Mechanism of Action of Progestins in the Treatment of Hormone-dependent Breast Cancer.

The in vitro interference of some gestagens with the binding of 3 H-17 β-oestradiol to cytosol specific receptors was investigated with a view to elucidating the mechanism of action of progestins in the treatment of human hormone-dependent breast cancer. A decrease (up to 85 %) of oestradiol binding capacity was observed with high concentrations of progesterone, clogestone and medrogestone. These findings are in good agreement with those previously obtained by the same progestins in our laboratory on rat uterine estrogen receptors in vitro or in vivo. These results provide support for the hypothesis that the mode of action of progestins in the therapy of mammary and perhaps uterine carcinomas is to some extent related to the inhibition of oestradiol binding to cytosol specific receptors.