Contragestive Effects of the Antigestagenic Agent, RU486, in Rats and Rabbits

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Abstract
The contragestive effects of the antiprogesterone RU486 in pregnant rats and rabbits were investigated. Administrations of increasing doses of RU486 in intact or castrated pregnant rats induced abortion in a dose dependent manner. RU486 had less effect when administered on the 16th day of gestation. Antiestrogens (tamoxifen, clomiphene citrate), an inhibition of 3 beta-hydroxysteroid dehydrogenase (epostane) showed a synergistic effect when used in combination with RU486. A decrease in serum progesterone levels was observed after RU486 administration. A larger dose of RU486 was necessary to induce abortion in pregnant rabbits compared to rats. The maximum blood concentration was greater and the area under the plasma level curve was larger in rats than in rabbits following administration of RU486.