Background: Patients (pts) with colorectal cancer (CRC) with microsatellite instability (MSI) have recently been shown to respond to anti–programmed death (PD)-1 drugs. Preclinical data suggest that trifluridine/tipiracil (FTD/TPI) treatment converts MSS CRC cells to MSI, sensitizing them to the activity of anti–PD-1 drugs. The aim of this phase 2 study was to evaluate this hypothesis. Methods: This was a multicenter, single-arm, safety lead-in, phase 2 study that used Simon’s 2-stage design to evaluate the safety and efficacy of FTD/TPI (35 mg/m² twice daily on days 1-5 and 8-12 every 28 days) plus nivolumab (3 mg/kg every 2 weeks) in pts with heavily pretreated MSS mCRC. Pts had histologically proven metastatic or locally advanced colorectal adenocarcinoma that was MSS (assessed by a local laboratory based on either previous or fresh biopsy), ≥1 measurable lesion for RECIST and immune-related response criteria (irRC) assessment, and failure of ≥2 previous lines of chemotherapy. Six pts were to be enrolled in the safety lead-in, and in order to proceed to Simon’s stage 2, ≥2 of the first 15 pts had to demonstrate a partial or complete response within 6 months based on irRC assessment. Results: The first 6 dose-limiting toxicity (DLT)-evaluable pts enrolled tolerated dosing with no DLTs. A total of 18 pretreated pts enrolled in the first stage (50% males; median age 56.5 yr), among whom 72% had colon cancer and 100% had MSS disease, 56% with RAS mutations. Pts received a median of 2.5 cycles of study therapy (range 1-8). The most common grade 3/4 adverse events (AEs) were neutropenia (28%); diarrhea (17%); and nausea, abdominal pain, fatigue, and anemia (11% each). No pts discontinued treatment due to AEs. No pts achieved a tumor response (either per RECIST or irRC), and the study did not progress to the second stage. Median (6-month) progression-free survival was 2.8 months (21%) per RECIST and 2.2 months (30%) per irRC. Conclusions: The combination of FTD/TPI and nivolumab was feasible and tolerated at the full dose of both compounds. Adding nivolumab to FTD/TPI did not provide additional clinical benefit in pts with previously treated MSS mCRC. Clinical trial information: NCT02860546