Nivolumab and Metformin in Patients With Treatment Refractory MSS Colorectal Cancer

Inclusion Criteria: * Patients must have histologically or cytologically confirmed stage IV colorectal adenocarcinoma with measurable disease * Prior treatment with 5 Fluorouracil (or capecitabine), oxaliplatin and irinotecan containing chemotherapy (needs to be treated with anti-epidermal growth factor receptor (EGFR) agent if extended RAS wild type) * Patients must have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions) as ≥ 20 mm with conventional techniques or as ≥ 10 mm with spiral computed tomography (CT) scan, magnetic resonance imaging (MRI), or calipers by clinical exam * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 (Karnofsky ≥ 70%) * Life expectancy of greater than 3 months * Absolute neutrophil count ≥ 1,500/µL * Platelets ≥ 100,000/µL * Hemoglobin ≥ 9 g/dL or ≥ 5.6 mmol/L without transfusion or erythropoietin (EPO) dependency (within 7 days of assessment) * Serum creatinine ≤ 1.5 x upper limit of normal (ULN) OR creatinine clearance ≥ 60 mL/min/1.73 m² for patients with creatinine levels \> 1.5 x ULN. Creatinine clearance should be calculated per institutional standard * Serum total bilirubin ≤ 1.5 x the upper limit of normal (ULN) OR direct bilirubin ≤ ULN for subjects with total bilirubin \> 1.5 ULN * Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase\[SGOT\])/alanine am…