Targeting CDK2 to combat drug resistance in cancer therapy

Abstract

Drug resistance remains a major obstacle in cancer treatment, leading to treatment failures and high mortality rates. Despite advancements in therapies, overcoming resistance requires a deeper understanding of its mechanisms. This review highlights CDK2's pivotal role in both intrinsic and acquired resistance, and its potential as a therapeutic target. Cyclin E upregulation, which partners with CDK2, is linked to poor prognosis and resistance across various cancers. Specifically, amplifications of CCNE1/CCNE2 are associated with resistance to targeted therapies, immunotherapy, endocrine therapies and chemo/radiotherapy. Given CDK2's involvement in resistance mechanisms, investigating its role presents promising opportunities for developing novel strategies to combat resistance and improve treatment outcomes.

Graphical Abstract

Plain language summary

Article highlights.

• CCNE and CycE drive resistance to chemotherapies in cancer.

• CycE and LMWE contribute to resistance against endocrine therapies; combining CDK2 inhibitor with endocrine therapy shows promise in overcoming the resistance.

• CDK2 inhibition can restore sensitivity to trastuzumab in HER2⁺ cancers by counteracting resistance linked to CCNE overexpression and SMAD3 phosphorylation.

• Elevated CDK2 activity allows cancer cells to resist CDK4/6 inhibitors by preserving RB phosphorylation; CDK2 plus CDK4/6 inhibitor may improve treatment efficacy.

• Combining CDK2 and PARP inhibition may enhance efficacy and overcome resistance in cancers with CCNE1 amplification.

• CDK2 inhibition can enhance immune responses by increasing IFN-I and promoting effector T-cell infiltration, potentially overcoming tumor immune escape.

1. Introduction

CDKs are a family of serine/threonine protein kinases that are activated through complex formation with cyclins. These kinases play essential roles in various cellular processes, including cell cycle regulation, transcription, metabolism and differentiation. Dysregulation of the cell cycle is a hallmark of cancer, making CDKs attractive targets for therapeutic intervention [1].

CDK2 has garnered significant research interest as a drug target due to its critical roles in cancer development and resistance to anti-cancer therapies. When complexed with CycE, CDK2 regulates progression through the G1 phase of the cell cycle by phosphorylating RB and fully activating E2F transcription factors. Additionally, CDK2 partners with CycA to facilitate entry into the S phase and progression through the G2 phase [2] (Figure 1).

Figure 1.

CDK2/Cyclin Complexes: Phosphorylation Substrates and Regulatory Roles in Cell Cycle Control.

This figure illustrates the phosphorylation targets of CDK2/CycE and CDK2/CycA complexes, detailing their involvement in cell cycle regulation. CDK2/CycE primarily regulates the G1 phase by phosphorylating substrates such as RB, which activates E2F, while CDK2/CycA functions during the S and G2 phases. The diagram also emphasizes the role of cell cycle checkpoints (e.g., ATR, ATM pathways) in the DNA damage response and illustrates important interactions with cell cycle regulators, including p21, p27 and c-MYC. Created with Biorender.

CDK2 plays a crucial role in oncogenesis through various signaling pathways. While CDK2 is rarely directly dysregulated in cancers, alterations in its binding partners - particularly CycE and CycA - often lead to CDK2 dysfunction within cancer cells. Overexpression or post-translational modifications of these cyclins are frequently observed in cancer, contributing to the dysregulation of CDK2 activity [3]. While somatic mutations in the endogenous CDK2 inhibitors CDKN1A (p21) and CDKN1B (p27) are rare, their dysregulation plays a significant role in cancer progression. For example, mutations in TP53, which primarily regulates p21, occur in over half of all cancers; and c-MYC, a repressor of p21, is frequently overexpressed in many tumors. Additionally, p27 can be deregulated through increased degradation by SKP2, abnormal cytoplasmic localization and phosphorylation by AKT, all of which compromise its inhibitory function on CDK2. This disruption results in uncontrolled CDK2 activity, promoting unchecked cell cycle progression and cancer growth [4,5].

CCNE1 amplification is notably associated with resistance to several chemotherapeutic agents, including anthracyclines, platinum and taxanes [6]. Additionally, amplified or overactivated CCNE1 has been linked to resistance against endocrine therapies and trastuzumab in breast cancer [7,8]. Furthermore, CDK2 and CycE can diminish the efficacy of CDK4/6 inhibitors, with CCNE1 amplification emerging as a significant factor in resistance to these inhibitors. This has spurred interest in co-targeting CDK2 and CDK4/6 as a strategy to overcome resistance across various cancers [9–11]. Beyond their impact on tumor cells, CDK2 inhibitors also have immunomodulatory effects, highlighting their potential to enhance immunotherapeutic strategies and address resistance in immunotherapy [12,13].

This review synthesizes the latest research on CDK2-mediated resistance, offering a comprehensive resource for researchers, clinicians and the pharmaceutical industry. By examining the mechanisms of CDK2 and CCNE1 amplification and their impact on various therapies, we aim to elucidate the complex interactions between these pathways and treatment resistance, while proposing future directions for overcoming these challenges.

