐competitive pan‐Akt kinase inhibitors, GSK2110183 and GSK2141795, as shown in Figure 10. Both demonstrate effective inhibition of the Akt signaling pathway and show significant antiproliferative effects in various tumor cell lines in vitro and in vivo. Biochemical tests reveal that GSK2110183 inhibits Akt1, Akt2, and Akt3 with IC₅₀ values of 0.08, 2.0, and 2.6 nM, respectively. GSK2141795 is more potent, with IC₅₀ of 0.066, 1.4, and 1.5 nM against the Akt isoforms. In addition, both inhibitors demonstrated significant cellular activity against phosphorylated GSK‐3β (pGSK‐3β) in LNCaP prostate cancer cells, with IC₅₀ values of 76 ± 16 nM for GSK2110183 and 34 ± 5 nM for GSK2141795. Research suggests the potential of Akt pathway‐targeting therapeutic agents in cancer.

FIGURE 10.

Lead compounds from GSK analogues.

Xiaowu Dong et al. [77] reported the development of a novel series of 3,4,6‐trisubstituted piperidine analogues as Akt inhibitors. The first design idea was to incorporate a privileged hinge‐binding moiety, comprising an N‐methyl pyrazole and a furan group, along with an amide substitution at the 3‐position of the piperidine scaffold and various substituted phenyl rings at the 4‐position. Among them, 17 and 18 demonstrate a significant effect on Akt1 inhibition, with IC₅₀ values of 7.2 and 6.4 nM, respectively. The design strategy is highlighted in Figure 11.

FIGURE 11.

Lead compound 3,4,6‐trisubstituted piperidine analogues.

However, replacing the piperidine core with a pyrrolidine ring reduced the inhibitory activity, resulting in less potency. Further investigation into the stereoisomers derived from the racemic mixtures of A5 and A6 showed that 19 (3S,4S) is the most active, with an IC₅₀ of 3 nM against Akt1, and it also displayed cellular activity in OVCAR‐8 with an IC₅₀ of 40 nM and in HCT116 with an IC₅₀ of 210 nM in cell lines. Additional isosteric modifications of the furan ring, involving several heteroaromatic groups, caused a notable decrease in both biochemical and cellular activities. To enhance potency, various aliphatic alkyl chains were added at the 6‐position of the piperidine scaffold (see Figure 11). This modification produced derivatives that showed notable activity, including 20 and 21, which had IC₅₀ values of 0.98 and 1.37 nM against Akt1, respectively. They also showed considerable activity across different cell lines, with IC₅₀ values of 60 and 50 nM in Lncap cells; 460 and 800 nM in OVCAR‐8 cells; and 360 and 1100 nM in HCT116 cells.

Jinxin Che et al. [78] reported a new series of 3,4‐disubstituted piperidine analogues has been reported, showing promising activity in inhibiting Akt. 21 was identified as a beneficial starting point for further structural tweaks. In their exciting study, the researchers explored substituting the 4‐chloro group on the methyl‐1H‐pyrazole moiety with different chlorine atoms, and they also attempted to replace the core furan ring with various phenyl groups. 22 and 23 demonstrated significant activity against Akt1, with IC₅₀ values of just 21.6 and 2.7 nM, respectively. 23 also demonstrated cellular efficacy against the cancer cell lines HGC‐27 (IC₅₀ = 180 nM), 786‐O (IC₅₀ = 950 nM), and KHOS (IC₅₀ = 7430 nM). Further optimization included the substitution of single‐halogen groups in the phenyl ring to enhance potency. Among them, 24 and 25 exhibited notable inhibition of Akt1, with IC₅₀ values of 1.8 and 4 nM, respectively, and effectively inhibited Akt2, with IC₅₀ values of 7.1 and 97.5 nM, respectively. Inhibition of hERG was demonstrated at the concentration of 3 µM shows 26.4% and 15.9% against 24 and 25, respectively (see Figure 12). Cellular assays demonstrated that the proliferation of HGC‐27, 786‐O, and KHOS was inhibited by 24, with IC₅₀ values of 150, 1010, and 7270 nM, respectively. Conversely, 25 exhibited IC₅₀ values of 680, 2500, and 14860 nM in the same cell lines, respectively. Table 3 provides a comparative analysis of the reported thiophene‐ and furan‐carboxamide compounds, emphasizing their structural features, biological activities and development phase.

FIGURE 12.

Lead compound derived from 3,4‐disubstituted piperidine analogues.

TABLE 3.

Comparative SAR of thiophene‐ and furan‐carboxamide‐based Akt inhibitors.

Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key SAR findings	Development stages
7	2‐Pyrimidyl‐5‐amidothiophene	6	23	3	NR	4‐Fluorophenyl group essential; balanced pan‐Akt inhibition.	Lead compound
8	Pyrazole–furan carboxamide	61	NR	NR	NR	Furan scaffold effectively occupied ATP‐binding pocket.	Lead compound
9	Pyrazole–thiophene carboxamide	71	NR	NR	NR	Thiophene scaffold showed activity comparable to furan.	Lead compound
15	Pyrrolo‐pyridinyl thiophene carboxamide	0.03	8	2	pGSK3β: 90 nM LNCaP: 18 nM	Meta‐fluoro substitution afforded highest potency.	Preclinical
16	Pyrrolo‐pyridinyl thiophene carboxamide	0.06	6	2	pGSK3β: 180 nM LNCaP: 66 nM	Para‐fluoro analogue retained excellent pan‐Akt activity.	Preclinical
GSK2110183 (Afuresertib)	Thiophene‐containing ATP‐competitive inhibitor	0.08	2.0	2.6	pGSK3β: 76 ± 16 nM	Highly potent pan‐Akt inhibitor.	Clinical
GSK2141795 (Uprosertib)	Thiophene‐containing ATP‐competitive inhibitor	0.066	1.4	1.5	pGSK3β: 34 ± 5 nM	Improved cellular potency over GSK2110183.	Clinical
17	3,4,6‐Trisubstituted piperidine	7.2	NR	NR	NR	Initial lead scaffold.	Lead compound
18	3,4,6‐Trisubstituted piperidine	6.4	NR	NR	NR	Similar activity to compound 17.	Lead compound
19	3,4,6‐Trisubstituted piperidine	3.0	NR	NR	OVCAR‐8: 40 nM HCT116: 210 nM	(3S,4S)‐Stereochemistry markedly enhanced potency.	Optimized lead
20	3,4,6‐Trisubstituted piperidine	0.98	NR	NR	LNCaP: 60 nM OVCAR‐8: 460 nM HCT116: 360 nM	C‐6 alkyl substitution greatly improved potency.	Optimized lead
21	3,4,6‐Trisubstituted piperidine	1.37	NR	NR	LNCaP: 50 nM OVCAR‐8: 800 nM HCT116: 1100 nM	Retained excellent biochemical activity.	Optimized lead
22	3,4‐Disubstituted piperidine	21.6	NR	NR	HGC‐27: 180 nM	Replacement of furan reduced potency.	Lead compound
23	3,4‐Disubstituted piperidine	2.7	NR	NR	HGC‐27: 180 nM 786‐O: 950 nM KHOS: 7430 nM	Optimized pyrazole substitution markedly improved activity.	Optimized lead
24	3,4‐Disubstituted piperidine	1.8	7.1	NR	HGC‐27: 150 nM 786‐O: 1010 nM KHOS: 7270 nM	Halogen substitution enhanced potency and selectivity.	Optimized lead
25	3,4‐Disubstituted piperidine	4.0	97.5	NR	HGC‐27: 680 nM 786‐O: 2500 nM KHOS: 14860 nM	Less potent than compound 24.	Optimized lead

4.2.1. Structure–Activity Relationship of the Thiophene Core Ring

In this series, we explore the SAR of the thiophene core ring, modified to include furan and phenyl rings, as well as essential modifications (Figure 13).

FIGURE 13.

Structure–activity relationship for the furan core ring.

4.2.1.1. A Ring

Introducing 1H‐pyrrolo[2,3‐d]pyridine and 4‐chloro‐1H‐pyrazole at the A Ring resulted in improved inhibitory activity, suggesting that these heterocyclic systems promote beneficial hydrogen‐bonding within the hinge‐binding region.

4.2.1.2. B Ring

The replacement of the thiophene ring with phenyl or furan moieties led to a marked improvement in both enzymatic and cellular activity. Furthermore, halogen substitution on the core scaffold further enhanced potency.

4.2.1.3. Side Chain

Replacing the amide side chain with methylamine (S) exhibits significant activity compared to analogues with 4,4‐disubstituted piperidine groups. The amide linkage was essential for maintaining strong biological activity.

4.2.1.4. C Ring

Replacing the phenyl group with halogen atoms resulted in an even greater increase in potency. This suggests that these modifications enhance hydrophobic interactions within the lipophilic pocket of the Akt enzyme, thereby increasing the overall effectiveness.

4.3. Thiadiazole and Thiazole Amine Core Ring

Qingping Zeng et al. [79] reported a novel series of 2‐aminothiadiazole derivatives as Akt inhibitors, in which the pyridine B‐ring was replaced with a thiadiazole scaffold, the C‐ring incorporated a 3‐methyl‐1H‐indazole, and the A‐ring, together with the side chain, contained an (S)‐3‐phenylpropane‐1,2‐diamine moiety of 26. This design evolved into a highly effective Akt1 inhibitor, with an IC₅₀ of 76 (±19) nM, and exhibited activity against PKA with an IC₅₀ of 54 (±12) nM (Figure 14). The researchers further modified the A‐ring by adding new variations. The m‐trifluoromethyl and p‐trifluoromethyl analogues (27 and 28) demonstrated high potency, inhibiting Akt1 with IC₅₀ values of 6.1 ± 0.1 and 5.5 ± 0.1 nM, respectively, and PKA with IC₅₀ values of 6.5 ± 2 and 13 ± 4 nM (Figure 14). Further structural investigation revealed that replacing the methyl group on the C‐ring with additional alkyl groups reduced potency while maintaining the unsubstituted methyl group's activity. In this series, 29, which has a p‐trifluoromethylphenyl group in the A‐ring, was a potent inhibitor of Akt1 (IC₅₀ = 6.0 ± 1 nM) and was more selective than PKA (IC₅₀ = 108 ± 15 nM). Additionally, replacing the C‐ring indazole with an isoquinoline moiety results in highly potent compounds, as revealed by 30 with Akt1 IC₅₀ = 3.0 ± 0.1 nM and PKA IC₅₀ = 8 (± 17) nM [80] (Figure 14).

FIGURE 14.

Lead compound from the 2‐aminothiadiazole series.

Subsequently, various five‐membered heterocycles were integrated into the core B‐ring using a method that enabled regioselective incorporation in specific regions. Among these, 31 with a thiazol‐2‐amine substitution showed strong inhibition of Akt1 (IC₅₀ = 4.9 ± 3.6 nM) and had a similar effect on PKA (IC₅₀ = 5.9 ± 4.2 nM) (Figure 15). Furthermore, substituting the C‐ring isoquinoline with various aromatic scaffolds resulted in the identification of compound 32. This compound shows strong Akt1 inhibition (IC₅₀ = 8.0 ± 2.6 nM) and improved selectivity over PKA (IC₅₀ = 326 ± 118 nM) (Figure 15). Replacing five‐membered heterocycles, including thiazol‐2‐amine, in the B‐ring led to reduced selectivity compared to the aminothiadiazole scaffold, which equally inhibited Akt1 and PKA. Benzo[d]oxazol‐2(3H)‐one (32), which took the place of the C‐ring isoquinoline, retained its Akt1 potency but was less potent overall than the thiadiazole series [81].

FIGURE 15.

The lead compound belongs to the thiazole series.

Shaohua Chang et al. [82] reported a new series of 2‐substituted thiazole carboxamides, demonstrating strong pan‐Akt inhibition that targets all three isoforms of the Akt kinase. The scaffold was improved by incorporating N‐methylpyrimidin‐2‐amine, 1H‐pyrazole, and 1H‐pyrrolo[2,3‐b]pyridine as hinge‐binding groups. To modify the activity and selectivity, structurally different substituted phenyl rings were incorporated into the side chain region. Among the synthesized derivatives 33, which has an N‐methylpyrimidin‐2‐amine at the hinge‐binding region and an ortho‐substituted phenyl ring, was very effective at inhibiting Akt1 (IC₅₀ = 33 nM), moderately effective at inhibiting Akt3 (IC₅₀ = 83 nM), and less effective at inhibiting Akt2 (IC₅₀ = 1950 nM) (Figure 15). It was additionally executed effectively in LnCaP cells from prostate cancer (IC₅₀ = 260 nM). 34 is substituted with a 1H‐pyrrolo[2,3‐b] pyridine to the hinge‐binding region and an ortho, para‐dichloro‐substituted phenyl ring. It possessed broader pan‐Akt activity, with IC₅₀ values of 25 nM (Akt1), 200 nM (Akt2), and 24 nM (Akt3). It also inhibited cells significantly in LnCaP cells (IC₅₀ = 230 nM) (Figure 15). These results demonstrate that thiazole carboxamides can be used to selectively or significantly inhibit Akt by making targeted modifications to the hinge‐binding and side‐chain regions. Table 4 presents a comparative analysis of the reported thiadiazole‐ and thiazole‐amine compounds, highlighting their structural features, biological activities, and stages of development.

TABLE 4.

Comparative SAR of thiadiazole and thiazole amine‐based Akt inhibitors.

Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key SAR findings	Development stages
26	2‐Aminothiadiazole	76 ± 19	NR	NR	NR	Initial lead; replacement of pyridine with thiadiazole afforded moderate Akt1 inhibition.	Lead compound
27	2‐Aminothiadiazole	6.1 ± 0.1	NR	NR	NR	m‐CF3 phenyl substitution markedly enhanced potency.	Optimized lead
28	2‐Aminothiadiazole	5.5 ± 0.1	NR	NR	NR	p‐CF3 phenyl analogue maintained high potency.	Optimized lead
29	2‐Aminothiadiazole	6.0 ± 1.0	NR	NR	NR	p‐CF3 substitution improved selectivity over PKA while maintaining Akt1 potency.	Optimized lead
30	2‐Aminothiadiazole	3.0 ± 0.1	NR	NR	NR	Replacement of indazole with isoquinoline further enhanced Akt1 inhibition.	Optimized lead
31	Thiazol‐2‐amine	4.9 ± 3.6	NR	NR	NR	Thiazole retained potency but showed poor selectivity versus PKA.	Lead compound
32	Thiazole derivative	8.0 ± 2.6	NR	NR	NR	Replacement of isoquinoline with benzo[d]oxazol‐2(3H)‐one improved selectivity but slightly reduced potency.	Optimized lead
33	2‐Substituted thiazole carboxamide	33	1950	83	LNCaP: 260 nM	N‐Methylpyrimidin‐2‐amine hinge binder produced strong Akt1 selectivity over Akt2.	Lead compound
34	2‐Substituted thiazole carboxamide	25	200	24	LNCaP: 230 nM	Pyrrolo[2,3‐b]pyridine hinge binder and dichlorophenyl substitution generated balanced pan‐Akt inhibition.	Optimized lead

4.3.1. Structure–Activity Relationship of the Thiadiazole Core Ring

In this series, the exploration of SAR was divided as follows (Figure 16):

FIGURE 16.

