Immune-related myositis resulting from combination therapy of ipilimumab and nivolumab in patient with metastatic renal cell carcinoma.

Immune-related myositis is one of the rare immune-related adverse events whose underlying precise mechanisms are not fully understood. Here, we describe a case of immune-related myositis that developed after four cycles of combination therapy with nivolumab plus ipilimumab for the treatment of metastatic renal cell carcinoma. Negative results of autoimmune antibodies, including anti-acetylcholine receptor and anti-muscle-specific kinase antibodies suggested a T-cell-mediated mechanism. After recovery with steroid therapy, the patient resumed nivolumab monotherapy and survived without any evidence of disease progression or refractory of myositis. Differential diagnosis between T-cell-mediated and B-cell-mediated immune-related myositis and its impact on optimal management are discussed.