Relative affinities of estramustine phosphate metabolites to the cytosol oestrogen receptor of rat testis.

A competitive binding assay was used for analysis of the affinities of ethinyl oestradiol, diethylstilboestrol and metabolites of estramustine phosphate to the oestrogen receptor of rat testis. The affinities were ranked as follows: ethinyl oestradiol greater than oestradiol-17 beta greater than diethylstilboestrol much greater than estramustine. Estromustine did not reveal any oestrogen receptor affinity. The results support the concept that estramustine and estromustine do not mediate the oestrogenic effects of estramustine phosphate. The oestrogenicity of estramustine phosphate is suggested to be mainly due to the metabolite oestradiol-17 beta.