Structure–activity relationship of the thiadiazole core ring.

4.3.1.1. A Ring (Hydrogen Bonding)

The isoquinoline ring enhanced activity against Akt1/PKB and PKA and was replaced with benzo[d]oxazol‐2(3H)‐one, which enhanced the selectivity of Akt1 over PKA.

4.3.1.2. Side Chain

(S)‐propane‐1,2‐diamine was essential for the activity.

4.3.1.3. B Ring (Core Ring)

Replacing the thiadiazole ring with a thiazole will enhance PKA's activity.

4.3.1.4. C Ring (Hydrophobic Interaction)

The trifluoromethyl group substitution on the para position of the phenyl ring enhanced activity.

4.4. Pyridine Core

4.4.1. 3,5‐Disubstituted Pyridine Core Ring

Qun Li et al. [83] reported a new series of trans‐3,4′‐bispyridinylethylenes that serve as ATP‐competitive inhibitors of protein kinase B (PKB/Akt) through a structure‐based approach. Adding various aliphatic and aromatic groups to this scaffold made it more potent and better suited to interact with Akt's ATP‐binding pocket.

In this series 35, substituted with indazole, emerged as the lead candidate. It was found to inhibit Akt1 (IC₅₀ = 14 nM) selectively and have a reduced effect on Akt2 (IC₅₀ = 257 nM) and Akt3 (IC₅₀ = 354 nM) when investigated at an ATP concentration of 10 µM. This data indicated that it might act as a suitable isoform‐selective Akt inhibitor (Figure 17).

FIGURE 17.

Lead compounds from the 3,5‐disubstituted pyridines series.

Further refinements were made to the structure by focusing on the pyridine part of the hinge‐binding area, replacing it with an isoquinoline ring, and removing a rotating bond between the two pyridinyl groups. These adjustments resulted in 36, which demonstrated significantly improved activity against various forms of the target protein. Its IC₅₀ values were 1.3 nM for Akt1, 6.8 nM for Akt2, and 35 nM for Akt3 (Figure 17). These results illustrate the importance of hinge‐region interactions in achieving both isoform selectivity and potency [84].

Gui‐Dong Zhu et al. [85] reported a novel series of potent and selective oxindole–pyridine‐based Akt inhibitors. Building on the disubstituted pyridine scaffold of 36, they investigated alternative heteroaromatic replacements for the metabolically labile isoquinoline moiety. They successfully identified a new class of oxindole–pyridine derivatives with improved pharmacological properties. The 3,3‐difluoroindolin‐2‐one‐substituted compound is one of these. 38 showed significant Akt1 inhibition (IC₅₀ = 1.5 nM) and moderate cytotoxicity in the MTT assay (F5.12‐Akt, IC₅₀ = 11.8 µM). The indolin‐2‐one‐substituted 37 also showed potent Akt1 inhibition (IC₅₀ = 3.2 nM) and enhanced cytotoxicity (MTT IC₅₀ = 940 nM) (Figure 17). It is essential to emphasize that 38 was approximately twice as effective against Akt1 as 37, but it was significantly less toxic. To further enhance activity and reduce toxicities, difluoro substitutions were systematically replaced with various heterocycles. Research led to the discovery of the furan‐substituted 39, which demonstrated remarkable potency against Akt1 (IC₅₀ = 0.17 nM) and reduced cytotoxicity (MTT = 400 nM) (Figure 17).

Keith W. Woods et al. [86, 87] investigated the function of the isoquinoline nitrogen and the significance of its regiochemical configuration in conferring Akt inhibitory activity. The research effort focuses on investigating feasible heteroaromatic alternatives for the isoquinoline scaffold, addressing its metabolic limitations, while retaining or enhancing potency and improving pharmacokinetic properties. Employing this scaffold‐hopping technique, the incorporation of an indazole–pyridine ring system (40) established an extremely potent analogue, with a K_i value of 0.16 nM against Akt1 at an ATP concentration of 10 µM (Figure 17). The findings indicated that careful modification of heteroaromatic interactions throughout the ATP‐binding pocket can significantly enhance both potency and selectivity while also addressing metabolic stability issues.

Sheela A. Thomas et al. [88] developed a novel series of 3,5‐disubstituted pyridines as Akt/PKB inhibitors. Studies on the structure–activity relationship (SAR) indicated that modification of the A‐ring was essential for potency. Using the Qun Li scaffold (35) as a guide, the incorporation of a naphthalene moiety led to compound 41, which strongly inhibited Akt1 with an IC₅₀ of 3.5 nM (Figure 17). Further modification based on the Keith W. Woods scaffold (42) led to the discovery of a 2‐methoxynaphthalene analogue with increased potency against Akt1, with an IC₅₀ of 1.1 nM, indicating that electron‐donating substitution can be a practical method for enhancing potency. Modifications to the side‐chain region underscored the crucial role of the ether linkage: replacing it with an amine (43) or sulfur (44) markedly lowered activity (IC₅₀ = 8.3 and 244 nM, respectively) (Figure 17). These results collectively highlight that adding a naphthalene group at A Ring enhances potency, and keeping the ether linkage is vital for maintaining the improved inhibitory effect against Akt1.

Gui‐Dong Zhu et al. [89] reported an indazole–pyridine series of Akt inhibitors designed to enhance potency while mitigating adverse hemodynamic effects, such as hypotension. In this series, substitution of the indazole ring with phenyl groups indicated an established SAR: ortho‐substituted analogues had less potency. At the same time, meta‐substituted derivatives retained their vigorous activity. Among these, 45 contains a meta‐iodo substituent, exhibited an inhibitory potency identical to that of the previously reported analogue from Keith W. Woods [86, 87]. To further the modification involved, an additional nitrogen atom was introduced to the methyl‐indazole ring, along with a meta‐trifluoromethyl (CF₃) group substituted to the phenyl ring. The modification led to 46, which demonstrated a very strong ability to inhibit Akt1, with an impressive IC₅₀ of just 0.6 nM (Figure 17). This makes it 2.5 times more potent than the Keith W. Woods analogue. These promising results highlight how careful modifications, such as heteroaryl changes and halogen substitutions, can greatly enhance the efficacy and pharmacological properties of Akt inhibitors.

4.4.2. 2,3,5‐Trisubstituted Pyridine Core

Hong Lin et al. [90] described a series of 2,3,5‐trisubstituted pyridines as a unique chemotype with strong Akt inhibition with a marked selectivity over ROCK1. The importance of 2‐position substitution at the pyridine core in determining selectivity was demonstrated by comparative profiling of Akt and ROCK1. Among the analogues investigated, the 2‐chloro‐5‐hydroxyphenyl derivative (47) demonstrated strong inhibition of Akt1 (IC₅₀ = 3 nM) while retaining minimal ROCK1 activity (IC₅₀ = 7600 nM) (Figure 18), thereby establishing a favorable selectivity index. Further modifications included replacing the S‐phenylalaninol side chain with S‐tryptophanol and a substituted furan group at the 3‐position of the pyridine ring. These structural modifications resulted in the development of the most important analogues, especially 48 and 49, each of which inhibits Akt1 at sub‐nanomolar concentrations with an IC₅₀ of 0.8 nM (Figure 18). Additionally, 49 inhibited pGSK3β (IC₅₀ = 500 nM) and showed antiproliferative activity in LNCaP (IC₅₀ = 190 nM) and BT474 (IC₅₀ = 340 nM) cells. The compound exhibited an IC₅₀ of 12 nM against Akt and an IC₅₀ of 1 nM against Akt3. These findings collectively show that strategic changes at the 2‐ and 3‐positions of the trisubstituted pyridine scaffold can produce inhibitors with increased potency and selectivity. This offers valuable insights for developing next‐generation Akt inhibitors that have less cross‐reactivity with ROCK1.

FIGURE 18.

Lead compound from the tri‐substituted and tetra‐substituted pyridine series.

In a separate study, the azaindazole scaffolds were examined as an alternative to indazole‐based Akt inhibitors. The aim was to achieve better cellular potency, kinase selectivity, and drug‐like properties. If you replaced the indazole ring with an azaindazole core, the analogues had better drug profiles. Among them, 50 stood out as a potential lead because it strongly inhibited all three types of Akt enzymes with IC₅₀ of 2 nM for Akt1, 39 nM for Akt2, and 10 nM for Akt3. It was also highly effective in terms of cellular efficacy, inhibiting pGSK3β (IC₅₀ = 160 nM) and producing antiproliferative effects against LNCaP (IC₅₀ = 96 nM) and BT474 (IC₅₀ = 370 nM) cancer cell lines (Figure 18). These results demonstrate that modification of the azaindazole‐based scaffold is a practical approach to enhance both potency and selectivity [91].

4.4.3. 2,3,5,6‐Tetra‐Substituted Pyridine Core

Further optimization of the trisubstituted pyridine scaffold aimed to enhance its effectiveness and achieve a more drug‐like profile by resolving barriers related to hERG activity and inhibiting CYP450. By incorporating an amino group into the pyridine core, the inhibition of both CYP450 and hERG was significantly reduced compared to the parent series. However, the inhibition of Akt remained significant. Substitution at the 6‐position of the pyridine ring was performed very effectively. In addition, the amino‐substituted 51 showed significant activity against all three isoforms of Akt with IC₅₀ values of 1 nM (Akt1), 19 nM (Akt2), and 2 nM (Akt3) (Figure 18). It is essential to consider that this compound also demonstrated activity in BT474 cells (IC₅₀ = 310 nM) and a much lower tendency to inhibit CYP3A4 (IC₅₀ = 5000 nM) [92]. Table 5 describes a comparative overview of the reported pyridinyl analogues, focusing on their structural modifications, biological activities, and respective stages of development.

TABLE 5.

Comparative SAR of pyridine‐based Akt inhibitors.

Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key SAR findings	Development stages
35	trans‐3,4′‐Bispyridinylethylene	14	257	354	NR	Indazole substitution produced selective Akt1 inhibition.	Lead compound
36	Isoquinoline–pyridine	1.3	6.8	35	NR	Isoquinoline replacement and rigidification significantly improved potency.	Optimized lead
37	Oxindole–pyridine	3.2	NR	NR	MTT (F5.12‐Akt): 940 nM	Indolin‐2‐one improved cellular activity.	Optimized lead
38	Difluoro‐oxindole–pyridine	1.5	NR	NR	MTT (F5.12‐Akt): 11800 nM	Difluoro substitution improved enzyme potency but reduced cellular potency.	Optimized lead
39	Furan‐substituted oxindole–pyridine	0.17	NR	NR	MTT (F5.12‐Akt): 400 nM	Furan substitution produced sub‐nanomolar Akt1 inhibition and improved cellular activity.	Optimized lead
40	Indazole–pyridine	0.16 (Ki)	NR	NR	NR	Scaffold hopping enhanced potency and metabolic stability.	Optimized lead
41	3,5‐Disubstituted pyridine	3.5	NR	NR	NR	Naphthalene substitution improved Akt1 inhibition.	Lead compound
42	3,5‐Disubstituted pyridine	1.1	NR	NR	NR	2‐Methoxynaphthalene further enhanced potency.	Optimized lead
43	3,5‐Disubstituted pyridine	8.3	NR	NR	NR	Ether‐to‐amine replacement reduced activity.	SAR analogue
44	3,5‐Disubstituted pyridine	244	NR	NR	NR	Ether‐to‐sulfur replacement markedly reduced potency.	SAR analogue
45	Indazole–pyridine	0.9	NR	NR	NR	Meta‐iodo substitution retained high potency.	Optimized lead
46	Indazole–pyridine	0.6	NR	NR	NR	Additional nitrogen and meta‐CF3 substitution substantially enhanced potency.	Optimized lead
47	2,3,5‐Trisubstituted pyridine	3	NR	NR	ROCK1 IC50: 7600 nM	Excellent Akt1/ROCK1 selectivity.	Lead compound
48	2,3,5‐Trisubstituted pyridine	0.8	NR	NR	NR	Modified side chain produced sub‐nanomolar Akt1 inhibition.	Optimized lead
49	2,3,5‐Trisubstituted pyridine	0.8	12	1	pGSK3β: 500 nM LNCaP: 190 nM BT474: 340 nM	Furan substitution improved pan‐Akt potency and cellular efficacy while maintaining ROCK1 selectivity.	Optimized lead
50	Azaindazole–pyridine	2	39	10	pGSK3β: 160 nM LNCaP: 96 nM BT474: 370 nM	Azaindazole improved cellular potency and drug‐like properties.	Optimized lead
51	2,3,5,6‐Tetrasubstituted pyridine	1	19	2	BT474: 310 nM CYP3A4 IC50: 5000 nM	Amino substitution reduced CYP450 and hERG liabilities while maintaining potent pan‐Akt inhibition.	Optimized lead

4.4.4. The Structure–Activity Relationship of the Pyridine Core Ring Is Examined

In this series, we explored the pyridine core ring with various substitutions for better activity for Akt inhibition, a SAR exploration of the pyridine core as follows (Figure 19).

FIGURE 19.

Structure–activity relationship of the pyridine core ring.

4.4.4.1. A Ring

Replacement of the pyridine ring with 3‐methyl‐1H‐indazole and 3‐(furan‐2‐yl) indolin‐2‐one functional groups significantly boosted the activity.

4.4.4.2. Side Chain

The Ether side chain was required for the activity. The replacement of the oxygen atom in the ether group with sulfur or an amine results in a loss of activity.

4.4.4.3. B Ring

The pyridine ring was shown to be an essential part of the activity. Substitution of the 2‐position in the pyridine ring with furan or methyl‐furan increased the inhibitory activity, and substitution of the 6‐position in the pyridine ring with an amino group resulted in a loss of activity.

4.4.4.4. Linker

The removal of the ethylene linker between the core and the substitution groups resulted in a significant increase in activity.

4.4.4.5. C Ring

Replacement of the phenyl ring with an indazole ring leads to a notable increase in the biological activity.

4.5. Imidazole‐Fused Pyridine

The research by Mark J. Bamford et al. [93, 94, 95] has reported the discovery and development of a new series of 1‐ethyl‐1H‐imidazo[4,5‐c]pyridine analogues, which are potential Akt kinase inhibitors. In this study, compounds containing oxadiazole identified as 52 and 53 from the GlaxoSmithKline (GSK) collection were used as starting points for lead optimization. These compounds were previously described as ATP‐competitive inhibitors that bind within the ATP‐binding pocket of kinases such as ROCK1 and MSK1.

Further biochemical tests showed that both compounds had encouraging inhibitory activity against the Akt isoforms. 52 notably inhibited Akt1 with an IC₅₀ of 79 nM, showing less effectiveness against Akt2 (IC₅₀ = 1000 nM) and Akt3 (IC₅₀ = 398 nM). Conversely, 53 inhibits Akt1 with an IC₅₀ of 126 nM but is considerably less potent against Akt2 (IC₅₀ = 3980 nM) and Akt3 (IC₅₀ = 1410 nM) (Figure 20). These findings suggest that imidazo[4,5‐c]pyridine scaffolds could be useful for selective Akt inhibitors, with 52 standing out as the most promising option due to its increased activity against Akt1. The study highlights the potential of repurposing existing kinase inhibitors as key components in developing therapies targeting the Akt signalling pathway.

