2-[4-[1-[17-[4-[[(4R,5S)-4,5-Bis(4-chlorophenyl)-4,5-dihydro-2-[4-methoxy-2-(1-methylethoxy)phenyl]-1H-imidazol-1-yl]carbonyl]-2-oxo-1-piperazinyl]-16-oxo-3,6,9,12-tetraoxa-15-azaheptadec-1-yl]-1H-1,2,3-triazol-4-yl]phenyl]-2H-indazole-7-carboxamide

Therapeutic targeting of the nuclear enzyme poly (ADP-ribose) polymerase 1 (PARP1) with PARP inhibitors (PARPis) in patients with a homologous recombination (HR)- deficient phenotype based on the mechanism of synthetic lethality has been shown tremendous success in cancer therapy. With the clinical use of various PARPis, emerging evidence has shown that some PARPis offer hope for breakthroughs in triple-negative breast cancer (TNBC) therapy, regardless of HR status. However, similar to other conventional cytotoxic drugs, PARPis are also subject to the intractable problem of drug resistance. Notably, acquired resistance to PARPis caused by point mutations in the PARP1 protein is hard to overcome with current strategies. To explore modalities to overcome resistance and identify patients who are most likely to benefit from PARP1-targeted therapy, we developed a proteolysis-targeted chimaera (PROTAC) to degrade mutant PARP1 in TNBC. Here, we investigated a PARP1 PROTAC termed "NN3″, which triggered ubiquitination and proteasome-mediated degradation of PARP1. Moreover, NN3 degraded PARP1 with resistance-related mutations. Interestingly, compared with other reported PARP1 degraders, NN3 exhibited a unique antitumor mechanism in p53-positive breast cancer cells that effectively promoted ferroptosis by downregulating the SLC7A11 pathway. Furthermore, NN3 showed potent activity and low toxicity in vivo. In conclusion, we propose PROTAC-mediated degradation of PARP1 as a novel strategy against mutation-related PARPi resistance and a paradigm for targeting breast cancer with functional p53 via ferroptosis induction.

Poly ADP-ribose polymerase (PARP) plays a critical role in many cellular processes such as DNA damage repair, gene transcription and cell apoptosis. Therefore, targeting PARP represents a promising strategy for cancer therapy. To date, numerous small molecule PARP1 inhibitors have been identified, but many of them suffer from limited clinical efficacy and serious toxicity. Hence, PARP1 inhibitor-based combination therapies, and other PARP1 modulators (e.g. PROTAC degraders, dual acting agents) have attracted great attention with significant advancements achieved in the past few years. In this review, we overviewed the recent progress on PARP1-based drug discovery with a focus on PARP1 inhibitor-based drug combination therapy and other PARP1-targeting strategies (e.g. selective PARP1 inhibitors, PARP1-based dual-target inhibitors, PROTAC PARP1 degraders, and prodrugs of PARP1 inhibitors). In addition, we also summarized the reported co-crystal structures of PARP1 inhibitors in complex with their target proteins as well as the binding interactions. Finally, the challenges and future directions for PARP-based drug discovery in cancer therapy are also discussed in detail.

Poly ADP-ribose polymerase 1 (PARP1) plays an essential role in DNA repair signaling, rendering it an attractive target for cancer treatment. Despite the success of PARP1 inhibitors (PARPis), only a few patients can currently benefit from PARPis. Moreover, drug resistance to PARPis occurs during clinical treatment. Natural and acquired resistance to PARPis has forced us to seek new therapeutic approaches that target PARP1. Here, we synthesized a series of compounds by proteolysis-targeting chimera (PROTAC) technology to directly degrade the PARP1 protein. We found that CN0 (compound 3) with no polyethylene glycol (PEG) linker can degrade the PARP1 protein through the proteasome pathway. More importantly, CN0 could inhibit DNA damage repair, resulting in highly efficient accumulation of cytosolic DNA fragments due to unresolved unrepaired DNA lesions when combined with daunorubicin (DNR). Therefore, CN0 can activate the cyclic GMP-AMP synthase/stimulator of the interferon gene (cGAS/STING) pathway of innate immunity and then spread the resulting inflammatory signals, thereby reshaping the tumor microenvironment, which may eventually enhance T cell killing of tumor cells.