2. Drug resistance

2.1. Epidemiology

Drug resistance is a major challenge in oncology, responsible for up to 90% of treatment failures and cancer-related deaths [14]. This phenomenon reflects the cancer cell's ability to survive, proliferate and metastasize despite therapeutic agents, indicating a significant tolerance level to the chosen treatment. Several complex factors contribute to drug resistance in cancer [14].

The limitations of conventional DNA-damaging chemotherapeutics and the identification of genomic drivers of oncogenesis have led to the development of molecularly targeted therapies. Despite the progress made with these targeted approaches, drug resistance remains a significant clinical issue [15] For instance, a substantial proportion (30–55%) of patients with non-small cell lung cancer (NSCLC) experience relapse and disease progression [16]. In ovarian adenocarcinoma, 50–70% of patients relapse within a year following initial chemotherapy [17]. Additionally, nearly 20% of children with acute lymphoblastic leukemia face recurrence [18].

2.2. CDK2 & its role in drug resistance in cancer

Resistance to cancer treatments often stems from complex molecular processes, with CDK2 playing a central and multifaceted role (Figure 1). CDK2's regulation of the cell cycle, DNA repair and transcription pathways significantly impact resistance to chemotherapy, radiotherapy and targeted therapies.

CDK2's involvement in cancer resistance mechanisms is primarily driven by its overexpression, interactions with cycE and the upregulation of oncogenes such as MYC. Additionally, mutations in key tumor suppressor genes like RB and TP53 further enhance CDK2 activity. These alterations disrupt cell cycle regulation, allowing cancer cells to evade therapeutic interventions. Table 1 provides an overview of CDK2's role in various resistance mechanisms across different cancer types.

Table 1.

Role of CDK2 in different resistance mechanisms in cancer.

Treatment	Primary treatment	Cancer type	Mechanism of resistance	Ref.
Chemotherapy	Platinum agents	HGSOC	CCNE1 overexpression	[19,20]
	Anthracycline and/or a taxane-containing regimen	TNBC		[21]
	Cisplatin	HPV-negative HNSCC	c-MYC and CycE upregulation	[22]
	Doxorubicin Paclitaxel	Prostate cancer	Sox2 overexpression leading to CycE upregulation	[23]
Endocrine therapy	Tamoxifen/letrozole	HR+ breast cancer	CCNE2 overexpression	[24]
Targeted therapy	Palbociclib/Ribociclib + endocrine therapy	ER+ breast cancer	CCNE1/CCNE2 overexpression	[25]
	Palbociclib	TNBC	CycE1 dysregulation post-mitosis	[26]
			CCNE1 and CDK2 overexpression	[27]
		Gastric cancer	CCNE overexpression	[28]
		ER+/HER2- breast cancer	Noncanonical CDK2/CycD1-mediated S phase entry	[29]
			CDK2/CycE-mediated phosphorylation of c-MYC	[30]
	CDK4/6 and aromatase inhibitors		CDK6, p-CDK2, and/or CycE1 upregulation	[31]
	CDK4/6 inhibitors	HR+/HER2- breast cancer	CCNE1, CCNE2 and CDK2 overexpression RB loss	[32]
	PF-3600 (CDK2/4/6 inhibitor)		MYC overexpression CCNE1/CCNE2 overexpression RB loss	[33]
	Abemaciclib/Palbociclib	GIST	CycD1 fusion Protein overexpression RB loss	[11]
	Dinaciclib	HGSOC	CCNE1 and AKT2 overexpression TP53 mutation	[34]
	Tyrosine Kinase Inhibitors	GIST (No clinical significance from palbociclib)	CCNE1 overexpression CDKN1A/p21 downregulation	[35]
	PI3K inhibitors (LY294002, PIK-90, PI-103, PIK-75)	Malignant glioma	CDK2 overexpression	[36]
	PARP inhibitor	BRCA1 mutated breast cancer	BRCA1 mutation leading to CycE1 high stability (disruption in degradation machinery)	[37]
	BRAF and Hsp90 inhibitors	Melanoma	MITF overexpression leading to CDK2 upregulation	[38]
	BRD4 inhibitor (JQ-1)	Medulloblastoma	MYC overexpression	[39]
Immunotherapy	Anti-PD1 (muDX400)	Colon adenocarcinoma Urothelial adenocarcinoma	Immune suppression via PD-1/PD-L1 Pathway	[13]
	Trastuzumab	HER2+ breast cancer	CCNE1 overexpression	[8]
Radiotherapy	X-rays	HNSCC	DSB repair by CDK2	[40]
	Gamma-rays	Glioblastoma	CDK2 overexpression	[41]

GIST: Gastrointestinal stromal tumor; HGSOC: High-grade serous ovarian cancer; HNSCC: Head and neck squamous cell carcinoma; TNBC: Triple-negative breast cancer.

Additionally, Figure 2 illustrates how CDK2/CycE contributes to resistance against various classes of drugs. Combining CDK2 inhibitors with other anticancer agents represents a promising strategy to enhance overall treatment effectiveness and overcome resistance.

Figure 2.

Role of CDK2/CycE in Resistance Mechanisms to Various Therapies.

Chemotherapy Resistance: ROS Scavenging: The CDK2/CycE can contribute to resistance by scavenging ROS, reducing oxidative stress-induced damage. Cyclin E Cleavage: Cyclin E is cleaved into a LMWE that binds more effectively to CDK2, enhancing resistance. Sox2 Overexpression: Increased Sox2 levels and its phosphorylation by CDK2/CycE1 contribute to resistance.

