Mogamulizumab is an anti-CCR4 monoclonal antibody prescribed to patients for the treatment of relapsed or refractory mycosis fungoides (MF) and Sézary syndrome (SS). CCR4 is expressed by both circulating pathogenic SS cells and in MF/SS lesional skin. By binding CCR4, mogamulizumab promotes antibody-dependent cellular cytotoxicity against malignant MF and SS cells. CCR4 is also highly expressed on regulatory T cells (Tregs) and type 2 helper T cells (Th2). As a result, mogamulizumab's therapeutic efficacy in MF/SS may be potentiated through by depleting local immunosuppressive Th2 and Tregs. In the large phase 3 mogamulizumab versus vorinostat in previously treated cutaneous T-cell lymphoma (MAVORIC) trial, mogamulizumab was observed to significantly increase progression-free survival in patients with SS. However, patients on mogamulizumab frequently develop an associated rash.

Mogamulizumab-associated rash (MAR) is heterogenous in its presentation, both clinically and histopathologically, and the clinical morphologic appearance does not always correlate with histopathologic findings. Furthermore, mixed pictures are also seen histopathologically. Importantly, MAR may mimic MF or SS clinically, potentially leading to premature discontinuation of the drug if the rash is thought to be disease progression.

A significant proportion of patients treated with mogamulizumab develop a rash, with studies reporting an incidence of 20% to over 60%. MAR was the second-most-common side effect (after infusion reactions) in the MAVORIC trial. Males and females appear to develop a rash at equal rates. The rash exhibits a heterogeneous clinical course, with variable and even delayed onset. The median time from initiating therapy to onset of rash varies significantly, with many studies reporting a median time over 3 months but with a range of less than 2 months to almost 4 years. Some patients also develop a rash several months after discontinuing mogamulizumab. The median number of treatment cycles also varies between patients, with some patients developing a rash after only 1 or 2 cycles, while others are treated tens of times.

Patients with SS who develop MAR have been noted to have an increased overall response rate than SS patients who did not develop MAR; the same has not been shown for MF.

Stevens-Johnson syndrome (SJS) / toxic epidermal necrolysis (TEN) has been reported in Japan in patients with adult T-cell leukemia/lymphoma receiving mogamulizumab. Such severe adverse reactions have not been reported in MF/SS to date.