The oestrogenic effects of gestodene, a potent contraceptive progestin, are mediated by its A-ring reduced metabolites

Gestodene (17 alpha-ethynyl-13 beta-ethyl-17 beta-hydroxy-4,15-gonadien-3-o ne) is the most potent synthetic progestin currently available and it is wi dely used as a fertility regulating agent in a number of contraceptive form ulations because of its high effectiveness, safety and acceptability. The o bservation that contraceptive synthetic progestins exert hormone-like effec ts other than their progestational activities, prompted us to investigate w hether gestodene (GSD) administration may induce oestrogenic effects, even though the GSD molecule does not interact with intracellular oestrogen rece ptors (ER). To assess whether GSD may exert oestrogenic effects through some of its neu tral metabolites, a series of experimental studies were undertaken using GS D and three of its A-ring reduced metabolites. Receptor binding studies by displacement analysis confirmed that indeed GSD does not bind to the ER, wh ereas its 3 beta,5 alpha-tetrahydro reduced derivative (3 beta GSD) interac ts with a relative high affinity with the ER. The 3 alpha,5 alpha GSD isome r (3 alpha GSD) also binds to the ER, though to a lesser extent. The abilit y of the A-ring reduced GSD derivatives to induce oestrogenic actions was e valuated by the use of two different molecular bioassays: (a) transactivati on of a yeast system cotransfected with the human ER alpha (hER alpha) gene and oestrogen responsive elements fused to the beta-galactosidase reporter vector and (b) transactivation of the hER alpha-mediated transcription of the chloramphenicol acetyl transferase (CAT) reporter gene in a HeLa cells expression system. The oestrogenic potency of 3 beta GSD was also assessed by its capability to induce oestrogen-dependent progestin receptors (PR) in the anterior pituitary of castrated female rats. The results demonstrated that 3 beta GSD and 3 alpha GSD were able to activ ate, in a dose-dependent manner, the hER alpha-mediated transcription of bo th the beta-galactosidase and the CAT reporter genes in the yeast and HeLa cells expression systems respectively. In both assays the 3 beta derivative of GSD exhibited a significantly greater oestrogenic effect than its 3 alp ha isomer, while unchanged GSD and 5 alpha GSD were completely ineffective. Neither 3 beta GSD nor 3 alpha GSD exhibited oestrogen synergistic actions . Interestingly, the pure steroidal anti-oestrogen ICI-182,780 diminished t he transactivation induced by 3 beta GSD and 3 alpha GSD in the yeast expre ssion system. Furthermore, administration of 3 beta GSD resulted in a signi ficant increase of oestrogen-dependent PR in the anterior pituitaries of ca strated rats in comparison with vehicle-treated animals. The characteristic s of the 3 beta GSD-induced PR were identical to those induced by oestradio l benzoate. The overall results demonstrate that 3 beta GSD and its 3 alpha isomeric al cohol specifically bind to the ER and possess a weak intrinsic oestrogenic activity, whereas unmodified GSD does not. The data contribute to a better understanding of the GSD mechanism of action and allow the hypothesis to be advanced that the slight oestrogen-like effects attributable to GSD are me dia non-phenolic, tetrahydro reduced metabolites.