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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 890/0 of previously untreated prostatic cancer patients and in 38% of reactivated cancer patients.