Radiation Induced Checkpoint Immunotherapy Response in Refractory Colorectal and Pancreatic Adenocarcinoma

Abstract Immune checkpoint blockade has limited efficacy in microsatellite stable (MSS) colorectal (CRC) and pancreatic (PDAC) cancer. Preclinical models have demonstrated the use of radiation to activate the innate immune response and stimulate responsiveness to immune checkpoint blockade. Here, we describe a Phase 2 trial of radiation therapy combined with combined anti-CTLA4 (ipilimumab) and anti-PD1 (nivolumab) antibodies in MSS CRC and PDAC. In the per protocol analysis disease control rate was 37% (10/27) in CRC and 29% (5/17) in PDAC with an overall response rate of 15% (4/27) and 18% (3/17), respectively. Whole exome and RNA sequencing of biopsies from 17 patients revealed low tumor mutational burden in all tumors, but a notable upregulation of interferon stimulated genes with concordant high expression of multiple repeat RNA transcripts in responders. Altogether, this study provides foundational human proof of concept of radiation with combination immune checkpoint blockade therapy in otherwise immunotherapy resistant cancers.