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

Abstract

Gestodene (17α-ethynyl-13β-ethyl-17β-hydroxy-4,15-gonadien-3-one) is the most potent synthetic progestin currently available and it is widely used as a fertility regulating agent in a number of contraceptive formulations because of its high effectiveness, safety and acceptability. The observation that contraceptive synthetic progestins exert hormone-like effects other than their progestational activities, prompted us to investigate whether gestodene (GSD) administration may induce oestrogenic effects, even though the GSD molecule does not interact with intracellular oestrogen receptors (ER). To assess whether GSD may exert oestrogenic effects through some of its neutral metabolites, a series of experimental studies were undertaken using GSD and three of its A-ring reduced metabolites. Receptor binding studies by displacement analysis confirmed that indeed GSD does not bind to the ER, whereas its 3β,5α-tetrahydro reduced derivative (3βGSD) interacts with a relative high affinity with the ER. The 3α,5α GSD isomer (3αGSD) also binds to the ER, though to a lesser extent. The ability of the A-ring reduced GSD derivatives to induce oestrogenic actions was evaluated by the use of two different molecular bioassays: (a) transactivation of a yeast system cotransfected with the human ERα (hERα) gene and oestrogen responsive elements fused to the α-galactosidase reporter vector and (b) transactivation of the hERα-mediated transcription of the chloramphenicol acetyl transferase (CAT) reporter gene in a HeLa cells expression system. The oestrogenic potency of 3βGSD 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 3βGSD and 3αGSD were able to activate, in a dose-dependent manner, the hERαmediated transcription of both the β-galactosidase and the CAT reporter genes in the yeast and HeLa cells expression systems respectively. In both assays the 3β derivative of GSD exhibited a significantly greater oestrogenic effect than its 3α isomer, while unchanged GSD and 5αGSD were completely ineffective. Neither 3βGSD nor 3αGSD exhibited oestrogen synergistic actions. Interestingly, the pure steroidal anti-oestrogen ICI-182,780 diminished the transactivation induced by 3βGSD and 3αGSD in the yeast expression system. Furthermore, administration of 3βGSD resulted in a significant increase of oestrogendependent PR in the anterior pituitaries of castrated rats in comparison with vehicle-treated animals. The characteristics of the 3βGSD- induced PR were identical to those induced by oestradiol benzoate. The overall results demonstrate that 3βGSD and its 3α isomeric alcohol 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 oestrogenlike effects attributable to GSD are mediated by its non-phenolic, tetrahydro reduced metabolites.