CLINICAL PHARMACOLOGY
Estramustine phosphate (Figure 1) is a molecule combining estradiol and nornitrogen mustard by a carbamate link. The molecule is phosphorylated to make it water soluble.
Estramustine phosphate taken orally is readily dephosphorylated during absorption, and the major metabolites in plasma are estramustine (Figure 2), the estrone analog (Figure 3), estradiol, and estrone.
Prolonged treatment with estramustine phosphate produces elevated total plasma concentrations of estradiol that fall within ranges similar to the elevated estradiol levels found in prostatic cancer patients given conventional estradiol therapy. Estrogenic effects, as demonstrated by changes in circulating levels of steroids and pituitary hormones, are similar in patients treated with either estramustine phosphate or conventional estradiol.
The metabolic urinary patterns of the estradiol moiety of estramustine phosphate and estradiol itself are very similar, although the metabolites derived from estramustine phosphate are excreted at a slower rate.