Figure 25.20 • Progesterone receptor antagonists and SPRM.

Mifepristone also causes a softening of the cervix, which aids in expulsion of the fertilized ovum. Mifepristone treatment is followed by the use of misoprostol, a prostaglandin E2 analog, to ensure a complete abortion. Mifepristone also has antagonist action at GR.

Another PR antagonist that is in development is CDB-2914. It is a norprogesterone derivative that has the same 11jS-aryl substituent as seen in mifepristone, although it has less GR antagonism than mifepristone.85 CDB-2914 is in phase III trials as an emergency contraceptive.

Compounds that have both agonist and antagonist properties at PRs, depending on the target tissue, have been called selective progesterone receptor modulators (SPRMs), which are analogous to the SERMs. Several are currently in development for various conditions, with asoprisnil being one that has been examined in phase III clinical trials for the treatment of uterine fibroids.86 Asoprisnil has antagonist properties in breasts and partial agonist actions in the uterus and vagina.87 Structurally, asoprisnil is a nortestosterone derivative with an 11j-aryl group bearing an oxime in the 4' position. o-Demethylation at C-17J produces a metabolite that also acts an SPRM.

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