Efficacy of Perioperative Pembrolizumab in Mismatch Repair Deficient/Microsatellite Unstable Localized Colorectal Cancers: Results of the Phase II Trial IMHOTEP

PURPOSE Mismatch repair deficiency (dMMR) or microsatellite instability (MSI) represents a distinct phenotype among solid tumors resulting in the generation of highly immunogenic neoantigens. Pembrolizumab has been approved in first-line unresectable or metastatic dMMR/MSI colorectal cancers (CRC). We aimed to assess efficacy and tolerance of perioperative pembrolizumab in dMMR/MSI CRC. PATIENTS AND METHODS The prospective multicenter phase II trial IMHOTEP enrolled patients with localized resectable dMMR/MSI CRC to receive one or two cycles of IV pembrolizumab 400 mg once every 6 weeks before surgery and 1-year total duration thereafter. The primary end point was pathologic complete response (pCR) rate (ypT0N0). Secondary objectives included safety, event-free survival, and overall survival. RESULTS IMHOTEP enrolled 81 patients with dMMR/MSI CRC who received at least one cycle of pembrolizumab from November 26, 2021, to February 22, 2023: median age was 66 (21-89) years, 46 (52%) were women, and 63 (71%) had clinical stage III disease at baseline. Out of the 72 patients included in the efficacy population, 38 patients (52.7% [95% CI, 41.4 to 63.9]) achieved a pCR. The exploratory post hoc analysis showed a pCR rate increased from 46% (23/50) after one cycle to 68.2% (15/22) after two cycles of neoadjuvant pembrolizumab ( P = .0125). With a median follow-up of 24.5 (95% CI, 23.3 to 25.6) months, three disease recurrences occurred. Grade ≥3 immune-related toxicities were reported in 14 (15.7%) patients including one grade 5 (myasthenia). CONCLUSION The IMHOTEP trial showed promising results, with pCR achieved after one or two cycles of neoadjuvant pembrolizumab in 53% of patients with dMMR/MSI CRC. To our knowledge, this prospective study is the first to demonstrate the feasibility and the safety of perioperative pembrolizumab.