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The Novel Progesterone Receptor Antagonists RTI 3021–012 and RTI 3021–022 Exhibit Complex Glucocorticoid Receptor Antagonist Activities: Implications for the Development of Dissociated Antiprogestins¹

Department of Pharmacology and Cancer Biology (B.L.W., G.P., P.G., D.P.M.), Duke University Medical Center, Durham, North Carolina 27710; Molecular Endocrinology Group (J.C.W., J.A.C.), NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709; Department of Biochemistry and Molecular Biology (M.B., W.V.V.), Louisiana State University Medical School, New Orleans, Louisiana 70112; Ligand Pharmaceuticals, Inc. (D.E.M.), San Diego, California 92121; Research Triangle Institute (C.E.C.), Chemistry and Life Sciences, Research Triangle Park, North Carolina 27709

Address all correspondence and requests for reprints to: D. P. McDonnell, Department of Pharmacology and Cancer Biology, Duke University Medical School, Box 3813, Durham, North Carolina 27710. E-mail: McDon016{at}acpub.duke.edu

We have identified two novel compounds (RTI 3021–012 and RTI 3021–022) that demonstrate similar affinities for human progesterone receptor (PR) and display equivalent antiprogestenic activity. As with most antiprogestins, such as RU486, RTI 3021–012, and RTI 3021–022 also bind to the glucocorticoid receptor (GR) with high affinity. Unexpectedly, when compared with RU486, the RTI antagonists manifest significantly less GR antagonist activity. This finding indicates that, with respect to antiglucocorticoid function, receptor binding affinity is not a good predictor of biological activity. We have determined that the lack of a clear correlation between the GR binding affinity of the RTI compounds and their antagonist activity reflects the unique manner in which they modulate GR signaling. Previously, we proposed a two step "active inhibition" model to explain steroid receptor antagonism: 1) competitive inhibition of agonist binding; and 2) competition of the antagonist bound receptor with that activated by agonists for DNA response elements within target gene promoters. Accordingly, we observed that RU486, RTI 3021–012, and RTI 3021–022, when assayed for PR antagonist activity, accomplished both of these steps. Thus, all three compounds are "active antagonists" 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, functioned 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 supporting the use of mechanism based approaches to identify nuclear receptor modulators. Of equal importance, RTI 3021–012 and RTI 3021–022 are two new antiprogestins 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.

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