Indazole – an emerging privileged scaffold: synthesis and its biological significance

Abstract

Nitrogen-containing molecules are an important class of heterocyclic compounds. Among these, indazole is one of the most important scaffolds with a broad scope because of its pharmaceutical and biological properties. Many novel molecules were reported in clinical trials for treating various diseases containing the indazole scaffold. Indazole has a vast potential for the discovery of novel pharmaceuticals such as anti-bacterial, anti-fungal, anti-leishmanial, antidiabetic, anti-cancer, anti-tuberculosis, anti-Parkinson's, anti-protozoal, anti-depressant, and anti-inflammatory. This review covers the structures of new indazoles, the current pipeline of indazole-containing compounds in the various stages of clinical trials, and naturally occurring indazoles, along with some marketed drugs. Furthermore, we have covered the different methods for the synthesis of 1H-indazole, 2H-indazole, and 3H-indazole with a wide variety of starting materials under different reaction conditions and their biological activities.

Indazoles as privileged scaffolds; synthetic strategies; clinical trial compounds; biological activities: anti-tubercular, antifungal, antibacterial, antileishmanial, anti-Parkinson's, anti-inflammatory, antidiabetic, and anticancer.

1. Introduction

Indazoles are aromatic heterocyclic compounds that are of significant interest due to their distinct structural features and properties. They consist of two nitrogen atoms. Fusing a pyrrole ring with a benzene ring to form indazole results in a stable, aromatic system with 10 π-electrons, following Hückel's rule of aromaticity. Indazole is also known by other names, such as benzpyrazole or isoindazolone. It is considered a bioisostere of indole. Indazole was first defined by Emil Fischer¹ and its occurrence in nature is quite rare. It exists in three tautomeric forms—1H, 2H, and 3H (Fig. 1). It reflects the compound's ability to exhibit different electronic characteristics depending on the position of the hydrogen atom attached to the nitrogen. 1H-Indazole is the most predominant and thermodynamically stable tautomer with the usual benzenoid feature of aromatic compounds, making it the dominant form in both chemical and biological contexts. Its aromaticity contributes to its stability, and this form is widely studied and used in a variety of chemical applications. The presence of two nitrogen atoms in the ring also imparts some unique electronic properties to the molecule, making indazoles interesting for their potential biological and pharmacological activities. Meanwhile 2H-indazoles possess ortho-quinoid characteristics. 3H-Indazoles, which lack heteroaromatic properties, are extremely rare. The relatively rare occurrence of indazole in nature could be due to the specific conditions required for its formation and stability.²

Fig. 1. Tautomeric structures of indazoles.

Several noteworthy reviews have previously described indazole derivatives in various contexts. For instance, Mal et al. (2022) presented a comprehensive overview on the synthetic strategies and pharmacological relevance of indazoles;³ Ghosh et al. (2020) focused on the catalytic functionalization of indazole scaffolds;⁴ Tandon et al. (2021) detailed the progress and challenges of indazole-based kinase inhibitors for cancer therapy.⁵ Other contributions include green synthesis perspectives (Kapoor and Yadav et al., 2024)⁶ and transition metal catalysed synthesis (Janardhanan et al., 2020).⁷ While these reviews have contributed valuable insights into either synthetic methods or therapeutic classes, they typically emphasize a particular angle—be it catalysis, target class, or disease model. In contrast, the current review aims to provide a holistic and updated consolidation of both synthetic strategies (including green, metal-free, and flow/microwave-assisted approaches) and the broad spectrum of biological activities of indazole derivatives. Notably, this article compiles indazoles across multiple therapeutic domains—such as anticancer, antibacterial, antifungal, anti-inflammatory, and neurodegenerative applications—alongside an extensive discussion on future prospects. This review outlines key challenges and future directions in indazole synthesis and drug discovery, aiming to serve as a forward-looking resource for researchers in medicinal and synthetic organic chemistry.

Indazole-containing alkaloids, such as nigellicine (found in Nigella sativa), nigeglanine (from Nigella glandulifera), and nigellidine (from Nigella sativa), are rare in nature (Fig. 2). Nigellidine is known for its carminative, stimulatory, and diaphoretic uses.⁸

Fig. 2. Natural products containing indazoles.

Indazole has a variety of medicinal applications, including chemotherapy, pain treatment, inflammation, and antiemetics. The FDA has already approved a very large number of novel indazole-containing drug molecules. Molecules such as pazopanib 7, an FDA-approved multikinase inhibitor used for renal cell carcinoma and soft tissue sarcoma, features an indazole core and is synthesized through scalable heteroaryl coupling techniques.⁹ Granisetron 11 and axitinib 13 are used in cancer chemotherapy. Granisetron 11 is used as an antiemetic because the most considerable drawback of some cancer drugs is vomiting and loss of the effect of the drugs used in cancer treatment. Granisetron is a selective 5-HT3 receptor antagonist containing an indazole nucleus as its key pharmacophore. Its synthesis typically involves regioselective N-alkylation of 1H-indazole derivatives, showcasing the role of heterocyclic ring functionalization in drug development.¹⁰ Benzydamine 14 is a drug that provides relief from pain. Bendazac 8 is used to treat inflammation. Fig. 3 shows some marketed medications (7–14) containing indazole rings.¹¹

Fig. 3. Marketed drugs containing the indazole scaffold (7–14).

In addition to these marketed drugs, many indazole-containing compounds (Fig. 4) are in various stages of clinical trials, showing promise for therapeutic applications.

Fig. 4. Indazole-containing drugs, currently in development and clinical trials (15–22).

GSK-724A (DG167) 15 was designed and developed by GSK and shows activity by selectively inhibiting the β-ketoacyl-ACP synthase enzyme in Mycobacterium tuberculosis, active against TB.¹² ABT-102 16 was developed by Abbott Laboratories for the treatment of leg, arm, and back pain in the phase I clinical trial. It showed its action by blocking the vanilloid type 1 (TRPV1) receptor. ABT-443 17 was developed by AbbVie, Inc., which has completed a phase I clinical trial and shows its action by selectively blocking the vanilloid receptor TRPV1.^13,14 PF-05241328 18 was developed by Pfizer which selectively inhibits the human voltage-dependent sodium channels (Nav17) and is used to treat pain. This drug recently completed phase-1 trials.¹⁵ SAM-315 19 was designed and developed by Pfizer and is in the phase-1 clinical trial. It selectively inhibits the serotonin 6 (5-HT) receptor and is used in the treatment of neurodegenerative disorders and Alzheimer's disease.¹⁶ Fasenra 20 was designed and developed by Roche, which showed its action by acting on α7-nAChR 5HT3 and is used for the treatment of Alzheimer's disease and schizophrenia. This drug is currently in phase II clinical trials.^17,18 Bindarit 21 was discovered by Angelini, which exhibited its action by inhibiting the CCL2 receptor, is used as an anti-inflammatory molecule and is found safe in type 2 diabetic nephropathy patients in phase II clinical trials.^19,20 LXR-623 22 was discovered by Pfizer targeting LXR-β and LXR-α. LXRs have an important role in lipid metabolism and can be used for the treatment of human atherosclerosis, inflammation, AD, skin diseases, and cancer. This drug is now in phase-1 clinical trials.²¹

2. Methods used for the synthesis of indazole

Indazole can be synthesized by various methods, along with a variety of reagents under different reaction conditions. Various methods reported for the synthesis of indazole have been reviewed here. Emil Fisher published the first report on the synthesis of indazole in 1800. They prepared indazolone 24 by heating the ortho-hydrazine benzoic acid 23. Later, they applied the same strategy to obtain the anhydride of ortho-hydrazino cinnamic acid 25 (Fig. 5). The various synthetic methods for synthesizing 1H-indazoles, 2H-indazole, and 3H-indazole have been described here.

Fig. 5. The first reported synthesis of indazolone and indazole.

