A case of irAE myositis with positive antistriational antibodies after anti-PD-L1 antibody administration: A case report.

Immune checkpoint inhibitors are widely used in clinical practice, necessitating appropriate management of immune-related adverse events (irAEs). Although severe neurologic irAEs are less common, they often lead to poor outcomes, requiring early detection and prompt intervention. An 88-year-old woman with invasive urothelial carcinoma received six cycles of gemcitabine plus carboplatin followed by avelumab, an anti-programmed cell death ligand 1 antibody, as maintenance therapy. One week later, she developed progressive limb weakness and was diagnosed with irAE myositis based on elevated creatine kinase (CK) levels and imaging findings. Early treatment with methylprednisolone pulse therapy, followed by prednisolone [1 mg/kg body weight (BW)], led to rapid improvement, and no relapse occurred after prednisolone completion at 4 months. IrAE myositis has clinical, pathological, and immunological features that differ from those of known inflammatory muscle diseases. In this case, the time to onset and the presence of antistriational antibodies were consistent with previous reports focusing on anti-programmed cell death 1 antibodies, whereas ocular symptoms, myocarditis, and myasthenia gravis, which are considered characteristic of irAE myositis, were not observed. Given the expected increase in high-grade neurological irAEs, accumulating case reports is essential to better understand the differences in clinical presentation and prognosis, which may vary depending on drug-specific effects and autoantibody profiles. Furthermore, this case suggests that some patients with irAE myositis may successfully taper or discontinue prednisolone earlier than traditionally expected.