Estrogenic and progestagenic activities coexisting in steroidal drugs: quantitative evaluation by in vitro bioassays...

The progestin-specific stimulation of alkaline phosphatase (AP) activity in cells of the T47D human breast cancer line was applied to the development of a sensitive microtiter plate bioassay for the quantitative evaluation of progestagenic and antiprogestagenic potencies of natural and synthetic compounds. Some of the steroids tested (viz. progesterone, medroxyprogesterone acetate, norethynodrel) behaved as full-agonists, capable of inducing AP activities to the same maximal levels (equal efficacy), while others (norethindrone, gestrinone, R5020, norgestrel, Org OD 14 and its 4-ene metabolite) behaved as partial agonists, eliciting lower maximal effects. Efficacy, EC50 values (concentrations at which they induce one-half of the maximal response) and "slope factors" serve to characterize agonistic effects. Relative progestagenic potencies among the full-agonists were evaluated by comparing EC50 concentrations. Several 19-nor synthetic progestins (norethynodrel, norethindrone, Org OD 14 and its 4-ene isomer, dl-norgestrel, levo-norgestrel, RU2323), but none of the tested progestins with the pregnane structure, showed intrinsic estrogenic activity, as evaluated by using a similar in vitro bioassay based on a previously reported estrogen-specific induction of AP in human endometrial adenocarcinoma cells of the Ishikawa Var-1 line. Maximal estrogenic effects of all the tested progestins with dual activity were as high as those of estradiol. However, these compounds widely varied in their EC50 values for estrogenic activity. Consequently, the in vitro bioassays can reveal differences in the ratio of progestagenic and estrogenic activities intrinsic to these compounds. The reduced capability of the partial agonists to exert progestagenic or estrogenic effects on AP expression may reflect an impeded, receptor-mediated action, a mechanism that would also account for their inhibitory effects on the induction of AP activity by full agonists. Partial progestagenic agonists were able to reduce the efficacy of a full agonist to their own partial maximal activity.