Comprehensive assessment of cytochromes P450 and transporter genetics with endoxifen concentration during tamoxifen treatment.

OBJECTIVES: Tamoxifen bioactivation to endoxifen is mediated primarily by CYP2D6; however, considerable variability remains unexplained. Our aim was to perform a comprehensive assessment of the effect of genetic variation in tamoxifen-relevant enzymes and transporters on steady-state endoxifen concentrations. PATIENTS AND METHODS: Comprehensive genotyping of CYP enzymes and transporters was performed using the iPLEX ADME PGx Pro Panel in 302 tamoxifen-treated breast cancer patients. Predicted activity phenotype for 19 enzymes and transporters were analyzed for univariate association with endoxifen concentration, and then adjusted for CYP2D6 and clinical covariates. RESULTS: In univariate analysis, higher activity of CYP2C8 (regression ß=0.22, P=0.020) and CYP2C9 (ß=0.20, P=0.04), lower body weight (ß=-0.014, P<0.0001), and endoxifen measurement during winter (each ß<-0.39, P=0.002) were associated with higher endoxifen concentrations. After adjustment for the CYP2D6 diplotype, weight, and season, CYP2C9 remained significantly associated with higher concentrations (P=0.02), but only increased the overall model R by 1.3%. CONCLUSION: Our results further support a minor contribution of CYP2C9 genetic variability toward steady-state endoxifen concentrations. Integration of clinician and genetic variables into individualized tamoxifen dosing algorithms would marginally improve their accuracy and potentially enhance tamoxifen treatment outcomes.

In vivo assessment of the metabolic activity of CYP2D6 diplotypes and alleles.

AIMS: A prospectively enrolled patient cohort was used to assess whether the prediction of CYP2D6 phenotype activity from genotype data could be improved by reclassification of diplotypes or alleles. METHODS: Three hundred and fifty-five patients receiving tamoxifen 20 mg were genotyped for CYP2D6 and tamoxifen metabolite concentrations were measured. The endoxifen : N-desmethly-tamoxifen metabolic ratio, as a surrogate of CYP2D6 activity, was compared across four diplotypes (EM/IM, EM/PM, IM/IM, IM/PM) that are typically collapsed into an intermediate metabolizer (IM) phenotype. The relative metabolic activity of each allele type (UM, EM, IM, and PM) and each EM and IM allele was estimated for comparison with the activity scores typically assigned, 2, 1, 0.5 and 0, respectively. RESULTS: Each of the four IM diplotypes have distinct CYP2D6 activity from each other and from the EM and PM phenotype groups (each P < 0.05). Setting the activity of an EM allele at 1.0, the relative activities of a UM, IM and PM allele were 0.85, 0.67 and 0.52, respectively. The activity of the EM alleles were statistically different (P < 0.0001), with the CYP2D6*2 allele (scaled activity = 0.63) closer in activity to an IM than an EM allele. The activity of the IM alleles were also statistically different (P = 0.014). CONCLUSION: The current systems for translating CYP2D6 genotype into phenotype are not optimally calibrated, particularly in regards to IM diplotypes and the *2 allele. Additional research is needed to improve the prediction of CYP2D6 activity from genetic data for individualized dosing of CYP2D6 dependent drugs.

Comparative evaluation of patients newly initiating first-generation versus second-generation tyrosine kinase inhibitors for chronic myeloid leukemia and medication adherence, health services utilization, and healthcare costs.

BACKGROUND: Chronic myeloid leukemia (CML) treatment guidelines recommend first-line therapy with either first- or second-generation tyrosine kinase inhibitors (1GTKI, 2GTKI), but do not specify which generation should be used first. OBJECTIVE: To examine the association between initiation of 2GTKI versus 1GTKI and medication adherence, health services utilization, and healthcare costs. METHOD: This was a retrospective cohort study utilizing administrative claims data from a single health plan within the US of commercial and Medicare patients newly initiating 1GTKI or 2GTKI therapy for CML between June 2010 and December 2011. Multivariate logistic regression was used to investigate the association between TKI therapy and adherence, defined as proportion of days covered ≥0.85. Multivariate logistic regression and generalized linear models examined the association between TKI and health services utilization and direct healthcare costs (plan and patient paid) during the 12 month follow-up period. RESULTS: Among the 368 patients included, there was no difference in adherence between patients initiating a 2GTKI compared to a 1GTKI (odds ratio = 0.88, 95% confidence interval [CI] 0.55-1.40). Initiating a 2GTKI was associated with increased outpatient visits (incidence rate ratio [IRR] = 1.12, 95% CI 1.06-1.20); however, there were no statistically significant differences in emergency room visits or inpatient visits between the treatment groups. Total costs were 1.3 times higher for 2GTKI initiators versus 1GTKI initiators ($86,509 versus $66,443; p = 0.001), with a significant difference in TKI pharmacy costs. CONCLUSIONS: Although there were no differences in adherence, hospitalizations, or emergency room visits among patients initiating a second- versus first-generation TKI, total all-cause costs and outpatient visits were higher for 2GTKI initiators. With the impending release of generic imatinib, these comparative data will become germane in the selection of a first-line TKI therapy. Because this study used claims from a single health plan, it may not be generalizable to the general population.