Endocrine Resistance: MYC-Induced Resistance: CDK2/CycE mediates resistance through MYC, a key oncogene. Downregulation of p21: Reduced levels of p21, a CDK inhibitor, facilitate resistance. Cyclin E Cleavage: The LMWE form of Cyclin E, resulting from cleavage, plays a role in endocrine resistance.

Resistance to CDK4/6 Inhibitors: c-MET/FAK/CDK2 Axis: Dysregulation in this pathway contributes to resistance. Noncanonical CDK2/Cyclin D1 Activation: Alternative activation of CDK2/Cyclin D1 can drive resistance. Loss of RB and PTEN: The absence of RB and PTEN is linked to resistance.

Resistance to PI3K Inhibitors and Anti-HER2 Therapies: CDK2 and PI3K Interaction: The interaction between CDK2 and PI3K mediates resistance mechanisms to these therapies.

Resistance to PARP Inhibitors: Replication Fork Repair: The CDK2/CycE aids in repairing replication forks, contributing to resistance to PARP inhibitors.

Radioresistance: CDK2/TRIM32/STAT3 Pathway: This pathway is crucial for radioresistance. DNMT1 Expression: Inhibition of CDK2 can lead to increased DNMT1 expression, which reverses immunoresistance.

Created with BioRender.

2.2.1. CDK2 & chemoresistance

Resistance to chemotherapy often involves CDK2 activation, particularly its association with CycE and CycA [6]. A recent study revealed that amplification of CycE1 in triple-negative breast cancer (TNBC) patients is associated with increased chemotherapy resistance and poorer overall survival [21]. Doxorubicin generates ROS that induce cytotoxicity in cancer cells. Preclinical studies indicate that elevated CycE expression in doxorubicin-resistant cells enhances their ability to scavenge ROS, thereby conferring resistance to doxorubicin. Additionally, CycE contributes to doxorubicin resistance by overexpressing Sox2 [23].

Most ovarian cancer patients initially respond to taxane- and platinum-based chemotherapies. However, they often develop resistance over time, leading to recurrence and metastasis [42]. One prevalent genetic alteration in ovarian cancer is CCNE1 amplification, which plays a crucial role in predicting chemotherapy resistance [43]. A study analyzing genome-wide copy number variations in tumor samples from 118 patients identified a significant correlation between amplification of the 19q12 genome region, which includes the CCNE1 gene and poor responses to primary treatments, including taxanes and platinum-based agents [44].

CCNE1 is also identified as a key oncogenic driver in high-grade serous ovarian cancer (HGSOC) [45]. Nearly half of HGSOC patients have homologous recombination deficiency, which supports the use of PARP inhibitors and platinum agents [46]. However, patients with CCNE1 amplification and homologous recombination proficiency are likely to resist platinum agents [45]. Chemotherapy resistance in ovarian cancer can also involve mechanisms such as “mitotic slippage”, where CCNE1 amplification facilitates mitotic exit, leading to increased rates of the mitotic slippage [47]. Additionally, taxane resistance may be driven by simultaneous hyperactivation of PI3K/AKT and CycE [48].

The role of CCNE2 in chemotherapy resistance, although less characterized than CCNE1, is also significant. CCNE2 amplification is present in 15–20% all breast cancer patients and 28.6% of luminal B breast cancer cases and is associated with higher invasiveness and chemoresistance [25].

Overall, chemotherapy resistance in cancers such as TNBC and ovarian cancer involves complex mechanisms related to CDK2 and CycE. Gene amplifications like CCNE1 and CCNE2 play crucial roles in promoting resistance and affecting patient responses to treatments. Understanding these molecular mechanisms is essential for developing targeted therapies to overcome resistance and improve cancer outcomes. Table 1 summarizes the role of CDK2 in the development of resistance to taxanes, platinum-based agents and antimetabolites in various cancer types.

2.2.2. CDK2-mediated resistance to endocrine & anti-HER2 therapy

Approximately 70% of breast cancer patients have ER⁺ tumors, where estrogen binding to ER triggers cell cycle progression and the expression of genes that promote tumor growth [49]. Endocrine therapy is effective in reducing recurrence by almost 40% in ER⁺ cases, but resistance frequently develops, presenting a significant clinical challenge [50]. Endocrine-resistant tumors often display alteration in cell cycle regulators, including c-MYC. A primary mechanism by which c-MYC induces endocrine resistance is through the activation of CDK2/CycE, facilitated by decreased levels of p21 (Figure 2, Endocrine Resistance) [51].

Overexpression of CycE in various subtypes of ER⁺ breast cancers is associated with resistance to anti-estrogen therapies [51]. Given that high levels of CycE suggest that CDK2 inhibition could be a viable strategy in estrogen-resistant contexts. Specifically, overexpression of LMWE has been linked to resistance to aromatase inhibitors [52]. Since LMWE has a higher affinity for CDK2, combining CDK2 inhibitors with aromatase inhibitors may be an effective approach to counteract this resistance [7,52].

