Background: CYP2D6 is the rate limiting enzyme responsible for the metabolic activation of tamoxifen (tam) to endoxifen. Compared to CYP2D6 poor metabolizers (PM), tam-treated CYP2D6 extensive metabolizers (EM) have higher endoxifen concentrations, more vasomotor symptoms (Goetz, MP J Clin Oncol 2005), and are more likely to discontinue tam (Rae, JM 2009. Pharmacogenomics J). Additionally, higher endoxifen concentrations are associated with a stepwise increase in tam side-effects (Lorizio, W Breast Cancer Res Treat 2012). The data regarding CYP2D6 genotype and recurrence is mixed. Venlafaxine is a weak CYP2D6 inhibitor not known to alter tam pharmacokinetics (PK) and commonly recommended for tam induced hot flashes. We conducted a multicenter pharmacological study to determine whether venlafaxine altered the PK of tam and to determine the distribution of CYP2D6 genotypes in this population Methods: Women taking tam for at least 4 weeks and for whom venlafaxine was recommended for the treatment of hot flashes were eligible. Blood samples were collected prior to and 8–16 weeks following initiation of venlafaxine for steady state tam and metabolites. Genotyping was performed for alleles associated with no (PM; *3, *4, *5,*6); reduced (intermediate, IM; *10, 17 and *41); and ultra-rapid (UM; *1×2) metabolism. Power calculations demonstrated that 17 patients with paired samples were required (two-sided alpha=0.05 t-test, 90% power) to detect a 25% change in endoxifen levels after at least 8 weeks of concurrent treatment. Results: 30 women (median age 48.5) initiated venlafaxine. CYP2D6 genotypes were within Hardy Weinberg Equilibrium (HWE). CYP2D6 UM allele frequency (6.7%) was higher while CYP2D6 *4 (13.3%) was lower than expected compared to an unselected population (0.5 and 21% respectively; Sachse Am. J. Hum. Genet. 1997), resulting in the absence of CYP2D6 PM/PM. Mean (min/max) baseline endoxifen concentrations (8.73; 1.5–20.5 ng/ml) were correlated with CYP2D6 phenotype as follows: intermediate (EM/PM, PM/IM): 6.8 (1.5–11.2); extensive (EM/EM, EM/IM): 9.4 (1.5–20.5) and ultra-rapid (UM/EM: 11.0; 7.8–14) (r2 = 0.35 p = 0.05). In patients with paired samples (n = 20), venlafaxine resulted in a 23% decrease in endoxifen (−2.06 ng/ml; 95% CI −0.69 to −3.04; p = 0.004), but not tam, NDMT, or 4HT concentrations. Following initiation of venlafaxine, CYP2D6 genotype was no longer associated with endoxifen concentrations (r2 = 0.28 p = 0.23). For women with reduced CYP2D6 metabolism [EM/PM (n = 9) or PM/IM (n = 1)], venlafaxine lowered endoxifen concentrations (−2.98 ng/ml) to a level (5.41 ng/ml) reported to be associated with a higher risk of recurrence in adjuvant tam treated patients (Madlensky, L Clin Pharmacol Ther 2011). Conclusions: In this study, women with tam-induced vasomotor symptoms requiring venlafaxine were comprised predominantly of CYP2D6 EM and UM metabolizers. Venlafaxine significantly decreased endoxifen concentrations. Although the optimal concentration of endoxifen is unknown, given prior data linking low endoxifen concentrations with recurrence, venlafaxine should be used with caution in tam treated patients. (Supported by R01CA133049) Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD10-08.
Prior to this study there had been no studies directly comparing the relative effects of duloxetine, escitalopram or sertraline on the functional activity of cytochrome P450 enzyme, 2D6. In this study metoprolol was used as the model substrate drug for CYP2D6. Single‐dose pharmacokinetics of metoprolol were measured before and after 17 days of treatment with escitalopram 20 mg/day, duloxetine 60 mg/day or sertraline 100 mg/day in young healthy male and female volunteers (n=54). The outcome measures were changes in metoprolol peak plasma levels (Cmax), area under the plasma concentration‐time curve (AUC) and clearance. The results were tested using paired t‐tests and independent t‐tests. The addition of each drug produced statistically significantly changes in metoprolol pharmacokinetics. The rank order for the change in metoprolol AUC was duloxetine (180%) > escitalopram (89%) > sertraline (48% and 67%). Duloxetine, compared to sertraline, produced statistically significantly larger changes in metoprolol pharmacokinetic parameters. The differences between the changes produced by escitalopram and sertraline were not significant. In summary, each drug produced mild to moderate inhibition of CYP2D6 as reflected in a change in the pharmacokinetics of metoprolol. Clinical Pharmacology & Therapeutics (2005) 79, P52–P52; doi:
As the science of genomics has matured, it is increasingly seen as a tool used in clinical practice to design new treatments and to tailor current treatments more effectively.
Background: Tamoxifen is a selective estrogen receptor modulator that is the most commonly used and cost effective agent for hormone sensitive breast cancer with documented efficacy in prevention, treatment of preneoplasia, and treatment in both the adjuvant and metastatic settings. However, tamoxifen is a prodrug, and up to half of those taking it may not receive the full benefit because of genetic differences in CYP2D6 that affect metabolism to its active form, endoxifen. Tamoxifen is FDA approved for use at both 20mg/day and 40 mg/day, however by convention is dosed at 20mg. We aimed to determine if endoxifen levels can be manipulated by genotype-guided dosing of tamoxifen.Methods: 118 patients on tamoxifen ≥ 4 months and not on potent CYP2D6 inhibiting medications enrolled in Lineberger Comprehensive Cancer Center (LCCC) trial 0801. Genotyping was performed using the CYP450 Amplichip® (Roche Diagnostics) for 2D6 alleles: *1-11, *15, *17, *19, *20, *29, *35, *36, *40, *41, *1XN, *2XN, *4XN, *10XN, *17XN, *35XN and *41 XN. Tamoxifen dose was increased from 20mg to 40mg in patients with any intermediate or poor metabolizing (IM or PM) alleles [but not in patients homozygous for extensive metabolizing (EM) alleles]. Endoxifen levels were drawn at baseline and 4 months later. Assuming that endoxifen levels in IM pts are 40% lower than EM at baseline (Jin et al., 2005) and with a one-sided significance level of 0.025 and a sample size of 40 patients with intermediate metabolizing CYP2D6 genotypes, this study would have 84% power to detect a 40% increase in endoxifen.Results: Of the 118 patients, 25 withdrew or were removed from study, leaving 93 who have completed this study and who are evaluable for the primary analysis. Genotyping results were presented at 2009 ASCO meeting (Irvin et al.). For this analysis, 19 (20%) are African-American, 69 (74%) are non-Hispanic white, 2 are Hispanic, and 3 are Asian. For the 93 evauable patients, genotyping revealed 31 (33%) EM/EM, 1 EM/UM (ultra-rapid), 20 (22%) EM/IM, 19 (20%) EM/PM, 4 (4%) IM/IM, 9 (10%) IM/PM, 9 (10%) PM/PM and 1 unknown. The primary outcome, the tamoxifen metabolite levels, including endoxifen, will be available in July.(Supported by NC University Cancer Research Fund, NCI SPORE, Laboratory Corporation of America, Roche Diagnostics) Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 410.
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