Mechanism of retention of estramustine in the rat prostate and results of a clinical trial of Estracyt in Japan.

To clarify the mechanism of action of Estracyt, we performed experiments using 3H-estramustine of high specific activity. 3H-Radioactivity accumulated selectively in the ventral prostate of castrated male rats after the administration of 3H-estramustine. Estramustine and its metabolites were retained in the ventral prostate for long time periods. The uptake of 3H-radioactivity was almost totally localized in the cytosol fraction, but not in a purified receptor fraction. The apparent equilibrium dissociation constant of the estramustine binding protein was 18.9 nM, and the apparent equilibrium Bmax value was 0.76 nmoles/mg of cytosol protein. In addition, we wish to report in this paper the results of clinical trials of Estracyt studied by a cooperative research group in Japan from 1977 to 1979. It was concluded that Estracyt was effective in 89% of previously untreated prostatic cancer patients and in 38% of reactivated cancer patients.