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Acquired resistance to immune checkpoint inhibitors (ICIs) remains a significant challenge in the treatment of metastatic melanoma. Phenotypic plasticity, such as dedifferentiation and transdifferentiation, is an increasingly recognized mechanism for treatment resistance. We present a case of a man in his 70s with metastatic melanoma who experienced progression through sequential treatments including pembrolizumab in combination with the HDAC inhibitor entinostat, and ipilimumab. During treatment a histologically distinct pleomorphic rhabdomyosarcoma (RMS) emerged at metastatic sites. Longitudinally acquired tumor samples representing both phenotypes were analyzed using whole-exome sequencing (WES), RNA sequencing (RNA-seq) and high-plex tissue imaging (spatial proteomics). WES revealed driver mutations (e.g. NRAS, NF1) and loss-of-heterozygosity (LOH) shared between phenotypes indicating a common ancestral clone. Phylogenetic analysis demonstrated an early divergence of the phenotypes, with each later acquiring unique mutations. RNA-seq showed mutually exclusive expression of lineage-specific markers as well as epithelial-mesenchymal transition and myogenic gene set enrichment in the RMS samples. High-plex imaging identified distinct tumor microenvironments, with RMS lesions enriched in CD163 macrophages.