Clinical pharmacology of tamoxifen and N-desmethyltamoxifen in patients with advanced breast cancer.

Serum concentrations of tamoxifen and its metabolite, N-desmethyltamoxifen (DMT) were determined in six post-menopausal patients with advanced breast cancer. Following a single 10-mg dose PO parent drug was detected in the serum, with a peak concentration of 17.5 ng/ml. Concentrations of the N-desmethyl metabolite were below the limit of detection (less than 2.5 ng/ml). After 21 days' oral therapy with 10 mg b.i.d. the serum concentration of tamoxifen had increased ten fold, while DMT was now present in comparable amounts. Two patients were further studied for a longer time period. There was little change in the serum concentration of tamoxifen, while the DMT increased two fold above its value at 21 days.