Effect of Tamoxifen on the Enzymatic Activity of Human Cytochrome CYP2B6

Abstract

Tamoxifen is primarily used in the treatment of breast cancer. It has been approved as a chemopreventive agent for individuals at high risk for this disease. Tamoxifen is metabolized to a number of different products by cytochrome P450 enzymes. The effect of tamoxifen on the enzymatic activity of bacterially expressed human cytochrome CYP2B6 in a reconstituted system has been investigated. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of purified CYP2B6 was inactivated by tamoxifen in a time-and concentration-dependent manner. Enzymatic activity was lost only in samples that were incubated with both tamoxifen and NADPH. The inactivation was characterized by a K I of 0.9 M, a k inact of 0.02 min Ϫ1, and a t 1/2 of 34 min. The loss in the 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity did not result in a similar percentage loss in the reduced carbon monoxide spectrum, suggesting that the heme moiety was not the major site of modification. The activity of CYP2B6 was not recovered after removal of free tamoxifen using spin column gel filtration. The loss in activity seemed to be due to a modification of the CYP2B6 and not reductase because adding fresh reductase back to the inactivated samples did not restore enzymatic activity. A reconstituted system containing purified CYP2B6, NADPH-reductase, and NADPH-generating system was found to catalyze tamoxifen metabolism to 4-OH-tamoxifen, 4Ј-OH-tamoxifen, and N-desmethyl-tamoxifen as analyzed by high-performance liquid chromatography analysis. Preliminary studies showed that tamoxifen had no effect on the activities of CYP1B1 and CYP3A4, whereas CYP2D6 and CYP2C9 exhibited a 25% loss in enzymatic activity.

CYP3A4 overexpression enhances apoptosis induced by anticancer agent imidazoacridinone C-1311, but does not change the metabolism of C-1311 in CHO cells

“…The latter strongly inhibited CYP3A4 while also being metabolized by it. However, there are few reports about the inhibition of specific P450 isoenzymes by antitumor agents, eg, by thiotepa [42,43] , tamoxifen [44,45] and the less well-known sorafenib [46] . Our results showed that cytochrome P450 CYP3A4 did not metabolize C-1311 in cells, confirming our hypothesis that overexpression of this enzyme might modulate the cellular response of CHO cells following C-1311 treatment, even though CYP3A4 does not participate in metabolic transformations.…”

Mechanism-Based Inactivation of Human Cytochrome P450 3A4 by Two Piperazine-Containing Compounds

“…SCH 66712 and EMTPP are structurally related with piperazine rings, substituted imidazoles, and fluorinated heteroaromatic rings. To our knowledge, there have been no reports of potent MBI of CYP2D6 and CYP3A4 by the same inactivator [though there are conflicting reports regarding inactivation by tamoxifen (Sridar et al, 2002;Zhao et al, 2002)]. Part of the lack of dual inactivators of both CYP3A4 and CYP2D6 is the difference in the presence of potential requisite nucleophile(s) in the active site.…”

CYP1B1 Is Not a Major Determinant of the Disposition of Aromatase Inhibitors in Epithelial Cells of Invasive Ductal Carcinoma

“…However, in another independent study, Sridar et al (2002) mentioned that tamoxifen had no effect on the activities of CYP1B1.…”

“…Although the inhibitory activities of AIs against human CYP1B1 have not been previously studied, the effect of tamoxifen, a nonestradiol antiestrogen and a chemopreventive agent widely used for breast cancer treatment, was tested against human CYP1B1 as a possible inhibitor. However, the results of these studies were inconsistent (Rochat et al, 2001;Sridar et al, 2002). Rochat et al (2001) have reported that tamoxifen reversibly inhibits CYP1B1 and is a noncompetitive inhibitor.…”