Indole–Pyrazoline Hybrid Derivatives as Potential PARP-1 Targeted Anticancer Agents: A Comprehensive Review of Design Strategies, Structure–Activity Relationships, and Future Perspectives

Cancer continues to rank among the leading causes of morbidity and mortality worldwide. The 2024 GLOBOCAN estimates, reported in a 2026 publication, indicate approximately 20.6 million new cancer cases and 9.8 million cancer deaths worldwide, with projections of a substantial rise by mid-century if current demographic and risk-factor trends persist [8]. Although targeted therapies, immunotherapy and advanced radiation techniques have improved outcomes for selected patient subsets, intrinsic and acquired resistance, dose-limiting toxicities and incomplete tumour eradication remain formidable clinical challenges [9].

One of the most successful strategies for addressing these limitations is the exploitation of synthetic lethality. Inhibition of poly (ADP-ribose) polymerase-1 (PARP-1), a key enzyme in the base-excision repair pathway, is synthetically lethal in cells deficient in homologous recombination (HR), most notably those carrying BRCA1/2 mutations [1,2]. Four PARP inhibitors—olaparib, rucaparib, niraparib and talazoparib—have received approval from major regulatory agencies, including the US Food and Drug Administration and the European Medicines Agency. Additional PARP inhibitors, including fuzuloparib and pamiparib, have been approved in China [10,11].

Parallel to the clinical success of PARP inhibitors, medicinal chemists have intensively explored molecular hybridization—the covalent combination of two or more privileged pharmacophores into a single molecular entity [12]. Hybrids frequently display improved potency, multi-target engagement or more favourable physicochemical profiles relative to their parent scaffolds. Among nitrogen heterocycles, the indole nucleus and the pyrazoline (4,5-dihydro-1H-pyrazole) ring occupy prominent positions. Indole is present in numerous natural products and approved drugs (vincristine, sunitinib, panobinostat) and offers versatile substitution points at N-1, C-2, C-3, C-5 and C-6 [13]. Pyrazoline derivatives have been extensively investigated as tubulin polymerization inhibitors, topoisomerase ligands and anti-inflammatory agents; their aromatic pyrazole counterparts are well validated in kinase inhibition [3,14].

The covalent linkage of an indole moiety (most commonly at C-3) with a pyrazoline ring has generated a distinct class of hybrids that often exhibit superior cytotoxic potency compared with the individual components [3–6,15]. These hybrids have been reported to disrupt microtubule dynamics and inhibit topoisomerase II, while certain series have been investigated for possible interactions with EGFR. The strength of the mechanistic evidence varies between studies [4–6,15,18]. Remarkably, however, systematic experimental evaluation of these hybrids against PARP-1 itself remains scarce. Although the pyrazoline and pyrazole rings share some structural features with the indazole core of niraparib, this resemblance provides only a design hypothesis and does not establish PARP-1 inhibitory activity [7].

This review therefore aims to bridge these two productive areas of anticancer research. We first summarise the biological rationale for PARP-1 targeting and extract key SAR lessons from pyrazole- and indazole-based PARP-1 inhibitors. We then survey the existing anticancer literature on indole–pyrazoline hybrids, organised by primary molecular target. Finally, we propose concrete design principles that could enable the rational exploration of indole–pyrazoline derivatives as selective or dual PARP-1 inhibitors, while highlighting the experimental gaps that must be closed before translational claims can be made.

2. PARP-1: BIOLOGICAL RATIONALE AND CLINICAL CONTEXT

2.1. Structure and catalytic function

PARP-1 is a multifunctional nuclear enzyme of approximately 116 kDa that catalyses the transfer of ADP-ribose units from NAD+ onto itself and acceptor proteins, generating linear or branched poly (ADP-ribose) (PAR) chains [1,17]. The enzyme comprises an N-terminal DNA-binding domain containing three zinc fingers, a central automodification domain, and a C-terminal catalytic domain that houses the NAD+-binding site. Upon detection of DNA single-strand breaks, PARP-1 undergoes rapid activation, PARylates itself and chromatin-associated proteins, and recruits additional repair factors of the base-excision repair machinery [1,17].