FIGURE 20.

Lead compounds from the 1‐ethyl‐1H‐imidazo[4,5‐c]pyridine series.

Dirk A. Heerding et al. [96] extended the work further to systematically carry out side‐chain modification on the C7 position of the scaffold to optimized Akt activity. Diverse piperidine‐replaced side chains were incorporated with 54, displaying significant inhibitory efficiency over the Akt isoforms, with IC₅₀ values of 56, 204, and 380 nM, respectively, on Akt1, Akt2, and Akt3. To enhance selectivity, the group investigated the back‐pocket binding region of Akt kinases. The C7 side chain in these studies was fixed as an aminopropyl ether, and several substituents were introduced to the C4 position of the imidazopyridine core. Various five‐membered aryl and heteroaryl groups were incorporated; however, these analogues exhibited only moderate activity against Akt. Structural considerations indicated that the five‐membered heteroaryl rings did not penetrate the back pocket of the kinase to a sufficient level to act on ROCK1 selectively.

In contrast, incorporating an alkyne spacer would be a more effective approach in terms of traversing the pocket opening to allow for the addition of a substituent at the triple bond terminus, thereby creating further interactions. 55 and 56, which were the synthesized derivatives, had sig nificantly better potency. 55 was an Akt1 inhibitor with an IC₅₀ of 6 nM, whereas 56 was more potent with an IC₅₀ of 2 nM (Figure 35). Both compounds demonstrated better selectivity against ROCK1, with IC₅₀ values of 501 nM (55) and 20 nM (56). In addition, cellular activity when tested against GSK3β resulted in IC₅₀ values of 1050 nM (55) and 1130 nM (56). The terminal dimethyl groups played a significant role in selectivity, as demonstrated by the variations in the activities of 55 and 56.

FIGURE 35.

Illustrates the SAR of M2698.

Subsequent structural modifications were aimed at enhancing cellular potency by incorporating nitrogen‐containing alkyl and aliphatic side chains. Based on this series, GSK690693, which incorporated an (S)‐3‐methylpiperidine, emerged as a highly potent candidate. GSK690693 showed IC₅₀ values of 2, 13, and 2 nM for Akt1, Akt2, and Akt3, respectively. It showed inhibitory effects on GSK3 (IC₅₀ = 140 nM) and on BT474 breast cancer cells (IC₅₀ = 50 nM), LNCaP prostate cancer cells (IC₅₀ = 20 nM), and HFF fibroblasts (IC₅₀ = 16300 nM) (Figure 35). The latter demonstrated a favorable therapeutic window by sparing normal fibroblasts relative to cancer cell lines. Collectively, these data demonstrate the importance of side‐chain optimization of the imidazo[4,5‐c]pyridine backbone at the C7 and C4 positions in optimizing both potency and kinase selectivity. The identification of GSK690693 marked a breAkthrough in the development of pan‐Akt inhibitors that could be used clinically in cancer treatment.

Meagan B. Rouse et al. [97] reported the design of a novel imidazopyridine amino‐furazan scaffold as a new analogue for potent Akt inhibition. Their primary goal was to enhance the pharmacodynamic and pharmacokinetic properties of the clinical candidate GSK690693 by modifying the C‐6 side chain. By substituting different alkyl groups at this position, they enhanced the effectiveness of the analogues, making them more selective for isoforms and more active in cellular assays. Among these, 57 (R‐isomer) demonstrated significant activity against Akt isoforms, with IC₅₀ values of 1 (0.4) nM for Akt1, 25 (5) nM for Akt2, and 1 nM for Akt3. Additionally, its cellular activity showed significant inhibition, with pGSK3β IC₅₀ = 40 nM (3.5 times more effective than GSK690693), BT474 IC₅₀ = 35 nM (twofold better), and LNCaP IC₅₀ = 27 nM. The stereochemistry at this position had a significant impact on activity. The S‐isomer (58) showed similar effectiveness against Akt1 (IC₅₀ = 2 (0.2) nM) and slightly better inhibition against Akt2 (IC₅₀ = 16 nM). It also proved effective against pGSK3β (IC₅₀ = 41 nM, 3.5 times more potent than GSK690693). The S‐isomer had a 70‐fold greater impact on BT474 (IC₅₀ = 1 nM) and a fourfold more substantial effect on LNCaP (IC₅₀ = 5 nM) (Figure 20) than the reference molecule. These findings highlight the significant influence of C‐6 side chain stereochemistry on isoform selection and cellular efficacy, with the imidazopyridine amino‐furazan series demonstrating substantial improvements over the previous standard, GSK690693.

Mark A. Ashwell et al. [98] described a systematic design and optimization of a new series of 3H‐imidazo[4,5‐b]pyridine derivatives that act as effective inhibitors of Akt kinases. The initial scaffold exploration was the incorporation of a pyridin‐2‐amine moiety at the 3‐position and different phenyl analogues at the 2‐position. 59 having a phenyl‐methenamine functional group, had moderate Akt1 inhibitory activity with an IC₅₀ of 930 nM.

To improve the compound potency, the free aniline group was then derivatized with an amide group to generate 60, which exhibited a significant improvement in Akt1 inhibition with an IC₅₀ of 14 nM (Figure 21). Based on these results, further structural optimization was undertaken at the 6‐position of the imidazopyridine core. Adding heteroaryl groups greatly improved kinase inhibition. Among these 61, contain a pyridine substituent, was highly effective against Akt1 (IC₅₀ = 8 nM) and Akt2 (IC₅₀ = 30 nM), while remaining selective for Akt3 (IC₅₀ = 660 nM). It was further optimised with modifications at the 7‐position and by adding various aromatic groups, leading to analogues with enhanced pan‐Akt activity. Notably, 62 proved to be the most promising, showing strong potency against Akt1 (IC₅₀ = 5 nM), Akt2 (IC₅₀ = 18 nM), and Akt3 (IC₅₀ = 170 nM) (Figure 21). Overall, these studies highlight the SAR of the 3H‐imidazo[4,5‐b]pyridine scaffold, demonstrating that rational changes at positions 2, 6, and 7 significantly improve potency and isoform selectivity. The work offers a useful framework for the development of next‐generation Akt inhibitors with therapeutic potential. Table 6 summarizes the reported imidazole‐fused pyridine analogues with respect to their structural modifications, biological activities, and progress across different stages of development.

FIGURE 21.

Lead compound from the 3H‐imidazo[4,5‐b]pyridine series.

TABLE 6.

Comparative SAR of imidazole‐fused pyridine‐based Akt inhibitors.

Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key SAR findings	Development stages
52	1‐Ethyl‐1H‐imidazo[4,5‐c]pyridine	79	1000	398	NR	Initial lead; selective Akt1 inhibition over Akt2/Akt3	Lead compound
53	1‐Ethyl‐1H‐imidazo[4,5‐c]pyridine	126	3980	1410	NR	Less potent than compound 76; reduced pan‐Akt activity.	Lead compound
54	C7‐substituted imidazo[4,5‐c]pyridine	56	204	380	NR	C7 piperidine side chain improved Akt inhibition.	Optimized lead
55	Alkyne‐substituted imidazo[4,5‐c]pyridine	6	NR	NR	pGSK3β: 1050 nM	Alkyne spacer improved potency and ROCK1 selectivity.	Optimized lead
56	Alkyne‐substituted imidazo[4,5‐c]pyridine	2	NR	NR	pGSK3β: 1130 nM	Terminal dimethyl substitution further enhanced Akt1 potency.	Optimized lead
GSK690693	Imidazo[4,5‐c]pyridine	2	13	2	pGSK3β: 140 nM BT474: 50 nM LNCaP: 20 nM HFF: 16300 nM	(S)‐3‐Methylpiperidine side chain produced potent pan‐Akt inhibition and a favorable therapeutic window.	Clinical candidate
57 (R)	Imidazopyridine amino‐furazan	1	25	1	pGSK3β: 40 nM BT474: 35 nM LNCaP: 27 nM	C6 R‐stereochemistry improved biochemical and cellular activity compared with GSK690693.	Optimized lead
58 (S)	Imidazopyridine amino‐furazan	2	16	NR	pGSK3β: 41 nM BT474: 1 nM LNCaP: 5 nM	S‐isomer showed superior cellular potency, demonstrating stereochemical importance.	Optimized lead
59	3H‐Imidazo[4,5‐b]pyridine	930	NR	NR	NR	Initial scaffold exhibited weak Akt1 inhibition.	Initial lead
60	3H‐Imidazo[4,5‐b]pyridine	14	NR	NR	NR	Amide derivatization markedly improved Akt1 potency.	Optimized lead
61	3H‐Imidazo[4,5‐b]pyridine	8	30	660	NR	Pyridine substitution improved Akt1/Akt2 potency while retaining Akt3 selectivity.	Optimized lead
62	3H‐Imidazo[4,5‐b]pyridine	5	18	170	NR	Further optimization at positions 6 and 7 afforded improved pan‐Akt inhibition.	Optimized lead

4.5.1. Structure–Activity Relationship of Imidazo[4,5‐c]pyridine Ring

In this series, we explored the discovery of GSK690693 and subsequent modifications to the Imidazo[4,5‐c]pyridine core ring (Figure 21).

4.5.1.1. A Ring (Key Functional Group)

The 1,2,5‐oxadiazol‐3‐amine group is essential for biological activity and forming hydrogen bonds with Akt's ATP‐binding site. This ring stabilizes the inhibitor in the active site, enabling effective kinase inhibition. It is a crucial position because any modifications or replacements result in a significant decrease in activity.

4.5.1.2. B Ring (Core Scaffold)

The core structure of 1H‐imidazo[4,5‐c]pyridine is more active as an inhibitor than 3H‐imidazo[4,5‐b]pyridine. The nitrogen arrangement in this fused heterocycle enhances binding affinity by optimizing molecular orientation in the active site. The fourth position substituted with 2‐methylbut‐3‐yn‐2‐ol results in a significant increase in inhibitory activity. Furthermore, the ethyl group is required to maintain activity, as it increases lipophilicity while facilitating the proper fit of the hydrophobic pocket.

4.5.1.3. Linker Region

The ether linker is effective at binding the core scaffold to the terminal moieties while maintaining conformational flexibility. It also facilitates beneficial inhibitory activity (Figure 22).

FIGURE 22.

Structure–activity relationship for GSK690693.

4.5.1.4. C Ring (Terminal Substituent)

The 4(S)‐methylpyridine ring enhances the potency of inhibition by increasing the interaction at Akt's allosteric site. Elongation at this site, especially by replacing with phenyl or alkylamine chains such as (S)‐ethylamine, enhances activity while reducing toxicity.

4.6. Piperidine/Piperazine/Phenyl/Pyrrolidine Core Ring

4.6.1. Substituted Piperidin‐4‐amine Core Ring

Tatiana McHardy et al. [99] described the design of potent and selective protein kinase B (PKB/Akt) inhibitors, whose rationale was to assess their antiproliferative effect and to understand the factors that determine their selectivity against other kinases. The studied chemical series included 7H‐pyrrolo[2,3‐d]pyridin‐4‐yl)piperidin‐4‐amine analogues with various benzyl substituents to assess the SARs.

The majority of analogues were inhibitory to PKB, but selectivity and potency varied with the nature of the benzyl substitution. The 2,4‐dichlorobenzyl‐substituted analogue produces compound A10, while the 2‐naphthyl‐substituted analogue results in compound A16. Both compounds demonstrate inhibitory potency comparable to the reference inhibitor CCT128930, with IC₅₀ values of 8.5 nM for A10 and 7 nM for A16 (Figure 23). These findings highlight the significance of hydrophobic and steric interactions from specific aromatic substitutions in boosting PKB inhibition. Adding an amide group at the fourth position of piperidine yields 63, which exhibits strong activity against PKBβ with an IC₅₀ of 2.2 (±1.2) nM (Figure 24).

FIGURE 23.

Inhibition of PKBβ by substituted (7H‐pyrrolo[2,3‐d]pyrimidin‐4‐yl)piperidin‐4‐amines.

FIGURE 24.

Shows the lead compound from the pyrrolo[2,3‐d]pyrimidine series.

To further examine the influence of heterocyclic core modification on PKB inhibition, the pyrrolo[2,3‐d]pyrimidine scaffold was progressively replaced with other heteroaromatic substitutions, such as 7‐azaindole (64–66), oxopurine (67–69), and pyrazolo[3,4‐b]pyridine (70 and 71). This substitution strategy aimed to find piperidine‐linked analogues with increasing potency and selectivity profiles (Figure 25). Within this series, 64 and 69 showed potent activity against PKB, with IC₅₀ values of 5.5 and 5 nM, respectively (Figure 23), and maintained significant activity compared to the parent scaffold. Surprisingly, when compared to the pyrrolo[2,3‐d]pyridines analogues, the 7‐azaindol derivatives (65 and 66), the 8‐oxopurine analogues (67 and 68), and the pyrazolo [3,4‐b]pyridines analogues (70 and 71) exhibited comparable selectivity; however, this improvement was accompanied by a modest reduction in overall PKB inhibitory efficacy was significantly reduced [99] (Figure 25).

FIGURE 25.

Illustrates the optimization of hinge region binding scaffolds for the PKB inhibitor.

John J. Caldwell et al. [100] reported a new series of piperidine analogues that selectively inhibit PKB over PKA through fragment‐based screening and hinge‐binding region enhancement. They employed various scaffolds, such as 6‐(piperidin‐1‐yl)‐9H‐purine, 4‐(piperidin‐1‐yl)‐1H‐pyrazolo[3,4‐d]pyrimidine, 4‐(piperidin‐1‐yl)‐1H‐pyrrolo[2,3‐b]pyridine, and 4‐(piperidin‐1‐yl)‐7H‐pyrrolo[2,3‐d]pyrimidine. Substitution at the piperidine ring demonstrated that the free amine analogue, 72 exhibited superior activity against PKBβ with an IC₅₀ of 180 ± 6 nM and PKA with an IC₅₀ of 550 ± 120 nM. The free amine substitution on the piperidine ring allowed it to be more potent than the methylamine substitute, while the 4‐(piperidin‐1‐yl)‐1H‐pyrazolo[3,4‐d]pyrimidine scaffold lacks inhibitory activity against PKBβ or PKA. The pyrazolo[3,4‐d]pyrimidine analogue, on the other hand, had a minimal impact on both PKBβ and PKA, suggesting that this scaffold is poorly suitable for these targets. To further enhance its potency, lipophilic substituents were added to interact with the P‐loop. The addition of the 4‐chlorophenyl group at the 4‐aminomethyl and 4‐aminopiperidine positions resulted in 73 and 74. These compounds were very effective at inhibiting PKBβ, with IC₅₀ values of 3 ± 0.9 and 5 ± 0.2 nM, respectively. They also performed effectively against PKA, with IC₅₀ values of 7 ± 1.5 and 9 ± 2 nM, respectively (Figure 26). Although these analogues exhibited higher activity, they lacked selectivity and were less potent in cellular assays. Further research, involving homologation of the lipophilic substituent, resulted in 75. This compound exhibited strong PKBβ inhibition with an IC₅₀ of 6 ± 1.5 nM, while significantly reducing PKA activity, with an IC₅₀ of 168 ± 36 nM, thereby achieving approximately 28‐fold selectivity for PKBβ over PKA.

