Anti-PD-1 +/- RT for MSI-H Solid Tumors

To determine if the out-of-field ORR is improved with the addition of radiation therapy to anti-PD-1 for patients with MSI-H/dMMR metastatic solid tumors. Determine the rates of in-field tumor control, disease control (stable disease, partial response, complete response), durability of disease response, progression-free survival, overall survival, and to assess quality of life and toxicity. Determine the chronology and profile of the radiation-associated immune response.

This is a randomized phase II study with a primary objective to compare the objective response rate (ORR) for anti-PD-1 therapy alone versus anti-PD-1 therapy and limited metastatic site radiation, in patients with microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) metastatic solid tumors. The anti-PD-1 agent, pembrolizumab, received recent FDA accelerated approval for the use in patients with metastatic MSI-H or dMMR solid tumors that have progressed following prior treatment or without satisfactory alternative treatment options. FDA approval for pembrolizumab was based on the results of five multi-cohort, multi-center, single-arm trials, which together showed an ORR of 39.6% among 149 patients with MSI-H/dMMR cancers. Importantly, there is mounting preclinical and clinic evidence supporting the safety and efficacy of combining radiation therapy with systemic immunotherapy, although no prospective comparative data, to the best of our knowledge. In this study, the investigators will focus on patients with MSI-H/dMMR tumors, given their baseline responsiveness to immune checkpoint inhibition, and test the hypothesis that ORR will be improved with radiation and anti-PD-1 therapy compared to anti-PD-1 therapy alone, through a randomized phase II trial design.