. "B11. Primate Models for the Study of Antiprogestins in Reproductive Medicine." Clinical Applications of Mifepristone (RU486) and Other Antiprogestins: Assessing the Science and Recommending a Research Agenda. Washington, DC: The National Academies Press, 1993.
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Clinical Applications of Mifepristone (RU 486) and other Antiprogestins: Assessing the Science and Recommending a Research Agenda
FIGURE B11.2 Multiple actions of antiprogestins.
expressing progestin activity (Figure B11.2), albeit less than 1/1,000 that of levo-norgestrel, a synthetic progestin (Gravanis et al., 1985; Koering et al., 1986; Wolf et al., 1989a) (Figure B11.3).
In addition, certain antiprogestins are known to exert antiglucocorticoid activity upon the adrenocorticotropic hormone (ACTH)/adrenal cortex axis, therein elevating ACTH and cortisol secretion. The degree of this effect is both dose and compound specific, as well as being more pronounced during nighttime hours, when there are inherent diurnal rises in circulating ACTH and cortisol (Healy et al., 1983; Kettel et al., 1991; Chwalisz et al., 1992).
In the context of estrogen-induced mitogenesis, as it occurs in proliferative endometrium, antiprogestins can also be antiestrogens (Figure B11.4). However, since such antiproliferative actions do not arise through competitive binding of antiprogestins to the estrogen receptor (apparently postreceptor binding mechanisms intervene), the inhibition of tissue growth is called a noncompetitive antiestrogenic activity. This is very different from that achieved by tamoxifen or clomiphene (van Uem et al., 1989; Wolf et al., 1989b; Chwalisz et al., 1991). Moreover, this antiproliferative action of antiprogestins is not a manifestation of a progestin-like agonist activity. As revealed by the concentration of estrogen receptors in endometrial tissue, antiprogestins elevate the estrogen-receptor concentration by about sixfold (Neulen et al., 1990), yet paradoxically mitogenesis due to estrogen-induced growth is strik-