Abstract
Pembrolizumab, a humanized monoclonal anti-programmed cell death protein 1 (PD1) antibody, has shown efficacy in various malignancies. This article presents a case of stage III squamous cell carcinoma of the bladder treated with pembrolizumab, resulting in the development of a rare overlap syndrome known as myocarditis, myositis, and myasthenia gravis (IM3OS). While immune checkpoint inhibitors like pembrolizumab demonstrate notable antitumor activity, they also pose the risk of severe immune-related adverse events. The case underscores the importance of the early detection of IM3OS and the potential dangers associated with delayed diagnosis, offering valuable insights for healthcare providers managing patients on pembrolizumab therapy.