Moreover, roscovitine, a CDK2/7/9 inhibitor, has been shown to downregulate the ERα isoform receptor, leading to reduced growth in letrozole and tamoxifen-resistant tumors [53]. On the other hand, patients with primary or acquired resistance to aromatase inhibitors have shown benefits from combining endocrine therapy with CDK4/6 inhibitors [31]. However, resistance to this combination therapy can still occur. It has been demonstrated that elevated levels of CDK6, p-CDK2 and CycE1 are associated with resistance to both aromatase inhibitors and the combined endocrine therapy and CDK4/6 inhibitor regimen [31]. In a preclinical model, combining dinaciclib (a CDK2/5/9 inhibitor) with palbociclib and letrozole resensitized letrozole-resistant breast cancer to the combination, effectively impairing tumor growth [31]. Mechanistically, this triple combination suppresses tumor growth by reducing CDK2-mediated c-MYC-Ser62 phosphorylation (Figure 1) [30,31].

Trastuzumab, a humanized monoclonal antibody targeting the HER2 receptor, is FDA-approved for treating HER2-positive breast and gastric cancers. It improves disease-free survival by approximately 30% in HER2+ breast cancer patients. However, about 70% of patients, whether in primary or metastatic stages are resistant to trastuzumab [54]. Overexpression of CCNE has been implicated in the resistance, with this overexpression regulated by noncanonical SMAD3 phosphorylation, which leads to higher rates of proliferation (Figure 2, Anti-HER2 resistance) [55]. In a study, HER2+ breast cancer cell line BT474R2, which overexpresses CCNE, showed higher pSMAD3 at T179 and S204 compared with the sensitive BT474 cell line. CDK2 phosphorylates SMAD3 at T179 and S213 in vivo and S204 in vitro, consistent with the observed phosphorylation pattern in BT474R2 cells. Fadraciclib, a CDK2/9 inhibitor, reduced the pSMAD3, leading to the restoration of TGFβ/SMAD3 signaling [55]. Additionally, fadraciclib has been shown to restore the sensitivity of HER2+/CCNE breast cancer cells to trastuzumab [8], suggesting that CCNE overexpression/amplification is a key molecular alteration contributing to trastuzumab resistance [8,56].

2.2.3. Role of CDK2 in resistance to CDK4/6 inhibitors

While endocrine therapy is effective in treating ER⁺ breast cancer, persistent activity of CycD1 and CDK4 highlights the need to target CDK4 [57]. The FDA-approved CDK4/6 inhibitors have shown clinical efficacy in ER⁺ breast cancer cases, significantly improving overall survival when combined with endocrine therapies [58]. In endocrine-sensitive cases, the PALOMA-2, MONALEESA-2 and MONARCH-3 studies demonstrated significant improvements in both progression-free survival (PFS) and objective response rate (ORR) when CDK4/6 inhibitors were combined with aromatase inhibitors, compared with the use of aromatase inhibitors alone [58,59]. However, CDK4/6 inhibitors exhibit varying effectiveness in endocrine-resistant breast cancer. Abemaciclib, for instance, has shown benefits in extending PFS, while ribociclib and palbociclib have not demonstrated significant advantages in patients with primary endocrine resistance. Resistance to CDK4/6 inhibitors remains a challenge, with approximately 25–30% of patients experiencing intrinsic resistance and eventual resistance developing in all cases over time [58–60].

Several mechanisms have been proposed (Figure 2, CDK4/6 inhibitor resistance) to explain resistance to CDK4/6 inhibitors [61], including:

• Intrinsic RB Loss: Loss of the RB tumor suppressor gene can bypass the need for CDK4/6 activity, reducing the effectiveness of inhibitors.

• Acquired Functional Loss of RB: Even in the presence of RB, its functional loss through mutations or alterations can lead to resistance.

• Loss of RB and PTEN: Concurrent loss of RB and PTEN can disrupt multiple regulatory pathways, contributing to resistance.

• Overactivation of CDK2: CDK2 overactivation can compensate for the inhibition of CDK4/6, allowing continued cell cycle progression.

• CCNE1 Amplification: Amplification of CCNE1 has been implicated in resistance by promoting cell cycle progression despite CDK4/6 inhibition.

• Aberrations in the c-MET/FAK/CDK2 Axis: Disruptions in the c-MET/FAK/CDK2 signaling axis can lead to resistance by bypassing the effects of CDK4/6 inhibition.

• c-MYC Upregulation: Increased levels of c-MYC can drive cell cycle progression and contribute to resistance.

Several preclinical and clinical studies have reported the significant interaction between CCNE1 gene amplification and the effectiveness of CDK4/6 inhibitors, supporting the role of CycE1 as a marker of resistance [62]. In ovarian and gastric cancer, the upregulation of CycE1 has been linked to resistance to palbociclib [28,63]. Additionally, biomarker analyses have identified an association between elevated CCNE1 mRNA levels and reduced response to CDK4/6 inhibitors in HR⁺/HER2^- metastatic breast cancer patients [61].

Overactivation of CDK2 has been observed in response to CDK4/6 inhibition, particularly in TNBC and ER⁺ breast cancer [29,64]. CDK2 can become active independently of CDK4/6, driving cells from the G1 to S phase. Consequently, CDK2 overactivation undermines the effectiveness of CDK4/6 inhibitors, such as palbociclib and ribociclib, as cancer cells with elevated CycE1 levels adapt to CDK4/6 inhibition by upregulating CDK2 activity [64].

