Combination Radiation Therapy and PARP Inhibition Enhances Responsiveness To Anti-PD-1 Therapy In Colorectal Tumor Models

Abstract

The role of T-cell checkpoint blockade remains limited in colorectal cancer to patients with microsatellite instable-mutated (MSI) disease. Radiation (RT) and PARP inhibition have separately been shown to enhance anti-tumoral immunity when combined with T-cell checkpoint inhibitors. Therefore, we hypothesized that RT and PARP-inhibition would promote a pro-immunogenic tumor microenvironment and enhance the in vivo efficacy of anti-PD-1 therapy in colorectal tumor models. We utilized CT26 and MC38 which represent validated murine models of microsatellite stable (MSS) and MSI-mutated tumors, respectively. The radiosensitization was assessed using clonogenic survival assays. Surface localization of MHC-1, PD-L1, and calreticulin were assessed 24 hours after treatment with RT +/- veliparib using FLOW cytometry. Expression of inflammatory and immunosuppressive cytokines including IFN-γ, CXCL10, IL6, and TGF-β were assessed using PCR at 1, 3, and 7 days after treatment with RT +/- veliparib. In vivo efficacy of combination therapy was assessed using unilateral flank tumor models treated with RT (8 Gy x 2) +/- veliparib (BID x 7 days) +/- anti-PD-1 antibody (x3 doses). T-cell depletion was achieved using an anti-CD8 antibody administered 3 x weekly. In vitro effects were analyzed using 1- and 2-way ANOVAs and in vivo tumor growth delay was analyze using mixed effect linear regression analyses. We identified veliparib as a potent radiosensitizer in both cell lines. Radiation dose-dependently increased MHC-1 and PD-L1 surface localization and this effect was enhanced with veliparib pretreatment in both cell lines. High-dose RT significantly increased delayed expression of INF-γ and CXCL-10 and veliparib pretreatment significantly enhanced these effects. Neither veliparib nor RT affected expression of TGF-β or IL-6 at any timepoint. Concurrent administration of veliparib and subablative RT (8 Gy x 2) significantly extended in vivo anti-PD-1 mediated tumor growth delay and survival in both tumor models. Moreover, these effects were greater in the MSI-mutated MC38 tumor model. Enhancement of anti-PD-1 mediated tumor growth delay with veliparib and RT was attenuated by CD8+ T-cell depletion. We provide preclinical evidence of a novel therapeutic approach to enhance local responsiveness to checkpoint inhibitors in both MSS and MSI colorectal cancers. Enhanced local tumor control with RT, PARP inhibition, and anti-PD-1 therapy could provide clinically meaningful benefits for patients with locally-advanced rectal cancer which could be readily tested in prospective clinical trials.