FIGURE 26.

Lead compound from the piperidine scaffolds.

Matt Addie et al. [101] have reported data on the development of a novel series of pyrrolo‐pyrimidine‐substituted analogues, which are ATP‐competitive inhibitors of Akt. Early studies have been done on α‐alkyl‐substituted benzylamide analogues; among them, the 76 (methyl, S‐isomer) and 77 (cyclopropyl) showed potent activity in inhibiting all Akt isoforms with IC₅₀ values of 4 nM and 5 nM against Akt1, 20 nM and 30 nM against Akt2, and 16 nM and 24 nM against Akt3. Both compounds demonstrated significant cellular activity against GSK3β, with IC₅₀ values of 134 and 208 nM (Figure 26). Research was followed by the introduction of α‐substituted benzylamide analogues with basic side chains, and each of them maintained potent inhibitory activity. 78 was the most potent of them with IC₅₀ values of 2 nM (Akt1), 14 nM (Akt2), 7 nM (Akt3), and 96 nM against cellular GSK3β.

Nonetheless, basic side chains were found to correlate with excellent clearance and low oral bioavailability. To solve this, non‐basic polar substituents were studied. In this series, 79 (R) and its stereoisomer AZD5363 (capivasertib, S) demonstrated potent activity with IC₅₀ values of 4 and 3 nM for Akt1, 24 and 8 nM for Akt2, and 22 and 8 nM for Akt3, as well as similar cellular activity against GSK3 at 91 and 89 nM.

Qian Liu et al. [102] reported a novel series of dual APN and Akt inhibitors derived from the clinical candidate of an Akt inhibitor, namely AZD5363/capivasertib. Their approach involved combining the AZD5363 scaffold with Bestatin by modifying the chlorophenyl ring, as well as the alkyl alcohol moiety of AZD5363, by replacing it with the carboxylate group of Bestatin. Further structural optimization was accomplished by adding several aromatic substitutions to the side chain. Among these synthesized 80 and 81 (Figure 27) exhibited vigorous dual inhibitory activity, showing potent inhibition against Akt1 with IC₅₀ values of 121 and 270 nM, respectively, and against APN with IC₅₀ values of 960 and 210 nM, respectively. A comparative overview of the reported pyrrolo‐pyridine and pyrrolo‐pyrimidine‐substituted compounds is shows in Table 7, with emphasis on their structural characteristics, biological profiles, and developmental status.

FIGURE 27.

Lead compounds from APN and Akt dual inhibitors.

TABLE 7.

Comparative SAR of pyrrolo‐pyridine and pyrrolo‐pyrimidine‐based Akt inhibitors.

Series/Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key SAR findings	Development stages
A10	7H‐Pyrrolo[2,3‐d]pyridine‐piperidine	8.5	NR	NR	NR	2,4‐Dichlorobenzyl substitution enhanced hydrophobic interactions and potency.	Lead compound
A16	7H‐Pyrrolo[2,3‐d]pyridine‐piperidine	7.0	NR	NR	NR	2‐Naphthyl group maintained high Akt inhibition.	Lead compound
63	Pyrrolopyridine piperidin‐4‐amine	2.2	NR	NR	NR	4‐Amide substitution markedly improved PKBβ potency.	Optimized lead
64	7‐Azaindole analogue	5.5	NR	NR	NR	Replacement of pyrrolopyrimidine retained strong potency.	Lead compound
69	8‐Oxopurine analogue	5.0	NR	NR	NR	Oxopurine scaffold maintained PKB inhibition.	Optimized lead
72	Piperidine analogue	180	NR	NR	NR	Free amine improved potency over methylamine analogue.	Initial lead
73	Piperidine analogue	3	NR	NR	NR	4‐Chlorophenyl substitution markedly increased potency but reduced selectivity.	Optimized lead
74	Piperidine analogue	5	NR	NR	NR	Similar potency to compound 97; limited kinase selectivity.	Optimized lead
75	Piperidine analogue	6	NR	NR	NR	Homologation improved ∼28‐fold selectivity over PKA.	Optimized lead
76	Pyrrolopyrimidine	4	20	16	pGSK3β: 134 nM	α‐Methyl (S)‐benzylamide produced potent pan‐Akt inhibition.	Optimized lead
77	Pyrrolopyrimidine	5	30	24	pGSK3β: 208 nM	Cyclopropyl analogue retained balanced pan‐Akt activity.	Optimized lead
78	Pyrrolopyrimidine	2	14	7	pGSK3β: 96 nM	Basic side chain increased potency but reduced oral bioavailability.	Optimized lead
79 (R)	Pyrrolopyrimidine	4	24	22	pGSK3β: 91 nM	Non‐basic polar substituent improved drug‐like properties.	Advanced lead
Capivasertib (AZD5363)	Pyrrolopyrimidine	3	8	8	pGSK3β: 89 nM	S‐isomer provided balanced pan‐Akt inhibition with improved pharmacokinetic properties.	Clinical/Approved*
80	Capivasertib–bestatin hybrid	121	NR	NR	APN IC50: 960 nM	Dual Akt/APN inhibition achieved through hybrid design.	Lead compound
81	Capivasertib–bestatin hybrid	270	NR	NR	APN IC50: 210 nM	Enhanced APN inhibition with reduced Akt potency.	Lead compound

4.6.2. Structure–Activity Relationship of Piperidin‐4‐amine Core Ring

This study elucidates the SAR of the Akt inhibitor capivasertib (AZD5363) by analyzing key structural features, activity trends, and optimization strategies reported across the existing published literature (Figure 28).

FIGURE 28.

Structure–activity relationship of capivasertib/AZD5363.

4.6.2.1. A Ring (Hinge Binding)

This 7H‐pyrrolo[2,3‐d]pyrimidine ring is necessary to bind in the hinged region of the enzyme Akt. This ring serves as a crucial hydrogen bonding site to the backbone of the kinase domain, thereby greatly inhibiting Akt activity. Modifications at this site tend to decrease potency, a factor that is crucial for enzyme selectivity and stability.

4.6.2.2. B Ring (Piperidine Moiety)

The piperidine ring that is attached to the core structure and plays a vital role in maintaining binding affinity and solubility. The presence of the 4‐position by amino or methylamino groups is much more active, presumably because of an augmentation of both electrostatic and hydrogen‐bonding interactions in the ATP‐binding site. The changes also enhance pharmacokinetics and ADME.

4.6.2.3. Linker Region

The linker region is also essential for positioning the molecule correctly within the binding pocket. Akt activity increases with the incorporation of polar alkyl groups, for example, (S)‐propanol over (R)‐propanol.

4.6.2.4. C Ring (Substitutions of Phenyl Ring)

Substitution of the phenyl rings can affect the selectivity and hydrophobic interactions considerably. The replacement of the phenyl ring in the allosteric site of the enzyme with 4‐chloro and 3,4‐dichloro has been observed to enhance the activity (Figure 28).

4.7. Piperazine‐Substituted Core Ring

Yang Liu et al. [103] describe a new class of 4‐piperazin‐1‐yl‐7H‐pyrrolo[2,3‐d]pyrimidine derivatives as a new potent Akt‐serine/threonine kinase inhibitor. Structural optimization was performed using 82 and halogen substitutes on the 5‐position of the pyrrolo[2,3‐d]pyrimidine scaffold and substituting the phenyl or naphthyl moieties conjugated to the 4‐position piperazine ring through an acetamide linker. This modification produced a series of analogues that were systematically tested for their ability to inhibit Akt. Among the synthesized 83 and 84 showed strong enzymatic inhibition with IC₅₀ values of 18 (±0.7) and 21.3 (±1.4) nM, respectively (Figure 29). Furthermore, these compounds showed strong antiproliferative effects in cells, with an IC₅₀ of 9.3 (±1.3) nM for the androgen‐sensitive LNCaP prostate cancer cells and 21.1 (±0.5) nM for the androgen‐insensitive PC‐3 cells. These results emphasize the potential of pyrrolo[2,3‐d]pyridine‐based inhibitors as promising options for targeted Akt inhibition in cancer therapy.

FIGURE 29.

Lead compounds from the piperazine scaffolds.

James F. Blake et al. [104] reported a novel series of pyrrolopyrimidine‐based protein kinase B (PKB/Akt) inhibitors, discovered through a combination of high‐throughput screening (HTS) and structure‐based drug design approaches. Because the kinase domains of Akt1, Akt2, and Akt3 are very similar, the x‐ray crystal structure of Akt2 was used in the early stages of optimization through rational design. Initial SAR studies focused on phenyl ring optimization. The para‐chloro analogue (85) showed strong Akt activity, with IC₅₀ values of 20 (±6) nM for Akt1, 118 (±47) nM for Akt2, and 179 (±29) nM for Akt3, as shown in Figure 29. Further optimization included testing different functional group modifications on the quinazoline scaffold. 86 features a fluoro group at the seventh position of the quinazoline ring, exhibits strong potency against Akt1 (IC₅₀ = 30 ± 12 nM). In contrast, alternative modifications on the quinazoline ring resulted in a marked reduction in Akt1 inhibitory activity. To increase potency, the quinazoline core was replaced with bioisosteric heterocycles. In this case, the pyrrolo‐pyrimidine analogue (87) showed strong activity against Akt1, with an IC₅₀ of 5 (±2) nM (Figure 29). Detailed SAR studies on the pyrrolo‐pyrimidine scaffold established the necessity of changes to the 5‐position to be active. They are exploring the replacement of an amino side chain with alkylamine groups and a 4‐chlorophenyl group, as well as other aromatic substitutions. Both 88 and 89 were found to have strong activity against Akt1 with IC₅₀ values of 1 ± 0 nM. Similar activity is exhibited by a methyl or fluoro group on the pyrrolo‐pyrimidine ring. However, extending the alkyl side chains on the amino group caused a reduction in activity. Additionally, various structural modifications to the 4‐chlorophenyl group lead to a reduction in Akt1 inhibitory activity.

Gui‐Dong Zhu et al. reported the discovery of a novel class of dihydrothieno‐ and dihydrofuropyrimidines as potent pan‐Akt inhibitors. The team hypothesized that enhancing selectivity for protein kinase A (PKA) might be possible by either increasing the steric bulk to the hinge‐binding core or by adjusting its polarity. To evaluate this, they incorporated dihydrothieno‐ and dihydrofuropyrimidine scaffolds, fused to the pyrimidine ring, within the hinge‐binding area. SAR studies concentrated on changes at the 5‐position of the pyrimidine ring with different alkyl groups, along with modifications on the 4‐chlorophenyl ring. Among the synthesized analogues, 90 which is a dihydrothienopyrimidine with a methyl group in the (S)‐configuration and a 4‐chlorophenyl substituent, showed significant potency. It had an Akt1 IC₅₀ of 1 (±0) nM, a PKA IC₅₀ of 13 (±1) nM, and demonstrated strong cellular activity in LNCaP cells with an IC₅₀ of 184 (±89) nM. Similarly, 91 with an (S)‐methyl group and a 3‐fluoro, 4‐chlorophenyl substituent, retained the strong potency of enzyme Akt1 with an IC₅₀ of 2 (±0) nM; PKA IC₅₀ of 8 (±2) nM; and LNCaP IC₅₀ of 176 (±15) nM (Figure 29). Further optimization of the amine side chain involved the incorporation of various aliphatic alkylamines and systematic substitution on the 4‐chlorophenyl moiety. Among these analogues 92 as a lead molecule emerged with N‐methylpropan‐2‐amine and 4‐chlorophenyl moiety, exhibiting Akt1 with an IC₅₀ of 1 (±0) nM, PKA IC₅₀ of 35 (±3) nM, and LNCaP cellular IC₅₀ of 137 (±9) nM. The study conclusively demonstrated that the dihydrothienopyrimidine scaffold, particularly in combination with a para‐chlorophenyl substituent, consistently yielded the most effective and selective Akt inhibitors. Simultaneously, the optimization of the dihydrofuropyrimidine series revealed 93 as a promising lead molecule. This analogue, which contains a 4‐chlorophenyl moiety and an N‐methylpropan‐2‐amine side chain, showed IC₅₀ values of 3 (±1) nM for Akt1, 52 (±9) nM for PKA, and 125 (±6) nM for LNCaP cells. These results collectively showed that both dihydrothieno‐ and dihydrofuropyrimidine scaffolds provide highly potent and selective Akt inhibitors, with the para‐chlorophenyl substitution pattern being recognized as the key factor in activity [105].

James F. Blake et al. [106] described the discovery of a novel series of 6,7,7‐dihydro‐5H‐cyclopenta[d]pyridines, which serve as protein kinase B (Akt) ATP‐competitive and selective inhibitors. SARs studies demonstrated that substituents at positions C5 and C7 of the scaffold had a significant impact on the potency and selectivity of the isoforms. Within this series, GDC‐0068, incorporated with 5‐methyl‐ and 7‐(R)‐hydroxyl‐substitutions, exhibited inhibition of all three Akt isoforms with IC₅₀ values of 5 (±7) nM for Akt1, 18 (±10) nM for Akt2, and 8 (±9) nM for Akt3, respectively, and showed better selectivity compared to protein kinase A. In comparison, the 94, substituted with a 5‐vinyl group, had significant activity against the Akt isoforms, achieving IC₅₀ values of 2 (±1) nM for Akt1, 6 (±2) nM for Akt2, and 1 (±0) nM for Akt3, respectively (Figure 30).

FIGURE 30.

Discovery of lead analogue NTQ1062.

To further increase cellular activity, secondary amine substituents were systematically modified with various aliphatic groups. This optimization resulted in the formation of 95 retained good pan‐Akt activity with an IC₅₀ of 2 (±1) nM for Akt1, 4 (±2) nM for Akt2, and 3 (±1) nM for Akt3 and showed good Akt cellular activity, measured by inhibiting p‐PRAS40 in LNCaP cells (IC₅₀ = 75 (+15) nM) (Figure 30). Further development, potency, and cellular effectiveness were optimized with subsequent modifications to the amine functional groups and aromatic substituents of the P‐loop binding region. Among these, 96 became a lead analogue, with IC₅₀ values of 3 (±1) nM against Akt1, 3 (±1) nM against Akt2, 2 (±1) nM against Akt3, and a potent cellular activity against p‐PRAS40 in LNCaP cells (IC₅₀ 65 (±1) nM). These results demonstrate the flexibility of the dihydrocyclopenta pyrimidine scaffold and how targeted modifications to key positions can balance isoform potency, kinase selectivity, and cellular efficacy, ultimately leading to promising clinical candidates.

