Problems in evaluating chronic toxicity of contraceptive steroids in dogs.

The long-term effects of oral contraceptive steroids including a combination of norethindrone and ethynylestradiol, a sequential regimen of dimethisterone and ethynylestradiol, and daily administration of megestrol acetate were studied in female beagle dogs at dose levels of 1, 10, or 25 times the projected human dose levels. The major findings included cystic endometrial hyperplasia and pyometra requiring hysterectomies and alopecia for the norethindrone-ethynylestradiol and dimethisterone-ethynylestradiol treated dogs. These groups did not have accentuated mammary development or treatment-related hyperplastic or neoplastic changes. For dogs given dimethisterone-ethynylestradiol, numerous acne-like lesions occurred in the skin of the mammary areas. Dogs given the higher dose levels of megestrol acetate had marked mammary stimulation, hyperplastic and neoplastic changes in the mammary glands, and clinical and pathologic changes typical of diabetes mellitus. Mammary changes of nodular hyperplasia, benign mixed tumor, and adenocarcinoma appeared as distinct entities although constant and intense mammary stimulation may be a common denominator. Such mammary changes have not been found in long-term studies in monkeys or rats with megestrol acetate, and the relevance of the canine mammary changes to projecting potential tumorigenesis in women is questioned.