S-1 as an alternative treatment after 5-fluorouracil-induced coronary vasospasm in a patient with small bowel adenocarcinoma: a case report

5-Fluorouracil (5-FU) is a chemotherapeutic agent used worldwide to treat various solid tumors. It may cause adverse cardiotoxic events, the most common of which is coronary vasospasm. Recently, the 2022 European Society for Medical Oncology guidelines for metastatic colorectal cancer recommended S-1 as an alternative therapy after 5-FU-induced cardiotoxicity; however, only limited data on Asian patients are available. Here, we report a case of safe administration of S-1 to a 72-year-old Japanese female patient with metastatic small bowel adenocarcinoma who developed 5-FU-induced coronary vasospasm. While receiving modified FOLFOX6 (5-FU plus leucovorin and oxaliplatin) as palliative chemotherapy, she experienced chest pain with electrocardiographic ST elevation. Chemotherapy was temporarily suspended, but treatment was resumed by switching from modified FOLFOX6 to SOX (S-1 plus oxaliplatin) as the tumor began to worsen. Owing to the adverse event of enteritis, the patient’s treatment was switched to S-1 monotherapy after cycle 3, and S-1 monotherapy was continued without any cardiotoxicity. S-1 may be a promising alternative therapy after 5-FU-induced cardiotoxicity.