2.1. Syntheses of 1H-indazole

Lokhande et al. have reported the efficient synthesis of 1H-indazoles. They treated salicylaldehyde 26 with hydrazine hydrochloride under reflux in acidic ethanol for 2–3 h to get 1H-indazole as a yellow solid. Acetic acid or hydrochloric acid served as an effective acidic medium, though aprotic solvents such as DMSO and DMF provided higher yields. The method has certain limitations, including the requirement for elevated temperatures and the occurrence of side reactions such as hydrazone and dimer formation. The presence of an ortho-hydroxy group was found to be essential for cyclization; O-substituted analogs failed to produce the desired indazole products.²²

Counceller et al. developed a mild, metal-free method for synthesizing 1H-indazoles using o-aminobenzoximes 27 through selective oxime activation with methane sulfonyl chloride using triethylamine as a base, giving 1H-indazole. The substrate scope tolerates a broad range of electron-donating and electron-withdrawing substituents, as well as both primary and secondary arylamines.²³ Inamoto et al. reacted o-halo acetophenones 28 with an excess of hydrazine monohydrochloride in dioxane, resulting in the formation of hydrazine which then underwent cyclization to yield 1H-indazole. The cyclization was facilitated by a Pd-catalyzed intramolecular carbon–nitrogen bond formation, using a catalyst system consisting of Pd(OAc)₂, ligand (dppf, dba, dppp, etc.), and Cs₂CO₃ (1.5 equiv.) as the base. This method, which proceeds under mild conditions, is suitable for a broad range of substrates, including those with acid or base-sensitive functional groups. The method is clean, efficient, and relies on metal–ligand cooperative catalysis, offering a valuable synthetic route to substituted 1H-indazoles. To date, a general and efficient synthetic route to structurally diverse 3-substituted indazoles has not been established, highlighting a critical gap in current synthetic strategies.²⁴ Souers et al. treated o-toluidine 29 with sodium nitrite (diazotization), and then the diazotized product reacted with potassium acetate in ethyl acetate at rt, giving 1H-indazole. This method was successfully employed in the development of urea-based indazoles as melanin-concentrating hormone receptor 1 (MCHR1) antagonists for the treatment of obesity. Substitution on the indazole scaffold was achieved by employing substituted toluidines as starting materials. These were further elaborated into an amide-based series featuring aromatic ether moieties as side chains, which are known to be critical for biological activity in this class of compounds.²⁵ Gaikwad et al. reacted ortho alkoxy acetophenone 30 with hydrazine hydrate and iodine in DMSO to get 1H-indazole as a product. The conversion was highly efficient and rapid, affording superior yields compared to other methods. The reagent employed was exceptionally mild, and notably, no byproducts were observed under the reaction conditions.²⁶ Gaikwad et al. have also reported the synthesis of 1H-indazole by reacting the hydroxyl group in the ortho position of acetophenone 31 with hydrazine hydrate in the presence of silica sulfuric acid acting as a catalyst, giving 1H-indazole. One of the key advantages of using silica sulfuric acid as a catalyst is that the reaction proceeds to completion within 2 hours simply by stirring at room temperature. Moreover, the catalyst can be easily recovered and reused multiple times without any significant loss in activity, making this method highly cost-effective compared to other catalytic systems.²⁷ Nyerges et al. treated aryl mesylate 32 with hydrazine hydrate, leading to the formation of arylhydrazone as an intermediate, followed by cyclization to give 1H-indazole.²⁸ Schumann et al. have prepared 1H-indazole by subjecting the o-toluidine 33 to diazotization reaction, where diazonium salt was formed by a phase transfer-catalyzed reaction from o-methyl-benzenediazonium tetrafluoroborates giving 1H-indazole²⁹ (Fig. 6).

Fig. 6. Various methods for the synthesis of 1H-indazole (26–33).

Counceller et al. have prepared 1H-indazole from intramolecular electrophilic amination of 2-aminophenyl ketoximes 34via a metal-free, simple, and high-yielding procedure under mild conditions.³⁰ Ainsworth et al. synthesised indazole from cyclohexanone 35 in three steps. After undergoing a condensation and ring closure reaction with ethyl formate and cyclohexanone, the aldol condensation product yielded tetrahydroindazole, which was oxidized using a Pd/C catalyst in refluxing decalin to indazole.³¹ Stephenson et al. efficiently constructed the 1H-indazole structure by combining aromatic substitution reactions with reduction and ring closure steps. The reaction of anthranilic acid 36 with sodium nitrite (diazotization) resulted in a diazonium salt intermediate which underwent reduction followed by cyclization, giving 1H-indazole.³² Dubost et al. conducted a palladium-catalyzed cross-coupling reaction between 2-bromobenzaldehyde 37 and benzophenone hydrazone, followed by cyclization in the presence of p-TsOH to give 1H-indazole.³³ Schumann et al. prepared 1H-indazole by reacting o-toluidine 38 with sodium nitrite, leading to an N-nitroso intermediate. N-Nitroso derivatives on refluxing in benzene gave 1H-indazole.³⁴ Paul et al. have treated o-aminobenzoxime 39 with triphenylphosphine, I₂, and imidazole as a base, giving 1H-indazole. The driving force for this reaction is the selective formation of the oxime-phosphonium ion intermediate in the presence of the amino group³⁵ (Fig. 7).

Fig. 7. Various methods for the synthesis of 1H-indazole (34–39).

Vasudevan et al. reported the synthesis of 3-amino-1H-indazoles from ortho-fluorobenzonitrile 40 upon reaction with hydrazine hydrate in butanol under reflux.³⁶ Liu et al. reported metal-free synthesis of 1H-indazole by reacting N-tosylhydrazones 41 with nitroaromatic compounds. This transformation proceeds under transition-metal-free conditions, offering a practical and environmentally benign alternative to metal-catalyzed methods. It demonstrates a broad substrate scope. It is successfully applied in the formal synthesis of the bioactive compound WAY-169916.³⁷ Zhang et al. reported a practical and efficient method for the synthesis of 1H-indazoles via [bis(trifluoroacetoxy)iodo]benzene (PIFA)-mediated oxidative C–N bond formation from readily accessible arylhydrazones 42. This protocol features a broad substrate scope, tolerates various functional groups, and offers a relatively green and reliable approach for the rapid construction of substituted 1H-indazoles under mild conditions (a metal-free).³⁸ According to Cho et al., 1-aryl-1H-indazoles were produced in good yields by treating 2-bromobenzaldehydes 43 with arylhydrazines in toluene at 100 °C, in the presence of a palladium catalyst and phosphorus chelating ligands like 1,1′-bis(diphenylphosphino)ferrocene and 1,3-bis(diphenylphosphino)propane in addition to NaO^tBu. This is an efficient and straightforward method that utilizes readily available starting materials and offers operational simplicity, making it highly practical for synthetic applications.³⁹ Yu et al. synthesised 1H-indazoles by Rh(iii)/Cu(ii)-co-catalyzed C–H amidation and N–N bond formation by treating arylimidates 44 with organo azides. This method employs readily available starting materials and is efficient, enabling the synthesis of diverse indazole derivatives. The method is environmentally friendly and scalable, using O₂ as the terminal oxidant and producing N₂ and H₂O as byproducts⁴⁰ (Fig. 8).

Fig. 8. Various methods for the synthesis of 1H-indazole (40–44).

2.2. Syntheses of 2H-indazole

Kumar et al. have prepared 2H-indazoles using one-pot, three-component reactions of 2-bromobenzaldehydes 45, primary amines, and sodium azide that are catalyzed by copper, which plays a key role in the formation of C–N and N–N bonds. This method is very tolerant of a wide range of functional groups and has a wide substrate scope.⁴¹ Khatun et al. made 2H-indazole from ortho-bromobenzaldehydes 46 by reacting with primary amines and sodium azide under ligand-free conditions. All three components are treated in one pot by using a nanoparticle CuO catalyst. The catalyst can be recycled up to three times, however, with a slight decrease in the yields each time. Nanoparticle-catalyzed reactions offer several advantages over conventional metal-catalyzed methods, including lower catalyst loading, higher atom economy, improved yields, shorter reaction times, and excellent catalyst recyclability. Moreover, they exhibit high dispersibility and enhanced stability.⁴² Rai et al. have prepared 2H-indazole using microwave conditions. They have treated 2-azidobenzaldehydes 47 with amines under microwave to give 2-H indazole. This aligns with the principles of green chemistry, offering high atom economy and operational simplicity without the need for harsh conditions. Limitations include expensive or air/moisture-sensitive ligands, low substrate scope, complicated scale-up and reproducibility⁴³ (Fig. 9).

