The antiprogestin RU38 486: receptor-mediated progestin versus antiprogestin actions screened in estrogen-insensitive T47Dco human breast cancer cells.

Despite the theoretical promise of synthetic antiprogestational agents as anticancer agents, experimental tools, midcycle contraceptives, and implantation inhibitors, none has been available for either basic or clinical studies. However, a candidate antiprogestin, RU38 486 [17 beta-hydroxy-11 beta-(4-dimethylaminophenyl)17 alpha-(1-propynl)estra-4,9 -dien-3-one], has recently been described that has antiprogestational and antiglucocorticoid activities in early clinical trials. Its mechanisms of action are unclear. Furthermore, development of this drug underscores an old bioassay problem: that biological screening of progestins and antiprogestins is complex because of the physiological requirement that progestational effects must be superimposed upon an estrogenized system. This has made it difficult to distinguish among progestational, antiprogestational, and antiestrogenic properties of unknown agents. Here we describe the use of T47Dco human breast cancer cells to circumvent these problems. T47Dco cells are rich in progesterone receptors (PR), but are resistant to estrogens and antiestrogens. Their PR are estrogen-independent, and this permits progestins to be studied in an estrogen-free system. We have used these cells to assess the receptor-binding properties and the biological actions of RU38 486. Since RU38 486 absorbs UV at approximately 300 nm, this wavelength was used to covalently photolink the drug to PR in situ. Like the synthetic progestin R5020, low concentrations (10 nM) of [3H]RU38 486 bind two PR subunits in nuclei of T47Dco; glucocorticoid receptors are not bound. RU38 486 has a high affinity for PR in vitro (Kd approximately 2 nM at 0-4 C), and in intact cells, low concentrations (6-8 nM) transform more than 95% of PR to a high affinity nuclear binding state. In contrast to progesterone, the compound is not metabolized, so that it chronically (3-6 days) suppresses PR replenishment. These biochemical properties of RU38 486 are typical of synthetic progestins, but distinguish it from pure glucocorticoids. To bioassay RU38 486, we have measured growth and insulin receptors, since in T47Dco, physiological concentrations of progestins inhibit proliferation and increase the number of cell surface insulin-binding sites. Like progestins, RU38 486 is growth inhibitory; unlike progestins, it fails to stimulate insulin receptors and partially blocks their stimulation by R5020. Thus, RU38 486 has dual progestin agonist/antagonist actions depending on the biological response measured.