Effect of megestrol acetate or melengestrol acetate on preneoplastic and neoplastic mammary growth in mice.

Pellets of megestrol acetate and melengestrol acetate were implanted subcutaneously every one and two months, respectively, into 3-5 month old SHN virgin mice. Spontaneous mammary tumourigenesis was significantly enhanced by both steroids. The formation of preneoplastic mammary hyperplastic alveolar nodules (HAN) was, in contrast, inhibited by both progestins. As these results closely resemble the effects of progesterone and of medroxyprogesterone acetate on the development of mammary tumours both in the SHN mouse and in the woman, it seems likely that this animal model may prove valuable for the study of human breast cancer that is no longer responsive to progestins.