Differences in the mechanism of action of medrogestone and cyproterone acetate.

In the intact adult rat, medrogestone was found to be effective in causing the involution of the prostate and seminal vesicle. Medrogestone, however, does not function through an antiandrogenic action. In the castrate animal medrogestone was unable to block the stimulating action of exogenous testosterone on the sex accessory organs as shown by effects on tissue weight and DNA synthesis. In addition, medrogestone was unable to block the uptake of labeled androgen into prostatic nuclei. The effectiveness of medrogestone on sex accessory tissue may be accounted for by its ability to lower plasma testosterone levels. This is achieved by inhibiting testosterone biosynthesis. The action of medrogestone was compared to that of cyproterone acetate and it was concluded that these drugs inhibit prostatic growth through two different mechanisms.