Role of PARP Inhibitors in Cancer Immunotherapy: Potential Friends to Immune Activating Molecules and Foes to Immune Checkpoints

SIMPLE SUMMARY: This review aims at analyzing an emergent topic regarding the possible combined use of poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi), anticancer drugs approved to treat tumors with defective repair of DNA damage, and immunostimulating agents, like Immune Checkpoint Inhibitors (ICIs), that represent the current treatment of a variety of malignancies. Preclinical investigation shows how PARPi, by positively impacting anti-tumor immune response through DNA damage-related mechanisms, might render cancer cells more responsive to immunotherapies, especially in the setting of genomic instability. In addition to well-recognized mechanisms that may be elicited by PARPi, recent experimental evidence is summarized further supporting the potential synergistic effects of PARPi and ICIs. We believe that an in-depth analysis of the tumor genome as well as of the characteristics of the tumor microenvironment at a single-patient level, along with an implementation of clinical trials that are presently at an early-stage, can contribute to identify more effective and individually-tailored treatments. ABSTRACT: Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) induce cytotoxic effects as single agents in tumors characterized by defective repair of DNA double-strand breaks deriving from BRCA1/2 mutations or other abnormalities in genes associated with homologous recombination. Preclinical studies have shown that PARPi-induced DNA damage may affect the tumor immune microenvironment and immune-mediated anti-tumor response through several mechanisms. In particular, increased DNA damage has been shown to induce the activation of type I interferon pathway and up-regulation of PD-L1 expression in cancer cells, which can both enhance sensitivity to Immune Checkpoint Inhibitors (ICIs). Despite the recent approval of ICIs for a number of advanced cancer types based on their ability to reinvigorate T-cell-mediated antitumor immune responses, a consistent percentage of treated patients fail to respond, strongly encouraging the identification of combination therapies to overcome resistance. In the present review, we analyzed both established and unexplored mechanisms that may be elicited by PARPi, supporting immune reactivation and their potential synergism with currently used ICIs. This analysis may indicate novel and possibly patient-specific immune features that might represent new pharmacological targets of PARPi, potentially leading to the identification of predictive biomarkers of response to their combination with ICIs.