Pembrolizumab-induced necrotizing myositis in a patient with metastatic non-small-cell lung cancer: a case report

A 57-year-old man presented with swelling and pain in the lower limbs, inability to walk and increasing dyspnea for 2 days. Because of refractory stage IV non-small-cell lung cancer, pembrolizumab was started 21 days before presentation. Since then, he experienced general discomfort, fatigue and bilateral weakness in the legs with exercise limitation. A diagnosis of pembrolizumab-induced grade III myositis was made based on muscle biopsy. Pembrolizumab is a humanized monoclonal antibody against PD-1. It has been approved for the treatment of metastatic melanoma and refractory non-small-cell lung cancer with increased expression of PD-L1 on the cell surface of tumor cells. With such a humanized monoclonal antibody, fewer adverse events are expected than with systemic chemotherapy. However, 13% of patients develop autoimmune side effects which can be severe (grade III, IV or V) in 5–10%. We discuss a case of pembrolizumab-induced myositis, with a brief overview of the literature. Only three cases of pembrolizumab-induced myositis have been reported in literature.