BMN673 Is a PARP Inhibitor with Unique Radiosensitizing Properties: Mechanisms and Potential in Radiation Therapy

SIMPLE SUMMARY: PARP inhibitors (PARPi) have broad applicability as monotherapy in tumors with HR defects. However, their combination with radiotherapy (RT) is less advanced owing to the rather modest radiosensitization typically achieved with most tested PARPi, including olaparib. Tumor resistance to PARPi or RT is a significant bottleneck in the clinic, and strategically designed combinations of PARPi and RT may offer approaches to overcome such resistance. Talazoparib (BMN673), as compared to other PARPi, is a superior radiosensitizer. In this review, we discuss our evolving understanding of the mechanistic underpinnings of BMN673 radiosensitization and outline the potential for combinations of BMN673 with RT to treat various forms of cancer. ABSTRACT: BMN673 is a relatively new PARP inhibitor (PARPi) that exhibits superior efficacy in vitro compared to olaparib and other clinically relevant PARPi. BMN673, similar to most clinical PARPi, inhibits the catalytic activities of PARP-1 and PARP-2 and shows impressive anticancer potential as monotherapy in several pre-clinical and clinical studies. Tumor resistance to PARPi poses a significant challenge in the clinic. Thus, combining PARPi with other treatment modalities, such as radiotherapy (RT), is being actively pursued to overcome such resistance. However, the modest to intermediate radiosensitization exerted by olaparib, rucaparib, and veliparib, limits the rationale and the scope of such combinations. The recently reported strong radiosensitizing potential of BMN673 forecasts a paradigm shift on this front. Evidence accumulates that BMN673 may radiosensitize via unique mechanisms causing profound shifts in the balance among DNA double-strand break (DSB) repair pathways. According to one of the emerging models, BMN673 strongly inhibits classical non-homologous end-joining (c-NHEJ) and increases reciprocally and profoundly DSB end-resection, enhancing error-prone DSB processing that robustly potentiates cell killing. In this review, we outline and summarize the work that helped to formulate this model of BMN673 action on DSB repair, analyze the causes of radiosensitization and discuss its potential as a radiosensitizer in the clinic. Finally, we highlight strategies for combining BMN673 with other inhibitors of DNA damage response for further improvements.