Combination Therapy with Radiation and PARP Inhibition Enhances Responsiveness to Anti-PD-1 Therapy in Colorectal Tumor Models.

PURPOSE: The majority of colorectal cancers are resistant to cancer immune checkpoint inhibitors. Ionizing radiation (IR) and several radiosensitizers, including PARP inhibitors, can enhance responsiveness to immune checkpoint inhibitors by potentially complementary mechanisms of action. We assessed the ability of radiation and PARP inhibition to induce proimmunogenic changes in tumor cells and enhance their in vivo responsiveness to anti-PD-1 antibodies. METHODS AND MATERIALS: We performed a candidate drug screen and used flow cytometry to assess effects of the PARP inhibitor veliparib on IR-mediated changes in MHC-1 antigen presentation and surface localization of immune-modulating proteins including PD-L1 and calreticulin in colorectal cancer tumor models. Reverse transcription polymerase chain reaction was used to assess the effects of veliparib and radiation on the expression of proinflammatory and immunosuppressive cytokines. The ability of concurrent PARP inhibition and subablative doses of radiation therapy to enhance in vivo responsiveness to anti-PD-1 antibodies was assessed using unilateral flank-tumor models with or without T-cell depletion. RESULTS: Veliparib was a potent radiosensitizer in both cell lines. Radiation increased surface localization of MHC-1 and PD-L1 in a dose-dependent manner, and veliparib pretreatment significantly enhanced these effects with high (8 Gy) but not with lower radiation doses. Enhancement of MHC-1 and PD-L1 surface localization by IR and IR+ veliparib remained significant 1, 3, and 7 days after treatment. IR significantly increased delayed tumoral expression of proinflammatory cytokines interferon-Ƴ and CXCL10 but had no significant effect on the expression of IL-6 or TGF-β. Concurrent administration of veliparib and subablative radiation therapy (8 Gy × 2) significantly prolonged anti-PD-1-mediated in vivo tumor growth delay and survival in both tumor models. Moreover, these effects were more pronounced in the microsatellite instability-mutated MC38 tumor model. Enhancement of anti-PD-1 mediated tumor growth delay with veliparib and IR was attenuated by CD8+ T-cell depletion. CONCLUSIONS: We provide preclinical evidence for a novel therapeutic strategy to enhance responsiveness of colorectal tumors to immune checkpoint inhibitors.