Fig. 9. Various methods for the synthesis of 2H-indazole (45–47).

Taher et al. have treated 2-nitrobenzyl triphenyl phosphonium bromide 48 with aryl isocyanates in the presence of sodium hydride or DBU as a base to give 2H-indazole. With the nitro group being transformed into the indazole N-1 atom, substituents on the aromatic ring can be readily varied at this position, enabling the synthesis of a wide range of 2-substituted indazoles.⁴⁴ Fang et al. quickly and effectively synthesized 2H-indazoles by treating sydnones with arynes 49 proceeding via a [3 + 2] cycloaddition followed by a retro-[4 + 2] elimination of CO₂, avoiding 1H-indazole byproducts and ensuring clean product formation. This reaction offers a mild, high-yielding, and selective route to indazole derivatives.⁴⁵ Using α-zirconium sulfophenylphosphonate-methanephosphonate Zr(CH₃PO₃)_1.2(O₃PC₆H₄SO₃H)_0.8 as a mediator, Rosati et al. reported a regioselective synthesis of 2,3-disubstituted tetrahydro-2H-indazoles, beginning from 2-benzoylcyclohexanone 50 and substituted hydrazines under moderate and solvent-free conditions.⁴⁶ Prasad et al. have reported a new heterogeneous Cu(ii)–HT catalyst-catalyzed, efficient, and simple synthesis of 2H-indazoles from 2-bromobenzaldehydes 51, primary amines, and sodium azide via successive condensation, C–N, and N–N bond formation. The recoverable heterogeneous Cu(ii)–HT catalyst demonstrated excellent activity for the target reaction under additive-free conditions, eliminating the need for costly ligands or other auxiliaries, making the process more economical and environmentally friendly. The catalyst can be easily recovered and reused for at least three cycles without a notable loss of activity, highlighting its efficiency and stability. Its robust performance and recyclability make it a promising method for large-scale industrial applications⁴⁷ (Fig. 10).

Fig. 10. Various methods for the synthesis of 2H-indazole (48–51).

Avila et al. made 2H-indazole starting from nitrobenzyl bromide 52, on reaction with an amine in tetrahydrofuran (THF), giving 2-nitrobenzylamines as an intermediate, which on further treatment with 5% KOH alcoholic solution at 60 °C for 6 hours gave 2H-indazoles (Davis–Beirut reaction). The advantage of this method is that no expensive or toxic metals have been used to mediate heterocycle formation and the reaction proceeds in the presence of a mild base at a relatively low temperature in an alcoholic solvent.⁴⁸ A novel pathway to isoindazoles was suggested by Kimball et al. via the neutral cyclization of (2-alkynylphenyl)triazenes 53 and CuCl (5 equiv.) in 1,2-dichloroethane as a solvent, at 50 °C for 12 h, giving 2H-indazole. The heating of the initial triazenes yields different products depending on the temperature and the type of alkyne ortho to the triazene functionality (coarctate reaction)⁴⁹ (Fig. 11).

Fig. 11. Various methods for the synthesis of 2H-indazole (52–53).

2.3. Syntheses of 3H-indazole

Liu et al. have reported the synthesis of 3,3-disubstituted 3H-indazoles from benzynes 54, (generated in situ from o-silylaryl triflates and fluoride ion) in the presence of CsF or TBAF on reaction with disubstituted diazo substrates that do not have hydrogen on the diazo carbon via a [3 + 2] cycloaddition reaction. This can rearrange to N-substituted-1H-indazoles by 1,3 shifts. This methodology offers a valuable new route to indazoles.⁵⁰ Toledano et al. have reported the synthesis of 3H-indazole by reacting aminomethyl-phenylamines 55 by N–N bond-forming oxidative cyclization with Na₂WO₄·2H₂O (1 equiv.) and H₂O₂ (10 equiv.) with 94% yield.⁵¹ Chen et al. treated benzynes derived from o-silylaryl triflates 56 with α-substituted α-diazomethylphosphonates through a 1,3-dipolar cycloaddition reaction. The phosphoryl group controlled the product distribution by acting as both a tuning group and a traceless group in the reaction. The advantages of this method include the generation of a sole product, more yield, fewer steps.⁵² Zhu et al. reported the synthesis of indazolones from nitrobenzyl alcohol 57, through the formation of o-nitrosobenzaldehyde in situ in the presence of 365 nm light in a photoreactor, which reacted with substituted amine in water at room temperature to give 1,2-dihydro-3H-indazol-3-ones. A green photochemical route to indazolones reduces the overall harshness of the reaction conditions, expanding the substrate scope⁵³ (Fig. 12).

Fig. 12. Various methods for the synthesis of 3H-indazole (54–57).

2.4. Patents filed for the synthesis of indazole

Several patents have been filed outlining the innovative synthetic methodologies for indazole derivatives, reflecting the ongoing efforts to improve yield, selectivity, and scalability for pharmaceutical applications.

Patent 1: U.S. Patent 3833606 – preparation of indazoles

An improved method for synthesizing 2-alkyl- and 2-aryl-2H-indazoles was reported by Mohan and co-workers (U.S. Patent 3833606), offering a valuable route to intermediates commonly employed in the preparation of dyes and optical brighteners. The process involves heating an N-(o-nitrobenzylidene)amine in an inert solvent in the presence of iron carbonyl, which acts as a source of carbon monoxide—either added directly or generated in situ. This facilitates an efficient intramolecular cyclization, leading to the formation of the indazole ring. The method offers several advantages, including high yields and product purity, eliminating the need for extensive purification steps. It also demonstrates broad functional group tolerance, accommodating various inert substituents on the aromatic ring, and is compatible with a wide range of substituted anilines and alkylamines, making it a versatile and industrially relevant approach for indazole synthesis (Scheme 1).⁵⁴

Scheme 1. Preparation of indazoles (U.S. Patent 3833606).

Patent 2: U.S. Patent 8022227 B2 – “method of synthesizing 1H-indazole compounds”

A noteworthy metal-free method for the synthesis of 1H-indazole derivatives was reported by Stambuli and co-workers (U.S. Patent 8022227B2), offering a practical and efficient approach under mild reaction conditions. The process converts o-aminobenzaldehydes or o-aminoketones into oxime intermediates using hydroxylamine, followed by cyclization with activating agents (e.g., methanesulfonyl chloride) in the presence of weak bases like triethylamine or 2-aminopyridine at 0–23 °C. This method enables the synthesis of both N-unsubstituted and N-substituted 1H-indazoles with broad functional group tolerance, including methoxy, halogen, nitro, and furan groups. With yields up to 94%, the protocol offers a scalable, environmentally friendly alternative to traditional metal-catalyzed or harsh-condition-based indazole synthesis (Scheme 2).⁵⁵

Scheme 2. Method of synthesizing 1H-indazole compounds (U.S. Patent 8022227B2).

Patent 3: U.S. Patent 6391872B1 – “indazole bioisostere replacement of catechol in therapeutically active compounds”

This patent, granted to Anthony Marfat and assigned to Pfizer Inc., discloses novel therapeutically active compounds in which the catechol moiety of existing biologically active molecules is replaced by an indazole group through bioisosteric substitution. The invention demonstrates that indazole can effectively mimic catechol in maintaining or enhancing biological activity such as cholinesterase inhibition, adrenergic activity, calcium channel inhibition, and antineoplastic effects. The patent emphasizes that the non-catechol substituents remain unchanged, and the indazole replacement retains or improves the therapeutic function of the original catechol-containing compounds. Multiple provisional applications and international filings support the claimed invention, which addresses a broad range of pharmaceutical applications (Scheme 3).⁵⁶

Scheme 3. Indazole bioisostere replacement of catechol in therapeutically active compounds: (U.S. Patent 8022227B2).

3. Biological activities

Indazole has garnered considerable attention from researchers due to its distinctive chemical structure and wide-ranging biological activity. Derivatives of indazole are found in many natural and synthetic compounds, demonstrating various pharmacological properties such as anti-inflammatory, antibacterial, anti-HIV, antiarrhythmic, antifungal, and antitumor effects (Fig. 13).

Fig. 13. Various biological activities shown by the indazole scaffold.

