Combining low-dose regorafenib with pembrolizumab for patients with MSI-H colorectal cancer: REGPEM-CRC-01.

TPS313 Background: Currently, pembrolizumab is one of the front-line therapies for patients with MSI-H CRC. However, approximately 40% of patients who received pembrolizumab experienced disease progression early in the course of disease (KEYNOTE 177). Therefore, there is still an unmet need to enhance the efficacy of checkpoint inhibitors in MSI-H CRC. MSI-H CRC has a higher level of expression of VEGF in blood compared to patients compared to its MSS counterpart (Hansen et al. Colorectal Dis. 2011). Consistently, exploratory analysis of CALBG-80405 and PARADIGM trial showed that patients with MSI-H CRC were more likely to benefit from anti-VEGF therapy than anti-EGFR therapy regardless of the side of the tumor. NSABP C-08 also suggested that anti-VEGF therapy may have biological activity even in as adjuvant therapy for patients with MSI-H colon cancer. Regorafenib is a potent VEGF inhibitor, with preclinical evidence showing its immune modulatory effect in the tumor microenvironment. In this trial, we hypothesize that adding low-dose regorafenib to pembrolizumab may induce synergistic activity beyond their independent clinical efficacy and create deep and durable responses for patients with MSI-H CRC. Methods: In the lead arm of this prospective randomized study, 22 patients will be enrolled through Hoosier Cancer Research Network (HCRN-GI23-643). Patients with treatment naïve MSI-H CRC will be enrolled in this front-line trial. One cycle of pembrolizumab and up to 3 cycles of chemotherapy prior to determination of MMR-D/MSI-H is allowed. Patients will receive regorafenib 60 mg daily in combination with pembrolizumab 200mg IV in cycle 1, followed by regorafenib 90mg in subsequent cycles to improve treatment tolerance. The primary outcome that will be measured is ORR, defined as the percentage of partial or complete response to the treatment within 12 months. ORR will be measured using RECIST 1.1. criteria. A formal one-sided hypothesis test will be conducted for futility, assuming that we will reject the null hypothesis of a target ORR only if we have strong evidence. In this study, we assume a null hypothesis that ORR is 0.60, which would reflect significant clinical improvement over the current standard of ORR = 0.43 from KEYNOTE 177. The alternative hypothesis is that ORR is less than 0.60. For the lead-in phase of the study, the emphasis is on controlling Type I error to be small, approximately 0.05. The test statistic will be the number of ORRs in the 22 patients, which we assume to follow a binomial distribution. Clinical trial information: NCT06006923 .