LC-MS/MS Method for Therapeutic Drug Monitoring of Abiraterone, Darolutamide, Apalutamide, Enzalutamide, and Metabolites in Prostate Cancer Patients

Accurate measurement of androgen receptor pathway inhibitors (ARPIs) and their active metabolites is essential for pharmacokinetic studies and therapeutic drug monitoring (TDM) in patients with prostate cancer (PC). However, their simultaneous determination in human plasma is analytically challenging because of the wide concentration ranges. This study aimed to develop and validate a sensitive and robust LC–MS/MS method for the quantification of abiraterone, Δ4-abiraterone, enzalutamide, N-desmethyl enzalutamide, darolutamide, keto-darolutamide, apalutamide, and N-desmethyl apalutamide in human plasma. Sample preparation was performed by protein precipitation, followed by chromatographic separation and detection using multiple reaction monitoring with isotopically labeled internal standards (total run time 6.5 min). The method was validated in accordance with regulatory guidelines by assessing selectivity, linearity, sensitivity, accuracy, precision, recovery, matrix effects, and stability. The assay demonstrated good linearity (≥0.997) across clinically relevant concentration ranges, with lower limits of quantification between 0.1 and 40 ng/mL, depending on the analyte. Intra- and inter-day precision and accuracy were within acceptable limits, and recovery and matrix effects were consistent across different plasma matrices. Stability experiments conducted in plasma and whole blood provided practical guidance for sample handling. The method was successfully applied to 79 plasma samples from 61 patients with metastatic PC. Measured concentrations were generally consistent with published pharmacokinetic data, while unexpectedly high ABI levels were observed. Sample collection occurred between 1 and 28 h after the last drug intake, enabling assessment of the analytical method across the entire pharmacokinetic profile.