Supporting these observations, a recent study compared the effects of a CDK2 inhibitor (PF-4091) with a CDK2/4/6 inhibitor (PF-3600) and identified a buffering mechanism between CDK2 and CDK4/6 that maintains RB hyperphosphorylation [65]. The study demonstrated that while acute inhibition of CDK2 quickly reduces the phosphorylation of CDK2 substrates, RB phosphorylation can be compensated by CDK4/6 activity. This compensation allows E2F to continue its transcriptional activity, leading to increased production of CycA2, which then binds to CDK2 and results in a rebound of CDK2 substrate phosphorylation. Conversely, co-inhibition of CDK2 and CDK4/6 prevents RB phosphorylation and eliminates this rebound effect. Additionally, incomplete chemical inhibition of CDK2 leaves residual active CDK2 that maintains RB phosphorylation [65].

An earlier investigation by the same group showed that PF-3600 can reverse palbociclib resistance in HR⁺ breast cancer with CCNE1 amplification or RB loss [33]. These findings suggest that CDK2/4/6 inhibitors, like PF-3600, have the potential to effectively overcome resistance in cancers with CCNE1 amplification [61].

Activation of CDK2 has been identified as a key mechanism of resistance to CDK4/6 inhibitors across various cancers, mediated through the c-MET/FAK/CDK2 axis [66]. MET/FAK signaling facilitates CDK2 activation and cell cycle progression independent of CDK4/6 activity, a phenomenon observed in glioblastoma and breast cancer. Data suggest that prolonged expression of SKP2 plays a crucial role in MET/FAK-mediated CDK2 activation by enhancing the degradation of p21 and other CIP/KIP CDK inhibitors [66].

Consistent with these findings, inhibition of FAK by small molecule inhibitors or the FRNK domain has been shown to alter levels of SKP2 and p21 in various cell lines [67]. Clinical data further support a connection between c-MET aberrations and resistance to abemaciclib in HR⁺/HER2^- breast cancer [68]. Given these insights, combining MET or FAK inhibitors with CDK2 inhibitors emerges as a promising strategy to manage CDK2 hyperactivity and enhance the effectiveness of CDK4/6-based therapies.

c-MYC overexpression can establish a positive feedback loop with CDK2, thereby contributing to resistance against CDK4/6 inhibitors [69]. Elevated c-MYC levels have been reported in palbociclib-resistant breast cancer cell lines [30], where CDK2 inhibition has been shown to impede c-MYC upregulation. Additionally, a recent in vivo study revealed that pre-exposure to cisplatin induces intrinsic resistance to palbociclib by upregulating both c-MYC and CycE [70].

The combination of palbociclib with a c-MYC bromodomain inhibitor, such as JQ1, resulted in synergistic antiproliferative effects. This finding underscores the potential of targeting both c-MYC and CDK2 to overcome resistance. Moreover, some ER⁺ breast cancers utilize noncanonical CDK2/ CycD1-mediated S phase entry as a compensatory mechanism for CDK4/6 inhibition [29]. This further highlights the critical role of CDK2 in mediating resistance and the therapeutic value of co-targeting CDK2 and CDK4/6 in cancer treatment.

Several CDK2 inhibitors are currently under investigation (Table 2) in combination with CDK4/6 inhibitors (or CDK2/4/6 inhibitors) and hormonal therapies:

Table 2.

CDK2 inhibitors in clinical trials.

CDK2 inhibitor	Structure	Kinase inhibition (nM)	Targeted cancers	Clinical toxicity	Schedule	Clinical trial	Ref.
Fadraciclib (Cyclacel)		CDK2 (4.5) CDK1 (578) CDK4 (232) CDK5 (20.5) CDK7 (193) CDK9 (26.2)	Solid Tumours CLL AML MDS	Reversible neutropenia, thrombocytopenia, febrile neutropenia, diarrhoea, hypomagnesemia, white blood cell lysis syndrome, constipation, decreased appetite, dehydration, fatigue, nausea and vomiting	Day 1, 2, 8, 9 and then every 3 weeks	Phase I/II - Recruiting	[71,72,73]
Ebvaciclib (PF-3600 Pfizer)		CDK2 (0.13) CDK1 (4.5) CDK4 (1.25) CDK6 (0.11) CDK9 (19.6)	Locally advanced/metastatic HR+/HER2-, TNBC, Platinum-resistant ovarian cancer	Nausea, anaemia and neutropenia	QD for 28 days	Phase II - Active, Not Recruiting	[74,75,76,77]
Inixaciclib (NUV-422 Nuvation Bio Inc)		CDK2 (7) CDK1 (73) CDK4 (2) CDK6 (1)	ER+ metastatic breast cancer, mCRPC, glioma	Blurred vision, eye redness and pain	QD for 28 days	Phase I/II - Terminated	[78]
Tagtociclib (PF-4091 Pfizer)		CDK2 (1.1) CDK1 (110) CDK4 (243) CDK6 (465) CDK9 (177)	SCLC, ovarian cancer, breast cancer	Nausea, diarrhoea, fatigue, vomiting, anaemia	BID	Phase I/II - Recruiting	[79,80]
BLU-222 (Blueprint-Medicines Corp)		CDK2 (2.6) CDK1 (233.6) CDK4 (377.4) CDK6 (275.2) CDK7 (6941.2) CDK9 (6115.1)	Advanced solid tumours, resistant HR+ HER2- breast cancer, endometrial and gastric cancer, CCNE1-amplified ovarian cancer	nausea, vomiting, anaemia, diarrhoea, fatigue, blurred vision, photophobia, vision change	BID for 28 days	Phase I/II - Recruiting	[81,82]
INX-315 (Incyclix Bio)		CDK2 (0.6) CDK1 (30) CDK4 (126) CDK6 (349) CDK7 (>10000) CDK9 (62)	CCNE1-amplified cancers and CDK4/6 inhibitor-resistant breast cancer	NA	QD for 28 days	Phase I/II - Recruiting	[83]
INCB0123667 (Incyte Corporation)	NA	CDK2 (0.87) CDK1 (195) CDK4 (46) CDK6 (206) CDK7 (355) CDK9 (3676)	CCNE1-amplified cancers	NA	QD for 28 days	Phase I - Recruiting	[84,85]
ARTS-021 (Allorion Therapeutics)	NA	CDK2 (1.4) CDK1 (942) CDK4 (477) CDK6 (1237) CDK7 (2834) CDK9 (7440)	CCNE1-amplified cancers	NA	QD for 28 days	Phase I/II - Recruiting	[86]

