Estrogen inhibition of MPA‐Induced mouse mammary tumor transplants

Estrogen compounds were used to treat mice bearing syngeneic transplants of medroxyprogesterone acetate(MPA)‐induced BALB/c mammary adenocarcinomas. Both MPA‐dependent and MPA‐independent tumor lines were used. These lines expressed estrogen (ER) and progesterone receptors (PR). We demonstrate that different doses of estradiol benzoate (EB) and 17‐β‐estradiol (E2) inhibit tumor growth and induce tumor regression in both MPA‐independent and ‐dependent tumors, even in the presence of MPA or progesterone (P). EB was unable to induce regression of (ER‐) hormone‐independent tumor lines. A few MPA‐dependent tumors became resistant to the estrogenic treatment; in subsequent passages some of these tumors retained their MPA‐responsiveness, although estrogen sensitivity was not recovered.