3.1. Anti-tubercular activity

Park et al. have reported sulphonamide containing indazoles as new antitubercular agents, and the active compound was identified by high-throughput screening of more than 100 000 synthetic compounds against tuberculosis. Among the molecules, the most potent compound 58 was reported with an MIC of 0.09 μM against M. tuberculosis.⁵⁷ Malapati et al. have reported the synthesis of new indazole derivatives as potent antibacterial agents. The most potent compound, 59, was reported with an MIC of 67.09 μM against Mycobacterium tuberculosis. Molecular docking was performed using the 5HJ7 protein structure. The predicted bound conformation of the active compound demonstrated a key interaction with the side chain of Glu153. Additionally, the binding was further stabilized through a combination of hydrophobic interactions and limited polar contacts with surrounding amino acid residues.⁵⁸ Nanaware et al. have reported that the new indazole derivatives have antitubercular effects. These derivatives were discovered by comparing with naturally available indazole alkaloids with an in silico technique, and the designed indazole scaffold was found to be a potent inhibitor of Enoyl-ACP (CoA) reductase and a COX-2 enzyme inhibitor. The most active designed compound 60 showed the highest binding affinities (−7.66 kcal mol⁻¹) when compared with nigellidine (−7.89 kcal mol⁻¹), nigellidine (−9.02 kcal mol⁻¹), and nigellicine (−8.02 kcal mol⁻¹) when docked with 2AQK. The binding interactions between 2AQK and indazole derivative 60 did not exhibit hydrogen bonding interactions. However, it formed a carbon–hydrogen bond with Ala527. Additionally, it engaged in alkyl and π-alkyl interactions with Leu63. Amide-π stacked interactions were observed with Gly526 and Met522, while π-alkyl interactions were noted with Tyr385, Phe381, Trp387, Leu352, Val523, Val116, Tyr355, Leu531, and Ala527. Moreover, it also formed alkyl interactions with Val116 and Leu93 (ref. 59) (Fig. 14).

Fig. 14. Indazole derivatives exhibiting anti-tubercular activity (58–60).

3.2. Anti-bacterial activity

Dunga et al. have reported novel substituted indazole-1,2,3-triazolyl-1,3,4-oxadiazole derivatives as antimicrobial agents. Among the synthesized compounds, the most active compound 61 was reported with an MIC of 4.0 μg mL⁻¹ against gram-positive bacteria S. pneumoniae. Molecular docking studies revealed key interactions, including hydrogen bonds with LysA:273, LysA:20, Pi-donor hydrogen bonding HisA:18, ValA:268, Pi-alkyl: LysA:20 in the active site of S. aureus with 2ZCS protein.⁶⁰ S. Murugavel et al. have reported the antibacterial activity of 3,3a,4,5-tetrahydro-2H-benzo[g]indazole fused carbothioamide derivatives as antibacterial agents. The most potent compound was 62 with a MIC value of 3.125 μg mL⁻¹ against S. aureus. It was found that this molecule showed its action by inhibiting DNA gyrase (PDB ID: 1KZN) by molecular docking. In the MNPBICT–1KZN complex, key interactions include hydrogen bonds between the indazole N1 and ASN46, and between nitro group oxygens and residues ARG76, GLY77, and THR165. Additionally, π-anion interaction occurs between the nitrobenzene ring and GLU50.⁶¹ Yap et al. have reported the synthesis of new indolenine-substituted pyrazoles and pyrimido[1,2-b]indazoles as antibacterial agents. Among the synthesized compounds the most active compound 63 was reported with an IC₅₀ of 6.25 μg mL⁻¹ against methicillin-resistant S. aureus.⁶² Yadav et al. have synthesized N-(6-indazolyl) benzenesulfonamide derivatives as potential antioxidants and antimicrobial agents. The potent compound 64 was reported with multi-pronged antimicrobial interactions IC₅₀ = 0.094 μmol mL⁻¹, and also potent antioxidant effects with antioxidant potential in DPPH (IC₅₀ = 0.094–0.467 μmol ml⁻¹) and ABTS assays (IC₅₀ = 0.101–0.496 μmol ml⁻¹).⁶³ Mareyam et al. reported a docking investigation of newly created indazole compounds against the topoisomerase-II DNA gyrase enzyme. When compared to conventional norfloxacin (10.7 kcal mol⁻¹), active compound 65 had the highest binding affinity (12.2 kcal mol⁻¹). Molecular docking studies revealed key interactions, including conventional hydrogen bonding between the sulfonyl (SO₂) group and DG4, π-donor hydrogen bonding involving the phenyl rings, π–π stacking interactions between the phenyl moiety and DA18, and π–sigma interactions between the phenyl ring and residues Thr1329 and DTE3.⁶⁴ Shaikh et al. have reported the synthesis of new novel 3-methyl-1H-indazole derivatives as potent antibacterial agents. All the synthesized compounds were found to be active against bacteria and showed antibacterial activity against B. subtilis and E. coli. The most active compound 66 was reported with a zone of inhibition of 22 mm against Bacillus subtilis and 46 mm against E. coli. The zone of inhibition at the concentration of 300 μg mL⁻¹ showed the best antibacterial activity against the bacteria B. subtilis and E. coli when compared with the standard drug, ciprofloxacin.⁶⁵ Khan et al. have reported the synthesis and antibacterial evaluation of new 4,5-dihydro-1H-indazole derivatives. Among the synthesized compounds, the most active compound 67 was reported with an MIC₅₀ of 3.85 mg ml⁻¹ against Salmonella typhimurium. In silico evaluation revealed that ligand binding is stabilized through π–π interactions between the ligands and the indole ring of Trp252, and a backbone acceptor interaction was observed between Glu150 and the N,N-dimethyl moiety of the ligand⁶⁶ (Fig. 15).

Fig. 15. Indazole derivatives exhibiting antibacterial activity (61–67).

3.3. Antifungal activity

Rodríguez-Villar et al. have reported the synthesis of new indazole derivatives as anti-candida agents. Among the synthesized compounds, the most potent compound 68 was reported with a MIC value of 75 μM and 100 μM against C. albicans and C. glabrata, respectively.⁶⁷ Wu et al. have reported the cascade cyclization of indazole aldehydes with propargylic amines to produce pyrazino[1,2-b]indazoles in a single pot. The active compound 69 showed 76.3% mycelium growth inhibition.⁶⁸ Vincent et al. have reported the synthesis of novel indazole derivatives that are found to be selectively active against the cytochrome bc1 inhibitor. Among them, compound 70 showed potency (IC₅₀ 0.289 μM) against fungi. The potency, selectivity, and improved stability of 70 allowed us to investigate the role of mitochondrial respiration in diverse aspects of fungal pathogenesis.⁶⁹ Jaromin et al. have reported that the promising compound 71 exhibited antifungal activity against biofilms of C. Auris clinical isolates, with MIC values between 0.67 and 1.25 μg mL⁻¹.⁷⁰ Pérez-Villanueva et al. have reported the synthesis of new indazole derivatives as antimicrobial and anti-inflammatory dual agents. The most potent compound, 72, was reported with an MIC of 3.807 mM (C. albicans)⁷¹ (Fig. 16).

Fig. 16. Indazole derivatives exhibiting antifungal activity (68–72).

