The novel progesterone receptor antagonists RTI 3021-012 and RTI 3021-022 exhibit complex glucocorticoid receptor antagonist activities: Implicationsfor the development of dissociated antiprogestins

We have identified two novel compounds (RTI 3021-012 and RTI 3021-022) that demonstrate similar affinities for human progesterone receptor (PR) and di splay equivalent antiprogestenic activity. As with most antiprogestins, suc h as RU486, RTI 3021-012, and RTI 3021-022 also bind to the glucocorticoid receptor (GR) with high affinity. Unexpectedly, when compared with RU486, t he RTI antagonists manifest significantly less GR antagonist activity. This finding indicates that, with respect to antiglucocorticoid function, recep tor binding affinity is not a good predictor of biological activity. We hav e determined that the lack of a clear correlation between the GR binding af finity of the RTI compounds and their antagonist activity reflects the uniq ue manner in which they modulate GR signaling. Previously, we proposed a tw o step "active inhibition" model to explain steroid receptor antagonism: 1) competitive inhibition of agonist binding; and 2) competition of the antag onist bound receptor with that activated by agonists for DNA response eleme nts within target gene promoters. Accordingly, we observed that RU486, RTI 3021-012, and RTI 302-1022, when assayed for PR antagonist activity, accomp lished both of these steps. Thus, all three compounds are "active antagonis ts" of PR function. When assayed on GR, however, RU486 alone functioned as an active antagonist. RTI 3021-012 and RTI 3021-022, on the other hand, fun ctioned solely as "competitive antagonists" since they were capable of high affinity GR binding, but the resulting ligand receptor complex was unable to bind DNA. These results have important pharmaceutical implications suppo rting the use of mechanism based approaches to identify nuclear receptor mo dulators. Of equal importance, RTI 3021-012 and RTI 3021-022 are two new an tiprogestins that may have clinical utility and are likely to be useful as research reagents with which to separate the effects of antiprogestins and antiglucocorticoids in physiological systems.