Plasma concentrations of estramustine phosphate and its major metabolites in patients with prostatic carcinoma treated with different doses of estramustine phosphate (Estracyt).

Plasma concentrations of estramustine phosphate and its major metabolites were measured in patients with prostatic carcinoma treated with increasing oral doses, 70-650 mg/day, of estramustine phosphate (Estracyt). Parent drug and estradiol were measured by radioimmunoassay, and estramustine and its estrone analogue (Leo 271) utilizing gas chromatography. The concentrations of estramustine phosphate and estramustine were below or close to the limits of the methods. A linear correlation was found between the daily dose of estramustine phosphate, and plasma concentrations of Leo 271-the main metabolite-and estradiol, which suggests that no capacity limiting processes are involved in the pharmacokinetics of estramustine phosphate in man. Plasma was also analyzed during prolonged treatment with 560 mg/day. The metabolite pattern was not changed by two to three years of estramustine phosphate treatment.