In cells with intact HR, the temporary accumulation of unrepaired single-strand breaks can be resolved during replication by HR-mediated pathways. In HR-deficient cells, however, PARP-1 inhibition leads to the persistence of unrepaired lesions, replication-fork collapse and the accumulation of toxic double-strand breaks that cannot be accurately repaired, culminating in cell death—the classic synthetic-lethal interaction [1,2,10].

2.2. Approved PARP inhibitors and structural features

Four PARP inhibitors are currently approved for clinical use: olaparib (phthalazinone core), rucaparib (tricyclic scaffold), niraparib (indazole core) and talazoparib (tricyclic core with a fluorophenyl substituent) [10,11]. Niraparib is of particular interest because its core contains an indazole ring. Indazole is a fused bicyclic aromatic system that is related to, but distinct from, both the aromatic pyrazole and the partially saturated pyrazoline rings [7]. Characteristic interactions observed in the binding modes of established PARP inhibitors include hydrogen-bond contacts with Gly863 and Ser904 (human numbering), often reinforced by π-stacking with Tyr907 and hydrophobic contacts within the nicotinamide-binding pocket [7,17]. These interactions are representative of the NAD+-site pharmacophore but are not necessarily identical for every inhibitor.

A recent comprehensive review of PARP-1 inhibitors bearing pyrazole or indazole cores systematically catalogued derivatives published over a fifteen-year period and extracted clear SAR trends [7]. Key observations include: (i) the importance of a hydrogen-bond donor–acceptor motif capable of mimicking the amide of NAD+; (ii) the beneficial effect of an appropriately positioned aromatic or heteroaromatic substituent that occupies the adenine-binding region; and (iii) the frequent advantage of rigidifying the core or introducing small polar substituents that improve aqueous solubility without compromising potency [7].

3. INDOLE–PYRAZOLINE HYBRIDS: EXISTING ANTICANCER PROFILE

Although direct experimental PARP-1 inhibition data for true indole–pyrazoline hybrids are currently sparse, a substantial body of work has established the scaffold as a productive platform for anticancer activity through other mechanisms [3,14]. The following subsections summarise the principal series, organised by primary molecular target.

3.1. Tubulin polymerization inhibitors

Tubulin polymerisation is one of the better experimentally characterised anticancer mechanisms reported for indole–pyrazoline hybrids. Zhang and co-workers designed a series of twenty-eight 1-methyl-1H-indole–pyrazoline hybrids [4]. The standout compound, e19 [5-(5-bromo-1-methyl-1H-indol-3-yl)-3-(3,4,5-trimethoxyphenyl)-4,5-dihydro-1H-pyrazole-1-carboxamide], inhibited tubulin assembly with an IC?? of 2.12 µM and displayed cellular IC?? values of 0.21–0.31 µM against HeLa, MCF-7, A549 and HepG2 cell lines [4]. Mechanistic studies confirmed G2/M arrest, microtubule-network disruption and apoptosis. Molecular docking placed the 3,4,5-trimethoxyphenyl moiety deep in the colchicine pocket, while the 5-bromoindole and the carboxamide formed additional polar and hydrophobic contacts [4].

Chen and colleagues prepared nicotinoyl-substituted pyrazolines bearing an N-methylindole moiety [5]. Compound 28 exhibited GI?? values of 29–90 nM against HeLa, HepG2 and MCF-7, inhibited tubulin polymerisation with an IC?? of 1.6 µM, and produced 61.52 % tumour-growth inhibition in a HeLa xenograft model [5]. Within the studies reviewed here, this remains one of the stronger in-vivo efficacy results reported for an indole–pyrazoline hybrid.

Hawash et al. examined related indole-3-pyrazole-5-carboxamides (aromatic pyrazole rather than 4,5-dihydropyrazole) [16]. Compound 18 displayed IC?? values of 0.6–2.9 µM against hepatocellular carcinoma lines and moderately inhibited tubulin polymerization (IC?? ≈ 19 µM) [16]. The difference in reported tubulin-inhibitory potency between these series suggests that ring saturation may influence biological activity. However, differences in substitution patterns, compound properties and experimental conditions prevent a direct attribution of this difference to ring saturation alone.

