Anti-PD-1 therapy in unresectable desmoplastic melanoma: the phase 2 SWOG S1512 trial

Desmoplastic melanoma is a distinct subtype of melanoma known to have preexisting immune infiltrates and high ultraviolet light damage, resulting in a high tumor mutational burden. We hypothesized that this may result in high response rates with single-agent anti-programmed death protein 1 (PD-1) therapy. SWOG S1512 was a two-cohort clinical trial testing the activity of pembrolizumab in patients with surgically resectable (cohort A) and unresectable (cohort B) desmoplastic melanoma. Here we report on the cohort B single-arm clinical trial, which enrolled 27 patients with unresectable desmoplastic melanoma receiving pembrolizumab 200 mg intravenously every 3 weeks for up to 2 years, with the primary endpoint of complete response rate. The complete response rate was 37% (95% confidence interval: 19-58%), and the post hoc endpoint of objective response rate was 89% (95% confidence interval: 71-98%). The estimated secondary endpoints of 3-year melanoma-specific progression-free survival and overall survival were 84% and 96%, respectively, with only one patient having died from melanoma progression. Ten patients (37%) experienced grade 3 or 4 adverse events, and nine patients (33%) discontinued treatment because of adverse events. Patients with advanced desmoplastic melanoma have a high response rate to single-agent PD-1 blockade therapy, supporting single-agent anti-PD-1 as the treatment of choice, but are limited by a frequency of toxicities that is numerically higher than in other patient populations. ClinicalTrials.gov identifier: NCT02775851.