EM-652 (SCH 57068), a third generation SERM acting as pure antiestrogen inthe mammary gland and endometrium

Breast cancer is the most frequent cancer in women while it is the second c ause of cancer death. Estrogens are well recognized to play the predominant role in breast cancer development and growth and much efforts have been de voted to the blockade of estrogen formation and action. The most widely use d therapy of breast cancer which has shown benefits at all stages of the di sease is the use of the antiestrogen Tamoxifen. This compound, however, pos sesses mixed agonist and antagonist activity and major efforts have been de voted to the development of compounds having pure antiestrogenic activity i n the mammary gland and endometrium. Such a compound would avoid the proble m of stimulation of the endometrium and the risk of endometrial carcinoma. We have thus synthesized an orally active non-steroidal antiestrogen, EM-65 2 (SCH 57068) and the prodrug EM-800 (SCH57050) which are the most potent o f the known antiestrogens. EM-652 is the compound having the highest affini ty for the estrogen receptor, including estradiol. It has higher affinity f or the ER than ICI 182780, hydroxytamoxifen, raloxifene, droloxifene and hy droxytoremifene. EM-652 has the most potent inhibitory activity on both ER alpha! and ERP compared to any of the other antiestrogens tested. An import ant aspect of EM-652 is that it inhibits both the AF1 and AF2 functions of both ER alpha and ERP while the inhibitory action of hydroxytamoxifen is li mited to AF2, the ligand-dependent function of the estrogen receptors. AF1 activity is constitutive, ligrand-independent and is responsible for mediat ion of the activity of growth factors and of the ras oncogene and MAP-kinas e pathway. EM-652 inhibits Ras-induced transcriptional activity of ER alpha and ERP and blocks SRC-l-stimulated activity of the two receptors. EM-652 was also found to block the recruitment of SRC-I at AF1 of ERP, this ligand -independent activation of AF1 being closely related to phosphorylation of the steroid receptors by protein kinase. Most importantly, the antiestrogen hydroxytamoxifen has no inhibitory effect on the SRC-1-induced ER beta act ivity while the pure antiestrogen EM-652 completely abolishes this effect, thus strengthening the need to use pure antiestrogens in breast cancer ther apy in order to control all known aspects of ER-regulated gene expression. In fact, the absence of blockade of AF2 by hydroxytamoxifen could explain w hy the benefits of tamoxifen observed up to 5 years become negative at long er time intervals and why resistance develops to tamoxifen. EM-800, the pro drug of EM-652, has been shown to prevent the development of dimethylbenz(a )anthracene (DMBA)-induced mammary carcinoma in the rat, a well-recognized model of human breast cancer. It is of interest that the addition of dehydr oepiandrosterone, a precursor of androgens, to EM-800, led to complete inhi bition of tumor development in this model. Not only the development, but al so the growth of established DMBA-induced mammary carcinoma was inhibited b y treatment with EM-800. An inhibitory effect was also observed when medrox yprogesterone was added to treatment with EM-800. Uterine size was reduced to castration levels in the groups of animals treated with EM-800. An almos t complete disappearance of estrogen receptors was observed in the uterus, vaginum and tumors in nude mice treated with EM-800. EM-652 was the most po tent antiestrogen to inhibit the growth of human breast cancer ZR-75-1, MCF -7 and T-47D cells in vitro when compared with ICI 182780, ICI 164384, hydr oxytamoxifen, and droloxifene. Moreover EM-652 and EM-800 have no stimulatory effect on the basal levels o f cell proliferation in the absence of E2 while hydroxytamoxifen and drolox ifene had a stimulatory effect on the basal growth of T-47D and ZR-75-1 cel ls. EM-652 was also the most potent inhibitor of the percentage of cycling cancer cells. When human breast cancer ZR-75-1 xenografts were grown in nud e mice, EM-800 led to a complete inhibition of the stimulatory effect of es trogens in ovariectomized mice while tamoxifen was less potent and even sti mulated the growth of the tumors in the absence of estrogens, thus illustra ting the stimulatory effect of tamoxifen on breast cancer growth. When incu bated with human Ishikawa endometrial carcinoma cells, EM-800 had no stimul atory effect on alkaline phosphatase activity, an estrogen-sensitive parame ter. Raloxifene, droloxifene, hydroxytoremifene and hydroxytamoxifen, on th e other hand, all stimulated to various extent, the activity of this enzyme . The stimulatory effect of all four compounds was blocked by EM-800, thus confirming their estrogenic activity in human endometrial tissue. When admi nistered to ovariectomized animals, EM-800 prevents bone loss, the effect o n bone mineral density, trabecular bone volume, and trabecular separation b eing 5-10 times more potent than raloxifene. EM-800 lowers serum cholestero l and triglyceride levels in the rat as well as in women. In a Phase II stu dy performed in patients; with breast cancer showing failure on tamoxifen, 1 patient had a complete response while 5 patients had a partial response a nd stable disease for at least three months has been observed in an additio nal 13 patients for a total of 19 positive responses out of 43 evaluable pa tients (44.2%). No significant secondary effect related to the drug was obs erved. a phase 3 international clinical trial is currently being performed in tamoxifen failure patients where EM-800 (SCH 57050) is compared to Arimi dex. The detailed information obtained at the preclinical level with EM-652 or EM-800 indicates that these orally active compounds are highly potent a nd pure antiestrogens in the mammary gland and endometrium while they preve nt bone loss and lower serum cholesterol and triglyceride levels. Preclinic al and clinical data clearly suggest the interest of studying this compound in the neoadjuvant and adjuvant settings and, most importantly, for the pr evention of breast and uterine cancer in which settings they should provide additional benefits on bone and lipids. (C) 1999 Published by Elsevier Sci ence Ltd, All rights reserved.