Clinical implications of CYP2D6 genotypes predictive of tamoxifen pharmacokinetics in metastatic breast cancer.
OBJECTIVE
The CYP3A and CYP2D6 enzymes play a major role in converting tamoxifen to its active metabolites. CYP3A is a highly inducible enzyme, regulated mainly by pregnane X receptor (PXR). This study assessed the association between genetic polymorphisms of CYP2D6 and PXR, and tamoxifen pharmacokinetics (PK) and clinical outcomes in patients with breast cancer.


METHODS
Plasma concentrations of tamoxifen and its metabolites were measured. Common alleles of CYP2D6 and PXR were identified in 202 patients treated with tamoxifen 20 mg daily for more than 8 weeks. Twelve of the 202 patients and an additional nine patients with metastatic breast cancer receiving tamoxifen were assessed for clinical outcome in correlation with genotypes.


RESULTS
Patients carrying CYP2D6*10/*10 (n = 49) demonstrated significantly lower steady-state plasma concentrations of 4-hydroxy-N-desmethyltamoxifen and 4-hydroxytamoxifen than did those with other genotypes (n = 153; 4-hydroxy-N-desmethyltamoxifen: 7.9 v 18.9 ng/mL, P < .0001; 4-hydroxytamoxifen: 1.5 v 2.6 ng/mL, P < .0001), whereas no difference by PXR genotypes was found. CYP2D6*10/*10 was significantly more frequent among nonresponders with MBC (100% v 50%, P = .0186). In Cox proportional hazard analysis, CYP2D6 genotype and number of disease sites were significant factors affecting time to progression (TTP). The median TTP for patients receiving tamoxifen was shorter in those carrying CYP2D6*10/*10 than for others (5.0 v 21.8 months, P = .0032)


CONCLUSIONS
CYP2D6*10/*10 is associated with lower steady-state plasma concentrations of active tamoxifen metabolites, which could possibly influence the clinical outcome by tamoxifen in Asian breast cancer patients.
Lim
Hyeong-Seok
HS
Research Institute and Hospital, National Cancer Center, Madu1-dong, Ilsan-gu, Goyang-si, Gyeonggi-do, Republic of Korea.
Ju Lee
Han
H
Seok Lee
Keun
K
Sook Lee
Eun
E
Jang
In-Jin
IJ
Ro
Jungsil
J
D000328:Adult; D000368:Aged; D001859:Bone Neoplasms / Q000556:secondary; D001943:Breast Neoplasms / Q000097:blood / Q000188:drug therapy* / Q000235:genetics*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics*; D018572:Disease-Free Survival; D005260:Female; D005838:Genotype; D006801:Humans; D008113:Liver Neoplasms / Q000556:secondary; D008207:Lymphatic Metastasis; D008875:Middle Aged; D010865:Pilot Projects; D011110:Polymorphism, Genetic; D016016:Proportional Hazards Models; D012142:Respiratory Tract Neoplasms / Q000556:secondary; D013629:Tamoxifen / Q000493:pharmacokinetics* / Q000627:therapeutic use
D013629:Tamoxifen; D003577:Cytochrome P-450 Enzyme System; C509164:CYP2D7 protein, human