Lack of interaction between enzalutamide and warfarin in a metastatic castration-resistant prostate cancer patient

Enzalutamide is an androgen receptor antagonist used for the treatment of metastatic castration-resistant prostate cancer. Enzalutamide is classified as a strong cytochrome P450 3A4 inducer, a moderate 2C9 and 2C19 inducer, and a time-dependent inducer of 1A2. Warfarin’s more potent enantiomer is primarily metabolized by cytochrome P450 2C9 and has a narrow therapeutic window. Enzalutamide is thought to decrease therapeutic warfarin concentrations per pharmacokinetic studies performed during drug development. This case report describes a 59–year-old man undergoing treatment with enzalutamide for metastatic castration-resistant prostate cancer with a history of femoral vein thrombosis. The patient was receiving a total weekly warfarin dose of 37.5 mg prior to starting enzalutamide. Enzalutamide was initiated and warfarin continued at a constant dose without decrease in the patient’s INR. The patient continued on enzalutamide and warfarin for 1 year without having any documented subtherapeutic INRs. This report illustrates one case in which the interaction between warfarin and enzalutamide was not clinically significant.