Exploring Pembrolizumab‐Induced <scp>IM3OS</scp> in a Patient With Bladder Cancer

Pembrolizumab, an immune checkpoint inhibitor (ICI), has transformed cancer treatment across various malignancies, including metastatic urothelial cancer. As the use of pembrolizumab becomes increasingly widespread, understanding its potential adverse effects is essential. Here, we present the case of a 79-year-old male with urothelial cancer of the bladder, who experienced local recurrence after surgery and conventional therapies. He subsequently began pembrolizumab treatment. Following the first infusion, the patient developed a series of complications over a span of 26 days, including acute hepatitis, chest pain, and progressive bilateral ptosis. A third-degree atrioventricular block was also detected, necessitating pacemaker implantation. Further neurophysiological evaluations confirmed a diagnosis of myasthenia gravis and myositis. The patient was treated with intravenous methylprednisolone, immunoglobulin therapy, and pyridostigmine bromide. Despite these interventions, his condition deteriorated, requiring intubation and mechanical ventilation 28 days post-treatment. This case highlights that pembrolizumab can trigger rare but severe neurological and cardiovascular adverse events. Prompt and thorough investigations to identify syndromes such as myocarditis-myositis-myasthenia gravis overlap syndrome (IM3OS) are vital for timely diagnosis and intervention.