Persistent Lichenoid Reaction and Eruptive Keratoacanthomas Following PD-1 Inhibitor Therapy.

Background: Immune checkpoint inhibitors, such as nivolumab, have become integral in treating metastatic malignancies by enhancing immune responses against tumors. However, they are associated with immune-related adverse events (irAEs), including cutaneous reactions. We report a case of persistent lichenoid dermatitis during therapy and eruptive keratoacanthomas (KAs) following the cessation of nivolumab. Case Presentation: A 77-year-old Caucasian man presented with a subcutaneous melanoma on the left lateral chest, treated with excision followed by adjuvant nivolumab. Seven weeks into therapy, he developed pruritic erythematous macules, forming patches on his forearms, later spreading to his trunk. Biopsy confirmed lichenoid dermatitis. Treatment with triamcinolone cream 0.1% provided limited relief. Despite completing nivolumab therapy, the rash persisted. Oral prednisone 10 mg daily for 14 days yielded temporary improvement. Nineteen weeks post-therapy, he developed tender nodules on his left lower extremity, diagnosed as KAs via biopsy. Further KAs emerged and were treated and resolved with intralesional 5-fluorouracil. Forty weeks post-therapy, the lichenoid dermatitis remained stable with topical and intermittent oral steroids. The patient opted against additional treatments, such as UV-B therapy, preferring to await spontaneous resolution. Conclusion: This case underscores the need for vigilance in identifying and managing dermatologic irAEs associated with programmed cell death protein 1 inhibitors. Persistent lichenoid dermatitis and eruptive KAs present unique challenges, requiring tailored therapeutic strategies. Further research is essential to optimize management of these adverse events and improve patient outcomes.