Abstract
Tamoxifen (TAM), used as the endocrine therapy of choice for breast cancer, undergoes metabolism primarily forming N-desmethyltamoxifen, 4-hydroxytamoxifen, α-hydroxytamoxifen, and tamoxifen-N-oxide (TNO). Our earlier studies demonstrated that flavin-containing monooxygenases (FMOs) catalyze the formation of TNO. The current study demonstrates that human FMO1 and FMO3 catalyze TAM N-oxidation to TNO and that cytochromes P450 (P450s), but not FMOs, reduce TNO to TAM. CYP1A1, CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 all reduced TNO, with CYP2A6, CYP1A1, and CYP3A4 producing the greatest reduction. A portion of TAM formed by CYP3A4-mediated reduction of TNO was further metabolized, but not TAM formed by the other P450s. TNO reduction by P450s is extremely rapid with considerable TAM formation detected at the earliest time point that products could be measured. TAM formation exhibited a lack of linearity with incubation time but increased linearly as a function of TNO and P450 concentration. TNO was converted into TAM by reduced hemoglobin (Hb) and NADPH-P450 oxidoreductase, suggesting involvement of the same heme-Fe2+ complex in both Hb and P450s. The findings raise the question of whether the reductive activity may be nonenzymatic. Results of this in vitro study demonstrate the potential of TAM and TNO to be interconverted metabolically. FMO seems to be the major enzymatic oxidant, whereas several P450 enzymes and even reduced hemoglobin are capable of reducing TNO back to TAM. The possibility that these processes may comprise a metabolic cycle in vivo is discussed in this article.
Footnotes
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This study was supported by National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health Grant ES00834. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS.
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Preliminary account of this investigation was presented at the Experimental Biology 2004 Meeting in Washington DC, 17-21 April, 2004 (Abstract 3429).
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.104.000802.
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ABBREVIATIONS: TAM, tamoxifen; P450, cytochrome P450; FMO, flavin-containing monooxygenase; Hb, hemoglobin; HLM, human liver microsome(s); MTM, methimazole; HPLC, high-pressure liquid chromatography; OR, NADPH-P450 oxidoreductase (P450 reductase); RLM, rat liver microsome(s); [3H]TAM, [N-methyl-3H]TAM; TAO, troleandomycin; TNO, tamoxifen N-oxide; TPE, triphenylethyleneamine; LC, liquid chromatography; MS, mass spectrometry; TLC, thin layer chromatography.
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↵† Deceased December 10, 2004.
- Received June 3, 2004.
- Accepted June 28, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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