Clinical Summary:

• Design/Population: Phase 3 STAMP trial randomized patients with surgically treated stage I-IIIb merkel cell carcinoma to receive either adjuvant pembrolizumab for up to 1 year or standard of care observation. 
• Key Outcomes: Adjuvant pembrolizumab resulted in a numerical improvement in relapse-free survival and a significant improvement in distant metastasis-free survival. Exploratory analyses suggested better outcomes with sequential rather than concurrent radiation and immunotherapy. 
• Clinical Relevance: These findings suggest that adjuvant pembrolizumab may reduce the risk of distant recurrence in selected high-risk patients and provide important insights into the optimal integration of radiation and immunotherapy.

Janice Mehnert, MD, NYU Langone Health Perlmutter Cancer Center, New York, New York, discusses updated results from the phase 3 STAMP trial evaluating adjuvant pembrolizumab in patients with surgically treated merkel cell carcinoma. 

While relapse-free survival remained numerically improved without reaching statistical significance, pembrolizumab consistently demonstrated a benefit in distant metastasis-free survival. Additional analyses suggested that sequential administration of radiation and immunotherapy may be associated with better outcomes than concurrent treatment, providing important guidance for future clinical trial design and treatment sequencing.

Dr Mehnert presented these results at the 2026 ASCO Annual Meeting in Chicago, Illinois.

Transcript: 

Hi, I'm Dr Janice Mehnert, I am a professor of medicine, associate director of clinical research, and director of melanoma and cutaneous oncology at the Perlmutter Cancer Center at NYU Langone Health.

I'm going to talk today about the results of EA6174 STAMP, which stands for Surgically Treated Adjuvant Merkel Cell Carcinoma with Pembrolizumab. This is a study that accrued patients between 2018 and 2023. It was designed to test the role of adjuvant immunotherapy with the anti–PD-1 antibody pembrolizumab (Keytruda) to understand whether it improves outcomes in patients who have already undergone surgery for merkel cell carcinoma.

The study randomized 293 patients in a 1:1 fashion between pembrolizumab, given every 3 weeks for 17 doses at 200 mg versus standard of care. Eligible patients ranged from stage I disease, if a sentinel lymph node biopsy had not been performed, through stage IIIB disease. Radiation therapy was permitted in both arms.

I first presented an earlier analysis of these results at the ESMO meeting in Berlin in October. At that time, we observed a numerical, but not statistically significant, difference in relapse-free survival between the two arms. At this meeting, with additional follow-up, the relapse-free survival data remained very similar. I had also previously presented data showing that distant metastasis–free survival was significantly improved, and that finding remained true with the updated analysis.

To me, this suggests that radiation is quite effective at controlling local recurrences, but that administering adjuvant immunotherapy may protect patients against life-threatening distant spread. At this meeting, we also looked at merkel cell carcinoma–specific outcomes by removing deaths that were unrelated to cancer. This was particularly important because this is an older patient population with greater frailty and more medical comorbidities. When we performed that analysis, the outcomes continued to improve in favor of the pembrolizumab arm.

Another very important analysis involved the role of radiation therapy. In this trial, radiation could be given either concurrently with pembrolizumab or sequentially. When we analyzed outcomes by radiation subgroup, we found that delivering radiation sequentially appeared to result in better relapse-free survival and distant metastasis–free survival than delivering it concurrently. That finding makes biological sense. Not only might patients experience additive toxicity when the treatments are given together, but radiation delivered concurrently with immunotherapy could potentially dampen the immune response in ways that we have not fully appreciated before.

In summary, I think the way I will take these results into clinical practice is that we should not necessarily recommend adjuvant immunotherapy for every patient. However, for patients with very high-risk features, I will have a risk-benefit discussion with them given the consistent distant metastasis–free survival benefit that we have observed in this trial.

One question that often comes up is: if you're not recommending adjuvant immunotherapy for everyone, how do you move forward from these results? I think the first and most obvious next step is more rigorous investigation of neoadjuvant approaches. This was an adjuvant study, but we have learned a great deal about neoadjuvant therapy in melanoma and cutaneous squamous cell carcinoma, and many of those lessons likely apply to merkel cell carcinoma as well. 

As a community, we probably need response-adapted neoadjuvant trials. For those patients who do undergo surgery first—and there will always be patients for whom surgery-first remains the appropriate approach—we also need better biomarkers to help us understand recurrence risk. Markers such as ctDNA may help us better define which patients are at highest risk and how we should tailor therapy. Understanding how to incorporate those tools into clinical practice will likely be the focus of future clinical trials.

Source: 

Mehnert J, Lee SJ, Gastman B, et al. Updated outcomes from STAMP: Surgically treated adjuvant Merkel cell carcinoma with pembrolizumab, a phase III trial—ECOG-ACRIN EA6174. Presented at the ASCO Annual Meeting. May 29 - June 2, 2026. Chicago, Illinois. Abstract LBA9505.