Pembrolizumab for previously treated, microsatellite instability–high/mismatch repair–deficient advanced colorectal cancer: final analysis of KEYNOTE-164

Background: Pembrolizumab demonstrated durable clinical benefit and manageable safety in previously treated advanced or metastatic microsatellite instability-high (MSI-H)/mismatch repair deficient (dMMR) colorectal cancer (CRC) in the phase 2 KEYNOTE-164 study. Results from the final analysis are presented. Methods: Eligible patients had unresectable or metastatic MSI-H/dMMR CRC and ≥2 prior systemic therapies (cohort A) or ≥1 prior systemic therapy (cohort B). Patients received pembrolizumab 200 mg intravenously every 3 weeks for ≤35 cycles. The primary end-point was objective response rate (ORR) assessed per Response Evaluation Criteria in Solid Tumors, version 1.1 by blinded independent central review. Secondary end-points included duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety and tolerability. Results: Sixty-one patients in cohort A and 63 patients in cohort B were enroled; median follow-up was 62.2 months and 54.4 months, respectively. ORR was 32.8% (95% CI, 21.3%–46.0%) in cohort A and 34.9% (95% CI, 23.3%–48.0%) in cohort B. Median DOR was not reached (NR) in either cohort. Median PFS was 2.3 months (95% CI, 2.1–8.1) in cohort A and 4.1 months (95% CI, 2.1–18.9) in cohort B. Median OS was 31.4 months (95% CI, 21.4–58.0) in cohort A and 47.0 months (95% CI, 19.2–NR) in cohort B. No new safety signals were observed. Nine patients who initially responded experienced disease progression off therapy and received second-course pembrolizumab. Six patients (66.7%) completed an additional 17 cycles of pembrolizumab, and 2 patients achieved a partial response. Conclusions: Pembrolizumab continued to show durable antitumor activity, prolonged OS, and manageable safety in patients with previously treated MSI-H/dMMR CRC. Clinical Trial Registry Information: ClinicalTrials.gov,