RU 486 completely inhibits the action of progesterone on cell proliferation in the mouse uterus.

The antiprogestagen RU 486 completely inhibited the progesterone-induced switch in cell proliferation from the luminal and glandular epithelia to the stroma in response to oestradiol-17 beta. It also inhibited the progesterone-induced differentiation of the uterine epithelium. Since the proliferative switch of the uterus and the differentiation of the epithelium are prerequisites for implantation, these inhibitory actions may, in part, explain the ability of RU 486 to prevent implantation. Furthermore, it also suggests that the proliferative response to oestradiol in the presence of progesterone may be a sensitive assay for compounds with anti-progestational activity.