Daoguang Zhang et al. [107] described the design and synthesis of a novel series of N‐heterocyclic‐based Akt inhibitors, incorporating a piperidine linker between various substituted benzyl groups and a piperazine amide scaffold. Structural modifications were further aimed toward the hinge‐binding region, where the typical pyrrolo‐pyrimidine core was replaced with several heteroaromatic systems. The 1H‐pyrazolo[3,4‐d]pyrimidine group has been employed as the hinge‐binding region in 97 and 98. These compounds possessed chloro and bromo substituents at the 3‐position, along with 3,4‐dichloro and 4‐chloro substitutions on the phenyl ring. These derivatives exhibited significant Akt1 inhibitory activity, with IC₅₀ values of 24.3 ± 1.6 nM (97) and 26.9 ± 1.7 nM (98). Both compounds also had potent antiproliferative effects on PC‐3 prostate cancer cells, with IC₅₀ values of 3.7 ± 0.9 and 9.8 ± 1.2 nM, respectively (Figure 30). It shows these compounds could be effective Akt‐targeted anticancer agents.

Wenhu Zhan et al. [108] reported a novel series of 4‐amino‐pyrimidine analogues as Akt inhibitors. Using an MD‐SVR model‐based approach, structural modifications were made to optimize these derivatives. Among this 99 demonstrated vigorous inhibitory activity against Akt1 with an IC₅₀ value of 7.7 nM. In addition, 99 exhibited antiproliferative effects, showing IC₅₀ values of 22 670 nM against OVKAR‐8 cells and 5150 nM against HCT116 cells (Figure 30).

Changyou Ma et al. [109] have developed and enhanced a novel series of ATP‐competitive Akt inhibitors featuring unique chemical scaffolds that exhibited excellent enzymatic activity and improved pharmacokinetic behavior in vivo. The researchers aimed to employ GDC‐0068, which focuses on modifying the hinge‐binding region of 5‐methyl‐6,7‐dihydro‐5H‐cyclopenta[d]pyrimidin‐7‐ol, replacing it with different heterocyclic cores. 100, which is substituted with 5‐methyl‐5,8‐dihydropyrido[2,3‐d]pyrimidin‐7(6H)‐one, shows significant activity against Akt isoforms, exhibiting an IC₅₀ of 2.4 nM for Akt1, 6 nM for Akt2, and 1.6 nM for Akt3. After separating the racemic mixture, it was found that the S‐isomer (101a) lost the original inhibitory activity.

The R‐isomer (101b) maintained vigorous activity across Akt1/2/3 (IC₅₀ = 1.6/24/0.3 nM) and showed promising cellular inhibition in LNCaP cells (IC₅₀ = 103 nM) (Figure 30). Following this, structural refinement through stereochemical modification yields 102, which enhances its potency while maintaining the same level of cell activity (Akt1/2/3 IC₅₀ = 1.5/11/0.2 nM) and standard cellular functionality (LNCaP IC₅₀ = 102 nM). To further enhance potency and pharmacokinetic properties, NTQ1062 was developed by incorporating a cyclopropyl‐fused ring into the piperazine ring, which significantly improved potency and pharmacokinetic performance. All Akt isoforms showed significant activity, with Akt isoforms likely (Akt1/2/3 IC₅₀ = 0.4/6.3/0.1 nM), whereas LNCaP, with an IC₅₀ of 50 nM, exhibited significant cellular inhibition (Figure 30). The amine isopropyl group was then modified with various heterocycles, resulting in 103, which contained a cyclopropyl moiety and showed identical efficacy against Akt isoforms (Akt1/2/3 IC₅₀ = 0.7/5.7/0.1 nM). A comparative assessment of the piperazine‐substituted analogues is presented in Table 8, focusing on their structural features, biological activities along with developmental status.

TABLE 8.

Comparative activity of piperazine‐substituted and related ATP‐competitive Akt inhibitors.

Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key optimization/SAR	Development stages
82	Pyrrolo[2,3‐d]pyrimidine–piperazine	18 ± 0.7	NR	NR	LNCaP: 9.3 nM; PC‐3: 21.1 nM	Halogen substitution at C5 improved Akt inhibition.	Lead
84	Pyrrolo[2,3‐d]pyrimidine–piperazine	21.3 ± 1.4	NR	NR	LNCaP: 9.3 nM; PC‐3: 21.1 nM	Aromatic substitution on piperazine maintained potency.	Lead
85	Quinazoline	20 ± 6	118 ± 47	179 ± 29	NR	para‐Chloro phenyl favored Akt1 selectivity.	Lead
86	7‐Fluoro quinazoline	30 ± 12	NR	NR	NR	Fluoro substitution retained activity.	Lead
87	Pyrrolopyrimidine	5 ± 2	NR	NR	NR	Quinazoline replaced by pyrrolopyrimidine markedly improved potency.	Optimized lead
88	Pyrrolopyrimidine	1 ± 0	NR	NR	NR	Amino side‐chain optimization produced maximal potency.	Optimized lead
89	Pyrrolopyrimidine	1 ± 0	NR	NR	NR	Similar potency to compound 112.	Optimized lead
90	Dihydrothienopyrimidine	1 ± 0	NR	NR	LNCaP:184 ±89 nM	(S)‐Methyl and para‐chlorophenyl enhanced potency and PKA selectivity.	Optimized lead
91	Dihydrothienopyrimidine	2 ± 0	NR	NR	LNCaP:176 ±15 nM	3‐F,4‐Cl phenyl maintained activity.	Optimized lead
92	Dihydrothienopyrimidine	1 ± 0	NR	NR	LNCaP:137 ±9 nM	N‐Methylpropan‐2‐amine improved potency and selectivity.	Lead
93	Dihydrofuropyrimidine	3 ±1	NR	NR	LNCaP:125 ±6 nM	Dihydrofuropyrimidine retained high potency.	Lead
GDC‐0068	Cyclopenta[d]pyrimidine	5 ±7	18 ±10	8 ±9	p‐PRAS40: NR	Clinical scaffold exhibiting balanced pan‐Akt inhibition.	Clinical candidate
94	Cyclopenta[d]pyrimidine	2 ±1	6 ±2	1 ±0	NR	5‐Vinyl substitution increased potency.	Optimized lead
95	Cyclopenta[d]pyrimidine	2 ±1	4 ±2	3 ±1	p‐PRAS40:75 ±15 nM	Secondary amine optimization improved cellular activity.	Optimized lead
96	Cyclopenta[d]pyrimidine	3 ±1	3 ±1	2 ±1	p‐PRAS40:65 ±1 nM	Further amine optimization produced balanced pan‐Akt inhibition.	Lead
98	Pyrazolo[3,4‐d]pyrimidine	24.3 ±1.6	NR	NR	PC‐3:3.7 ±0.9 nM	Chloro substitution enhanced activity.	Lead
99	Pyrazolo[3,4‐d]pyrimidine	26.9 ±1.7	NR	NR	PC‐3:9.8 ±1.2 nM	Bromo analogue retained potency.	Lead
97	4‐Aminopyrimidine	7.7	NR	NR	OVCAR‐8:22670 nM; HCT116:5150 nM	MD‐SVR‐guided optimization.	Lead
100	Pyrido[2,3‐d]pyrimidinone	2.4	6	1.6	NR	Replacement of cyclopentapyrimidine hinge improved PK.	Lead
101 (R)	Pyrido[2,3‐d]pyrimidinone	1.6	24	0.3	LNCaP:103 nM	R‐enantiomer retained activity; S‐isomer inactive.	Optimized lead
102	Pyrido[2,3‐d]pyrimidinone	1.5	11	0.2	LNCaP:102 nM	Stereochemical optimization improved Akt2 potency.	Optimized lead
NTQ1062	Cyclopropyl‐fused pyrido[2,3‐d]pyrimidinone	0.4	6.3	0.1	LNCaP:50 nM	Cyclopropyl‐fused piperazine markedly enhanced potency and PK.	Clinical candidate
103	Cyclopropyl analogue	0.7	5.7	0.1	NR	Heterocyclic side‐chain optimization maintained pan‐Akt inhibition.	Lead

4.7.1. Structure–Activity Relationship of Piperazine Core Ring

This study reveals the SAR of the next‐generation Akt inhibitor NTQ1062 by examining key structural features, potency trends, and optimization strategies (Figure 31).

FIGURE 31.

Illustrates the SAR of NTQ1062.

4.7.1.1. A Ring

The 5,8‐dihydropyrido[2,3‐d]pyrimidin‐7(6H)‐one and 5‐methyl‐7H‐pyrrolo[2,3‐d]pyrimidine scaffolds enhanced Akt inhibition. Incorporating an (R)‐methyl group into the A‐ring further enhanced the activity.

4.7.1.2. B Ring

The piperazine core was found to be necessary for activity because it was a key structural part that kept the right shape and binding orientation. Fusing the piperazine core with a cyclopropyl ring enhanced the activity.

4.7.1.3. Linker Region

The amide linker was crucial for maintaining activity, as it formed the most suitable environment for hydrogen bonding and molecular alignment within the Akt binding pocket. Substitution of the linker with an (S)‐methylamine bridge was essential for activity. Likewise, incorporating isopropylamine, cyclopropyl, and 2‐amino‐2‐methylpropan‐1‐ol moieties improved activity.

4.7.1.4. C Ring

A 4‐chlorophenyl ring notably improved Akt inhibitory potency, emphasizing that halogen substitution improves hydrophobic interactions and binding affinity (Figure 31).

4.8. Phenyl‐Substituted Core Ring

Gordon Saxty et al. [110] have employed a fragment‐based lead discovery (FBLD) approach, coupled with Free Wilson analysis, to develop (5‐methyl‐4‐phenyl‐1H‐pyrazole) derivatives that act as protein kinase B (PKB/Akt) inhibitors. The original pyrazole framework with the simplest phenyl substituent showed only weak micromolar activity. To improve potency, a systematic optimization of the series was performed through structural changes, including the addition of aminoalkyl linkers, extension of the aromatic ring, and replacement of halogen groups. The modifications significantly enhanced the inhibition effectiveness, and 104 proved to be the most effective analogue. It inhibited Akt with an IC₅₀ of 18 (±7) nM (Figure 32), indicating that a carefully rational elaboration of fragments may convert weakly active ones into highly potent Akt inhibitors.

FIGURE 32.

The lead compound belongs to the phenyl scaffold.

Alastair Donald et al. [111] performed a structure‐based study of the evolution of 6‐phenylpurine derivatives as protein kinase B (PKB/Akt) inhibitors. The study was conducted by replacing the 7‐azaindole ring with a purine scaffold, resulting in the discovery of 6‐phenylpurine as a promising lead compound for Akt inhibition. Among these derivatives, 105 showed the greatest potency against PKBβ with an IC₅₀ value of 0.9 (±0.3) nM (Figure 32). However, the compound did not exhibit a corresponding increase in cell‐based growth inhibition despite this exceptional biochemical potency, indicating possible deficiencies in physicochemical or pharmacokinetic properties. To address these issues, structural modifications were focused on lipophilicity by introducing a piperidine substituent. This approach resulted in 106, which showed excellent bifunctionality with IC₅₀ values of 2 (±0.3) nM against PKBβ and 33 nM against PKA (Figure 32). These results demonstrate the feasibility of using the 6‐phenylpurine scaffold as a hinge‐binding core for Akt inhibition and that optimization of lipophilicity plays an essential role in achieving high kinase inhibition potency.

Tao Liu et al. [112] have reported a novel series of diphenyl methylamine analogues as potential Akt inhibitors. The analogue was designed in silico and subsequently tested using virtual screening. Initial results showed that both 107 and 108, exhibited excellent pharmacophore fitting and molecular docking scores. When large alkyl groups, such as an isopropyl group in 107 and cyclopropyl group in 108, were substituted for the primary amine, they exhibited IC₅₀ values of 160 and 470 nM against Akt1 (Figure 32). Later they replaced the purine ring with different heterocycles, and the primary amine was substituted with various bulkier alkyl groups. Among them, 109 with a pyrazole ring and t‐butyl substitution to primary amine groups shows potent activity against Akt1 with an IC₅₀ of 38 nM and antiproliferative activity against OVKAR‐8 with an IC₅₀ of 8100 nM, against HL60 with an IC₅₀ of 5300 nM, and against HCT‐116 with an IC₅₀ of 8900 nM. Table 9 presents a comparative overview of the reported phenyl‐substituted compounds, highlighting their structural modifications, biological activities, and respective stages of development.

TABLE 9.

Comparative activity of phenyl‐substituted Akt inhibitors.

Lead compounds	Core scaffolds	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Key SAR findings	Development stages
104	5‐Methyl‐4‐phenyl‐1H‐pyrazole	18 ± 7	NR	NR	NR	Fragment‐based optimization of pyrazole with aminoalkyl linker and aromatic extension markedly improved Akt inhibition.	Lead compound
105	6‐Phenylpurine	NR	0.9 ± 0.3	NR	Weak despite excellent biochemical potency	Purine hinge binder produced sub‐nanomolar enzyme inhibition but poor cellular efficacy, indicating physicochemical/PK limitations.	Optimized lead
106	Piperidine‐substituted 6‐phenylpurine	NR	2 ± 0.3	NR	NR	Piperidine substitution reduced lipophilicity and improved kinase selectivity (PKA IC50 = 33 nM).	Optimized lead
107	Diphenyl methylamine	160	NR	NR	NR	Isopropyl substitution on the amino group improved binding compared with the parent analogue.	Initial lead
108	Diphenyl methylamine	470	NR	NR	NR	Cyclopropyl substitution decreased Akt1 potency relative to compound 131.	Initial lead
109	Pyrazole–diphenyl methylamine	38	NR	NR	OVCAR‐8: 8100 nM HL60: 5300 nM; HCT116: 8900 nM	Replacement of the purine with a pyrazole ring and tert‐butyl substitution significantly enhanced enzymatic potency but produced only modest cellular activity.	Optimized lead

4.8.1. Structure–Activity Relationship of the Phenyl Core Ring

4.8.1.1. A Ring (Hinge Region)

9H‐purine and 1H‐pyrazole at the hinge give significant activity both motifs provide the necessary H‐bond donors/acceptors to interact with the kinase hinge. Purine tends to be a strong hinge binder (rigid, with multiple heteroatoms), which contributes to potency but can increase polarity and risk metabolic liabilities. In contrast, 1H‐pyrazole is smaller and can improve ligand efficiency while retaining hinge interaction.

4.8.1.2. B Ring (Phenyl Core)

The phenyl core is essential as the central scaffold, as it enforces geometry between the hinge (A Ring) and distal elements (C Ring/sidechain) and provides π‐stacking and hydrophobic contacts in the ATP pocket.

4.8.1.3. Side Chain (Linker/Substituent Off the Phenyl)

Amino substitution on the side chain enhances activity likely by (a) forming additional H‐bonds with solvent or nearby residues, (b) improving cell permeability via protonation (if basic), and (c) improving solubility/PK balance. Basic amines also allow salt formation (pharmacokinetic leverage) and increase clearance via metabolism; placement (primary vs. secondary vs. tertiary) and basicity (pKa) should be tuned.

4.8.1.4. C Ring (Distal Phenyl)

4‐Chloro substitution on the distal phenyl enhances activity, likely due to improved hydrophobic interactions in the lipophilic subpocket and subtle electronic effects (modulating ring electron density and conformation).

