Pembrolizumab-associated erythema nodosum in the treatment of metastatic melanoma.

A 66-year-old woman receiving pembrolizumab for metastatic melanoma presented with tender red nodules on her shins and forearms. Biopsy was consistent with erythema nodosum (EN). The eruption responded to oral minocycline and potent topical steroids. Subsequent investigations detected bihilar lymphadenopathy, biopsied as granulomatous lymphadenitis, confirming the diagnosis of pembrolizumab-associated sarcoidosis. Pembrolizumab was stopped for two cycles and was restarted without recrudescence of EN or bihilar lymphadenopathy. Immunotherapy-associated sarcoidosis is a rare but recognized adverse event related to therapy with immune checkpoint inhibitors. EN is an uncommon manifestation of immunotherapy-induced sarcoidosis. New-onset bihilar lymphadenopathy in the context of immunotherapy requires prompt histological evaluation to differentiate between immunotherapy-associated sarcoidosis and metastatic progression. We review the literature related to immunotherapy-associated EN.