Background: Preclinical data suggests metformin can improve immune exhaustion of tumor infiltrating lymphocytes and potentiate the effects of PD-1 blockade. By normalizing the hypoxic TME, metformin was shown to improve cytotoxic T cell function and efficacy of anti-PD-1 antibody in highly aggressive B16 melanoma and MC38 colon adenocarcinoma tumor models. Based on this preclinical rationale we conducted a phase II study with nivolumab and metformin combination in treatment refractory MSS metastatic colorectal cancer (mCRC). Methods: Nivolumab 480 mg IV every 4 weeks and Metformin 1000 mg po twice daily was administered in 28-day cycles following a 14-day metformin only lead-in phase.Eligible patients included stage IV metastatic treatment refractory MSS mCRC (patients must have received oxaliplatin, irinotecan, and fluoropyrimidine), age ≥18 years, ECOG PS 0-1, adequate organ function, no prior anti PD-1 agent. The primary endpoint was overall response rate (ORR). Secondary endpoints included overall survival (OS) and progression free survival (PFS). Simon’s two-stage Minimax design was employed (H₀: ORR=4%; H₁: ORR=15%; alpha = 0.1; power =80%). If ≥1 objective response was observed in the first evaluable 18 patients, 10 additional patients would be included in the cohort. ≥3 objective responders in 28 patients would be required to be considered positive study. Pre-treatment and on-treatment research biopsies and correlative peripheral blood specimens were collected. Results: A total of 24 patients were enrolled, 6 patients were replaced per protocol, and 18 patients had evaluable disease. Of the 18 evaluable patients 11/18 (61%) were female, median age 58 [IQR 50-67]. 2 patients had prolonged stable disease (4 and 10 cycles). No patients had objective response based on RECIST 1.1. Median OS and PFS was 5.1 months [95% CI (2-11.7)] and 2.3 months [95% CI (1.7-2.4)], respectively. Most common grade 3 and 4 toxicities were anemia (n=2) and diarrhea (n=2). Conclusions: In treatment refractory MSS mCRCnivolumab and metformin combination was well tolerated. Two patients achieved stable disease, but no objective response was seen; therefore, the study did not proceed with the second stage of enrollment. Immunologic correlative analysis of this study is ongoing. Clinical trial information: NCT03800602