A case of trastuzumab-induced dermatomyositis.

Human epidermal growth factor receptor 2 (HER-2) is a checkpoint, controlling cell proliferation and differentiation. Trastuzumab, a humanized monoclonal antibody directed against HER-2, is nowadays standard treatment for breast cancer patients whose tumors express HER-2. It is generally well tolerated, with a small number of patients developing mild adverse reactions. Dermatomyositis is a rare adverse event of trastuzumab therapy not well described in the literature. We herein present a case of a patient treated for hormone-sensitive invasive ductal carcinoma, who presented with symptoms of proximal muscle weakness, arthralgias, skin rash, and generalized fatigue. The symptoms started after the sixth cycle of trastuzumab and progressively deteriorated. The patient's medical and family history was unremarkable. Disease progression as a possible cause of dermatomyositis had been ruled out, and laboratory evaluation revealed moderate elevation of serum muscle proteins and acute-phase reactants. Trastuzumab treatment was discontinued, and 3 months later, the patient was free of symptoms without any further intervention.