3.4. Dual acting anti-bacterial as well as antifungal activity

Saminathan et al. have reported the synthesis of halogen-substituted N-phenylbenzo[g]indazole derivatives as antimicrobial agents. Molecular docking studies of the synthesized compounds were carried out and they were found to be active bacterial cholesterol inhibitors that inhibit lanosterol-14α-demethylase. The most potent compound, 73, showed both antibacterial and antifungal activities. Molecular docking analysis of the 73–2ZCS complex exhibits four hydrogen bonds and two π–cation electrostatic interactions. Hydrogen bonding involves ARG171 and ARG265 with the ligand's sulfur atom (S1), and HIS18 with the nitrogen atom (N3) of 4CLPBIC. Additionally, π–cation interactions are observed between ARG45 and ARG171 with the aromatic rings (C1–C6 and C19–C24) of the ligand contributing to complex stabilization.⁷² Naaz et al. have reported the synthesis and antibacterial evaluation of sulfonamide derivatives of indazole. The molecules were docked with Bacillus anthracis dihydropteroate synthase enzyme (BaDHPS: PDB ID-3TYE). Among the synthesized compounds, the most active compound 74 showed good antibacterial activity against B. cereus and S. aureus (MIC, 6.2 μg mL⁻¹) as well as E. coli and P. aeruginosa (MIC, 3.1 μg mL⁻¹). Molecular docking revealed multiple hydrogen bond interactions involving residues Arg68, Asn120, Ser221 (twice), and Arg254. Additionally, non-covalent interactions, including π–π stacking and π–cation interactions, were observed with Arg254 (twice), Lys220, and Phe189 (twice), contributing to the overall stability of the complex.⁷³ The synthesis of new heterocycles with the indazolylthiazole moiety as antibacterial agents has been described by Nadia et al. Among the synthesized compounds, the most active compound 75 was reported with MIC values of 11.2 μg mL⁻¹ (streptococcus mutant), 18.29 μg mL⁻¹ (Pseudomonas aeruginosa), and 40.74 ± 0.5511 (C. albicans) when compared with ampicillin (MIC: 13.5 μg mL⁻¹) and ciprofloxacin (MIC: 18.7 μg mL⁻¹).⁷⁴ Kumar et al. reported the synthesis of new sulfonamide and carbamate derivatives of 5-nitro-1H-indazole derivatives as antimicrobial agents. Among the synthesized compounds the most potent compound 76 was reported with a zone of inhibition of 19.4 mm against A. niger fungi and 17.4 mm a S. aureus bacteria⁷⁵ (Fig. 17).

Fig. 17. Indazole derivatives with anti-bacterial and antifungal dual activity (73–76).

3.5. Anti-leishmanial, antiprotozoal, and antiparasitic activities

Mohamed Abdelahi et al. have reported the synthesis of novel 3-chloro-6-nitro-1H-indazole derivatives as promising antileishmanial candidates. Among the synthesized compounds, the most active compound 77 was reported with an IC₅₀ value of 11.23 μM against L. infantum. The molecular docking analysis of the TryR-77 complex revealed a total of five key interactions, including carbon–hydrogen bonds and π–π T-shaped interaction, contributing to the stable binding of compound 77 within the active site of trypanothione reductase (TryR).⁷⁶ Rodríguez-Villar et al. have reported the synthesis of 2-phenyl-2H-indazole derivatives by an ultrasound-assisted one-pot procedure as antiprotozoal agents. All the synthesized compounds are active against E. histolytica, G. intestinalis, and T. vaginalis. Among the synthesized compounds, the most active compound 78 was reported with an IC₅₀ < 0.070 μM against protozoa.⁷⁷ Martín-Escolano et al. have reported the synthesis of novel 5-nitroindazole derivatives. Among the synthesized compounds, the most potent compound 79 was reported with an IC₅₀ of 4.7 ± 0.3 against the T. cruzi Arequipa strain for the development of a new anti-Chagas agent⁷⁸ (Fig. 18).

Fig. 18. Indazole derivatives with anti-leishmanial, antiprotozoal, and antiparasitic activities (77–79).

3.6. Anti-inflammatory activity

Here, we have tried to explain various anti-inflammatory activities shown by indazoles (NCE), discovered by medicinal chemistry. Lavrentaki et al. have reported the synthesis of indazole carboxamides of N-substituted pyrrole derivatives against lipoxygenase. Among the synthesized compounds, the most active compound 80 was reported with an IC₅₀ value of 22 μM as a 5-lipoxygenase inhibitor, which is used in the treatment of inflammation.⁷⁹ Liu et al. have reported 3-(indol-5-yl)-indazole derivatives for the treatment of acute lung injury. They act by targeting the myeloid differentiation protein 2/toll-like receptor 4 (MD2–TLR4) complex. Among the synthesized compounds, the most active compound 81 showed IC₅₀ values of 0.53 μM and IC₅₀ (TNF-α) = 0.89 μM. Molecular docking of compound 81 with the MD2–TLR4 complex revealed that its 2,6-dichlorobenzamide moiety deeply embeds within the MD2 hydrophobic pocket, interacting with residues Ile52, Phe121, Phe126, and Ile153. The indole fragment extends to the rim of the pocket and forms a critical hydrogen bond with Lys125 (MD2), while the indazole ring reaches the MD2–TLR4 interface, establishing an additional hydrogen bond with Glu439 (TLR4).⁸⁰ Hou et al. have reported the synthesis of new indazole derivatives as ASK1 inhibitors. Among the synthesized compounds, the most potent compound 82 has an IC₅₀ of 4462 nM, and this can be used for the treatment of inflammatory bowel disease (IBD). Docking analysis revealed that compound 82 binds in the ATP-binding site of ASK1, with the amide carbonyl forming a key hydrogen bond with Val757, anchoring the molecule in the hinge region. The isopropyl-triazole moiety extends deep into the binding pocket, where its N1 nitrogen forms a hydrogen bond with Lys709, while the cyclopropyl-imidazole points toward the solvent-accessible area.⁸¹ Elie et al. have reported the synthesis of (aza)indazole series as selective COX-2 inhibitors. From the synthesized series, the most active compound 83 was reported with effective COX-2 inhibitory activity, with IC₅₀ values of 0.409 μM, which is used to treat the inflammatory disease.⁸² The synthesis and structure–activity relationships (SARs) of a number of 3,6-disubstituted indazole compounds as inhibitors of hepcidin production have been reported by Fukuda et al. These compounds were shown to have the ability to reduce serum hepcidin levels and are used to treat anemia linked to inflammation. The most active of the synthesized compounds, compound 84, was found to have an IC₅₀ of 0.13 μM.⁸³ Jayakodiarachchi et al. have reported a novel set of substituted indazole-ethanamines and indazole-tetrahydropyridines as potent serotonin receptor subtype 2 (5-HT2) agonists. It was found that these derivatives have serotonin-2 receptor agonists and have an anti-inflammatory effect. Among the synthesized molecules, the most active molecule, 85, was reported to affect calcium flux 5-HT2A with an EC₅₀ value of 189 nM and used as an anti-inflammatory agent⁸⁴ (Fig. 19).

Fig. 19. Anti-inflammatory activity shown by indazole derivatives (80–85).

