Long-term inhibitory effects of a novel anti-estrogen on the growth of ZR-75-1 and MCF-7 human breast cancer tumors in nude mice

The effects of the novel anti-estrogen EM-343 on the growth of 2 hormone-responsive human breast cancer tumors have been examined in athymic nude mice. At the low daily dose of 5 mu g, EM-343 administered subcutaneously for 6 months completely blocked the stimulatory effect of endogenous estrogens on the growth of ZR-75-I and MCF-7 tumors implanted in nude mice, In addition, uterine weight decreased by 60% while ovarian weight increased by 37%. Estrogen receptor (ER) levels measured by [H-3]-labeled estrogen binding were markedly reduced (by 96%, 96% and 92%) in ZR-75-I and MCF-7 tumors, and in the mouse uterus, respectively, Accompanying the decrease in ER, progesterone receptor levels were reduced by 79%, 87% and 76%, respectively, in the above-mentioned tissues following EM-343 treatment. Our data show the pure anti-estrogenic properties of EM-343 and its high potency as an inhibitor of growth of human ZR-75-I and MCF-7 breast tumors in nude mice. (C) 1997 Wiley-Liss, Inc.