Abstract 4603: Role of endoxifen as an antiestrogen: Effects on oncogene expression and polyamine pathway

Abstract

Abstract The nonsteroid antiestrogen tamoxifen [trans-1-(4-b-dimethylaminoethoxyphenyl)-1,2-diphenylbut-1-ene] is the most commonly used endocrine treatment for estrogen receptor α (ERα)-positive breast cancer in pre- and post-menopausal women, and it has helped to reduce breast cancer death rate by one third. Tamoxifen is extensively metabolized, and several metabolites have been detected in human serum. It is metabolized to 4-hydroxytamoxifen and N-desmethyltamoxifen by the action of CYP2D6 and CYP3A4/5 enzymes, respectively. N-desmethyltamoxifen and 4-hydroxy-tamoxifen are further converted to endoxifen by the action of these enzymes. Steady state levels of serum tamoxifen/metabolite are achieved within 4 weeks of continuous administration of the drug. The therapeutic efficacy of tamoxifen is determined by the distribution of the drug into tissues and the availability of the parent drug and its active metabolites in target tissues. Recent reports indicate that endoxifen might act as an antiestrogen. In order to examine the mechanism of action of endoxifen on an ERα-positive breast cancer cell line, MCF-7, we determined the effects of 100, 250, 500 and 1000 nM concentrations of this agent on cell growth, c-myc oncogene expression and activity of the polyamine biosynthetic enzymes. MCF-7 breast cancer cells were treated for 1, 3 or 5 days with endoxifen. Endoxifen had a growth inhibitory effect on MCF-7 cells by day 3 and 5 of treatment, as measured by cell number. Estradiol increased cell number by >2-fold compared to untreated control by day 5 of treatment, whereas endoxifen inhibited the estrogenic effect. c-myc gene expression was increased by a significant 2- to 3-fold by 4 nM estradiol in 2 and 4 hours of treatment, while 250 and 500 nM endoxifen suppressed the increase in c-myc gene expression, as measured by real-time qPCR experiment. In the absence of estradiol, endoxifen slightly (1.5 fold) increased c-myc gene expression. Endoxifen decreased ornithine decarboxylase (ODC) and S-adenosine methyl decarboxylase (AdoMetDC) activities in a concentration- and time-dependent manner; however, it had little effect on spermidine/spermine N1-acetyl transferase (SSAT) activity. The change in biosynthetic enzyme activities were reflected in polyamine pools, as decreased putrescine and spermidine levels. There were no changes in spermine level. As expected, treatment of the cells with 4 nM estradiol induced ODC and AdoMetDC activities, and slightly decreased SSAT activity. Addition of endoxifen in combination with 4 nM estradiol led to even more dramatic decrease in ODC and AdoMetDC activities and reduction of putrescine and spermidine levels. These results support our hypothesis that endoxifen can act as an antiestrogen and suppress cell growth stimulatory effects of estradiol in ERα-positive breast cancer cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4603.