The formation of estrogen-like tamoxifen metabolites and their influence on enzyme activity and gene expression of ADME genes

Abstract

Tamoxifen, a standard therapy for breast cancer, is metabolized to compounds with anti-estrogenic as well as estrogen-like action at the estrogen receptor. Little is known about the formation of estrogen-like metabolites and their biological impact. Thus, we characterized the estrogen-like metabolites tamoxifen bisphenol and metabolite E for their metabolic pathway and their influence on cytochrome P450 activity and ADME gene expression. The formation of tamoxifen bisphenol and metabolite E was studied in human liver microsomes and Supersomes™. Cellular metabolism and impact on CYP enzymes was analyzed in upcyte® hepatocytes. The influence of 5 µM of tamoxifen, anti-estrogenic and estrogen-like metabolites on CYP activity was measured by HPLC MS/MS and on ADME gene expression using RT-PCR analyses. Metabolite E was formed from tamoxifen by CYP2C19, 3A and 1A2 and from desmethyltamoxifen by CYP2D6, 1A2 and 3A. Tamoxifen bisphenol was mainly formed from (E)- and (Z)-metabolite E by CYP2B6 and CYP2C19, respectively. Regarding phase II metabolism, UGT2B7, 1A8 and 1A3 showed highest activity in glucuronidation of tamoxifen bisphenol and metabolite E. Anti-estrogenic metabolites (Z)-4-hydroxytamoxifen, (Z)-endoxifen and (Z)-norendoxifen inhibited the activity of CYP2C enzymes while tamoxifen bisphenol consistently induced CYPs similar to rifampicin and phenobarbital. On the transcript level, highest induction up to 5.6-fold was observed for CYP3A4 by tamoxifen, (Z)-4-hydroxytamoxifen, tamoxifen bisphenol and (E)-metabolite E. Estrogen-like tamoxifen metabolites are formed in CYP-dependent reactions and are further metabolized by glucuronidation. The induction of CYP activity by tamoxifen bisphenol and the inhibition of CYP2C enzymes by anti-estrogenic metabolites may lead to drug–drug-interactions.Electronic supplementary materialThe online version of this article (10.1007/s00204-017-2147-y) contains supplementary material, which is available to authorized users.

Factors Affecting Tamoxifen Metabolism in Patients With Breast Cancer: Preliminary Results of the French PHACS Study

“…Although there is not much evidence in the literature that ENDO could be metabolized by CYP3A4, it has been suggested that norendoxifen, another metabolite of TAM, could be formed via N‐demethylation of ENDO, which is a phase I reaction involving CYP isoenzymes . In addition, a recent in vitro study showed that an estrogen‐like metabolite of TAM (bisphenol tamoxifen) can be formed from ENDO in presence of CYP3A4 . More studies are needed to investigate the relative implication of CYP3A4 in ENDO metabolism compared with other enzymes.…”

“…23,24 In addition, a recent in vitro study showed that an estrogen-like metabolite of TAM (bisphenol tamoxifen) can be formed from ENDO in presence of CYP3A4. 25 More studies are needed to investigate the relative implication of CYP3A4 in ENDO metabolism compared with other enzymes.…”

Tamoxifen inhibits cell proliferation by impaired glucose metabolism in gallbladder cancer

“…23 Future studies should expand research efforts on comprehensive understandings of TAM functions in cancers. It is interpretable that TAM might have some metabolites with oestrogen-like action at the oestrogen receptor.…”

“…22,23 Since TAM could inhibit cell growth by inhibiting glycolysis, we It is quite reasonable because plenty of studies indicated low concentration TAM treatment show slight oestrogen-like activity in short time.…”

“…It is quite reasonable because plenty of studies indicated low concentration TAM treatment show slight oestrogen-like activity in short time. 22,23 Since TAM could inhibit cell growth by inhibiting glycolysis, we…”