Pembrolizumab (PD-1) cuts MCC-specific relapse events in resected Merkel cell carcinoma, with signal for better distant control

Pembrolizumab is an anti–PD-1 checkpoint inhibitor tested as adjuvant therapy in the randomized Phase III STAMP trial (EA6174; NCT03712605) after complete resection of stage I (no SLNB) to IIIB Merkel cell carcinoma: pembro 200 mg IV q3w ×17 (n=147) vs SOC (observation and/or protocol-directed RT as planned) (n=146; median follow-up 47 months, data cutoff 2/19/26). The co-primary RFS endpoint defined as recurrence or death from any cause (RFS_a) was numerically better but not statistically significant (one-sided p=0.10), while MCC-specific RFS (RFS_b) improved with HR 0.66 (90%CI 0.45–0.96; one-sided p=0.032) with 1-year/2-year RFS_b 84%/77% vs 73%/67% on SOC; DMFS also improved (one-sided p=0.05). By design, SOC in this setting is typically surveillance with selective adjuvant radiation rather than routine systemic therapy—RT improves local/regional control in historical series but does not reliably prevent distant relapse, which is where this pembro signal is most relevant.