The influence of CYP2B6, CYP2C9 and CYP2D6 genotypes on the formation of the potent antioestrogen Z‐4‐hydroxy‐tamoxifen in human liver

Abstract

Aims To investigate in a large panel of 50 human liver samples the contribution of CYP2C9, CYP2D6, and CYP3A4 to the overall formation of the potent antioestrogen Z-4-hydroxy-tamoxifen, and how various genotypes affect its formation from tamoxifen.Methods The formation of Z-4-hydroxy-tamoxifen from 10 mM tamoxifen was studied in human liver microsomes (n=50), characterized for CYP2B6, CYP2C9, CYP2D6 and CYP3A4 expression, and CYP2B6, CYP2C9 and CYP2D6 genotype. The effect of chemical and monoclonal antibody inhibitors, and the formation in supersomes expressing recombinant CYP isoforms was also investigated. Z-4-hydroxy-tamoxifen was quantified using LC-MS analysis. Results Z-4-hydroxy-tamoxifen was formed by supersomes expressing CYP2B6, CYP2C9, CYP2C19 and CYP2D6, but not CYP3A4. In agreement with these data, the mean formation of Z-4-hydroxy-tamoxifen was inhibited 49% by sulphaphenazole (P=0.001), 38% by quinidine (P<0.05) and 13% by monoclonal antibody against CYP2B6 (MAB-2B6, P<0.05). Furthermore, Z-4-hydroxytamoxifen formation significantly correlated with both CYP2C9 expression (r s =0.256, P<0.05) and CYP2D6 expression (r s =0.309, P<0.05). Genotypes of CYP2D6, CYP2B6 and CYP2C9 had an effect on metabolite formation in such a way that samples with two nonfunctional CYP2D6, or two variant CYP2C9 or CYP2B6 alleles, showed lower enzyme activity compared with those with two functional or wild-type alleles, (5.0 vs 9.9 pmol mg x1 protein min x1 , P=0.046, 5.1 vs 9.9 pmol mg x1 protein min x1 , P=0.053, and 6.8 vs 9.4 pmol mg x1 protein min x1 , P=0.054, respectively). CYP2D6 and CYP2C9 contribute on average 45 and 46%, respectively, to the overall formation of Z-4-hydroxy-tamoxifen. Conclusions CYP2B6, CYP2C9 and CYP2D6 genotypes all affected Z-4-hydroxytamoxifen formation and can predict individual ability to catalyse this reaction.

New insights into the metabolism of tamoxifen and its role in the treatment and prevention of breast cancer

“…The SSRIs are twice as effective as the "placebo" effect at reducing menopausal symptoms in randomized clinical trials [138][139][140], so there is naturally an increased usage of SSRIs with long-term tamoxifen treatment to maintain compliance. Unfortunately, the metabolism of tamoxifen to hydroxylated metabolites [141][142][143] and the metabolism of SSRIs [39,[144][145][146][147] both occur via the CYP2D6 gene product. Indeed Stearns and coworkers [97] showed that the SSRI inhibitor paroxetine reduced the levels of endoxifen during adjuvant tamoxifen therapy and endoxifen levels decrease by 64% in women with wild type CYP2D6 enzyme.…”

Influence of CYP2C9 genotypes on the formation of a hepatotoxic metabolite of valproic acid in human liver microsomes

“…Individual human liver microsome samples were also obtained from a tissue bank, as detailed elsewhere 37 and the CYP2C9 genotyping of these samples was determined as described previously. 38 The distribution of the CYP2C9 genotypes in our panel of 39 individual human liver microsome samples is listed in Table 2.…”

Bias Analysis to Guide New Data Collection

“…While tamoxifen-treated women with 2 The International Journal of Biostatistics, Vol. 8 [2012] (Stearns et al, 2003;Coller et al, 2002;Gjerde et al, 2008;Jin et al, 2005), it is also possible that women with two functional CYP2D6*4 alleles may carry other non-functional mutations that reduce the concentration of these metabolites. From a genetic epidemiology perspective, the problem involves non-differential misclassification of metabolizer phenotype because the classification system relies on only one of the alleles with functional consequences.…”

“…Accurate markers of tamoxifen resistant tumors would allow personalization of combined therapies (Riggins et al, 2007), with the goal of preventing these recurrences. Tamoxifen is metabolized mostly by the gene product of CYP2D6 (Stearns et al, 2003;Coller et al, 2002) to secondary metabolites that bind the estrogen receptor 100-fold more readily than tamoxifen itself (Desta et al, 2004), so they are the most important inhibitors of tumor cell growth (Lim et al, 2005). In the past five years, it has been suggested that ER+ breast cancer patients with nonfunctional alleles of CYP2D6 may be poor candidates for adjuvant tamoxifen therapy (Desta and Flockhart, 2007;Jordan, 2007).…”