Erythema Multiforme Associated With Pembrolizumab Treatment for Recurrent Urothelial Carcinoma: A Case Report.

Immune checkpoint inhibitors (ICIs) have significantly improved outcomes in patients with advanced malignancies; however, their use is associated with immune-related adverse events (irAEs) that may affect multiple organ systems. Cutaneous manifestations are the most common and often represent the earliest signs of immune dysregulation. Erythema multiforme (EM) is a rare, but potentially underrecognized, cutaneous adverse event associated with ICI therapy. We report the case of an 84-year-old woman with recurrent urothelial carcinoma who developed EM following the sixth cycle of pembrolizumab. The eruption presented as symmetrically distributed, annular erythematous plaques with central crusting on the upper extremities and trunk, without mucosal involvement or systemic symptoms. Histopathologic examination revealed vacuolar interface dermatitis with lymphocytic infiltration, supporting the diagnosis. The patient was successfully managed with systemic and topical corticosteroids, along with supportive skin care, resulting in complete resolution of lesions. Pembrolizumab was subsequently discontinued, and no recurrence of cutaneous findings was observed. This case highlights the importance of recognizing EM as a rare, immune-related cutaneous adverse event associated with pembrolizumab. Early identification and appropriate management are essential to prevent progression and minimize interruptions in oncologic therapy. Increased awareness among clinicians may facilitate timely diagnosis and improve patient outcomes as the use of immunotherapy continues to expand.