Controlled release low dose medroxyprogesterone acetate (MPA) inhibits the development of mammary tumors induced by dimethyl-benz(a) anthracene in the rat.

Medroxyprogesterone acetate (MPA) is well recognized to have beneficial effects for the treatment of advanced breast cancer which are comparable to those achieved with other forms of endocrine therapy. Using mammary tumors induced in the rat by dimethylbenz(a)anthracene (DMBA) as a model, we have studied the possibility that low dose MPA could prevent the development of these tumors. Single subcutaneous injection of Depo-Provera (crystalline suspension of MPA) or MPA encapsulated in biodegradable microspheres of 50:50 poly[DL-lactide-co-glycolide] was given 7 days before oral DMBA. While 63% of intact animals developed palpable mammary tumors within 85 days after DMBA administration, tumor incidence decreased to 28% and 23% in animals who had received 30 mg and 100 mg of Depo-Provera, respectively. The same amounts of MPA delivered in microspheres caused a further decrease in tumor incidence to respective values of 7% and 6%. Average tumor area, on the other hand, decreased from 4.89 cm2 in intact rats to about 0.75 (0.57-0.88) cm2 and approximately 0.20 (0.14-0.22) cm2 in the Depo-Provera and microsphere-treated groups, respectively. Using the 50:50 formulation of poly[DL-lactide-co-glycolide] designed to release MPA at a constant rate for a 4-month period, the serum MPA concentration at 3 months was measured at 4.99 +/- 0.43 ng/ml. Such data suggest that administration of a low dose controlled-release formulation of MPA in 50:50 poly[DL-lactide-co-glycolide] microspheres could well be an efficient and well tolerated approach for the prevention of breast cancer in women.