Dermatomyositis following Biosimilar Trastuzumab in a Breast Cancer Patient: A Case Report

Trastuzumab, as a recombinant IgG1 kappa, is a humanized monoclonal antibody against human epidermal growth factor receptor 2. Accordingly, it is widely used in breast cancers at early and advanced stages. Dermatomyositis is a rare adverse event of trastuzumab therapy, which is not well documented yet. In this study, a patient was treated for invasive ductal carcinoma with some symptoms of rash and generalized fatigue. These symptoms started after the fifth cycle of trastuzumab, which were gradually deteriorating. This patient’s medical and family histories were unremarkable. The progression of the disease was ruled out as a possible cause of dermatomyositis, and the laboratory evaluation revealed a moderate increase in serum muscle protein (CPK). So, trastuzumab treatment was discontinued, and by passing 1 month from the start of prednisolone and hydroxychloroquine, the patient had no symptoms.