3.2. Topoisomerase II inhibitors

Song et al. synthesised 3-(1H-indol-3-yl)-2,3,3a,4-tetrahydrothiochromeno[4,3-c]pyrazole derivatives [6]. Compounds 7d and 7f inhibited MGC-803 gastric cancer cells with IC?? values of 15.43 and 20.54 µM, respectively, and acted as non-intercalative catalytic inhibitors of Topo II [6]. Haider and co-workers prepared indole-tethered pyrazoline derivatives; the leads 7a and 7b inhibited A431 cells with IC?? values of 3.17 and 5.16 µM, suppressed colony formation and migration, caused S-phase arrest, regulated apoptotic proteins, and directly inhibited Topo IIα [15]. The reported acute oral toxicity and cardiac histopathology findings suggested a more favourable profile than doxorubicin under the experimental conditions evaluated; however, these findings do not establish overall safety or clinical tolerability [15].

3.3. EGFR and related kinase engagement

Khalilullah et al. synthesised pyrazolinyl-indole derivatives and submitted them to the NCI-60 panel [18]. Compound HD05 [1-(3-(4-chlorophenyl)-5-(1H-indol-3-yl)-4,5-dihydro-1H-pyrazol-1-yl)-2-phenoxyethanone] produced the broadest spectrum of activity; at 10 µM it achieved 78.76 % growth inhibition of leukaemia cell lines (versus 9 % for imatinib under identical conditions) [18]. Molecular docking into the ATP-binding site of EGFR supported a plausible binding mode; however, direct biochemical confirmation of EGFR inhibition was not reported, and the observed cytotoxicity cannot be attributed solely to EGFR engagement on the basis of the published data [18]. Related pyrazole–indole hybrids have also been shown to inhibit CDK-2 and induce apoptosis, illustrating the broader capacity of the indole–azole hybrid family to engage multiple kinases [19]. Note that the Hassan series comprises aromatic pyrazole–indole hybrids rather than true pyrazolines [19].

3.4. Antiproliferative activity and apoptotic convergence

Venugopala et al. designed and evaluated a series of 5-(indol-3-yl)-4,5-dihydro-1H-pyrazole-1-carboxamide derivatives [20]. Compounds bearing a hydroxy-phenyl substituent displayed potent antiproliferative activity across A549, MCF-7, HepG2 and DU-145 lines [20]. While the study employed docking to explore possible interactions, including with the aryl hydrocarbon receptor, the primary experimental read-outs were cellular cytotoxicity; direct experimental confirmation of AhR engagement as the dominant mechanism requires further validation. Several reported indole–pyrazoline and related hybrids induce apoptosis, although the molecular events and supporting experimental evidence vary across individual compound series [4–6,15,16].

4. BRIDGING INDOLE–PYRAZOLINE CHEMISTRY TO PARP-1 INHIBITION

4.1. Structural considerations and design hypotheses

Although experimental PARP-1 IC?? values for true indole–pyrazoline hybrids are largely absent from the literature, several structural observations provide a basis for design hypotheses [7]:

• The pyrazoline and pyrazole rings share some topological features with the indazole core of niraparib and certain investigational PARP-1 inhibitors; however, they are not interchangeable scaffolds and any predicted PARP-1 activity must be confirmed experimentally [7].
• The carboxamide or nicotinoyl substituent frequently installed at pyrazoline N-1 in the tubulin-targeting series could, in principle, be re-optimised to serve as a hydrogen-bond donor–acceptor motif of the type required for NAD+-site recognition [4,5,7].
• The indole C-3 attachment provides a rigid, planar aromatic extension that might be directed toward the adenine-binding region or solvent-exposed surfaces of PARP-1, by analogy with hydrophobic or heteroaromatic appendages found in known PARP-1 ligands [7,13].
• Substitution at indole C-5 (bromo, chloro, methoxy) and N-1 (methyl versus hydrogen) offers independent handles for modulating potency, selectivity and physicochemical properties [4,5,13].

