Outcomes of patients with resected stage III/IV acral or mucosal melanoma, treated with adjuvant anti-PD-1 based therapy

Importance: Acral (AM) and mucosal melanomas (MM) are rare subtypes with a poor prognosis. In those with advanced disease, anti-PD-1 (PD1) therapy has reduced activity compared to that seen in non-acral cutaneous melanoma. Objective: To determine the efficacy of adjuvant PD1 in resected AM or MM. Design: An international, retrospective cohort study Setting: Data up to November 2021 collected from 20 centres across 10 countries. Participants: One hundred and ninety four patients with resected stage III or IV1 AM or MM who received adjuvant PD1 were included and compared to matched patients from the Melanoma Institute Australia (MIA) database using a propensity score matching analysis. Main outcomes and measures: Recurrence -free survival (RFS), distant metastasis -free survival (DMFS) and overall survival (OS) were investigated. Results: Forty five of 139 (32%) AM and 9 of 55 (16%) MM patients completed adjuvant therapy. The main reason for early treatment cessation in both groups was disease recurrence: 51 (37%) and 30 (55%) in the AM and MM groups, respectively. In the AM group adjuvant PD1 was associated with a longer RFS [HR -0.69 (0.52-0.92, p = 0.0127)], DMFS [HR0.58 (0.38-0.89, p = 0.0134)] and OS [HR of 0.59 (0.38-0.92, p -value 0.0196)] when compared to the historical cohort. In the MM group there was no statistical difference in RFS [HR1.36 (0.69-2.68,p -value 0.3799], DMFS or OS. Conclusion and relevance: After adjuvant PD1, both AM and MM have a high risk of recurrence. Our data suggests a benefit to using adjuvant PD1 therapy in resected AM but not in resected MM. Additional studies to investigate the efficacy of adjuvant PD1 for MM are needed.