Myasthenia gravis with myositis induced by pembrolizumab therapy in a patient with metastatic urothelial carcinoma.

Pembrolizumab is a long-awaited drug for the treatment of metastatic urothelial carcinoma. It is an immune checkpoint inhibitor, which has been shown to trigger new autoimmune disorders. We report a case of pembrolizumab-induced myasthenia gravis that occurred in an 84-year-old Japanese female with metastatic urothelial carcinoma in multiple organs. She developed right ptosis 3 days after the second pembrolizumab treatment. Although prednisolone was administered, her symptoms did not change and dysphagia appeared. She needed the steroid pulse therapy for treatment eventually. On the other hand, 9 weeks after the first pembrolizumab treatment, reductions in the sizes of liver and adrenal metastases was observed. However, unfortunately, the severe immune-related adverse events did not allow her to continue the administration of pembrolizumab for the treatment of multiple metastatic urothelial carcinoma. To our knowledge, this is the first case of myasthenia gravis associated with pembrolizumab treatment against metastatic urothelial carcinoma. In this case, a relationship between immune-related adverse events associated with myasthenia gravis and tumor responses to pembrolizumab was suspected, because marked reductions in the sizes of some metastatic lesions were observed after the administration of only two courses of pembrolizumab. © The Japan Society of Clinical Oncology 2020.