EM-652 exerts pure antiestrogenic activity in the mammary gland and endometrium, while
tamoxifen, the
antiestrogen most widely used for the treatment of
breast cancer, exerts mixed antiestrogenic-estrogenic activity in these tissues. Our objective was to compare the agonistic and antagonistic effects of
EM-652 with
tamoxifen and 5 other
antiestrogens on the growth of ZR-75-1 human breast xenografts in ovariectomized nude mice. During the 23 weeks of treatment at a daily oral dose of 50 microg,
EM-652 was the only compound that decreased
tumor size relative to pretreatment values, whereas the 6 other
antiestrogens only decreased to various extents the progression rate stimulated by
estrone. Under
estrone stimulation, all groups of animals had more than 60% of their
tumors in the progression category except for the EM-652-treated group, where only 7% of the
tumors progressed. In the absence of
estrone stimulation, progression was seen in 60%, 33%, 21% and 12% of
tumors in the
tamoxifen-,
idoxifene-,
toremifene- and
raloxifene-treated groups, respectively, while only 4% of
tumors progressed in the EM-652-treated group. The agonistic and antagonistic actions of each
antiestrogen were also measured on endometrial epithelial cell thickness. Our present findings indicate that
EM-652, in addition to being the most potent
antiestrogen on human
breast tumor growth, has no agonistic effect in breast and endometrial tissues. Since previous data have shown benefits of
EM-652 on bone density and
lipid profile, this compound could be an ideal candidate for
chemoprevention of breast and
uterine cancers, while protecting against
osteoporosis and
cardiovascular disease.