Sequential hormone therapy with medroxyprogesterone acetate for 7, 12-dimethylbenz [alpha] anthracene-induced rat mammary tumors.

Two types of sequential hormone therapy with medroxyprogesterone acetate (MPA) were examined in female 93 Sprague--Dawley (SD) rats with 7,12-dimethylbenz [alpha] anthracene (DMBA)-induced mammary tumors. Estradiol (E2) priming induced progesterone receptors (PgR) in most cases and, after induction of PgR, MPA often showed an augmented antitumor effect. For this reason, E2 priming + MPA had a more marked antitumor effect on DMBA tumors than MPA alone. Tamoxifen (TAM) (0.1 mg/kg) priming induced PgR more frequently than TAM (0.4 mg/kg) priming. Furthermore, treatment with TAM priming (0.1 mg/kg) + MPA showed a more marked antitumor effect than with TAM priming (0.4 mg/kg) + MPA or MPA alone. In the above two priming therapies with MPA, the latter is more practical for the clinical treatment of breast cancer than the former because the mechanism of action of E2 is usually thought to show tumorigenic activity on breast cancer, while the mechanism of action of TAM is thought to show an antitumor effect. It is suggested that the sequence of administration of MPA after TAM priming may be favorable for the treatment of breast cancer.