High affinity and low PARP-trapping benzimidazole derivatives as a potential warhead for PARP1 degraders

PARPi have been explored and applied in the treatment of various cancers with remarkable efficacy, especially BRCA1/2 mutated ovarian, breast, prostate, and pancreatic cancers. However, PARPi renders inevitable drug resistance and showed high toxicity because of PARP-Trapping with long-term clinic tracking. To overcome the drug resistance and the high toxicity of PARPi, many novel methods have been developed including PROTACs. Being an event-driven technology, PROTACs needs a high affinity, low toxicity warhead with no steric hindrance in binding process. Veliparib shows the lowest PARP-Trapping effect but could hardly to be the warhead of PROTACs because of the strong steric hindrance. Other PARP1 inhibitors showed less steric hindrance but owns high PARP-Trapping effect. Thus, the development of novel warhead with high PARP1 affinity, low PARP1-Trapping, and no steric hindrance would be valuable. In this work, we reserved benzimidazole as the motif to reserve the low PARP1-Trapping effect and substituted the pyrrole by aromatic ring to avoiding the steric hindrance in PARP1 binding cave. Thus, a series of benzimidazole derivates were designed and synthesized, and some biological activities in vitro were evaluated including the inhibition for PARP1 enzyme and the PARP-Trapping effect using MDA-MB-436 cell line. Results showed that the compound 19A10 has higher PARP1 affinity(IC₅₀ = 4.62 nM)) and similar low PARP-Trapping effect compared with Veliparib(IC₅₀ (MDA-MB-436) >100 μM). Docking study showed that the compound 19A10 could avoiding the steric hindrance which was much better than Veliparib. So, the compound 19A10 could potentially be a perfect warhead for PARP1 degraders. Besides, because of the depletion of the PARP1 and the decreasing of the binding capability, we suppose that the PROTACs using 19A10 as the warhead would be no-PARP-Trapping effect. Furthermore, QSAR study showed that to develop novel compounds with high PARP1 binding affinity and low PARP-Trapping, we can choose the skeleton with substituent R₁H, R₂ = piperiazine, and R₃ with large tPSA. And, if we want to develop the compounds with high PARP1 binding affinity and high PARP-Trapping which can possibly improve the lethality against tumor cells, we can choose the skeleton with substituent R₁F, R₂ = 3-methy-piperiazine, and R₃ with large tPSA.