5α-Reduction of norethisterone enhances its binding affinity for androgen receptors but diminishes its androgenic potency

Abstract Norethisterone (NET), a 19-nor synthetic progestin, undergoes enzyme-mediated 5α-reduction and exerts potent androgenic effects in target organs. To investigate its mode of androgenic action we examined, in a comparative manner, the in vitro metabolism of NET and testosterone (T), as well as the binding affinities to androgen receptors (AR) and the androgenic potency of NET, T, and their 5α-reduced derivatives. Bioconversion of [ 3 H]-NET and [ 3 H]-T was studied in rat prostate homogenates, AR binding affinity was assessed in rat ventral prostates using [ 3 H]-mibolerone as the radioligand, and the androgenic potency was evaluated by the increase of β-glucuronidase activity in the mouse kidney, and by the growth of accessory sex organs in castrated male rats. The results demonstrated that 5α-NET displayed a higher AR binding affinity but a significantly lower androgenic potency than unchanged NET. The bioconversion studies indicated that the metabolism of NET was similar to that of T, although to a lesser extent, thus ruling out the possibility that the synthetic progestin metabolizes rapidly into less active derivatives. To investigate the nature of the paradoxical effect of 5α-reduction upon the NET molecule, the interaction with AR and the androgenic potency of T, 19-nortestosterone (19norT), 17α-ethynyl testosterone (ET) and their 5α-reduced derivatives were examined. The results of AR binding studies revealed that 5α-reduction of T and ET significantly enhanced their affinities, and that the 5α-derivative of 19norT displayed a similar binding affinity to that exhibited by 19norT. In terms of biological activity, the results showed that 5α-reduction of T and 19norT significantly increased their androgenic potency, whereas 5α-reduction of ET resulted in a significant diminution of its androgenicity in a manner similar to that observed with the 5α-reduction of NET. When NET and 19norT were simultaneously administered with 5α-dihydrotestosterone they exhibited a potent synandrogenic activity, an effect that was cancelled by their 5α-reduction. Interestingly, ET displayed an antiandrogenic activity, an effect that was also suppressed by its 5α-reduction. The overall results demonstrated a distinctive, paradoxical effect of 5α-reduction upon the NET molecule, which was different from that seen in naturally occurring androgens, and which suggests that the presence of the 17α-ethynyl group plays a key role in this phenomenon. The data provided further evidence that the metabolism of synthetic contraceptive progestins modulates the expression of their hormone-like actions.