Para‐halogens (Cl, F, Br) often increase potency; size and lipophilicity determine the optimal halogen (Cl often balances size and lipophilicity well) (Figure 33).

FIGURE 33.

Structure–activity relationship of the phenyl core ring.

4.9. Pyrrolidine‐Substituted Core Ring

Blaise Lippa et al. [113] described the discovery of a novel class of pyrrolo‐pyrimidine‐based inhibitors targeting the Akt kinase. In this investigation, the 1,3,4‐triazole moiety was utilized as the hinge‐binding region, and various substitutions were examined to enhance inhibitory effectiveness. The methyl‐substituted derivative (110) exhibited significant potency among these analogues, displaying an IC₅₀ of 151 (± 32) nM against Akt kinase (Figure 34). The researchers further improved upon these findings by incorporating a spiroindoline unit fused with a phenyl ring at the hinge‐binding region, which substantially enhanced the activity. Compounds in this series exhibited limited selectivity when evaluated against the related kinase PKA; however, certain analogues demonstrated significant enhancements in potency. For instance, 111 inhibited Akt1 with an IC₅₀ of 2.4 (± 0.6) nM, achieving a cellular potency of 50 (± 19) nM. It demonstrates that this design strategy is effective in developing pyrrolo‐pyrimidine‐based Akt inhibitors.

FIGURE 34.

Displays the lead compounds from the pyrrolo‐pyrimidine series.

Kevin D. Freeman‐Cook et al. [113] from Pfizer reported the development of a novel series of ATP‐competitive Akt inhibitors based on an extended 3‐aminopyrrolidine scaffold. The design approach mainly focused on selectivity for Akt compared to its related PKA kinase. Initially, in the series 112, incorporating the (3R)‐aminopyrrolidine core, demonstrated a higher degree of selectivity, with an IC₅₀ of 180 nM for Akt1 and 3200 nM for PKA, corresponding to a selectivity factor of 17.8. The initial attempts at optimization focused on the derivatization of the basic amine functionality, utilizing parallel chemistry techniques to generate amides, sulfonamides, and ureas. Although these modifications increased structural diversity, the majority of analogues were less selective for Akt compared to PKA. Among this group, the sulfonyl phenyl‐substituted 113 had moderate potency against Akt1, with an IC₅₀ of 600 nM for Akt kinase, and against PKA kinase, the IC₅₀ was 800 nM (Figure 34).

The crystal structure of the PKA in complex with 113 was analyzed, and structural insights were obtained. The analysis revealed that modifying the C3 part of the pyrrolidine ring causes issues in PKA but not in Akt, which helps explain how to design C3 modifications in the pyrrolidine ring to increase selectivity. A loss of activity was observed with many of the analogues, although alternative substitutions were widely explored. Ultimately, efforts converged on the design of a non‐racemic amide series, which produced substantial improvements in both potency and selectivity. 114 was the most potent in this series, with an Akt kinase IC₅₀ of 0.5 nM and cellular activity against Akt at 310 nM, while demonstrating an exceptional selectivity index of 900 (Figure 34). These findings emphasize the value of precise stereochemical and structural modifications in tailoring aminopyrrolidine‐based scaffolds for highly selective Akt inhibition [114].

Lizbeth DeSelm et al. [115] reported a novel series of quinazoline carboxamides, which are dual inhibitors of p70S6K/Akt kinases. Their initial approach was to identify potent analogues of p70S6K inhibitors that were amenable to subsequent optimization and refinement. Various hit identification approaches were assessed, including high‐throughput screening, fragment‐based screening, and analoging; however, a focus on kinase screening was ultimately adopted, as it appeared to be the most effective in advancing the discovery forward. Among them, 115 (racemic) exhibited potent activity against p70S6K, with an IC₅₀ of 1 nM, against Akt1 with an IC₅₀ of 15 nM, and cellular activity against pS6 with an IC₅₀ of 25 nM. Later, they separated the two isomers, with the (R) isomer being inactive, and 116 (S) demonstrating potent activity against p70S6K with an IC₅₀ of 0.6 nM, against Akt with an IC₅₀ of 4 nM, and cellular activity against pS6 with an IC₅₀ of 45 nM, respectively (Figure 34). To enhance cellular efficacy, they further modified the molecules and developed M2698. This analogue significantly increased the dual p70S6K/Akt inhibition, having IC₅₀ nM values of 1.1 nM against p70S6K, 4 nM against Akt1, and 11 nM in the pS6 cell assay.

Rui Xu et al. [116] subsequently advanced this research through a structure‐based rational design approach, reporting the discovery and optimization of spirocyclic sulfonamides as potent and selective Akt inhibitors. Building on the earlier spirochromane scaffold, their work focused on improving selectivity against protein kinase A (PKA) while retaining potent Akt inhibition. A SAR was initiated by designing azabicyclic spiro linker‐based 117 exhibited significant potency against Akt1 with an IC₅₀ of 2 nM, but only moderate selectivity over PKA was 28‐fold (Figure 34).

To overcome these limitations, several modifications were introduced to the hinge‐binding region. Among them, quinazoline and pyrimidine‐substituted compounds were found to have significantly increased specificity, with a maximum 280‐fold activity compared to PKA, while retaining high potency toward Akt1. Further optimization concentrated on conformational restriction strategies aimed at constraining the rotation of sp³–sp³ bonds by interconnecting the propyl chain and ethoxyethyl side chain of 117 via a pyrrolidine ring. The resulting modification yields 118, which retains Akt1 inhibition with an IC₅₀ of 9 nM and simultaneously lowers the PKA concentration to an IC₅₀ of 3900 nM, resulting in a significant increase in selectivity.

The systematic alteration of the sulfonamide substituents also led to further modifications, allowing both potency and selectivity. Among them, 119 exhibited subnanomolar Akt1 activity with an IC₅₀ of 4 nM and outstanding selectivity of over 850‐fold over PKA. The replacement of sulfonamide with isoxazole yields 120, resulting in an Akt1 with an IC₅₀ of 3 nM and selectivity exceeding 6000‐fold compared to its PKA counterpart (Figure 34). Overall, this work demonstrates that the spirocyclic sulfonamide serves as an excellent framework for establishing highly selective Akt inhibitors, which exhibit better selectivity over many other kinases and possess drug‐like properties, surpassing those of previous ATP inhibitors.

4.9.1. Structure–Activity Relationship of the Pyrrolidine Core

4.9.1.1. A Ring (Hinge Region)

Quinazoline‐8‐carboxamide and 7H‐pyrrolo[2,3‐d], the pyrimidine at the hinge enhances activity, and both scaffolds provide the correct H‐bond acceptor/donor pattern to engage the kinase hinge; quinazoline offers a larger, more planar scaffold, while the fused pyrrolopyrimidine provides a compact, polar hinge binder.

4.9.1.2. Side Chain (Linker/Substituent)

(S)‐Methylamine stereochemistry is essential for activity the S‐configuration likely places the amine in the correct vector to form a key H‐bond/salt bridge with receptor residues or to position the molecule for optimal binding. Azetidine or methylamine substitution enhances dual inhibition of p70S6K and Akt. Small, constrained basic moieties (azetidine) can provide favorable entropic profiles and tune pKa for optimal target engagement and cell permeability.

4.9.1.3. Core

Pyrrolidine ring presence enhances Akt inhibition. The pyrrolidine likely contributes to favorable conformational geometry, hydrophobic contacts, and fills a small lipophilic pocket; it may also rigidify the molecule and improve cell potency.

4.9.1.4. C Ring (Hydrophobic)

4‐trifluoromethyl (para‐CF₃) and 3‐chloro (meta‐Cl) substitutions on the phenyl increase both biochemical potency and cellular activity. CF₃ increases lipophilicity and can improve binding into hydrophobic subpockets and metabolic stability; the 3‐Cl influences the electronic/steric profile, as well as the vectoring of the para substituent (Figure 35).

5. Allosteric Inhibitor

5.1. 5,6‐Diphenyl‐Substituted Heterocycle

Craig W. Lindsley et al. [117] have discovered that Akt isozymes (Akt1, Akt2, and Akt3) possess unique sensitivity profiles to small‐molecule inhibitors, which enable the strategic design of isoform‐selective or dual‐isoform inhibitors. In their research, they built a library of 5,6‐diphenylpyrazin‐2(1H)‐ones for evaluating isozyme selectivity. Among them, 121 was shown to be an Akt1‐selective inhibitor, with an IC₅₀ of 760 nM against Akt1 and significantly reduced activity against Akt2 (25 400 nM) and Akt3 (>50 000 nM) (Figure 36). Modifying the structure of this scaffold by replacing the methyl group with an isobutyl group resulted in 122, a significant Akt2‐selective inhibitor with an IC₅₀ of 325 nM. It had minimal activity against Akt1 (21 200 nM) and Akt3 (21 870 nM) [117] (Figure 36). Later incorporation of the tetrazole moiety led to 123 and 124 with significant activity against the Akt isoform, with IC₅₀ values of 63 and 20 nM for Akt1, 65 and 144 nM for Akt2, and 1228 and 1613 nM for Akt3, respectively. But they form a zwitterionic character. To overcome the zwitterionic character of earlier compounds, N‐alkylation of the tetrazole moiety was attempted; however, this modification resulted in a greater than 15‐fold loss of Akt1/Akt2 potency and poor physicochemical properties. Subsequently, optimizing through tricyclic substitution, 125 (a pyrazole derivative) was discovered. It inhibited Akt1, Akt2, and Akt3 with IC₅₀ values of 85, 300, and 2400 nM, respectively. Similarly, 126 (an imidazole analogue) obtained IC₅₀ values of 58 nM for Akt1, 210 nM for Akt2, and 2119 nM for Akt3 (Figure 36). These results show that designing effective dual Akt inhibitors and isozyme‐specific inhibitors is achievable by making strategic changes to the scaffold. Furthermore, optimizing 126 modified its drug‐like profile by incorporating amide‐substituted moieties. This approach was successful in enhancing solubility, physicochemical properties, and cellular activity while maintaining potency. Among the resultant analogues, 127 exhibited promising allosteric inhibition, with IC₅₀ values of 138 nM for Akt1, 212 nM for Akt2, and 7200 nM for Akt3. It validated its capacity as a dual Akt1/Akt2 selective inhibitor (Figure 36).

FIGURE 36.

Shows the lead compounds from the 2,3,5‐trisubstituted pyridine series.

Researchers further refined these findings by examining the 2,3‐diphenylquinoxaline scaffold to boost its selectivity for specific isoforms and enhance its metabolic stability. Replacing the quinoxaline with a quinazoline core led to a significant decline in activity, resulting in the vital role of the original heteroaryl structure in maintaining efficacy. Additional modifications involved the addition of pyridine‐linked 2H‐tetrazole substituents and the investigation of replacing quinoxaline with 1‐(piperidin‐4‐yl)‐1,3‐dihydro‐2H‐benzo[d]imidazol‐2‐one analogues, as well as other substituents. In this series, 128 was the most important analogue, with IC₅₀ values of 177 nM for Akt1 and 248 nM for Akt2. It had minimal impact on Akt3 (IC₅₀> 50 000 nM) (Figure 36). These outcomes validate that the structural integrity of the quinoxaline‐based scaffold is crucial for its activity and that peripheral modifications can be employed to enhance dual Akt1/Akt2 inhibition while minimizing Akt3 activity [118]. Later, they reported that optimized 123 yielded potent dual Akt1/Akt2 allosteric inhibitors with improved solubility, physicochemical properties, and cellular activity.

Zhicai Wu et al. [119] reported the discovery of a new class of pyridopyrimidines that are potent inhibitors of Akt1 and Akt2. The initial optimization approach was to replace the terminal amine with a thiomethyl group on the pyridopyrimidine ring and combine with the acyclic amines in the piperidine‐substituted region. In this series, 129 containing a 1‐(2‐aminophenyl) propan‐1‐one substituent, showed significant inhibitory activity, with IC₅₀ values of 15 (±3) nM for Akt1 and 90 (±19) nM for Akt2 (Figure 36). The outcomes align with those of the additional compounds documented by Zhijian Zhao [120]. Additional studies on 4‐substituted piperidines, encompassing various heterocycles and aliphatic groups, led to the identification of compound 61, which contains a 2‐(4H‐1,2,4‐triazol‐3‐yl)pyridine moiety and exhibits strong inhibitory activity. Remarkably, 130 inhibits Akt1 with an IC₅₀ of 3.8 (± 1) nM and Akt2 with an IC₅₀ of 26 (± 5) nM. In cell‐based tests, it maintained significant activity against Akt1 (IC₅₀ = 9.3 (± 2.6) nM), while its activity against Akt2 was much lower (IC₅₀ = 589 (± 8) nM), indicating increased cellular selectivity for Akt1 (Figure 36).

The subsequent optimization involved examining the 2‐methylthio group and attempting several substitutions. Among them, incorporating a pyridyl triazole terminal group along with a methylamine substituent led to 131, which was very effective against Akt1 (IC₅₀ = 6.0 (±0.3) nM) and Akt2 (IC₅₀ = 94.4 (±2.6) nM). In cellular studies, 131 inhibited Akt1 with an IC₅₀ of 20.3 (± 10.1) nM, whereas its inhibition of Akt2 was less potent, with an IC₅₀ of 899 (± 202) nM (Figure 36). Overall, the findings highlight the importance of western terminal modifications and 2‐position substituents in affecting both the efficacy and isoform selectivity of pyridopyrimidine‐based Akt inhibitors. 130 and 131 are two promising leads.

John C. Hartnett et al. [121, 122] described a new series of 2,3,5‐trisubstituted pyridines as potent allosteric inhibitors of Akt1 and Akt2. Their design approach consisted of substituting the acidic tetrazole moiety with non‐acidic heterocycles and systematically optimizing the eastern basic amine substituent. In this series, the replacement of tetrazole by an amino‐thiadiazole ring was beneficial. The most potent of these analogues, 132 in which the A‐ring had an amino‐thiadiazole substitution and the C‐ring had a triazole substitution, had enzyme IC₅₀ values of 85 nM (Akt1) and 22 nM (Akt2). 132 showed significant activity in cellular assays with IC₅₀ values of 516 nM (Akt1) and 330 nM (Akt2) (Figure 37).

FIGURE 37.

Lead compound from the 1,6‐naphthyridin‐5(6H)‐one series.

Likewise, 133 with an amino‐thiazole at the A‐ring and a 4‐amino‐pyrazole[3,4‐d]pyrimidine at the C‐ring, exhibited similar enzyme‐level potency (Akt1 IC₅₀ = 126 nM, Akt2 IC₅₀ = 22 nM). 133 exhibited better cellular activity compared to other analogues, with IC₅₀ values of 621 nM (Akt1) and 416 nM (Akt2), and was thus chosen to continue with the research. Later, in a series of optimizations, the A‐ring was altered to include an oxadiazole heterocycle, resulting in Akt1 and Akt2 inhibitor 134, with enzyme IC₅₀ values of 180 and 121 nM, respectively. In cell experiments, 134 has an IC₅₀ of 288 nM (Akt1) and 1061 nM (Akt2) (Figure 37). Nevertheless, these derivations were not superior to the amino‐thiadiazole derivatives, which proved the latter to be the most advantageous scaffold in the described series.

