Pembrolizumab-Induced Erosive Lichenoid Reaction in a Patient With Metastatic Lung Adenocarcinoma: A Case Report

Introduction: Immune-related adverse events induced by immune checkpoint inhibitors are quite common. Cutaneous lichenoid immune-related adverse events are among the most frequent. However, oral lichenoid adverse reactions are extremely rare. We herein describe a patient who was treated with pembrolizumab for metastatic lung cancer and developed an erosive oral lichenoid reaction induced by immunotherapy. Case presentation: An 87-year-old man treated with pembrolizumab for metastatic lung adenocarcinoma developed multifocal erosions of the oral mucosa with multiple small, irregular, hyperkeratotic areas. Histopathological examination showed epithelial necrosis and a dense band-like layer of an inflammatory infiltrate of lymphocytes and histiocytes within the upper dermis. Direct immunofluorescence was negative for both IgG and C3. A diagnosis of erosive oral lichenoid reaction of the mucosa induced by immunotherapy was established. Pembrolizumab treatment was withheld and concomitant topical and systemic steroids were started. After 1 month, the drug-related toxicity was ameliorated and immunotherapy was re-introduced. Discussion: Only one other case of pembrolizumab-induced erosive lichen planus of the oral mucosa has been described to date. Previously reported drug-induced lichenoid rashes were mainly localized on the skin. Clinically, the main differential diagnoses of lichenoid erosive lesions are bullous immune-related disorders. In our patient, histological examination combined with negative results of both direct immunofluorescence and enzyme-linked immunosorbent assays confirmed the diagnosis of erosive lichenoid drug reaction. Conclusion: Clinicians should be aware of lichenoid involvement of the oral mucosa because related pain and food intake difficulties may seriously compromise treatment compliance. Prompt treatment of oral drug-related reactions may prevent interruption of immunotherapy and improve patients’ quality of life.