3.7. Anti-Parkinsons activity

Lan et al. have reported the synthesis of new resveratrol-indazole hybrids as amyloid-β aggregation and MAO-B dual inhibitors. Among the synthesized compounds, the most potent compound 86 was reported with an IC₅₀ value of 1.14 μM (hMAO-B), with good ability to inhibit Aβ and self-aggregation (58.9% at 20 μM). Compound 86 exhibited selective MAO-B inhibition due to its unique binding mode. In MAO-B, the indazole and benzene rings engage in π–π stacking interactions with Tyr435, Tyr398, and Tyr326 within the substrate and entrance cavities. Conversely, in MAO-A, the compound lacks significant interactions, likely owing to the absence of the hydrophobic entrance subpocket exclusive to MAO-B.⁸⁵ Jismy et al. have reported pyrimido[1,2-b]indazole derivatives as selective MAO-B inhibitors. The most active compound 87, with an IC₅₀ value of 0.062 μM, showed selective inhibition of MAO-B with an SI of >1613. Molecular docking of the acetyl-substituted phenyl compound into hMAO-B (PDB: 2V5Z) revealed key hydrophobic interactions with residues Ile316, Leu171, and Cys172, despite the absence of hydrogen bonding. The acetylphenyl group was oriented toward the FAD cofactor, and its binding mode was stabilized by π–alkyl and π–sulfur interactions. These results align with experimental data and explain the potent inhibitory activity against hMAO-B.⁸⁶ Tzvetkov et al. have reported the synthesis and comparative study of novel indazole carboxamides vs. methanamine derivatives that are selective and reversible MAO-B inhibitors. Among the synthesized compounds the most potent compound 88 was reported with (K_i 0.17 nM, SI 25907), acting as a selective MAO-B inhibitor, and having IC₅₀ 78.3 ± 1.7 mM (hAChE). Compounds orient with the di-halogenated phenyl or pyridine ring toward the FAD cofactor, occupying the hydrophobic substrate cavity. The ligands engage the hydrophobic entrance cavity (around PRO102, PHE103) and hydrophilic linker region, aligning well with known MAO-B inhibitors like safinamide. Molecular docking studies using hMAO-B (PDB: 2V5Z) and X-ray data revealed that carboxamides and methanimines adopt similar binding orientations, with the di-halogenated ring facing the FAD cofactor. Carboxamides formed two key water-mediated H-bonds (N2⋯HOH1180 and CO⋯HOH1247), enhancing stability, while methanimines retained only the N2⋯HOH1180 interaction. The 3,4-dichlorophenyl ring improved hydrophobic contacts over 5,6-dichloropyridine, and N1-methylation increased ligand efficiency. Key interactions with residues TYR398, TYR435, CYS172, and GLN206 support the superior potency of carboxamides and guide further CNS-targeted MAO-B inhibitor design.⁸⁷ Rullo et al. have reported the synthesis of new indazole derivatives by bioisosteric replacement. Bio-isosteric replacement led to the discovery of neuroprotective MAO-B inhibitors, followed by a hybridization strategy. The most potent compound 89 was reported as a selective human MAO B inhibitor with an IC₅₀ value of 52 nM, SI > 192.⁸⁸ Feng et al. have reported the synthesis of new indazole derivatives and among the synthesized compounds, the most active compound, 90, was reported with an IC₅₀ value of 0.005 μM (JNK3 inhibitor). JNK3 inhibitors are being developed as potential treatments for neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. Compound 90 binds JNK3 in a type I mode, forming hydrogen bonds with Met149 and a water-bridged H-bond with Lys93. The aniline ring occupies hydrophobic pocket I, and the thiophenyl ring fits near pocket II, both twisted relative to the aza-indazole core. These interactions, along with the solvent-exposed amide chain, enhance JNK3 binding and selectivity over JNK1 but reduce overall kinase selectivity due to extensive hydrophobic contacts.⁸⁹ According to Shuai et al., new indazole chemotypes that are JNK3 isoform-selective inhibitors have been discovered. The most potent of these, compound 91, was shown to have an IC₅₀ of 85.21 nM for usage as a JNK3 inhibitor in Parkinson's disease. Docking studies showed that in JNK1 and JNK2, the Met108 side chain blocks hydrophobic pocket I, preventing key interactions seen in JNK3. While 25c binds deeply in JNK2 forming halogen and hydrogen bonds, it lacks important solvent-exposed interactions crucial for JNK3 selectivity. In JNK3, 25c interacts well with hydrophobic pockets I and II and solvent-exposed residues, driving its selective inhibition. Comparison with inhibitor SR3576 suggests that subtle binding differences influence isoform selectivity and potency.⁹⁰ Ning et al. have reported the discovery of potent, orally bioavailable new indazole derivatives against Parkinson's disease. Among the synthesized compounds, the most active compound 92 was reported with IC₅₀ values of 0.64 and 0.04 μM against human indoleamine 2,3-dioxygenase 1 (hIDO1) and tryptophan 2,3-dioxygenase (hTDO), respectively⁹¹ (Fig. 20).

Fig. 20. Anti-Parkinson activity shown by indazole derivatives (86–92).

3.8. Anti-Alzheimer's activity

González-Naranjo et al. have reported the synthesis of 5-substituted indazole derivatives with a multitarget profile, including cholinesterase and BACE1 inhibition. Among the synthesized compounds, the most potent compound 93 reported IC₅₀ values on various targets, IC₅₀ > 10 μM (AChE), IC₅₀ 0.57 μM (BuChE), and IC₅₀ 1.9 μM (BACE1).⁹² According to Khan et al., a hybrid technique was used to synthesize indazole-based thiadiazole-bearing thiazolidinone derivatives. The synthesized molecules were screened for their inhibition profile against targeted acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activities. The most active compound 94 was reported with IC₅₀ values of 0.86 μM (AChE) and 0.89 μM (BuChE) when compared with standard donepezil drugs, IC₅₀ 1.26 μM for (AChE) and 1.35 μM (BuChE).⁹³ Nirogi et al. have reported the discovery and preclinical characterization of new oxadiazole-indazole derivatives. The most active compound 95 was reported with EC₅₀ = 44 nM, E_max 43%, F = 34%, and is found active against AD. After completing phase 1 clinical trials, the anticipated effective concentration was reached with no significant safety issues. After completing phase 2 permitting long-term safety testing, there are no issues regarding further development.⁹⁴ Lüken et al. have reported the synthesis of new indazole derivatives by bioisosteric replacement. Compound 96 was reported with an IC₅₀ of 72 ± 6 μM and a K_i value of 93 ± 22 for GluN2B.⁹⁵ Manh et al. have reported that a series of indazole-based QC inhibitors were investigated as anti-Alzheimer's agents. Among the synthesized molecules, the most potent molecule, 97, was reported with an IC₅₀ value of 3.2 nM, suppressing the formation of pE-Aβ3-40 by 25%, which has potential use in the treatment of AD disease⁹⁶ (Fig. 21).

Fig. 21. Indazole derivatives showing anti-Alzheimer's activity (93–97).

3.9. Antidiabetic activity

Rafique et al. have reported the synthesis of new N-sulfonohydrazide-substituted indazole derivatives as α-amylase inhibitors. The most potent compound, 98, was reported with an IC₅₀ value of 1.23 ± 0.006 μM when compared with the standard acarbose (IC₅₀ value of 1.20 ± 0.09 μM).⁹⁷ Taha et al. have reported indazole-based Schiff base analogues as new anti-diabetic α-glucosidase and α-amylase inhibitors. Among the synthesized molecules, the most active compound 99 was reported with IC₅₀ values of 0.40 ± 0.01 μM (α-glucosidase) and 0.70 ± 0.01 μM (α-amylase) compared to the standard drug acarbose (IC₅₀ = 12.90 ± 0.10 and 12.80 ± 0.10 μM, respectively). Compound 99 exhibited hydrogen bonding interactions with the active site residues Asn430, Asp432, and Asp433 of α-amylase via its three hydroxyl groups, while its carbonyl group formed an additional hydrogen bond with Arg319.⁹⁸ Shamim et al. have reported indazole Schiff bases as α-glucosidase inhibitors. The most potent molecule 100 was reported as an α-glucosidase inhibitor having IC₅₀ = 9.43 ± 0.1 μM compared with standard acarbose 750 ± 10 μM, which can be used for the treatment of type-2 diabetes. The docking interactions of the potent molecule revealed strong binding affinity toward the α-glucosidase active site, involving two hydrogen bonds with Ser308 and Thr301, a π–anion interaction with Glu304, and three hydrophobic contacts with Pro309, Val305, and Arg312, collectively contributing to its high inhibitory potential.⁹⁹ Mphahlele et al. have reported the synthesis of a new series of 3,5,7-trisubstituted indazole derivatives as glucosidase inhibitors. Among the synthesized molecules, compound 101 exhibited stronger inhibition of the α-glucosidase enzyme, IC₅₀ = 0.42 ± 0.019, than acarbose, IC₅₀ = 0.82 ± 0.006. Docking analysis of compound 101 showed key interactions including π–alkyl/alkyl contacts with Ala284 and Leu283, an alkyl interaction with Phe525 via the 5-bromo group, and hydrogen bonding with Asp616. Its phenyl ring also formed π–π T-shaped and π–π-sulfur interactions with Trp481, Met519, and Asp616, supporting strong binding affinity¹⁰⁰ (Fig. 22).

Fig. 22. Anti-diabetes activity shown by indazole derivatives (98–101).

