Pembrolizumab in gastrointestinal (GI) malignancies with defective DNA mismatch repair (dMMR): A single institution experience.

792Background: Defective DNA mismatch repair is associated with increased mutation load and expression of neoantigens. Increased activity of PD-1 inhibitor is reported in patients with GI cancers with dMMR. Here, we describe our experience at Huntsman Cancer Institute with PD-1 inhibition in subjects with GI malignancies with dMMR. Methods: Retrospective chart review of patients with metastatic GI malignancies with dMMR, treated with pembrolizumab at Huntsman Cancer Institute between July 15, 2015 and September 26, 2016. Results: A total of 9 patients were identified that received at least 1 dose of pembrolizumab (colorectal n=5; cholangiocarcinoma n=2; pancreatic n=2). Of the 9 patients, 5 received concurrent cytoxic chemotherapy. Median number of doses given was 3 (range 1 to 17). Three patients received pembrolizumab on clinical trial with concomitant mFOLFOX6. Five patients have restaging scans at the time of abstract submission. The first patient has cholangiocarcinoma and experienced a -73% response...