Pharmacokinetic/Pharmacodynamic Relationship of Enzalutamide and Its Active Metabolite N-Desmethyl Enzalutamide in Metastatic Castration-Resistant Prostate Cancer Patients.

INTRODUCTION:Enzalutamide (ENZA) is an oral androgen receptor inhibitor approved by the Food and Drug Administration and the European Medicines Agency for the treatment of metastatic and nonmetastatic castration-resistant prostate cancer (CRPC). ENZA is extensively metabolized by cytochrome P450 3A4 into N-desmethyl ENZA (NDE), an active metabolite. We aimed to explore the pharmacokinetic/pharmacodynamic relationship for ENZA and NDE in metastatic CRPC patients from a real-world setting. PATIENTS AND METHODS:Trough plasma concentration (Ctrough) of ENZA and NDE were assayed using liquid chromatography coupled with UV detection. The relationship between ENZA, NDE, and composite (ENZA with NDE) plasma concentration and requirement of ENZA dose reduction was investigated using the Mann-Whitney test. A survival univariate analysis was conducted to explore association between progression-free survival (PFS), overall survival (OS), and plasma Ctrough (ENZA, NDE, and composite). RESULTS:Twenty-two metastatic CRPC patients treated with ENZA (median age, 75.5 years; 13 patients (59%) with Eastern Cooperative Oncology Group status 0-1) were prospectively included. Mean plasma Ctrough of ENZA and NDE were 12.4 ± 3.0 μg/mL and 8.8 ± 2.1 μg/mL, respectively. Neither PFS nor OS were statistically associated with ENZA, NDE, or composite plasma Ctrough. In 4 patients (18%) who required ENZA dose reduction because of severe clinical toxicity, an increased ENZA plasma Ctrough was observed compared with 18 remaining patients (16.1 ± 2.4 μg/mL vs. 11.6 ± 2.6 μg/mL, respectively; P = .027). CONCLUSION:The low interindividual variability in ENZA and NDE Ctrough and the lack of relationship with survival do not support the need for plasma drug monitoring. Severe asthenia might be related to higher exposure and could be improved by decreasing ENZA dosing.