Limited predictive value of achieving beneficial plasma (Z)-endoxifen threshold level by CYP2D6 genotyping in tamoxifen-treated Polish women with breast cancer

Abstract

Background: Tamoxifen, the most frequently used drug for treating estrogen receptor-positive breast cancer, must be converted into active metabolites to exert its therapeutic efficacy, mainly through CYP2D6 enzymes. The objective of this study was to investigate the impact of CYP2D6 polymorphisms on (Z)-endoxifen-directed tamoxifen metabolism and to assess the usefulness of CYP2D6 genotyping for identifying patients who are likely to have insufficient (Z)-endoxifen concentrations to benefit from standard therapy.

Methods: Blood samples from 279 Polish women with breast cancer receiving tamoxifen 20 mg daily were analyzed for CYP2D6 genotype and drug metabolite concentration. Steady-state plasma levels of tamoxifen and its 14 metabolites were measured by using the ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method.

Results: In nearly 60 % of patients, including over 30 % of patients with fully functional CYP2D6, (Z)-endoxifen concentration was below the predefined threshold of therapeutic efficacy. The most frequently observed CYP2D6 genotype was EM/PM (34.8 %), among which 83.5 % of patients had a combination of wild-type and *4 alleles. Plasma concentration of five metabolites was significantly correlated with CYP2D6 genotype. For the first time, we identified an association between decreased (E/Z)-4-OH-N-desmethyl-tamoxifen-β-D-glucuronide levels (r (2) = 0.23; p < 10(-16)) and increased CYP2D6 functional impairment. The strongest correlation was observed for (Z)-endoxifen, whose concentration was significantly lower in groups of patients carrying at least one CYP2D6 null allele, compared with EM/EM patients. The CYP2D6 genotype accounted for plasma level variability of (Z)-endoxifen by 27 % (p < 10(-16)) and for the variability of metabolic ratio indicating (Z)-endoxifen-directed metabolism of tamoxifen by 51 % (p < 10(-43)).

Conclusions: The majority of breast cancer patients in Poland may not achieve a therapeutic level of (Z)-endoxifen upon receiving a standard dose of tamoxifen. This finding emphasizes the limited value of CYP2D6 genotyping in routine clinical practice for identifying patients who might not benefit from the therapy. In its place, direct monitoring of plasma steady-state (Z)-endoxifen concentration should be performed to personalize and optimize the treatment.

Figures

Fig. 1

Significant association of the CYP2D6 genotype with plasma concentration and molecular ratio of tamoxifen metabolites. a (Z)-endoxifen, b The MR of (Z)-endoxifen/sum of the remaining measured compounds, c (Z)-4-OH-tamoxifen, d MR of (Z)-4-OH-tamoxifen/sum of the remaining compounds, eN-desmethyl-tamoxifen, f 4′-OH-N-desmethyl-tamoxifen, g (E/Z)-4-OH-N-desmethyl-tamoxifen-β-D-glucuronide. The number of patients (N) was as follow: EM/UM (18), EM/EM (87), EM/IM (43), EM/PM (97), IM/IM (3), IM/PM (11), PM/PM (20). The horizontal line indicates the median plasma concentration, the box covers 25th-75th percentiles and the maximum length of each whisker is 1.5× the interquartile range; dots outside the whiskers are outliers. Linear model (ordinary least squares) was fitted independently in six functional groups with EM/EM as a reference to examine whether there was an association between the measured metabolite concentration and CYP2D6 phenotype. Student’s t statistics was used to test if fitted coefficients were different than 0. The Bonferroni corrected p-value of less than 6.9 × 10−4 was considered significant. Significant associations are marked with asterisk (*)

Fig. 2

The steady-state concentration of plasma (Z)-endoxifen. The mean concentration of the metabolite in each CYP2D6 genotype group and the predefine threshold level of 6 ng/ml are indicated

Fig. 3

The comparison between plasma steady-state concentration of (Z)-endoxifen level and corresponding metabolic ratio. The metabolic ratio (MR) was estimated as (Z)-endoxifen plasma concentration divided by the sum of concentrations of the remaining measured compounds. The MR value of 0.0146 was delineated as the correlation coefficient with the level of 6 ng/ml. The assigned CYP2D6 functional category, based on CYP2D6 genotype, is indicated in color

Fig. 4

The (Z)-endoxifen-directed metabolism of tamoxifen according to the association of its metabolites concentration with CYP2D6 genotype. The intensity of gray shading corresponds with the number of CYP2D6 genotype predicted functional groups with a significant (p < 6.9 × 10−4) association with metabolite plasma concentration. White indicates no association and black indicates association with three genotypes, as indicated by linear modeling (see Fig. 1)