Immune Checkpoint Inhibitors in Merkel Cell Carcinoma of the Skin: A 2025 Comprehensive Review

Objective: Merkel cell carcinoma (MCC) is a rare and aggressive form of skin cancer. Although immunotherapy has transformed MCC management, published data remain limited. This comprehensive review evaluates current evidence on immune checkpoint inhibitors (ICIs) in MCC, in relation to other treatment modalities such as surgery and radiotherapy. Methods: Peer-reviewed articles published between January 2000 and August 2025 were searched manually in four databases: Scopus, ScienceDirect, PubMed and MEDLINE, using the keywords “Merkel cell carcinoma” AND “immunotherapy” AND “immune checkpoint inhibitors”. The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) methodology was employed. Results: ICIs can be given in different settings: (A) Neoadjuvant: The CheckMate 358 trial reported a 54.5% response rate among 33 radiologically evaluable patients treated with nivolumab, each showing over 30% tumor reduction. (B) Adjuvant: (1) The ADMEC-O phase II trial demonstrated improved disease-free survival with adjuvant nivolumab. (2) The ADAM phase III trial evaluates adjuvant avelumab in node-positive patients post-surgery/radiation, with common side effects including nausea, fatigue, and itching. (3) STAMP, a phase III trial, investigates pembrolizumab in stage I–III MCC. Both ADAM and STAMP have completed accrual and results are pending. (C) Primary therapy: KEYNOTE-017 and JAVELIN trials reported a 60% overall response rate and ~40% 3-year progression-free survival with first-line pembrolizumab or avelumab. Both agents also show promise as salvage therapies. Conclusions: ICIs demonstrate encouraging outcomes in MCC across various treatment stages. Continued research is essential to optimize treatment timing and integrate multimodal therapies.