A-ring reduced metabolites of 19-nor synthetic progestins as subtype selective agonists for ER alpha

It has previously been demonstrated that 19-nor contraceptive progestins un dergo in vivo and in vitro enzyme-mediated A-ring double bond hydrogenation . Bioconversion of 19-nor progestins to their corresponding tetrahydro deri vatives results in the loss of progestational activity and acquisition of e strogenic activities and binding to the ER. Herein, we report subtype-selec tive differences in ligand binding and transcriptional potency of nonphenol ic synthetic 19-nor derivatives between ER alpha and ER beta. In this study , we have examined both ER- and PR-mediated transcriptional activity of a n umber of A-ring chemically reduced derivatives of norethisterone and Gestod ene. Double bond hydrogenation decreased the transcriptional potency of nor ethisterone and Gestodene through both PR isoforms with a 100- to 1,000-fol d difference, respectively. In terms of the effects of norethisterone and G estodene and their corresponding 5 alpha -dihydro (5 alpha -norethisterone and 5 alpha -Gestodene), or 3 alpha ,5 alpha -tetrahydro or 3 beta ,5 alpha -tetrahydro derivatives (3 alpha ,5 alpha -norethisterone/3 alpha ,5 alpha -Gestodene and 3 beta ,5 alpha -norethisterone/3 beta ,5 alpha -Gestodene, respectively) on estrogen-mediated transcriptional regulation, the 3 beta ,5 alpha -tetrahydro derivatives of both norethisterone and Gestodene showe d the highest induction when HeLa cells were transiently transfected with a n expression vector for ER alpha. This activity could be inhibited with tam oxifen. These compounds did not activate gene transcription via ER beta, an d none of them showed antagonistic activities through either ER subtype. Th e 3 beta ,5 alpha -tetrahydro derivatives of both norethisterone and Gestod ene were active in other cells in addition to HeLa cells and activated repo rter expression through the oxytocin promoter. In summary, two ER alpha sel ective agonists have been identified. These compounds, with ER alpha vs. ER beta selective agonist activity, may be useful in evaluating the distinct role of these receptors as well as in providing useful insights into ER act ion.