AML: Acute myeloid leukemia; BID: Twice per day; CLL: Chronic lymphocytic leukemia; MDS: Myelodysplastic syndrome; mCRPC: Metastatic castration-resistant prostate cancer; NA: Not available; QD: Once per day; SCLC: Small cell lung cancer; TNBC: Triple-negative breast cancer. Data for structure, targeted cancers, reported clinical toxicity, F (%), route of administration, schedule and clinical trial stage were collected from https://go.drugbank.com/ and https://clinicaltrials.gov/.

• AZD8421 (CDK2 inhibitor) is under investigation in combination with camizestrant and CDK4/6 inhibitors (NCT06188520).

• PF-4091 (CDK2 inhibitor) is being tested in combination with palbociclib and endocrine therapy fulvestrant or letrozole (NCT04553133).

• PF-3600 (CDK2/4/6 inhibitor) is being evaluated alongside endocrine therapy (NCT03519178).

• BLU-222 is under study in combination with ribociclib and fulvestrant (NCT05252416).

• INX-315 is being tested with CDK4/6 inhibitors and endocrine therapy (NCT05735080).

These trials aim to explore the potential benefits of targeting CDK2 alongside CDK4/6 and hormonal therapies in improving treatment outcomes.

CDK2 and CycE can undermine the efficacy of CDK4/6 inhibitors and some cancers often rely on CCNE1 to survive. Consequently, co-targeting CDK2 alongside CDK4/6 presents a viable strategy for overcoming such resistance. This approach has shown promise in addressing resistance across various cancer types, including HR⁺/HER2^- breast cancer [9,10], ovarian cancer [10] and advanced gastrointestinal stromal tumor [11].

2.2.4. CDK2-mediated resistance to PI3K/AKT inhibitors

AKT phosphorylates CDK2 at threonine 38 as part of the PI3K/AKT signaling pathway and promotes the transition from G2 to M phase. Conversely, CDK2 is also responsible for phosphorylating AKT at Ser477 and Ser479, a necessary step for the full activation of AKT. Overactivation of this pathway is a key driver of cancer proliferation and survival [87].

Preclinical studies have shown that CDK2 and PI3K cooperate to induce therapeutic resistance in malignant glioma (Figure 2, PI3K inhibitor resistance) [36]. Inhibition of CDK2 with NU6102 effectively prevents the full activation of the PI3K/AKT pathway. Additionally, combining CDK2 inhibitors with PI3K and AKT inhibitors has demonstrated improved efficacy in CCNE1-amplified xenograft models of uterine serous carcinoma and ovarian cancer [34,88]. Thus, dual inhibition of PI3K and CDK2 represents a promising strategy for overcoming resistance and enhancing treatment efficacy in a range of cancers.

2.2.5. CDK2-driven resistance to PARP inhibitors

PARP inhibitors have emerged as a significant tool in targeted therapy, providing a promising initial response even in patients without homologous recombination deficiency. However, 40–70% of patients develop resistance, leading to cancer recurrence. PARP inhibition is particularly effective in cancers with BRCA1 or BRCA2 loss of function, as it exploits the inability of these cells to repair DNA damage through homologous recombination (Figure 1) [89].

CCNE1 amplification is often mutually exclusive with homologous recombination deficiency. This is because CCNE1-amplified cancer cells may depend on homologous recombination to repair replication fork collapse (Figure 2, PARP inhibitor Resistance), as indicated by the high levels of CDK2, CycE1 and CycA2 at the site of stalled replication forks [90]. Consequently, patients with HGSOC exhibiting CCNE1 amplification are less likely to benefit from PARP inhibitors [45]. Interestingly, CDK2 inhibition at sub-efficacious doses has been shown to enhance the sensitivity of resistant cancer cells to DNA-damaging agents and PARP inhibitors when CCNE1 is amplified [90]. Thus, targeting CDK2 in combination with PARP inhibitors could offer a strategic approach to overcome resistance and improve treatment outcomes in cancers exhibiting CCNE1 amplification.