4.2. Lessons from pyrazole/indazole PARP-1 SAR

Drawing on the systematic analysis of Bastos et al. and related studies, the following design considerations are relevant for any future attempt to develop indole–pyrazoline compounds as PARP-1 inhibitors [7]:

• A primary or secondary carboxamide (or closely related H-bond donor–acceptor pair) capable of interacting with Gly863 and Ser904 is a recurring feature of high-affinity NAD+-site binders [7].
• An aromatic or heteroaromatic substituent occupying the adenine-binding pocket frequently improves both enzymatic potency and cellular activity; the 3,4,5-trimethoxyphenyl motif successful in tubulin inhibitors would likely need replacement by groups better matched to the PARP-1 surface [4,7].
• Rigidification of the core (for example, by fusing additional rings or introducing constrained linkers) has often enhanced potency in other PARP-1 series, although excessive lipophilicity must be avoided [7].
• Small polar substituents (hydroxyl, primary amine, morpholine) on the periphery can improve solubility and reduce off-target binding [7].

These observations constitute design hypotheses only. They do not establish that any existing or proposed indole–pyrazoline hybrid will inhibit PARP-1, nor do they imply that an N-1 carboxamide automatically confers the correct geometry or affinity for PARP-1 inhibition.

4.3. Dual-target considerations

Because many of the most potent indole–pyrazoline hybrids already engage tubulin or topoisomerase II, a dual PARP-1 / tubulin or PARP-1 / Topo II profile is conceptually attractive [4–6,15]. Dual inhibitors could, in principle, simultaneously exploit synthetic lethality in HR-deficient cells and disrupt mitotic progression or DNA topology in HR-proficient cells [1,2]. Achieving balanced dual activity would require careful titration of relative affinities and rigorous biochemical profiling against both target classes—work that has not yet been reported for this scaffold class.

5. SYNTHETIC ACCESSIBILITY

Many indole–pyrazoline derivatives can be prepared through established condensation and cyclisation routes, although reaction conditions, yields and subsequent functionalisation depend on the individual substrate and target structure [3–6,14,15]. A commonly employed sequence comprises:

• Formylation of a substituted indole (Vilsmeier–Haack) or use of a commercial indole-3-carbaldehyde.
• Optional N-methylation of the indole.
• Claisen–Schmidt condensation with a substituted acetophenone (or heteroaryl methyl ketone) to afford the corresponding chalcone.
• Cyclization of the chalcone with hydrazine hydrate, semicarbazide or a substituted hydrazine under conventional heating or microwave irradiation to construct the pyrazoline ring.
• Optional acylation of the pyrazoline N-1 nitrogen with acid chlorides, anhydrides or isocyanates to install the carboxamide or related functionality.

In selected series, microwave irradiation has been reported to shorten reaction times and improve isolated yields relative to conventional heating [15]. The precise benefit depends on the substrate and should be evaluated case by case.

6. CURRENT CHALLENGES AND TRANSLATIONAL GAPS

Despite the attractive structural rationale, several important limitations must be acknowledged:

• Absence of direct experimental PARP-1 inhibition data for true indole–pyrazoline hybrids. Docking predictions, while useful for hypothesis generation, do not substitute for biochemical enzyme assays [7,18].
• Variable assay conditions across published series complicate cross-study comparisons of cellular potency [3,14].
• Limited normal-cell selectivity data and incomplete pharmacokinetic characterisation for most reported hybrids [5,15].
• Only a minority of compounds (most notably compound 28 of Chen et al.) have been advanced to detailed in-vivo efficacy and safety studies [5].
• The risk that hybridisation increases molecular weight and lipophilicity beyond desirable drug-like ranges must be monitored quantitatively [12].