Zhicai Wu et al. [121] have described a new series of 3,4‐diphenylpyridine analogues as potent allosteric Akt inhibitors. Their design approach involved combining A and B Rings to produce a pyridine‐pyridine fused system, followed by systematic modification of the NH group of the pyrazole moiety. In the same series, 134 containing an alkyl‐imidazole substitution, was selective with an IC₅₀ of 21 (±6) nM against Akt1 and much weaker with an IC₅₀ of 839 (±53) nM against Akt2. Later optimization led to the description of alternative fused‐ring analogues, such as pyridopyrimidines, which resulted in 135, demonstrating moderate inhibitory efficacy with IC₅₀ values of 81 (±48) nM for Akt1 and 259 (±150) nM for Akt2 (Figure 37). Additional replacements on the pyrimidine ring tended to result in reduced potency, however. Additional scaffold optimization focuses on the terminal benzimidazolone group (C Ring). Incorporation of an amino‐pyrazole[3,4‐d]pyrimidine ring resulted in a 136 exhibiting significant potency and dual activity against Akt1 and Akt2. In the enzyme assays, the IC₅₀ values were 14 (±0.1) nM (Akt1) and 99 (±8) nM (Akt2). In cellular assays, 137 retained substantial activity, with IC₅₀ values of 295 nM (Akt1) and 468 nM (Akt2) (Figure 37). These results emphasize that fused heteroaromatic systems and terminal heterocyclic modifications are crucial to improving potency and selectivity, and 137 is the most promising analogue of the series.

Mark T. Bilodeau et al. [123] have described a series of naphthyridine and naphthyridinone analogues as potent allosteric dual Akt1/Akt2 inhibitors. Instead, they modified the underlying scaffold to 1, 8‐naphthyridine and replaced the C‐ring with pyrazole‐linked positional pyridine groups. In the same series, 135 contained a 4‐pyridinone replacement, exhibited significant dual Akt inhibitory action with enzyme IC₅₀ values of 34 nM (Akt1) and 83 nM (Akt2) and showed significant potency in cellular assays with IC₅₀ values of 132 nM (Akt1) and 613 nM (Akt2) (Figure 37). Additional optimization focuses on modifying core scaffolds by exploring naphthyridine isomers as well as increasing molecule polarity and the pyrazole of the C‐ring was replaced with a triazole, which improved potency. The most promising analogue was then replaced with the 1,6‐naphthyridin‐5‐one derivative (136). This compound exhibited enzyme activity with IC₅₀ values of 3.5 nM for Akt1 and 42 nM for Akt2, as well as cellular IC₅₀ values of 16 nM (Akt1) and 266 nM (Akt2) (Figure 37). These results demonstrate that modifying the scaffold and substituting heteroaryl groups can greatly improve both enzymatic and cellular inhibition of Akt, establishing 136 as an effective dual inhibitor of Akt1 and Akt2.

Tony Siu et al. [124] described a novel series of pyrido‐pyrimidine analogues to enhance cellular potency within the group of allosteric Akt1 and Akt2 inhibitors. However, in contrast to ATP‐competitive inhibitors, which tend to have lower kinase selectivity, they bind to the pleckstrin homology (PH) domain of Akt, making them less prone to induce off‐target effects and related toxicities. Within this series, several heterocyclic linkers were incorporated between the two nitrogen atoms of the pyrimidine ring. In this group, the N, N‐dimethyl‐2‐piperazin‐1‐yl ethan‐1‐amine‐substituted analogue (137) exhibited excellent potency with an enzyme IC₅₀ of 4 nM (Akt1 and Akt2), as well as with a cellular IC₅₀ of 10 nM (Akt1) and 39 nM (Akt2) (Figure 37). Later research was carried out on the functionality of the basic nitrogen atom in the piperazine substituent, leading to the design of methyl‐piperazine regioisomers. Among them, 138 exhibited significant potency as an enzyme, with IC₅₀ values of 9 nM (Akt1) and 27 nM (Akt2), and a cellular IC₅₀ of 36 nM (Akt1) and 52 nM (Akt2), respectively.

The results indicate the essential role of replacing the piperazine ring and positioning the nitrogen in regulating enzyme and cellular activity, which establishes pyrido‐pyrimidines as an effective scaffold for developing selective and potent allosteric Akt inhibitors.

Later, they planned to introduce different polar group changes to the C‐ring of the lead compound x. Research was conducted to reduce hERG channel binding liabilities and assess their impact on cellular potency. They incorporated different positional nitrogen‐containing aromatic rings, including a pyridone‐containing moiety 139 shows better activity against the enzyme of Akt1 with an IC₅₀ of 1 nM and Akt2 with an IC₅₀ of 8 nM and the cell of Akt1 with an IC₅₀ of 62 nM and Akt2 with an IC₅₀ of 284 nM (Figure 37), improved the activity against the enzyme Akt1 but later collapsed in the enzyme Akt2 and cellular Akt1 and Akt2, as well as hERG, with an IC₅₀ of more than 10000 nM (Figure 37). To tackle the challenge of hERG channel activity, the researchers employed a clever strategy: modifying the molecules to make them more polar by incorporating small heteroatoms into the aromatic rings. This idea was combined with their earlier finding that water‐soluble piperazines boost potency. These stood out as powerful, low‐nanomolar inhibitors of Akt1 and Akt2 in cells, while showing virtually no binding to hERG (>10 000 nM) promising step toward both effectiveness and safety in drug design [125].

Yiwei Li et al. [126] have reported a novel series of [1,2,4]triazolo[3,4‐f][1,6]naphthyridine allosteric dual inhibitors of Akt1 and Akt2. They aimed to develop compounds with reduced hERG affinity and increased effectiveness against the Akt1 and Akt2 isozymes. To achieve these goals, they preferred to investigate the naphthyridine compounds again, which had been more effective against the Akt1 isozyme, and, through SAR exploration, improve their activity against the Akt2 isozyme. They thought that combining the core structures of 70 [123] and 63 to form a tricyclic structure would enhance the inhibitory action on the targeted Akt Isozymes. The researchers subsequently developed a series of fused 1,2,4‐triazoles with various functional and alkyl substitutions. The hydroxyl‐substituted analogue (140) showed strong enzyme inhibition, with IC₅₀ values of 2.6 nM for Akt1 and 6.8 nM for Akt2, and reduced hERG binding (IC₅₀ = 5312 nM) (Figure 37). In cell assays, it maintained activity with EC₅₀ values of 27 nM (Akt1) and 167.7 nM (Akt2). The unsubstituted analogue (141) outperformed in cellular potency, with EC₅₀ values of 5 nM (Akt1) and 41 nM (Akt2), while retaining strong enzyme activity (IC₅₀ = 4 nM for Akt1 and 10 nM for Akt2) and excellent hERG safety (>10 000 nM). Research highlighted that, in this scaffold, removing the hydroxyl group improved cell‐based efficacy without compromising enzyme potency or safety. Table 10 summarizes the reported 5,6‐diphenyl analogues in terms of their structural features, biological activities, and progress across different stages of development.

TABLE 10.

Comparative summary of 5,6‐diphenyl‐substituted heterocyclic Akt inhibitors.

Lead compounds	Core scaffolds	Binding modes	Akt1 IC50 (nM)	Akt2 IC50 (nM)	Akt3 IC50 (nM)	Cellular activities	Major SAR findings
121	5,6‐Diphenylpyrazin‐2(1H)‐one	Allosteric	760	25 400	>50 000	NR	Akt1‐selective inhibitor; methyl substituent favors Akt1 selectivity.
122	5,6‐Diphenylpyrazin‐2(1H)‐one	Allosteric	21 200	325	21 870	NR	Isobutyl substitution switched selectivity toward Akt2.
123	Tetrazole analogue	Allosteric	63	65	1228	Improved after optimization	Tetrazole markedly increased Akt1/Akt2 potency but introduced zwitterionic character.
124	Tetrazole analogue	Allosteric	20	144	1613	NR	Highest Akt1 potency among tetrazole analogues.
125	Tricyclic pyrazole	Allosteric	85	300	2400	Improved physicochemical profile	Tricyclic scaffold maintained dual Akt1/Akt2 inhibition.
126	Tricyclic imidazole	Allosteric	58	210	2119	Improved after amide optimization	Imidazole core improved potency and drug‐like properties.
127	Amide analogue	Allosteric	138	212	7200	Improved solubility and cellular activity	Amide substitution enhanced physicochemical properties while maintaining dual Akt1/Akt2 inhibition.
128	Quinoxaline derivative	Allosteric	177	248	>50 000	NR	Quinoxaline scaffold preserved strong Akt1/Akt2 selectivity over Akt3.
129	Pyridopyrimidine	ATP‐competitive	15 ± 3	90 ± 19	NR	NR	Initial lead with aminophenylpropanone substituent.
130 (optimized)	Pyridopyrimidine	ATP‐competitive	3.8 ± 1	26 ± 5	NR	Akt1: 9.3 ± 2.6 nM; Akt2: 589 ± 8 nM	Pyridyl‐triazole substitution markedly enhanced Akt1 potency and cellular selectivity.
131	Pyridopyrimidine	ATP‐competitive	6.0 ± 0.3	94.4 ± 2.6	NR	Akt1: 20.3 ± 10.1 nM; Akt2: 899 ± 202 nM	Methylamine substitution retained Akt1 selectivity.
132	2,3,5‐Trisubstituted pyridine	Allosteric	85	22	NR	Akt1: 516 nM; Akt2: 330 nM	Aminothiadiazole was superior to tetrazole replacement.
133	2,3,5‐Trisubstituted pyridine	Allosteric	126	22	NR	Akt1: 621 nM; Akt2: 416 nM	Aminothiazole maintained dual inhibition.
134	Oxadiazole analogue	Allosteric	180	121	NR	Akt1: 288 nM; Akt2: 1061 nM	Oxadiazole inferior to aminothiadiazole scaffold.
135	1,8‐Naphthyridine	Allosteric	34	83	NR	Akt1: 132 nM; Akt2: 613 nM	Increased scaffold polarity improved cellular activity.
136	1,6‐Naphthyridin‐5‐one	Allosteric	3.5	42	NR	Akt1: 16 nM; Akt2: 266 nM	Scaffold optimization dramatically enhanced potency.
137	Pyridopyrimidine	Allosteric	4	4	NR	Akt1: 10 nM; Akt2: 39 nM	Piperazine linker greatly improved enzyme and cellular potency.
138	Pyridopyrimidine	Allosteric	9	27	NR	Akt1: 36 nM; Akt2: 52 nM	Nitrogen position in piperazine regulated potency.
139	1,6‐Naphthyridin‐5‐one	Allosteric	1	8	NR	Akt1: 62 nM; Akt2: 284 nM	Pyridone substitution minimized hERG liability (>10 000 nM).
140	Triazolo‐naphthyridine	Allosteric	2.6	6.8	NR	Akt1 EC50: 27 nM; Akt2 EC50: 167.7 nM	Hydroxyl substitution reduced hERG binding (5312 nM).
141	Triazolo‐naphthyridine	Allosteric	4	10	NR	Akt1 EC50: 5 nM; Akt2 EC50: 41 nM	Hydroxyl removal improved cellular potency while maintaining excellent hERG safety (>10 000 nM).

5.1.1. Structure–Activity Relationship of 5,6‐Diphenylpyridine Analogues

5.1.1.1. A Ring

The pyridine core ring was significantly more active than the pyrazine ring, which was likely crucial for preserving the optimal electronic properties. At the fifth and sixth positions of the pyridine ring, diphenyl substituents were necessary to enhance the activity. At the second and third positions of the pyridine ring fused with a [1,2,4]triazolo [4,3‐a]pyridine‐3‐ol ring resulted in increased activity.

5.1.1.2. B Ring

The presence of a phenyl ring was essential for activity.

5.1.1.3. Linker

The piperidine ring served as a vital linker, ensuring notable activity and maintaining the correct spatial alignment of the pharmacophoric groups. Replacing the piperidine linker with a secondary amine resulted in a loss of activity.

5.1.1.4. C Ring

A triazole ring having two isomers incorporated led to a considerable enhancement in activity. Further substitution of the pyridine ring and pyridin‐2(1H)‐one on the C ring resulted in increased activity (Figure 38).

FIGURE 38.

Structure–activity relationship of 3,4 disubstituted phenyl scaffold.

6. Miscellaneous Inhibitors

Christine B. Breitenlechner et al. [127] reported a structure‐based optimization of azepane derivatives designed to selectively inhibit protein kinase B (PKBβ) and protein kinase A (PKA). It was discovered that balanol structures containing an ester function group were plasma‐unstable, thus limiting their pharmacological applicability. To improve the stability of metabolism while maintaining high potency, the research team used various substitutions, such as amide analogues, based on molecular modeling studies. Within the series, 145 shows a significant effect against enzymes with IC₅₀ values of 2 nM for PKA and 4 nM for PKBβ, respectively [128] (Figure 39). These results show that a rational, structure‐guided approach can avoid metabolic instability and effectively target key kinase targets with nanomolar potency.

FIGURE 39.

Lead compounds inhibits Akt kinase.

Jong Hee Ko et al. [129] described the design and synthesis of a new series of 5‐arylamino‐6‐chloro‐1H‐indazole‐4,7‐diones that inhibit protein kinase B (Akt). To investigate the effect of Akt inhibition, different types of substituted phenyl rings were incorporated into the scaffold and connected via either sulfur or amine linkages. The synthesis of 146 involved the incorporation of a para‐substituted ethoxy group onto the phenyl ring along with a secondary amine linker. The result significantly enhanced the compound's ability to inhibit Akt1, with an IC₅₀ of 4900 nM (Figure 39). Structure–activity relationship (SAR) analysis revealed that halogen substitutions at the ortho, meta, or para positions of the phenyl ring were generally inactive, indicating that simple halide substitution is not favorable for Akt inhibition. In contrast, derivatives incorporating an amine linker consistently showed greater inhibitory activity than those containing a sulfur linker. Collectively, these results identified indazole‐4,7‐diones as a promising new scaffold for further optimization toward potent and selective Akt inhibitors.

Nicholas C. Kallan et al. [130] reported a novel series of spirochromane derivatives as pan‐Akt inhibitors, identified through high‐throughput screening (HTS) and further optimized by modifying the sulfonamide substituent. Initial modifications involved substitution of the sulfonamide NH with small alkyl groups or alkyl ethers. Although alkyl groups provided comparable potency to the corresponding ethers, the latter were favored due to their lower ClogP values. For instance, the isopropoxyethyl analogue (147) exhibited Akt1 with an IC₅₀ of 300 nM, while the ethoxyethyl analogue (148) possessed an identical activity (IC₅₀ = 1000 nM) (Figure 39). The ethoxyethyl side chain has been demonstrated to possess the optimal balance of potency, size, and lipophilicity and was consequently kept for further SAR investigation. Molecular modelling indicated that the sulfonamide group interacts within the limited hydrophobic P‐loop region of Akt, guiding subsequent optimization strategies. Incorporating small lipophilic groups into aromatic and heteroaromatic rings enhanced the activity. The incorporation of a 2,5‐dimethylphenyl moiety (149) significantly enhanced potency, resulting in an IC₅₀ of 40 nM against Akt1 (Figure 39). By lowering the chromanone ketone and adding different heteroaromatic substituents, the structure has been improved even more. The x‐ray analysis of the crystal structure of phenol‐based analogues revealed that the phenolic OH group formed weak hydrogen bonds with the hinge region. To overcome this limitation, various hinge‐binding motifs were evaluated, leading to the development of a potent analogue, 150 featuring a pyrazole‐fused heterocycle, which exhibited IC₅₀ values of 9 nM against Akt1 and 30 nM against PKA, a substantial increase in potency and selectivity.

