Additive inhibitory effects of bromocryptine (CB-154) and medroxyprogesterone acetate (MPA) on dimethylbenz[a]anthracene (DMBA)-induced mammary tumors in the rat.

Treatment for 18 days of rats bearing dimethylbenz[a]anthracene-induced mammary tumors with the synthetic medroxyprogesterone acetate (MPA) or the inhibitor of prolactin secretion 2 alpha-bromocryptine (CB-154) inhibited total tumor area to 30 +/- 7% of the original volume. Combination of the two drugs, on the other hand, caused further inhibition to 10 +/- 5% of the pretreatment tumor area. The most striking effect of combination of the two drugs is a doubling of complete responses (no detectable tumor) from 30% when either drug was used alone to 60% in animals treated with the combination therapy. Both estradiol and progesterone receptors were further decreased when MPA was added to CB-154. The present data demonstrate that combination of the synthetic progestin MPA and the inhibitor of prolactin secrection CB-154 exerts maximal inhibitory effects on the growth of the DMBA-induced mammary tumor, the most widely used in vivo model of human breast cancer.