A rigorous development pathway for any future PARP-1-directed indole–pyrazoline series should therefore include: (i) reproducible synthesis and full spectroscopic characterisation; (ii) orthogonal biochemical PARP-1 and PARP-2 enzyme assays with appropriate positive controls; (iii) cellular target-engagement read-outs (PARylation assays, γH2AX foci); (iv) selectivity profiling against related NAD+-utilising enzymes and a representative kinase panel; (v) normal-cell cytotoxicity comparisons; and (vi) early ADME/PK evaluation before any in-vivo efficacy studies [1,7,10].

7. FUTURE PERSPECTIVES AND DESIGN RECOMMENDATIONS

The following recommendations are offered for the rational exploration of next-generation indole–pyrazoline compounds as potential PARP-1 inhibitors. They are design hypotheses that require experimental validation:

• Retain a pyrazoline N-1 carboxamide (or closely related H-bond donor–acceptor) as a candidate NAD+-site recognition motif, by analogy with known PARP-1 inhibitors [7].
• Systematically vary the C-3 aryl substituent of the pyrazoline, prioritising groups known to occupy the adenine-binding region of PARP-1 rather than the 3,4,5-trimethoxyphenyl colchicine-site pharmacophore [4,7].
• Explore both N-H and N-methyl indole variants, and introduce modest polarity at C-5 or C-6 to balance potency and solubility [4,5,13].
• Consider constrained or fused analogues (e.g., tetrahydrothiochromeno-fused systems already shown to inhibit Topo II) as dual-target leads [6].
• Incorporate matched-pair analysis and quantitative structure–property relationship modelling from the earliest stages of optimisation [12].
• Prioritise compounds that display biochemical PARP-1 IC?? values in the low-nanomolar range and cellular PARylation inhibition at concentrations well below those causing non-specific cytotoxicity [1,7].

CONCLUSION

Indole–pyrazoline hybrids constitute a synthetically accessible and structurally tunable scaffold that has demonstrated anticancer activity in studies involving tubulin and topoisomerase II, alongside reports of other potential mechanisms that require differing levels of experimental confirmation [3–6,15,16,18]. The structural features shared by the pyrazoline/pyrazole core and the indazole pharmacophore of certain clinically validated PARP-1 inhibitors, together with the frequent presence of a carboxamide functionality at pyrazoline N-1, provide a plausible rationale for exploring this hybrid class toward PARP-1 engagement [7]. Realisation of this potential, however, will require a disciplined experimental programme that moves beyond docking hypotheses to rigorous biochemical validation, selectivity profiling and early pharmacokinetic optimisation. With such an approach, the indole–pyrazoline framework may yield selective or dual-acting agents that expand the therapeutic reach of synthetic-lethal strategies in oncology [1,2,10].

