Pembrolizumab-induced nonischemic cardiomyopathy: a case report of a rare immune-related adverse event.

Background: Immune checkpoint inhibitors (ICIs) have improved survival in advanced cancers but may cause immune-related adverse events (irAEs). Cardiovascular irAEs, though rare, can be fatal, with myocarditis as the most serious manifestation. While typically occurring early in therapy, late-onset cases are increasingly recognized. We report a rare case of pembrolizumab-induced myocarditis complicated by severe cardiomyopathy and left ventricular (LV) thrombus. Case Description: A 63-year-old woman with stage IV lung adenocarcinoma on pembrolizumab-based therapy presented with chest pain and elevated troponin. Echocardiography suggested Takotsubo cardiomyopathy, and coronary angiography revealed no obstruction. Ten days later, she returned with worsening chest pain, markedly elevated troponins, and severely reduced ventricular function, raising suspicion for immune-mediated myocarditis. She was treated with high-dose intravenous methylprednisolone, discharged with a wearable cardioverter-defibrillator, and pembrolizumab was permanently discontinued. Two months later, she developed critical limb ischemia. Imaging demonstrated widespread thromboembolic disease, including a large LV apical thrombus. She was managed medically with anticoagulation due to high surgical risk. At 1-year follow-up, her ejection fraction (EF) had improved to 50-55%, and she remained in remission without further systemic therapy. Conclusions: This case highlights late-onset pembrolizumab-induced myocarditis progressing to severe cardiomyopathy and thromboembolic complications. Prompt recognition, immunotherapy discontinuation, and corticosteroid therapy were associated with cardiac recovery. Clinicians should remain vigilant for delayed cardiovascular irAEs in patients on ICIs. Multidisciplinary care and heightened awareness may improve outcomes and guide future management strategies.