Study on the effects of an OCT2/MATE1 substrate (metformin) and inhibitor (cimetidine) on the exposure of trifluridine/tipiracil (Lonsurf) in patients with metastatic colorectal cancer (mCRC).

Interventions: Patients treated with Lonsurf will be treated with metformin and cimetidine for 7 and 5 consecutive days respectively.
Primary outcome(s): 1. To determine the possible drug interaction between Lonsurf and metformin on Lonsurf plasma pharmacokinetics (AUC) in patients with metastatic colorectal cancer.
2. To determine the possible interaction between Lonsurf and cimetidine on Lonsurf plasma pharmacokinetics (AUC) in patients with metastatic colorectal cancer.
Study Design: N/A: single arm study, Open (masking not used), Active, Crossover