PT  - JOURNAL ARTICLE
AU  - Yan Zheng
AU  - Dongxiao Sun
AU  - Arun K. Sharma
AU  - Gang Chen
AU  - Shantu Amin
AU  - Philip Lazarus
TI  - Elimination of Antiestrogenic Effects of Active Tamoxifen Metabolites by Glucuronidation
AID  - 10.1124/dmd.107.016279
DP  - 2007 Oct 01
TA  - Drug Metabolism and Disposition
PG  - 1942--1948
VI  - 35
IP  - 10
4099  - http://dmd.aspetjournals.org/content/35/10/1942.short
4100  - http://dmd.aspetjournals.org/content/35/10/1942.full
SO  - Drug Metab Dispos2007 Oct 01; 35
AB  - 1-[4-(2-Dimethylaminoethoxy)-phenyl]-1,2-diphenylbut-1-(Z)-ene (tamoxifen, TAM) is a nonsteroidal antiestrogen that has been commonly used for the prevention and treatment of estrogen receptor-positive breast cancer. TAM is extensively metabolized into several primary active metabolites including 4-hydroxy-TAM (4-OH-TAM) and endoxifen. Glucuronidation is the major phase II metabolic pathway important in their excretion. Whereas high antiestrogenic activity has been reported for both 4-OH-TAM and endoxifen, studies examining the effect of glucuronide conjugation of these metabolites have not previously been performed. In the present study, the antiestrogenic activities of glucuronidated TAM metabolites were determined by examining their effect on the induction of the estrogen-responsive progesterone receptor (PGR) gene. 17β-Estradiol (E2)-mediated PGR gene expression in MCF-7 cells was determined by real-time reverse transcriptase-polymerase chain reaction for each TAM metabolite isomer. E2 (1 × 10-10 M) induction of PGR mRNA was 6-fold after a 12-h incubation; only unconjugated TAM metabolites inhibited this effect. A virtually identical dose-dependent inhibition of E2-induced PGR gene expression was found for both the trans- and cis-isomers of 4-OH-TAM and endoxifen, with maximal inhibition attained at 1 × 10-6 M of TAM metabolite. The glucuronide conjugates of all 4-OH-TAM and endoxifen isomers exhibited no effect on E2-mediated induction of PGR expression at all concentrations of TAM metabolite examined in this study. These data indicate that isomers of both 4-OH-TAM and endoxifen exhibit roughly equipotent antiestrogenic effects on E2-induced gene expression and that glucuronide conjugates of the same metabolites effectively negate this activity. This result may have important implications in terms of both whole-body and target tissue-specific glucuronidation pathways and individual responses to TAM therapy and cancer prevention. The American Society for Pharmacology and Experimental Therapeutics