Sudha Korwar et al. [131] examined pyranonaphthoquinone (PNQ) lactone natural products, which include 7‐deoxykalafungin, as an innovative class of covalent and selective Akt inhibitors. The research elucidated that the 3,6‐dihydro‐2H‐pyran ring in PNQ lactones is essential for providing significant and selective inhibition of Akt kinase activity. 7‐deoxykalafungin exhibited potent inhibitory activity against Akt1 with an IC₅₀ of 280 nM (Figure 39).

Salaski, E.J., et al. [132], had previously reported synthetic PNQ lactone analogues based on this foundation. In this series, 151 possesses the 3,6‐dihydro‐2H‐pyran ring but no substitution at the C5 position, was more potent than 7‐deoxykalafungin. 151 inhibited Akt1 with an IC₅₀ of 44 nM (Figure 39), which is approximately sixfold more potent than earlier. These results show the PNQ lactone scaffold as a promising natural product‐derived framework for the creation of effective and selective covalent Akt inhibitors.

 Martino Forino et al. [133] reported novel Akt (protein kinase B) inhibitors using in silico approaches. Based on the crystal structure of Akt1 kinase, several potential inhibitors were identified through predicted docking within the ATP‐binding site [114, 134, 135]. Among these, 152 exhibited significant activity against Akt1 with an IC₅₀ of 2600 nM and a K _i of 1100 nM (Figure 39).

E. Ibáñez et al. [136] reported the development of a novel series of imidoselenocarbamates as potential multi‐kinase inhibitors. Within this series, the structural modifications at the heteroaryl substituents have a significant influence on biological activity. Among them, the dipyridine‐substituted 154 exhibits significant inhibitory activity against Akt1, with an IC₅₀ value of 6700 nM (Figure 39). In contrast, the di‐quinazoline‐substituted 153 exhibited comparatively weaker inhibitory activity against Akt1 with an IC₅₀ value of 74 700 nM. These results reveal that the electronic and steric properties of heteroaryl substituents play a critical role in modulating the kinase inhibitory profile of quinoline imidoselenocarbamates.

Thuy Nguyen et al. [137] described the development of a new class of covalent, substrate‐competitive kinase inhibitors designed to enhance inhibitory potency against Akt1. In this series, phenylalanine‐substituted analogues were investigated as possible Akt inhibitors. Among them, 155 showed significant activity against Akt1 with an IC₅₀ of 580 nM (Figure 39). It also inhibited growth in HCT116 and H460 cancer cell lines with IC₅₀ values of 21 500 and 22 900 nM, respectively.

Dezhi Yang et al. [138] identified a series of 1,2,4,7‐tetra‐substituted indole analogues as novel Akt inhibitors. Structural modifications include the substitutions of various aromatic rings into the amino side chain, the substitution of various functional groups at the seventh position, and the incorporation of oxadiazole and methyl formate moieties at the second position. Several of the synthesized compounds demonstrated potent inhibition of Akt1 kinase as well as significant cellular efficacy in PC‐3 prostate cancer cells. The S‐isomer of 156 suppressed Akt1 by 72.5% with a cellular IC₅₀ value of 3800 nM in PC‐3 cells. Similarly, 157 (S‐isomer) suppressed Akt1 by 73% with an IC₅₀ of 5000 nM in PC‐3 cells. 158 (S‐isomer) suppressed Akt1 by 73.5%, with an IC₅₀ of 3200 nM. 159 (R‐isomer) showed a significant Akt1 inhibitory activity (78.6%) and cellular potency (IC₅₀ = 5300 nM) in PC‐3 cells (Figure 39). These results emphasize the potential of tetra‐substituted indole derivatives as scaffolds for the development of selective Akt inhibitors.

Xiaowu Dong et al. [139] applied the Catalyst/HypoGen program to develop a novel pharmacophore model for pyridinyl‐bridged PKB/Akt1 inhibitors. The important pharmacophoric features identified in the Hypo_1 model correlated with the interaction patterns predicted by molecular docking studies, allowing for an improved understanding of the compounds binding interactions with PKB/Akt1 [140, 141]. Within the series, 160 and 161 exhibited significant inhibitory activity against PKB/Akt1, with IC₅₀ values of 5400 and 3900 nM, respectively (Figure 39).

Wenhu Zhan et al. [142] continued this work by incorporating various heterocyclic substitutions at the sixth position of the chroman‐4‐one scaffold, which improved activity. 162, containing a morpholine substituent, inhibited Akt1 (IC₅₀ = 6200 nM) and exhibited significant activity against HL‐60 (IC₅₀ = 16650 nM) and HepG2 (IC₅₀ = 12500 nM) cell lines. Likewise, 163 containing an N‐methyl‐1‐phenylmethanamine moiety, demonstrated enhanced Akt1 inhibition (IC₅₀ = 5500 nM) and significant cytotoxicity against HL‐60 (IC₅₀ = 7560 nM) and HepG2 cells (IC₅₀ = 7200 nM) (Figure 39).

7. Conclusion

Understanding the SARs of inhibitors targeting the Akt protein kinase has been guiding the rational development of highly potent and selective therapeutic agents. The effectiveness and specificity of Akt inhibitors are significantly impacted by systematic structural alterations to their core heterocyclic scaffolds. These modifications are particularly relevant at the hinge‐binding region, the central heterocyclic core that occupies the ATP‐binding pocket, the linker or side chain that extends into the solvent‐accessible region, and the hydrophobic substituents that interact with the lipophilic pocket. These structural components collectively determine essential parameters such as binding affinity, isoform selectivity, and pharmacokinetic behavior. Studies using SAR across different chemical groups have shown that heterocyclic structures, such as isoquinoline‐5‐sulfonamide, thiophene/furan, thiadiazole/thiazole, pyridine, 5,6‐diphenylpyridine, imidazole‐fused pyridine, piperidin‐4‐amine/piperazine‐phenyl, and pyrrolidine derivatives, are important for stabilizing the inhibitor in Akt's ATP‐binding cleft. The stabilization process is facilitated by crucial hydrogen bonds and hydrophobic interactions.

However, decades of SAR‐driven optimization have not translated into broad clinical success, and this gap must be confronted directly rather than treated as a matter of further incremental potency gains. Several Akt inhibitors that showed strong biochemical and cellular activity failed or underperformed in the clinic, and the reasons are only partly chemical.

Class toxicity is dose‐limiting. Because Akt lies downstream of insulin signaling and controls glucose uptake (via GLUT4 translocation) and glycogen synthesis in normal tissue, on‐target inhibition produces a mechanism‐based toxicity profile hyperglycemia, diarrhea, and rash essentially every clinical Akt inhibitor, from MK‐2206 to ipatasertib and capivasertib. This is not an off‐target liability that better selectivity can remove; it is intrinsic to inhibiting a node that is physiologically essential outside the tumor. Continuous‐dosing regimens (e.g., ipatasertib) tend to produce higher rates of grade ≥3 hyperglycemia and diarrhea than intermittent schedules (e.g., capivasertib's 4‐days‐on/3‐days‐off regimen), illustrating that pharmacokinetic scheduling, not only scaffold design, is central to achieving a workable therapeutic index. Rare but serious complications, including diabetic ketoacidosis, have been reported post‐marketing, underscoring that metabolic monitoring is not a minor logistical addendum but a determinant of whether an otherwise active compound remains usable. Pathway feedback and reactivation blunt durable efficacy. Akt inhibition relieves negative feedback on upstream receptor tyrosine kinases and on PI3K itself, leading to compensatory reactivation of PI3K/Akt signaling and of parallel mitogenic pathways (e.g., MAPK). Allosteric inhibitors are particularly susceptible to this rebound because they preferentially stabilize the inactive kinase conformation, leaving actively phosphorylated Akt populations able to persist or reaccumulate; ATP‐competitive agents such as capivasertib, which inhibit both active and inactive conformations, were developed partly to counter this liability, but feedback reactivation of the wider network is not eliminated by scaffold choice alone.

Resistance is genomically and adaptively heterogeneous. Clinical benefit from Akt inhibitors is concentrated in molecularly defined subgroups tumors bearing PIK3CA, AKT1, or PTEN alterations while unselected populations show modest or inconsistent benefit; this was a central reason several trials of Akt inhibitors as unselected or monotherapy agents failed to meet their endpoints despite a sound mechanistic rationale. Acquired resistance further arises through secondary PTEN loss, compensatory activation of receptor tyrosine kinases (e.g., HER3 and IGF‐1R), and metabolic rewiring that reduces tumor dependence on Akt signaling, meaning a scaffold optimized against a single time‐point structural target does not guarantee durable pathway suppression in vivo.

Taken together, favorable in vitro SAR profiles are necessary but not sufficient for clinical success. The Akt inhibitor field's more informative lesson is that chemical optimization must be pursued alongside patient selection strategy, feedback‐pathway biology, and toxicity management from the earliest stages of design, rather than treating these as downstream clinical considerations. Table 11 summarizes the cell lines investigated in the study together with their corresponding RRIDs, enabling standardized identification and improving the transparency and reproducibility of the reported research.

TABLE 11.

List of cell lines along with their research resource identifier (RRID).

Cell line	Species/Tissue	RRID
LNCaP	Human, prostate carcinoma	CVCL_0395, CVCL_1379
BT474	Human, breast carcinoma	CVCL_0179
OVCAR‐8	Human, ovarian carcinoma	CVCL_1629
HCT116	Human, colon carcinoma	CVCL_0291
PC‐3	Human, prostate carcinoma	CVCL_0035
786‐O	Human, renal cell carcinoma	CVCL_1051
KHOS	Human, osteosarcoma	CVCL_2545, CVCL_2544
HGC‐27	Human, gastric carcinoma	CVCL_1279
HL‐60	Human, promyelocytic leukemia	CVCL_0002
HepG2	Human, hepatocellular carcinoma	CVCL_0027
H460	Human, large‐cell lung carcinoma	CVCL_0459
HFF	Human, normal foreskin fibroblast	CVCL_XB54
F5.12‐Akt	Mouse, IL‐3‐dependent pro‐B lymphocyte line	CVCL_0262

8. Future perspectives

The SAR analysis of Akt inhibitors has identified a clearly defined pharmacophoric framework, which can inform subsequent lead optimization efforts (Figure 40). The hinge‐binding region, designated as the A ring, which is characterized by electron‐donating groups (EDGs) including nitrogen and hydroxyl functionalities, is critical for hydrogen bonding; consequently, strategic optimization of this region is recommended to improve binding affinity and isoform selectivity. Furthermore, the heterocyclic core, or B ring, functions as a central scaffold, facilitating ATP mimicry, electronic interactions, and structural rigidity; therefore, scaffold diversification and bioisosteric replacement represent promising approaches for enhancing potency and pharmacokinetic profiles.

FIGURE 40.

Structural optimizing strategies and future prospective for better Akt analogue.

Advancing Akt inhibitors beyond the current plateau requires reframing the goal from “more potent, more selective scaffold” toward “clinically deployable agent with a defined patient population, a manageable toxicity profile, and a rational combination strategy.” Several specific directions emerge from this reframing.

8.1. Toxicity‐Aware Design and Scheduling

The issues of hyperglycemia, rash, and diarrhea arise from mechanisms rather than being specific to the scaffold itself. Hence, future advances are more likely to arise from pharmacokinetic engineering, such as development of intermittent dosing schedules, tissue‐selective delivery methods, or prodrug strategies that decrease exposure in insulin‐sensitive tissues, than from small modifications to hinge‐binding electronics. Physicochemical optimization of the solvent‐exposed region (polar linkers, controlled lipophilicity) should be evaluated not only for solubility and permeability but explicitly for its effect on peripheral (non‐tumor) tissue exposure.

8.2. Addressing Feedback Activation

Scaffold diversification of the heterocyclic core (B ring) should be paired with evaluation of each candidate's conformational selectivity (active‐ vs. inactive‐state binding), since this property determines susceptibility to feedback‐driven pathway reactivation. Preclinical pipelines should routinely include pathway‐reactivation assays (e.g., measuring compensatory p‐Akt, p‐ERK, or receptor tyrosine kinase phosphorylation after prolonged exposure) alongside standard potency and selectivity assays, so that reactivation liabilities are identified before clinical development rather than inferred retrospectively from trial failures.

8.3. Biomarker‐Based Patient Selection

Given that clinical benefit clusters in tumors with PIK3CA, AKT1, or PTEN alterations, future trial design should treat prospective biomarker stratification as a primary design element rather than a post hoc subgroup analysis. This includes refining companion diagnostics for low‐frequency but functionally important alterations (e.g., AKT1 E17K), and investigating whether combined biomarker signatures (pathway alteration plus baseline metabolic risk factors such as HbA1c) can simultaneously predict efficacy and anticipate toxicity risk, enabling more precise dose and monitoring decisions at treatment initiation.

8.4. Rational Combination Therapy

Because monotherapy activity is often modest and short‐lived, combination regimens designed around resistance and feedback biology are likely to be more productive than further single‐agent optimization. Promising directions include pairing Akt inhibitors with endocrine therapy or SERDs in hormone receptor‐positive breast cancer to address endocrine resistance, with CDK4/6 inhibitors or in the post‐CDK4/6 progression setting, with androgen‐axis inhibitors in prostate cancer, and with agents targeting compensatory receptor tyrosine kinase or MAPK signaling to pre‐empt feedback reactivation. Combination strategies also require explicit toxicity‐overlap assessment (e.g., additive hyperglycemia or gastrointestinal toxicity), which should be modeled early rather than discovered in combination‐phase trials.

8.5. Structure‐Based and Computational Integration

Molecular docking, molecular dynamics, and structure‐based design remain valuable, but should be applied to specific, clinically motivated questions such as distinguishing active‐ versus inactive‐state binders, modeling isoform‐selectivity trade‐offs relevant to the PTEN‐low or AKT1‐mutant contexts, and predicting off‐tissue exposure rather than pursued as generic potency‐optimization exercises disconnected from the reasons earlier candidates failed.

An iterative, hypothesis‐driven cycle linking scaffold modification to feedback and resistance biology, toxicity mechanism, and biomarker‐defined trial design is more likely to yield Akt inhibitors with durable clinical utility than continued refinement of binding affinity in isolation.

Author Contributions

Mayur S. Dhangar: writing – original draft, conceptualization, data curation, literature search, and visualization. Mahesh B. Palkar: conceptualization, visualization, data curation, manuscript editing, and supervision.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.