3.10. Anti-cancer activity

Cao et al. have reported novel indazole derivatives containing 1,2,3-triazole as potential anti-prostate cancer drugs. The most active compound 102 was reported with an IC₅₀ value of 4.42 ± 0.06 μmol L⁻¹ against the PC-3 cell line in a dose-dependent manner and showed fairly good safety both in vivo and in vitro.¹⁰¹ Wei et al. have reported the synthesis of a series of indazole derivatives as potential anticancer agents. In vitro screening of their antiproliferative activity revealed that compound 103 exhibited potent growth-inhibitory effects against several cancer cell lines, with IC₅₀ values of 0.95 ± 0.12 μM, 0.23 ± 0.03 μM, 0.80 ± 0.05 μM, 0.34 ± 0.02 μM, and 1.15 ± 0.10 μM against A549, 4T1, HepG2, MCF-7, and HCT116 cell lines, respectively.¹⁰² Ren et al. have reported the synthesis of novel indazole analogues as tubulin polymerization inhibitors with potent anticancer activities. Among the synthesized compounds, the most active compound 104 was reported with an IC₅₀ of 0.064 ± 0.007 μM (B16-F10 cells).¹⁰³ A new series of heterocycles with the indazolylthiazole moiety as anticancer drugs has been reported by Dawoud et al. With low IC₅₀ values and high SI values, these compounds demonstrated anticancer efficacy against HepG-2 and Caco-2 cell lines. With an IC₅₀ value of roughly 5.9 μg mL⁻¹ and an SI value of 26, compound 105 was found to be the most powerful. In docking studies with p53, it demonstrated the most promising binding profile. It not only reproduced the crucial hydrogen bond with Asp228, similar to the reference ligand, but also formed two additional hydrogen bonds with Pro151 and Cys220.⁷⁴ Abdelsalam et al. have reported the synthesis and anticancer evaluation of fused indazoles as potential EGFR Inhibitors. Among them, compound 106 showed IC₅₀ = 29.40 ± 1.76 μM (HepG2) and 16.33 ± 1.48 μM (HCT116), compared to that of erlotinib's IC₅₀ value of 10.19 ± 0.51 μM and 13.22 ± 0.71 respectively. Compound 106 effectively mimics the key binding interactions of erlotinib within the EGFR ATP-binding pocket. It forms two crucial hydrogen bonds—between the N1 and N3 atoms of the quinazoline ring and the amino acid residues Met769 and Thr766, respectively—thereby stabilizing its orientation in the active site. Additionally, a hydrogen bond between its methoxy substituent and Lys721, along with a π–π–π–π-cation interaction between the aromatic moiety and Phe699, further reinforces the binding affinity.¹⁰⁴ Novel 6-substituted aminoindazole compounds have been described by Hoang et al. as potential anticancer drugs. With an IC₅₀ value of 0.4 ± 0.3 μM in human colorectal cancer cells (HCT116), compound 107 has strong anti-cancer action in contrast to etoposide, which has an IC₅₀ value of 1.27 ± 0.8 μM. Compound 107 demonstrated strong binding to the IDO1 active site through key interactions: a hydrogen bond between its N2-indazole moiety and Ser167, π–π stacking of the fluorophenyl/pyridinyl ring with Phe226, and a hydrogen bond between its 6-NH group and the heme's 7-propionate¹⁰⁵ (Fig. 23).

Fig. 23. Anti-cancer activity shown by indazole derivatives (102–107).

Indoleamine 2,3-dioxygenase (IDO1) is a heme-containing enzyme primarily involved in the metabolism of tryptophan into kynurenine. To date, IDO1 inhibitors have been extensively developed to help reactivate the anticancer immune response. Based on the structure of the IDO1 active site, Hoang et al. have reported the synthesis of novel indazole compounds as IDO1 inhibitors. The most effective synthetic chemical, 108, was shown to have an IC₅₀ of 2.78 ± 0.21 μM (FaDu) in cancer.¹⁰⁶ Wang et al. have reported the synthesis of novel indazolylpyrazol acetamide derivatives as potent VEGFR-2 inhibitors. Among the synthesized derivatives the most active compound 109 was reported with IC₅₀ – 1.6 nM (VEGFR-2) and IC₅₀ – 0.36 ± 0.11 μM (HGC-27). Compound 109 binds tightly to the VEGFR-2 ATP-binding site, forming H-bonds with Glu917 and Cys919 via the indazole ring. Its terminal amide interacts with Glu885 and Asp1046, aiding the insertion of the 4-methyl-3-trifluoromethylphenyl group into the hydrophobic pocket to stabilize the DFG-out conformation. Additional H-bonding with Lys868 and π–π stacking with Phe1047 further strengthen the binding.¹⁰⁷ New compounds based on indazol-pyrimidines were created by H. M. Al-Tuwaijri et al. as specific anticancer agents. They were created and evaluated for their ability to inhibit three distinct malignant cell lines in vitro. The most active compound 110 among the synthesized compounds, with IC₅₀ values in the range of 1.841–4.99 μM, produced a higher safety profile than the reference drug towards normal cells (MCF10a). Docking results with caspase-3 showed strong binding to the caspase-3 S2 domain via key non-covalent interactions. Its indazole NH forms H-bonds with Gly94 and Glu95, while the pyrimidine ring engages in π–π stacking with Tyr165 and Phe217. Arg186 forms π–π–π–π-cation and π–π–π–π–π–π-alkyl interactions with the phenyl and morpholine rings, and Met33 forms a π–π–π–π–π–π-cation interaction with the indazole phenyl ring, enhancing overall binding affinity.¹⁰⁸ Several indazole derivatives containing acrylamide have been synthesized by Yang et al. and have been shown to selectively inhibit FGFR4 in both wild-type and gatekeeper mutants via covalent bonds. Among the synthesized molecules, the most potent compound 111 was reported with an IC₅₀ of 2.4 nM (FGFR4) and an IC₅₀ value of 21 nM (Huh7).¹⁰⁹ Puri et al. have synthesized potential anticancer agents, N1-alkylated 1H-indazole-3-carboxamide derivatives. The most active compound 112 has a GI₅₀ of 2.34 μM against the MCF-7 cell line.¹¹⁰ Elsayed et al. have reported synthetic indazole derivatives with antiangiogenic and antiproliferative anticancer activities. Among the synthesized molecules, the most active compound 113 was reported to have an IC₅₀ value of 5.4 nM against VEGFR-2, which will help in the development of new cancer drugs. All form strong hydrogen bonds with the hinge residue Cys919. Their urea moieties interact with Asp1046 (main chain NH) and Glu885 (side chain), while the chloro-phenyl ring occupies the hydrophobic pocket, engaging in lipophilic interactions with Ile889, Ile892, and Leu1019, enhancing binding affinity¹¹¹ (Fig. 24).

Fig. 24. Anti-cancer activity shown by indazole derivatives (108–113).

