Progesterone, 5-pregnene-3,20-dione, and desoxycorticosterone were found to be partial but significant inhibitors of estrone-induced growth of the uterus of hypophysectomized rats but they did not depress the amount of growth elicited by testosterone.

The presence of both the 2-carbon side chain at position 17 and an oxygenated function at position 3 is necessary for the inhibitory activity of progesterone. The introduction of a hydroxyl group in the progesterone molecule at positions 6 (ß), 11 (α), 16 (α) or 17 (α) destroyed its growth-inhibitory activity.

11ß-Hydroxyprogesterone is weaker than progesterone in the inhibition of estrone-induced uterine growth; unlike progesterone it has the capacity to depress growth elicited by testosterone.

The introduction of a flourine atom in the 9 (α) position resulted in a striking increase of the growth-inhibitory activity of 11ß-hydroxyprogesterone and hydrocortisone.

9α-Flouro-11ß-hydroxyprogesterone is a partial but significant inhibitor of growth of the uterus in hypophysectomized rats injected simultaneously with large doses of estrone or testosterone. It is more powerful than progesterone in transforming the vaginal epithelium to mucus cells in the presence of estrone. Administered alone, the only detectable growth was a slight increase in size of the preputial glands.

9α-Flouro-11ß-hydroxyprogesterone suppresses in part the growth of the adrenals, ovaries, and uterus in intact rats at a dose level which does not cause loss of body weight or inhibit body growth. Many rats treated with this compound in small amount had extensive mucification of the vagina resembling that of late pregnancy.

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