LY23 Granulomatous mogamulizumab-associated rash mimicking disease progression in erythrodermic mycosis fungoides

Abstract

Mogamulizumab, a monoclonal antibody targeting CCR4, is licensed for relapsed or refractory mycosis fungoides and Sézary syndrome. Mogamulizumab-associated rash (MAR) is a common adverse effect and may closely resemble active cutaneous lymphoma, creating diagnostic uncertainty. We present the case of a 77-year-old man with Sézary syndrome who presented with widespread ill-defined confluent erythematous patches and plaques consistent with T4 disease. He reported no systemic symptoms. Baseline investigations demonstrated Sézary cells on peripheral blood film, with flow cytometry revealing a circulating CD4+ CD7+ level of 1.3 × 109 cells L−1 and a CD4 : CD8 ratio of 32. A matching clonal T-cell population was identified in skin and bone marrow aspirate. Positron emission tomography–computed tomography showed no fluorodeoxyglucose-avid disease. His baseline modified Severity-Weighted Assessment Tool score was 77.6 (patch 61.8, plaque 8), and his Skindex-29 score was 62, indicating severe quality-of-life impairment. Previous treatments included narrowband ultraviolet B, oral psoralen–ultraviolet A and bexarotene. Mogamulizumab was commenced and 10 months later he developed thick, erythematous, infiltrated plaques on the scalp, ears and trunk. The patient also developed a patch of alopecia on the scalp. There was clinical concern for disease relapse and the rash was biopsied. Histopathology revealed marked dermal interstitial histiocytic and granulomatous inflammatory infiltrate with abundant eosinophils. Periodic acid–Schiff staining was negative for fungal organisms. Immunohistochemistry demonstrated CD3-positive T cells with mixed CD4 and CD8 expression, preservation of CD2, and CD30 positivity in < 5% of cells. Following clinicopathological correlation, a diagnosis of granulomatous-type MAR was made. Mogamulizumab was discontinued due to the severity of cutaneous involvement, and the patient was treated with topical clobetasol propionate and methotrexate 20 mg weekly, resulting in clinical improvement. His relapse-free survival is now 12 months. Cutaneous side effects are commonly reported with mogamulizumab; however, cases of scalp damage with alopecia are rarer. Recognition of MAR is essential to avoid misdiagnosis and initiate inappropriate escalation of lymphoma-directed therapy.