Antitumor effects of droloxifene, a new antiestrogen drug, against 7,12-dimethylbenz(a)anthracene-induced mammary tumors in rats.

The antitumor effects of droloxifene (DROL, (E)-alpha-[p-[2-(dimethylamino)ethoxy]-phenyl]-alpha-ethyl-3-stilbeno l), a new antiestrogen drug, on 7,12-dimethylbenz(a)anthracene (DMBA)-induced estrogen-dependent mammary tumors in rats were studied and compared with those of tamoxifen (TAM). Mammary tumors appeared from about 2 months after p.o. administration of DMBA to female rats, and all of them were estrogen receptor (ER) and progesterone receptor positive. DROL and TAM (p.o.) inhibited the growth of the tumors. Both drugs inhibited the binding of 125I-estradiol-17 beta to ER in the cytosol of the tumor in vitro, and the effect of DROL was much stronger than that of TAM. When 3H-estradiol-17 beta (3H-E2) was given s.c. to rats with the mammary tumors, 3H-E2 accumulated in the tumors, uteri and vaginae in which the ER levels are known to be high, but was low in the heart, in which the ER levels are normally low. DROL and TAM decreased the levels of 3H-E2 in the tumors, uteri and vaginae, but had no effect in the hearts. When DROL or TAM was given p.o. to rats daily for 7 consecutive days after administration of DMBA, they inhibited the induction of mammary tumors by DMBA. From these results, DROL inhibits the growth and the initiation of DMBA-induced mammary tumors by inhibiting the binding of estrogen to its receptor.