André T, Shiu KK, Kim TW, et al. - In microsatellite-instability–high (MSI-H)–dMMR (mismatch-repair–deficient) advanced or metastatic colorectal cancer, researchers assessed the effectiveness of programmed death 1 blockade vs chemotherapy as first-line treatment. Participants in this phase 3, open-label trial were 307 patients with metastatic MSI-H–dMMR colorectal cancer who were not previously treated. These patients were randomized, in a 1:1 ratio, to receive pembrolizumab at a dose of 200 mg every 3 weeks or chemotherapy (5-fluorouracil–based therapy with or without bevacizumab or cetuximab) every 2 weeks. Progression-free survival and overall survival were primary endpoints. According to findings, a significantly longer progression-free survival was conferred by pembrolizumab vs chemotherapy when administered as first-line therapy for MSI-H–dMMR metastatic colorectal cancer. In addition, treatment with pembrolizumab was associated with fewer treatment-related adverse events.