A Case of Drug‐Induced Hypersensitivity Syndrome Caused by Apalutamide

Introduction Apalutamide, an androgen receptor antagonist for prostate cancer, rarely causes drug‐induced hypersensitivity syndrome (DIHS). Case Presentation A 75‐year‐old male with prostate cancer and multiple bone metastases developed grade 2 rash and grade 3 liver dysfunction according to the Common Terminology Criteria for Adverse Events (CTCAE) 3 weeks after starting apalutamide with a GnRH antagonist, followed by a 3‐day fever. Ten days later, symptoms worsened to grade 3 rash and grade 4 liver dysfunction. He met five diagnostic criteria for DIHS. Hormonal therapy was discontinued, and prednisolone plus intravenous immunoglobulin (IVIG) was administered. Fatigue resolved within 2 days, rash by day 6, and liver function improved to grade 2 by day 10. The patient is currently on abiraterone and a GnRH antagonist without adverse events. Conclusion This report highlights the importance of caution and regular blood tests when using apalutamide owing to the risk of DIHS.