REFERENCES

1. Lord CJ, Ashworth A. PARP inhibitors: synthetic lethality in the clinic. Science. 2017;355 (6330):1152–1158. doi:10.1126/science.aam7344.
2. Mateo J, Lord CJ, Serra V, et al. A decade of clinical development of PARP inhibitors in perspective. Ann Oncol. 2019;30 (9):1437–1447. doi:10.1093/annonc/mdz192.
3. Matiadis D, Sagnou M. Pyrazoline hybrids as promising anticancer agents: an up-to-date overview. Int J Mol Sci. 2020;21 (15):5507. doi:10.3390/ijms21155507.
4. Zhang YL, Qin YJ, Tang DJ, et al. Synthesis and biological evaluation of 1-methyl-1H-indole–pyrazoline hybrids as potential tubulin polymerization inhibitors. ChemMedChem. 2016;11 (13):1446–1458. doi:10.1002/cmdc.201600137.
5. Chen K, Zhang YL, Fan J, et al. Novel nicotinoyl pyrazoline derivatives bearing N-methyl indole moiety as antitumor agents: Design, synthesis and evaluation. Eur J Med Chem. 2018;156:722–737. doi:10.1016/j.ejmech.2018.07.044.
6. Song Y, Feng S, Feng J, et al. Synthesis and biological evaluation of novel pyrazoline derivatives containing indole skeleton as anti-cancer agents targeting topoisomerase II. Eur J Med Chem. 2020;200:112459. doi:10.1016/j.ejmech.2020.112459.
7. Bastos IM, Rebelo S, Silva VLM. A review of poly (ADP-ribose)polymerase-1 (PARP1) role and its inhibitors bearing pyrazole or indazole core for cancer therapy. Biochem Pharmacol. 2024;221:116045. doi:10.1016/j.bcp.2024.116045.
8. Sung H, Filho AM, Laversanne M, et al. Global cancer statistics 2024: GLOBOCAN estimates of incidence and mortality worldwide for 34 cancers in 186 countries. CA Cancer J Clin. 2026;76 (4):e70090. doi:10.3322/caac.70090.
9. Vasan N, Baselga J, Hyman DM. A view on drug resistance in cancer. Nature. 2019;575 (7782):299–309. doi:10.1038/s41586-019-1730-1.
10. Curtin NJ, Szabo C. Poly (ADP-ribose) polymerase inhibition: past, present and future. Nat Rev Drug Discov. 2020;19 (10):711–736. doi:10.1038/s41573-020-0076-6.
11. Das PK, Matada GSWP, Pal R, Maji L, Dhiwar PS, Manjushree BV, Viji MP. Poly (ADP-ribose) polymerase (PARP) inhibitors as anticancer agents: An outlook on clinical progress, synthetic strategies, biological activity, and structure-activity relationship. Eur J Med Chem. 2024;274:116535. doi:10.1016/j.ejmech.2024.116535.
12. de Sena Murteira Pinheiro P, Franco LS, Montagnoli TL, Fraga CAM. Molecular hybridization: a powerful tool for multitarget drug discovery. Expert Opin Drug Discov. 2024;19 (4):451–470. doi:10.1080/17460441.2024.2322990.
13. Dadashpour S, Emami S. Indole in the target-based design of anticancer agents: a versatile scaffold with diverse mechanisms. Eur J Med Chem. 2018;150:9–29. doi:10.1016/j.ejmech.2018.02.065.
14. Haider K, Shafeeque M, Shaikh Y, Yar MS. A comprehensive review on pyrazoline based heterocyclic hybrids as potent anticancer agents. European Journal of Medicinal Chemistry Reports. 2022;5:100042. doi:10.1016/j.ejmcr.2022.100042.
15. Haider K, Sharma S, Pokharel YR, et al. Synthesis, biological evaluation, and in silico studies of indole-tethered pyrazoline derivatives as anticancer agents targeting topoisomerase IIα. Drug Dev Res. 2022;83 (7):1555–1577. doi:10.1002/ddr.21976.
16. Hawash M, Guntekin Ergun S, Kahraman DC, et al. Novel indole-pyrazole hybrids as potential tubulin-targeting agents; Synthesis, antiproliferative evaluation, and molecular modeling studies. J Mol Struct. 2023;1285:135477. doi:10.1016/j.molstruc.2023.135477.
17. Langelier MF, Planck JL, Roy S, Pascal JM. Structural basis for DNA damage-dependent poly (ADP-ribosyl)ation by human PARP-1. Science. 2012;336 (6082):728–732. doi:10.1126/science.1216338.
18. Khalilullah H, Agarwal DK, Ahsan MJ, et al. Synthesis and anti-cancer activity of new pyrazolinyl-indole derivatives: pharmacophoric interactions and docking studies for identifying new EGFR inhibitors. Int J Mol Sci. 2022;23 (12):6548. doi:10.3390/ijms23126548.
19. Hassan AS, Moustafa GO, Awad HM, et al. Design, synthesis, anticancer evaluation, enzymatic assays, and a molecular modeling study of novel pyrazole–indole hybrids. ACS Omega. 2021;6 (18):12361–12374. doi:10.1021/acsomega.1c01604.
20. Venugopala KN, Habeebuddin M, Aldhubiab BE, Asif AH. Design, synthesis, and in vitro evaluation of novel indolyl dihydropyrazole derivatives as potential anticancer agents. Molecules. 2021;26 (17):5235. doi:10.3390/molecules26175235.