Received for publication June 4, 2004.
Revised June 28, 2005.
Accepted for publication June 28, 2005.

Oxidation of tamoxifen by human FMO1 and FMO3 to tamoxifen-N-oxide and its novel reduction back to tamoxifen by human cytochrome P450s and hemoglobin

Abstract

ABSTRACT Tamoxifen(TAM), used as the endocrine therapy of choice for breast cancer, undergoes metabolism forming primarily N-desmethyltamoxifen, 4-hydroxytamoxifen, -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 cytochrome P450s (CYPs), but not FMOs, reduce TNO to TAM. CYPs 1A1, 1A2, 2A6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4 all reduced TNO, with CYPs 2A6, 1A1, and 3A4 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 CYPs. TNO reduction by CYPs 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 CYP 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 CYPs. The findings raise the question of whether the reductive activity may be non-enzymatic. Results of this in vitro study demonstrate the potential of TAM and its N-oxide (TNO) to be interconverted metabolically. FMO appear to be the major enzymatic oxidants, while several CYP 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.

Key words: cytochrome b5, cytochrome P450 catalyzed oxidations, cytochrome P450 function, drug development, flavin-containing monooxygenase, hemoproteins, human CYP enzymes, NADPH cytochrome P450 reductase, P450 mechanism, prodrugs

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