Extinction of experimental mammary cancer. I. Estradiol-17beta and progesterone.

Rats which had chemically induced (712-dimethylbenz(a)anthracene) b reast cancer were treated with estradiol-17-beta (20 mcg) plus progesterone and strong stimulation of the ovary with gonadotropin. Pregnancy and progesterone alone (4 mg) strongly accelerated growth of breast cancer and resulted in many foci of tumor. Estradiol-17-beta delayed the appearance of mammary cancer and the tumors were less widespread; however the incidence was 100%. Estradiol-17-beta plus progesterone resulted in a decreased incidence of breast cancer (52%). 20 mcg estradiol-17-beta was the dose that converted the 4 mg of progesterone from an accelerator of tumor to a suppressor. 1 unit of equine gonadotropin resulted in breast cancer suppression in 30% of the rats. Estradiol-17-beta plus progesterone produced extensive mammary growth and depressed ovarian function; however ovarian function returned within 6-9 days after cessation of the drugs.