Mogamulizumab Efficacy is Underscored by its Associated Rash that Mimics Cutaneous T-cell Lymphoma: A Retrospective Single-Centre Case Series

Background Mogamulizumab is a humanized antibody against chemokine receptor type 4 recently FDA approved for relapsed/refractory mycosis fungoides (MF) and Sezary syndrome (SS). The most commonly reported adverse event in the phase III licensing trial was drug eruption (28%), now termed mogamulizumab associated rash (MAR). Clinical recommendations about MAR and its treatment differ between the current package insert and post-approval insights reported from two single-centre studies that focused on its characterization, but less so on outcomes and clinicopathologic differentiation from CTCL. Objectives To describe our experience in the diagnosis of MAR and treatment of patients with cutaneous T-cell lymphoma (CTCL) with mogamulizumab. Methods This is a single-centre retrospective case series. Results We found a higher incidence of MAR in CTCL patients (17/24; 68%) than previously reported. MAR development is associated with complete (11/17) or partial (4/17) responses with an overall response rate (ORR) of 88%, versus 28% (2/7) ORR in patients without MAR. MAR diagnosis may be obscured by its ability to mimic key CTCL features both clinically and histologically, but an absence of TCR clonality and relatively decreased CD4:CD8 ratio compared to baseline lesions strongly favors MAR over recurrent disease. Conclusions MAR has the potential to create a significant management problem for patients on mogamulizumab. Misidentification of MAR as recurrent CTCL may detrimentally result in the premature discontinuation of mogamulizumab in patients whose disease is historically hard to treat. Thorough clinicopathologic investigation of new lesions during treatment with mogamulizumab is required to inform ideal treatment decisions and achieve better outcomes.