Pharmacokinetics and bioavailability of tamoxifen in postmenopausal healthy women

A randomised cross-over study in 24 postmenopausal women was selected to establish bioequivalence of two tamoxifen (CAS 10540-29-1) formulations. In addition, this study compiled pharmacokinetic parameters for the current 30 mg regimen in postmenopausal women, the target population of tamoxifen therapy. Mean C(max) values of 59.1 ± 8.9 (T) and 63.6 ± 11.1 (R) ng/ml were attained 3.6 ± 1.2 (T) and 3.2 ± 1.1 (R) h after administration of 30 mg tamoxifen for the test (T) and the reference (R) formulation. The mean AUC00((0-480)) of tamoxifen was calculated as 3299.7 ± 761.2 (T) and 3370.1 ± 701.9 (R) ng x h/ml. The corresponding AUC00((0-480)) of the active metabolite, N-desmethyl-tamoxifen, exceeded that of the parent drug with 4359.7 ± 830.5 (T) and 4306.3 ± 835.2 (R) ng x h/ml, whereas maximal concentrations of the metabolite were distinctly decreased with 14.4 ± 3.3 (T) and 14.3 ± 2.4 (R) ng/ml. The pharmacokinetic parameters evaluated in this study are well in line with already known pharmacokinetic data generated with young male volunteers and postmenopausal patients with breast cancer. Precise analytics and an extremely long blood sampling period facilitated an accurate determination of tamoxifen's half-life in postmenopausal women with 210.1 ± 60.8 (T) and 209.8 ± 59.9 (R) h. Based on the extremely long half-life, the suitability of a cross-over design is discussed and recommended for further studies.