A simple HPLC-UV method for quantification of enzalutamide and its active metabolite N-desmethyl enzalutamide in patients with metastatic castration-resistant prostate cancer

- A simple and sensitive HPLC-UV has been validated to simultaneously assay enzalutamide and N-desmethyl enzalutamide in plasma.
- Plasma concentration of enzalutamide and N-desmethyl enzalutamide was assayed in metastatic castration-resistant prostate cancer patients.
- The assay is suitable for the therapeutic drug monitoring of enzalutamide in clinical settings.

Enzalutamide is currently approved for the treatment of patients with metastatic castration-resistant prostate cancer (mCRPC). To date, a single liquid chromatographic-tandem mass spectroscopy method is available to measure plasma enzalutamide concentrations in mCRPC patients. In this work, an accurate and sensitive HPLC-UV method has been developed for the simultaneous determination of enzalutamide and its active metabolite, N-desmethyl enzalutamide in plasma from mCRPC patients. Before precipitation of proteins with acetonitrile, samples were spiked with nilutamide (internal standard). Separation of analytes was achieved under isocratic elution on a C18 Kinetex column. The mobile phase consisted of a mixture of ammonium acetate buffer (pH = 4.6, 20 mM) and acetonitrile (60:40, v/v), and was delivered at a flow rate of 1.5 mL/min throughout a 9-min run. UV detection was performed at 270 nm. The method was linear over a concentration range of 0.50-50.0 μg/mL for both analytes. Within- and between-day imprecision and accuracy were ≤10% at concentrations 0.75, 5.00, and 50.0 μg/mL. This method has been implemented to assay steady-state trough plasma concentrations (n = 30) of enzalutamide and N-desmethyl enzalutamide in 16 mCRPC patients. Overall, this HPLC-UV method is well-suited for routine application in clinical laboratories to perform therapeutic drug monitoring of enzalutamide in mCRPC patients.