In order to create new irreversible inhibitors of both the wild-type and gatekeeper mutant FGFR4, Shao et al. developed and synthesized aminoindazole derivatives. Excellent potency against FGFR4 17/0.5 nM, FGFR4^V550L 0.25 nM, and FGFR4^V550M 1.6 nM is exhibited by the most active molecule, 114. To elucidate the binding interactions of our inhibitors with FGFR4, a structural analysis of the co-crystal complex of compound 114 and FGFR4 was performed. Compound 114 covalently binds to Cys552 in FGFR4 and adopts a U-shaped conformation, similar to the lead compound. Its aminoindazole core forms essential hydrogen bonds with Glu551 and Ala553, while the acrylamide group facilitates covalent bond formation by interacting with Ala553 and Arg483, guiding the molecule toward Cys552. The 2,6-dichloropyridine moiety engages Asn557, and the 4-piperidinylmorpholine group projects into the solvent-exposed region. Importantly, the measured distances to Leu550 (4.5 Å) and Met550 (4.0 Å) suggest the absence of steric clashes, supporting 114's activity against FGFR4^V550L/M mutants.¹¹² Frejat et al. have synthesised a series of new indazole compounds as powerful antiproliferative apoptotic agents through a multi-targeted approach against CDK-2/EGFR/c-Met. Among the synthesized molecules, the most active compound 115 was reported with a GI₅₀ value of 1.07 μM, and was compared with doxorubicin (GI₅₀ = 1.10 μM). Docking studies of the compound with CDK2, EGFR, and c-Met revealed higher binding energies compared to their respective co-crystallized ligands. For CDK2, the compounds formed multiple hydrogen bonds and π–H interactions with key active site residues, indicating strong binding affinity and excellent active site occupancy. In the case of EGFR, although the test compounds exhibited slightly lower binding scores than the native ligand, their binding energies were still significant. Notably, compound 115 demonstrated the strongest interactions with critical residues and showed superior binding compared to the other test compounds, closely mimicking the co-crystallized ligand. For c-Met, all the compounds displayed a common π–π interaction with TYR1230 and a hydrogen bond acceptor interaction with MET1160, similar to the interactions observed with the native ligand. The compound showed the best structural overlay with the co-crystallized ligand, supporting their potential as effective c-Met inhibitors.¹¹³ Reddy et al. synthesised 1-aryl-3-arylamino-2-propen-1-ones as anti-cancer agents. One of them is with indazole-substituted derivative 116 which was reported with IC₅₀ values of 0.75 μM and 2.5 μM against DU145 and K562 cell lines, respectively.¹¹⁴ Chabukswar et al. have reported novel indazole derivatives by computational methods. These scaffolds exhibited better binding affinity and significant interactions with VEGFR-2 enzymes (PDB ID: 4AGD); the most potent compound, 117, was reported with (−6.99 kcal mol⁻¹). 4AGD was found as a potent VEGFR-2 inhibitor.¹¹⁵ Wang et al. have reported a series of 3-amino-1H-indazole derivatives targeting the PI3K/AKT/mTOR pathway of HGC-27 cells, showing the in vitro activity of compound 118 with IC₅₀ values HGC-27 = 0.43 ± 0.17 μM, with low tissue toxicity and good pharmacokinetic properties.¹¹⁶ Wei et al. have reported the synthesis of new indazole spiro-pentacylamide derivatives as acetyl-CoA carboxylase inhibitors. The active compound 119 was reported with an ACC-1 IC₅₀ value of 1.202 μM and ACC-2 IC₅₀ of 0.944 μM.¹¹⁷ A new class of amide compounds substituted with 6-bromo-1-cyclopentyl-1H-indazole-4-carboxylic acid has been described by Sawant et al. as anticancer medicines. The strongest of the produced compounds, 120, showed cytotoxicity against HEP3BPN-11, MDA-453 and HL-60, with 88.12 ± 0.6, 60.56 ± 0.8, and 69.10 ± 0.11 for each compound, respectively. Docking studies reveal key interactions with conserved TNFα residues. Specifically, 120 forms hydrogen bonds with Tyr-151 and Ser-60, while 11d interacts with Leu-120 and Ser-60. These findings are consistent with crystallographic data, which validate the importance of Tyr-151, Leu-120, and Ser-60 in TNFα-inhibitor stabilization, supporting their role in indazole-mediated TNFα inhibition.¹¹⁸ Qin et al. have reported novel indazole derivatives as type II TRK inhibitors. The most active compound, 121, was reported to be active against multiple TRK mutants and cancer cell lines with a Km-12 IC₅₀ value of 4.1 nM, Ba/F3-TRKAG595R IC₅₀ value of 41.5 nM, and Ba/F3-TRKAG667C IC₅₀ value of 1.4 nM for the tropomyosin receptor kinase (TRK)-type inhibitor used in cancer treatment. Docking studies with the TRKA DFG-out conformation revealed that the phenyl ring of the compound extends into the allosteric hydrophobic pocket, while amide or urea linkers form hydrogen bonds with Glu-560 and Asp-668. The indazole core engages in two hydrogen bonds with hinge region residues Glu-590 and Met-592, and participates in π–π stacking interactions with Phe-589 and Phe-669 (ref. 119) (Fig. 25).

Fig. 25. Anti-cancer activity shown by indazole derivatives (114–121).

3.11. Miscellaneous

Huang et al. reported the synthesis of novel indazole derivatives as selective ROCK2 inhibitors for the treatment of psoriasis. Among the synthesized compounds, the most active candidate, 122, demonstrated an IC₅₀ value of 3.7 ± 0.8 nM, with 19-fold selectivity over ROCK1. The findings indicate that compound 122 holds promise as a more potent treatment for psoriasis compared to belumosudil (KD025), offering a potential solution to the unmet medical needs of psoriasis patients. This could lead to better treatment options and potentially improve their quality of life. The binding mode of compound 122 with ROCK2 (PDB ID: 6ED6) revealed multiple stabilizing interactions, including hydrogen bonds with Lys121 and Met172, a π–π stacking interaction with Phe136, and a π–π–π-cation interaction with Lys121. Additionally, a hydrogen-bond-assisted salt bridge was observed with Asp232, further contributing to the compound's affinity and binding stability.¹²⁰ Alim et al. reported the inhibitory effects of a few molecules of indazoles, substituted with halogens at different positions, on CA-I and CA-II isoenzymes. The most active compound, 123, exhibited a K_i value (binding affinity) of 0.383 ± 0.021 μM for hCA-I, while compound 124 showed a K_i value of 0.409 ± 0.083 μM for hCA-II. Carbonic anhydrase-1 is linked to retinal and cerebral edema, whereas carbonic anhydrase-II is associated with glaucoma and epilepsy.¹²¹ Scala et al. have reported the synthesis of novel tetrahydroindazolylbenzamide derivatives as anti-HIV agents. The biological data revealed that compound 125 was reported with the capacity to significantly selectively limit HIV proliferation with minimal cytotoxicity (EC₅₀ 2.77 μM; CC₅₀ 118.7 μM; SI = 68).¹²² Wang et al. have introduced a new indazole derivative, Adjudin 150, as a non-hormonal male contraceptive. It effectively induces reversible male infertility without altering or disrupting the serum levels of follicle-stimulating hormone, testosterone, or inhibin B. The contraceptive effect of Adjudin 126 is believed to occur through modifications in microtubule-associated proteins (MAPs)¹²³ (Fig. 26).

Fig. 26. Miscellaneous activities shown by indazoles (122–126).

4. Future perspectives

Despite significant advances, a general and efficient method for synthesizing structurally diverse substituted indazoles remains elusive. Developing broadly applicable protocols targeting this important pharmacophore is a key future objective. Such methods should combine high yields, mild reaction conditions, operational simplicity, scalability, and environmentally friendly features, such as metal-free or recyclable catalysts and the use of green oxidants like molecular oxygen. Further research into heterogeneous and nanoparticle catalysts with high turnover, stability, and recyclability under mild conditions can improve cost-effectiveness and reduce environmental impact. Expanding the substrate scope to include sensitive functional groups and heteroatom-containing moieties will enhance the synthetic utility for complex, drug-like molecules. Integrating recent advances in photochemistry, flow chemistry, microwave-assisted synthesis, and computational design with traditional synthetic methods can accelerate the discovery of novel indazole derivatives. Concurrent progress in biological screening, fragment-based drug design, and scaffold hopping will reinforce the role of indazole scaffolds as privileged frameworks in medicinal chemistry. Additionally, deeper mechanistic insights—particularly into metal–ligand cooperative catalysis and electrocyclization pathways—will facilitate the rational design of new catalysts and innovative synthetic strategies. Indazole derivatives are promising scaffolds in drug discovery with broad biological activities. Future research should focus on expanding structural diversity to improve potency and selectivity, and on mechanism-guided design using computational and SAR studies. Developing multifunctional agents that target multiple pathways can address complex diseases and resistance. Comprehensive pharmacokinetic and safety profiling will support clinical translation, which is critical for exploring new therapeutic areas such as neurodegenerative and infectious diseases. Lastly, integrating advanced technologies like photochemistry, flow synthesis, high-throughput screening, and AI-driven design will accelerate the discovery of novel indazole-based drugs.

5. Conclusion

In this review, we have comprehensively highlighted the synthesis strategies, structural diversity, and broad spectrum of biological activities associated with indazole derivatives, including compounds currently undergoing various stages of clinical evaluation. The compilation of synthetic methodologies—ranging from classical approaches to modern green and catalytic techniques—along with detailed accounts of pharmacological applications, offers a valuable framework for researchers aiming to design novel indazole-based therapeutic agents. Moving forward, a critical challenge lies in advancing the sustainable synthesis of indazole derivatives. Future research should focus on the development of green, scalable, and economically viable methodologies that minimize environmental impact. Approaches such as metal-free catalysis, the use of bio-based solvents, recyclable catalysts, and energy-efficient techniques like microwave or flow-assisted synthesis hold great promise. Overcoming these challenges will not only enhance the synthetic accessibility of indazole scaffolds but also facilitate their industrial translation, paving the way for the next generation of eco-conscious drug candidates.

Author contributions

Anuradha Singampalli- writing – original draft, conceptualization; Pardeep Kumar- writing – original draft, conceptualization; Rani Bandela- writing – review & editing; Sri Mounika Bellapukonda- writing – review & editing; Srinivas Nanduri- funding acquisition, writing – review & editing; Venkata Madhavi Yaddanapudi- supervision, conceptualization.

Conflicts of interest

There are no conflicts to declare. The authors declare no competing financial interests.

Data availability

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