2.2.6. CDK2-mediated resistance to Hsp90 & BRAF inhibitors

RAF mutations are present in approximately 60% of malignant melanoma cases. The development of BRAF inhibitors, such as vemurafenib and dabrafenib, has significantly improved response rates in BRAF-mutated melanoma. However, resistance to these inhibitors often emerges rapidly [91]. A study by Azimi et al. examined various melanoma cell lines, including patient-derived xenograft (PDX) models and found that a subset of these cell lines was resistant to the Hsp90 inhibitor XL888 [38]. CDK2 has been implicated in driving resistance to both BRAF and Hsp90 inhibitors. The research identified that resistance to the inhibitors was associated with amplification or overexpression of MITF [38]. These findings suggest that patients with MITF amplification or overexpression, which indicates resistance, could potentially benefit from CDK2 inhibitors. This insight highlights a promising avenue for enhancing melanoma treatment strategies by incorporating CDK2 inhibitors to address resistance mechanisms associated with BRAF and Hsp90 inhibition.

Table 1 outlines the role of CDK2 in driving resistance across various treatments, including anti-HER2, endocrine therapy, CDK4/6 inhibitors, tyrosine kinase inhibitors, BRAF and Hsp90 inhibitors, BRD4 inhibitors and PARP inhibitors, in different solid tumors.

2.2.7. CDK2-associated resistance to immunotherapy

Immunotherapy has emerged as a cornerstone of cancer treatment, offering long-lasting responses in some patients. However, a significant number of patients either fail to respond or develop resistance [92]. Targeting CDK2 presents a promising approach to improve immunotherapy outcomes due to its involvement in tumor immunobiology. Recent research has shown that inhibiting CDK2 reduces E2F-mediated DNMT1 expression, which in turn boosts IFN-I responses (Figure 3, Immunoresistance). This enhancement leads to increased activation of effector T cells and their infiltration into tumor tissues [12]. Dinaciclib has been shown to reverse resistance to anti-PD-1 by inducing Immunogenic Cell Death (ICD) and enhancing the immune response. Moreover, when Dinaciclib is combined with anti-PD-1 therapy, the immune system will be able to overcome the tumor's immunosuppressive defences and attack the tumor more effectively [93].

Figure 3.

Drug resistance mechanisms in cancer and solutions to overcome therapy resistance.

On the left side, the infographic details key factors contributing to drug resistance: Tumor Heterogeneity: Genetic diversity within cancer cells leads to varied responses to treatment. Tumor Growth Kinetics: Slow-growing tumors often exhibit higher resistance to therapy. Undruggable Genomic Drivers: Genes like MYC and TP53 promote cancer progression but cannot be directly targeted. Selective Therapeutic Pressure: Treatment can inadvertently lead to the survival and expansion of resistant cells. Immune System and Tumor Microenvironment: These factors can mediate resistance through various mechanisms. Genetic Mutations: Mutations within cancer cells can contribute to resistance. Physical Barriers: Structures like the basement membrane can impede drug delivery to cancer cells.

On the right side, the infographic presents solutions to address these resistance mechanisms: Early Detection: Advanced diagnostic tools enable the identification of cancer at more treatable stages. Targeted Treatments: Small molecule inhibitors and other targeted therapies address specific vulnerabilities in cancer cells. Mapping Cancer Dependencies: Technologies like CRISPR help identify critical vulnerabilities in cancer cells. Combination Therapies: Approaching cancer with multiple treatment modalities can enhance effectiveness. Therapeutic Monitoring and Adaptive Interventions: Continuously assessing and adjusting treatment based on tumor response can improve outcomes and manage resistance.

Created with BioRender.

CDK2, through the CDK2-RB-E2F-DNMT1 pathway, influences tumor immune biology, contributing to immune evasion and resistance to immunotherapy (Figure 3, Immune system and tumor microenvironment). Therefore, inhibiting CDK2 could potentially overcome these barriers and improve the efficacy of cancer immunotherapy. Integrating CDK2 inhibitors with existing immunotherapeutic strategies might offer new avenues to enhance patient responses and combat resistance.

2.2.8. CDK2 in radioresistance

Radiotherapy is a fundamental treatment modality for many cancers, yet resistance to this approach poses a significant challenge. Radioresistance often develops following repeated exposures to ionizing radiation [94]. Recent studies have illuminated CDK2's role in mediating this resistance.

In a study of Head and Neck Squamous Cell Carcinoma (HNSCC), knockdown of CDK2 was shown to enhance radiosensitivity [40]. Similarly, in glioblastoma multiforme (GBM), depletion of CDK2 resulted in increased radiosensitivity and promoted apoptotic effects [41]. Furthermore, research on TNBC has identified CDK2's involvement in radioresistance through its phosphorylation of TRIM32, a tripartite motif family protein [95]. This modification leads to TRIM32's nuclear translocation and activation of the STAT3 pathway, which is known to contribute to cancer radioresistance (Figure 2, Radioresistance). These findings suggest that targeting the CDK2/TRIM32/STAT3 pathway may be an effective strategy to overcome radioresistance in TNBC and potentially other cancers. Overall, CDK2's role in radioresistance across various cancers, including HNSCC, GBM, osteosarcoma and TNBC, highlights the potential of CDK2-targeted therapies to enhance the effectiveness of radiotherapy.

