Ya-Chu Kuo, Hsuan-Yu Huang, Hsuan-Hui Ho, Li-Jiuan Shen, Ching-Hua Kuo
Tamoxifen requires biotransformation into metabolites to exert antiestrogenic activity, and variability in metabolite exposure contributes to interindividual differences in clinical outcomes. Volumetric absorptive microsampling (VAMS) provides a convenient and minimally invasive alternative for therapeutic drug monitoring (TDM). However, analytical methods enabling comprehensive quantification of tamoxifen and its metabolites in VAMS remain limited, and translation of VAMS-derived concentrations into plasma-equivalent values has not been well established. In this study, an LC-MS/MS method was developed and fully validated for simultaneous quantification of tamoxifen and six metabolites in VAMS samples. Water prerinsing prior to methanol extraction improved analyte recoveries to 89.2-111.2%. Stability evaluation of VAMS samples revealed poor room-temperature stability for most analytes, whereas all analytes remained stable for at least 100 days at -80 °C. Experimentally derived plasma-to-blood concentration ratios (Cp/Cb, 1.41-1.83) were further applied to estimate plasma-equivalent concentrations from VAMS samples. Clinical applicability was evaluated using paired VAMS and plasma samples collected from ten tamoxifen-treated breast cancer patients. Strong correlations between VAMS-derived and measured plasma concentrations were observed (R2 = 0.77-0.98). Bland-Altman analysis further demonstrated good agreement, with more than 90% of values falling within the ±1.96 SD limits. These results demonstrate that the proposed workflow provides a reliable approach for comprehensive tamoxifen metabolite quantification in VAMS samples and supports plasma-equivalent concentration estimation for comparison with existing plasma-based therapeutic data. The proposed strategy may facilitate the clinical implementation of VAMS-based TDM for personalized tamoxifen therapy.