Poly (ADP-ribose) polymerases (PARPs) are a family of 18 isoforms of proteins which involved in a variety of cellular functions, including DNA transcription, DNA damage response, maintenance of genome stability, and cell death [1,2].Poly (ADP-ribose) polymerase-1 inhibitor (PARPi) was the first example of successful clinical entry using PARPi-BRCA synthetic lethality. It plays an important role in DNA single-strand repair, but when inhibited it attenuates DNA single-strand repair and further leads to DNA double-strand breaks (DSB). Thus, in cancers with defective homologous recombination, including those with BRCA1 or BRCA2 deficiency, PARP-1 inhibitors achieve anticancer effects by causing cell death through synthetic lethality. In addition, PARPi is the essentially nicotinamide adenine dinucleotide (NAD+) analogs that bind to the NAD + binding pocket of PARP-1 to form a DNA-PARP complex which cannot be subsequently dissociated, a process known as "PARP-trapping" [3]. The "PARP-trapping" results in the long-term persistence of the DNA-PARP complex and PARP1 dysfunction to prevent DNA damage repair. Currently, many PARPis have been explored and applied in the treatment of various cancers with remarkable efficacy, especially BRCA1/2 mutated ovarian, breast, prostate, and pancreatic cancers [[4], [5], [6]], including Olaparib, Rucaparib, Niraparib, and Talazoparib [4,[7], [8], [9]] and many inhibitors were still candidate in clinical trials, such as veliparib (ABT-888).

However, clinic antitumor therapy of PARPis renders 40 %–70 % of patients inevitably resistant [10].The possible resistance mechanisms include the following: a) restoration of HRR function in HRR-deficient cancer cells; b) decreased expression of the target enzyme PARP-1; c) increased drug efflux, and mutation of the binding site [[11], [12], [13]]. Disappointingly, the emergence of resistance is hard to avoid. Because PARPis are thought to greatly enhance genomic instability, which may increase susceptibility to new mutations [14]. At the same time, PARPis can triggering the modulation of multiple immuno-related signaling pathways such as the cGAS-STING, ATM/ATR/Chk1, STAT3, etc. [[14], [15], [16], [17], [18], [19]] More seriously, PARPis can prevents PARP1 auto-PARylation and causes it to be trapped at DNA lesions. This PARP1–PARPi–DNA complex interferes with subsequent DNA replication and therefore is high toxicity [20]. With long-term follow-up, the Annual meeting on women's cancer, 2019–2021, issued the tracking PARPi result of final OS for gBRCAm and non-gBRCAm Cohorts which showed that the Hazard ratio of PARPi for non-gBRCAm was about 1.06, representing high risk of unsafety. In 2022, the FDA withdraw or remove all the new indication applications of Nilaparib, Olaparib and Rucaparib. To overcome the drug resistance and the high toxicity of PARPis, many novel methods have been developed, for example, combinational therapy [[21], [22], [23]], dual-target molecular design [24] and PROTACs [25], etc.

Proteolysis-targeting chimeras (PROTAC) was a novel and valuable tool for the chemical knockdown of a protein of interest [[26], [27], [28], [29]]. The concept was artificially targeting a protein to an E3 complex for ubiquitination and degradation. PROTAC molecules (PROTACs) are small molecules consisting of a warhead, an E3 recruiting ligand and an interval linker, which brings the E3 ligase and the target protein into proximity where the protein is ubiquitinated and subsequently degraded through the proteasome pathway. The degraders developed through this technique have significant advantages over the traditional small-molecule drugs, including targeting ‘‘undruggable targets’’ and overcoming the drug resistance by eliminating pathogenesis-related proteins with higher selectivity, lower toxicity, and lower dose [24,[30], [31], [32], [33]]. In 2019, Yu's group [14] was firstly attempted to synthesized a series of PROTACs using different PARPi (Niraparib, Olaparib, Rucaparib, Veliparib) as the warhead which combined with a PEG-mixed linker and CRBN as the E3 recruiting ligand. They found that iRucaparib-AP6 showed highly efficient and specific PARP1 degradation, and, most importantly, low PARP-Trapping effect. Encouraged by the exploration of Yu's group, more and more new PARPi PROTACs were developed, for example, Niraparib as the warhead and MDM2 as the E3 ligand [25], Niraparib as the warhead and CRBN as the E3 ligand [24], Olaparib as the warhead and CRBN or VHL as the E3 ligand [34].

Being an event-driven technology, PROTACs needs a high affinity and low toxicity warhead bringing the target protein close to an E3 ligase and then leads degradation. As the toxicity of PARPi was mainly from PARP-Trapping, the ideal PARPi warhead would be characterized by low PARP-Trapping. Among the PARP1 inhibitors, the PARP-Trapping capability was Talazoparib ≫ Niraparib ≈ Olaparib ≈ Rucaparib ≫ Veliparib, and the difference in PARP-Trapping activity between the inhibitors was greater than 10000-fold [35]. Furthermore, the current clinic trail results also showed that Veliparib was the highest safeness PARP1 inhibitor. Theoretically, Veliparib was an optimizing warhead for PROTACs. But, for Veliparib, the sp3 hybridized 2- Nitrogen atom in pyrrole makes the linker group nearly vertical to the aromatic plane, thus occurs strongly steric hindrance. In docking simulation by MOE, the linker group was strongly hindered by Asn868, Arg 878, Asp770 and Ile 895 in PARP1 binding cave (Fig. 1A). Thus, the development of novel warhead that mimic PARP1 genetic deletion (that is, inhibit PARP1 with low PARP1-Trapping) will likely have substantial therapeutic potential.

Considering the excellent low PRAP trapping by Veliparib, we reserved benzimidazole as the motif and substituted the pyrrole by aromatic ring to avoiding the steric hindrance by Asn868, Arg 878, Asp770 and Ile 895 in PARP1 binding cave (Fig. 1B,C). Docking study showed that these compounds substituted the pyrrole by aromatic ring could avoiding the steric hindrance by Asn868, Arg 878, Asp770 and Ile 895 in PARP1 binding cave, and at the same time around another potential steric hindrance by Glu763. So, a series of benzimidazole derivates were designed and synthesized, and some biological activities in vitro were evaluated including the inhibition for PARP1 enzyme. In 2019, David Maag's group [35] evaluate the PARP-trapping capability of traditional PARPis and found that the cell line MDA-MB-436 was one of the most suitable cell lines to assess the PARP-trapping capability of PARPi. So, in this work we evaluate the PARP-Trapping capability of these compounds using MDA-MB-436 cell line as the model cell. The result showed that these compounds probably have the potential to be a good warhead for PARP1- PROTACs.