3. CDK2 inhibitors: current status

Although many CDK inhibitors have been assessed in preclinical settings, only a few have progressed to clinical trials for their potential in treating CCNE1-amplified cancers, either as monotherapy or in combination with conventional or targeted therapies. As shown in Table 2, while these compounds are potent against CDK2, they generally exhibit low to moderate selectivity for CDK2 over other members of CDK family, particular the homology CDK1. Nonetheless, understanding their safety, tolerability and efficacy in human cancer trials is essential. In the early Phase I/II clinical trials, reported toxicities include nausea, anemia, neutropenia and visual disturbances. while efficacy data has yet to be disclosed.

4. Conclusion

CDK2 has been recognized as a key driver of resistance to a wide range of cancer therapies, including chemotherapy, radiotherapy, and targeted treatments like CDK4/6, PI3K/AKT, and PARP inhibitors. Its involvement in resistance across multiple cancer types underscores its critical role in treatment failure and the need for novel strategies to overcome it. CDK2 overactivation, often linked to CCNE1 amplification, enables cancer cells to bypass key cell cycle checkpoints, reducing the efficacy of these treatments. The development of CDK2 inhibitors, alongside the growing understanding of its role in therapeutic resistance, presents promising avenues for improving outcomes in resistant cancers. Co-targeting CDK2, along with other therapeutic pathways, could offer a more comprehensive strategy to overcome resistance and enhance treatment efficacy in cancers with aberrant CDK2 activity.

5. Future perspective

Resistance to conventional and targeted cancer therapies remains a significant challenge, often leading to treatment failure and disease progression. CDK2 and its interaction with CycE have emerged as critical factors in resistance mechanisms across various therapies, including chemotherapy, radiotherapy, endocrine therapy, anti-HER2 therapy, targeted therapy and immunotherapy. Table 1 summarizes the role of CDK2 and CycE in mediating resistance to various targeted therapies across different cancer types. It highlights how CDK2 and CycE contribute to resistance through distinct pathways for each type of targeted therapy. Targeting the CDK2/CycE axis presents a promising strategy to overcome resistance and enhance therapeutic outcomes across diverse cancer types.

Development of CDK2 Inhibitors: The development of CDK2 inhibitors, including AZD8421, PF-4091, BLU-222 and INX-315, is a significant area of current clinical research. These inhibitors are being tested for their potential to selectively target CDK2, particularly in cancers that no longer respond to existing therapies (Table 2). The challenge lies in achieving high specificity for CDK2 to avoid off-target effects on other CDKs or unintended disruption of normal cellular processes. If successful, CDK2 inhibitors could represent a major advancement in the treatment of cancers that are resistant to current therapeutic options.

Combination therapies: There is growing interest in combining CDK2 inhibitors with other targeted therapies or immunotherapies (Figure 3, Combination therapies). Such combination strategies could help overcome resistance mechanisms and enhance patient outcomes by leveraging the complementary actions of different therapeutic agents. Integrating CDK2 inhibitors into treatment regimens that include agents targeting other pathways or immune responses may offer new opportunities for effective cancer management.

Biomarker discovery: Identifying biomarkers that predict response to CDK2 inhibition is essential for personalizing cancer treatment. Biomarkers that indicate sensitivity or resistance to CDK2 inhibitors will enable clinicians to tailor therapies to individual patients, improving the likelihood of successful outcomes. Research into biomarkers will also develop refined patient selection criteria, ensuring that those most likely to benefit from CDK2-targeted therapies are prioritized (Figure 3, Therapeutic monitoring and adaptive interventions).

Challenges and collaborative efforts: Despite the promising prospects, several challenges persist. The scarcity of advanced-stage selective CDK2 inhibitors and the confidentiality surrounding emerging inhibitors present significant obstacles. Addressing these issues requires enhanced collaboration and open data sharing within the scientific community. These efforts are crucial for the discovery and development of effective, selective CDK2 inhibitors and will help advance cancer therapy.

Future possibilities: Ongoing research into CDK2 inhibitors, coupled with the identification of predictive biomarkers, will drive the development of personalized and effective cancer therapies. As advancements continue, CDK2-targeted treatments could become integral components of treatment regimens, providing new opportunities to enhance patient outcomes and tackle the issue of drug resistance.

In conclusion, the development of CDK2 inhibitors and the exploration of their role in therapies and biomarker identification represent exciting areas of research. Collaborative efforts and continued innovation will be crucial in realizing the potential of these therapies and enhancing the efficacy of cancer treatments.

Author contributions

S Kasirzadeh: conceptualization, design, collecting and assembling data, writing-original draft, writing-review and writing-editing. J Lenjisa: conceptualization, formal analysis, writing-review and writing-editing. S Wang: formal analysis, interpretation, funding acquisition, resources, writing-review and writing-editing. All authors approved the manuscript.

Financial disclosure

This paper was not funded.

Competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.