Regressed melanocytic nevi secondary to pembrolizumab therapy: an emerging melanocytic dermatologic effect from immune checkpoint antibody blockade

Background: Immune checkpoint antibody blockade is an emerging therapeutic option for treating certain cancers including melanoma. This therapy is associated with dermatologic and systemic toxicities, some of which are more severe than others and may require withholding therapy. Case reports: We report two patients with melanocytic nevi that regressed with pembrolizumab therapy. The first patient had stage IV BRAF K601E/L584F mutant melanoma that developed a regressed melanocytic nevus while being treated with pembrolizumab. The second patient had stage III BRAF V600R mutant melanoma that was treated with pembrolizumab and dabrafenib, and also developed a regressed melanocytic nevus. Both patients had good response to therapy and stable disease at 8 and 12 months of treatment, respectively. Results: Regressed melanocytic nevi were observed in both patients treated with pembrolizumab for advance-stage melanoma. Immunohistochemical analysis of a regressed melanocytic nevus was associated with an inflammatory infiltrate rich in CD8+ T cells and CD163+, CD11c+ histiocytes. Conclusion: Regressed melanocytic nevi are an emerging dermatologic effect from pembrolizumab therapy.