Pilot study on the growth inhibition by estramustine phosphate (Estracyt) of rat mammary tumours sensitive and insensitive of oestrogen.

Rat mammary tumours induced by 7,12-dimethylbenz(a)anthracene had a higher concentration of 3H and 14C than muscle after injection of estramustine phosphate labelled with 3H in the oestrogen moiety and 14C in the alkylating moiety. Thin-layer chromatography showed that dephosphorylated estramustine phosphate was present in the tumours but no free oestradiol-17beta. The uptake of the drug in the tumours was parallelled by a dose dependant retardation of tumour growth and a prevention of tumour number increase. Estramustine phosphate also retarded growth of mammary tumours resistant to treatment with oestradiol-17 beta. It is concluded that estramustine phosphate has a greater effect on tumour growth than oestrogen.