Analysis of mutational burden and adaptive immune response in desmoplastic melanomas treated with PD-1/L1 inhibitors.

9558 Background: Desmoplastic melanoma (DM) is a rare subtype of melanoma characterized by a dense fibrous stroma, resistance to chemotherapy and no actionable driver mutation for targeted therapy. We investigated the efficacy of PD-1/L1 inhibitors and correlation with genetic landscape and tumor immune microenvironment in DM. Methods: Retrospective analysis of 1054 pts with melanoma treated with anti-PD-1/L1, resulting in 57 pts with unresectable or metastatic DM. Available baseline biopsies were analysed by digital quantitative immunohistochemistry (IHC) for CD8 and PD-L1 and by whole exome sequencing (WES), compared to available tissue from non-DM pts treated with anti-PD1/L1 at UCLA. Results: At a median follow up of 20 mo, 40 pts (70%, 95% CI 57-82) had an objective response by RECIST 1.1 criteria, including 18 (45%) CRs with no relapse observed to date. Responses were similar in DM subsets (23 pure, 29 mixed and 5 indeterminate). Kaplan-Meier estimated 1-year and 2-year overall survival were 85% (95% CI 78-98) and 74% (95% CI 64-89). WES revealed a median of 1282 (interquartile range 517-1692) non-synonymous somatic mutations per tumor in DM tumors (n = 17), significantly higher (p = 0.02) than the median of 462 (interquartile range 230-1150) in non-DM (n = 23). Mutations in NF-1 were the most common (13/17) followed by loss-of-function TP53 and ARID2, and > 82% of single nucleotide mutations were UV damage signatures. IHC analysis from 19 DM and 13 non-DM revealed a strikingly higher percentage of PD-L1 positive cells in the tumor parenchyma in DM (p = 0.04), highly associated with CD8 density and PD-L1 expression in the tumor invasive margins, indicating an active adaptive immune response. No genetic mechanisms known to cause constitutive PD-L1 expression were detected in these samples. Conclusions: Patients with advanced DM derive significant clinical benefit from PD-1/L1 inhibitors, likely related to the high mutational burden and a highly active adaptive immune response as the main mechanism of immune escape prior to therapy. Our results challenge the general conception that dense fibrous stroma around the malignant cells interferes with immune cell infiltration and efficacy of immunotherapy.