CYP3A4*22 is related to increased plasma levels of 4-hydroxytamoxifen and partially compensates for reduced CYP2D6 activation of tamoxifen.

AIM: To evaluate the impact of CYP3A4*22 in the formation of endoxifen (EDF) and hydroxytamoxifen (HTF), under different CYP2D6 genotypic backgrounds. MATERIALS & METHODS: 178 patients were enrolled in the study. CYP2D6 and CYP3A4 genotyping and tamoxifen (TAM) and metabolites quantification were performed. RESULTS: EDF concentrations were lower in poor (2.77 ng ml(-1)) and CYP2D6 intermediate metabolizers (5.84 ng ml(-1)), comparing to functional group (EM-F) (10.67 ng ml(-1), p < 0.001). HTF and TAM levels were respectively 47 and 53% higher in CYP3A4*22 carriers compared with *1/*1 patients in the whole group. Patients with impaired CYP2D6 metabolism and carriers of CYP3A4*22 had EDF levels comparable to CYP2D6 EM-F group (9.06 and 10.67 ng ml(-1), p = 0.247). CONCLUSION: The presence of CYP3A4*22 might compensate the reduction of EDF concentrations related to CYP2D6 inactivity, especially due to increased HTF concentrations.

Development, validation and clinical application of a HPLC-FL method for CYP2D6 phenotyping in South Brazilian breast cancer patients.

OBJECTIVE: To develop and validate a method for determination of dextromethorphan (DMT) and dextrorphan (DTP) in plasma samples using HPLC-FL and to apply it to CYP2D6 phenotyping of a population from the South of Brazil. METHODS: Samples were prepared by hydrolysis and liquid-liquid extraction. Analysis was conducted in a reversed phase column, with isocratic elution and fluorescence detection. One hundred and forty patients being treated with tamoxifen were given 30 mg of dextromethorphan and their CYP2D6 phenotypes were determined on the basis of [DMT]/[DTP] metabolic ratios in plasma samples collected after 3h. RESULTS: Total chromatography running time was 12 min. Precision (CV%) was below 9.7% and accuracy was between 92.1 and 106.9%. The lower limits of quantification were 1 ng mL(-1) for DMT and 10 ng mL(-1) for DTP. Mean extraction yield of analytes was 86.6%. Mean age of patients was 55.7 years. Phenotype frequencies were as follows: 7.1% poor metabolizers, 13.6% intermediate metabolizers, 77.1% extensive metabolizers and 2.1 ultra-rapid metabolizers. Metabolic ratios for patients on strong (n=11) and weak (n=16) CYP2D6 activity inhibitors were different from each other and also different from ratios for patients not taking enzyme inhibitors (n=113). CONCLUSIONS: A sensitive method for determination of dextromethorphan and its metabolite in plasma samples was developed and successfully applied, providing evidence of the impact that CYP2D6 inhibitors have on the enzyme's metabolic capacity.