TOPLINE: 

Adjuvant pembrolizumab prolongs distant metastasis-free survival (MFS) among patients with Merkel cell carcinoma (MCC), according to the largest trial to date in this setting. However, immunotherapy also increases adverse events, and the impact on overall survival (OS) is not yet known.

METHODOLOGY:

• MCC has high rates of recurrence and distant metastases after resection, with 5-year survival rates historically ranging from 30% to 60%. Immune checkpoint inhibitors, including pembrolizumab, are central to treating metastatic MCC. Whether they could have a role in the post-surgery setting remains unclear.
• The phase 3 STAMP Trial is investigating adjuvant pembrolizumab against standard-of-care observation among patients with completely resected MCC (stage I with a positive sentinel lymph node biopsy up to stage IIIb).
• A total of 293 patients (median age, 69 years; 68% male) were randomized to pembrolizumab every 3 weeks for 17 doses (n = 147) or observation (n = 146). About 80% of patients in both arms received standard radiation therapy, and all were monitored for recurrence throughout the trial.
• The primary endpoints are relapse-free and OS, with distant MFS as a secondary endpoint and follow-up running to 5 years after trial registration.

TAKEAWAY: 

• After a median follow-up of 40 months, there was no significant difference between the arms in 24-month relapse-free survival — though there was a trend in favor of pembrolizumab, at 73% vs 66% with standard care (hazard ratio [HR], 0.80; P = .105).
• There was, however, a significant difference in distant MFS, at 88% with pembrolizumab vs 80% with standard care at 18 months (HR, 0.58; P = .032). OS data were not yet mature.
• Immunotherapy increased the rate of grade 3 or worse treatment-related adverse events, which occurred in 31.2% of patients receiving pembrolizumab vs only 4.3% of those receiving standard care (related to radiotherapy).
• The most common grade 3 or higher adverse events with pembrolizumab included decreased lymphocyte count (7.2% vs 0.7%), gastrointestinal events (3.6% vs 1.4%), and elevated pancreatic enzymes (3.6% vs 0%). One death, from pneumonitis, occurred in the pembrolizumab arm.

IN PRACTICE: 

“This is the first phase 3 report of adjuvant immunotherapy in Merkel cell carcinoma, demonstrating that pembrolizumab given after surgery does result in a trend toward improved relapse-free survival, and significantly prolonged distant metastasis-free survival,” lead researcher Janice Mehnert, MD, of NYU Langone Health, New York City, said during a conference presentation.

With the rising incidence of MCC, she added, “these results are going to become incredibly relevant.” However, final OS data will be necessary to inform practice, Mehnert said.

SOURCE: 

Mehnert presented the findings on April 23 at the European Association of Dermato-Oncology (EADO) Congress 2026, in Prague, Czech Republic.

DISCLOSURES: 

The trial was funded by the US National Cancer Institute. Mehnert disclosed relationships with Merck, Regeneron, Sanofi, and other pharmaceutical companies.