No abstract

Purpose: To characterize pregnane X receptor (PXR) polymorphic variants in healthy Asian populations [Chinese, Malay and Indian (n = 100 each)], and to investigate the association between PXR haplotypes and hepatic mRNA expression of PXR and its downstream target genes, CYP3A4 and ABCB1, as well as their influence on the clearance of doxorubicin in Asian breast cancer patients. Experimental Design: PXR genotyping was done by direct DNA sequencing, and PXR haplotypes and haplotype clusters were derived by expectation-maximization algorithm. Genotype-phenotype correlations were done using Mann-Whitney U test and Kruskal-Wallis test. Results: Significant interethnic variations were observed in PXR pharmacogenetics among the threeAsian ethnic groups.The expression of PXR mRNA in liver tissues harboring the PXR*1B haplotype clusters was 4-fold lower compared with the non-PXR*1B (*1A + *1C) haplotype clusters [PXR*1B versus PXR*1A ; P = 0.015; PXR*1B versus PXR*1C ; P = 0.023]. PXR*1B-bearing liver tissues were associated with significantly lower expression of CYP3A4 (PXR*1B versus non-PXR*1B, P = 0.030) and ABCB1 (PXR*1B versus non-PXR*1B, P = 0.060) compared with non^PXR*1B-bearing liver tissues. Doxorubicin clearance in breast cancer patients harboring the PXR*1B haplotypes was significantly lower compared with patients carrying the non-PXR*1B haplotypes [PXR*1B versus non-PXR*1B, CL/BSA (L h Conclusions: This study showed that PXR*1B was associated with reduced hepatic mRNA expression of PXR and its downstream targets, CYP3A4 and ABCB1. Genotype-phenotype correlates in breast cancer patients showed PXR*1B to be significantly associated with lower doxorubicin clearance, suggesting that PXR haplotype constitution could be important in influencing interindividual and interethnic variations in disposition of its putative drug substrates.

Among the four novel SLC22A16 polymorphisms identified, the c.146A>G and c.1226T>C polymorphisms exhibited interethnic variations in allele and genotype frequencies. This exploratory study suggests that the c.146A>G variation could contribute to the variations in the pharmacokinetics of doxorubicin and doxorubicinol in Asian cancer patients. Further in vitro studies are required to determine the functional impact of these novel polymorphisms on doxorubicin pharmacokinetics in cancer patients.

The influence of three high frequency ABCB1 polymorphisms (c.1236C>T, c.2677G>A/T, and c.3435C>T) and the ABCG2 c.421C>A polymorphism on the disposition of doxorubicin in Asian breast cancer patients receiving adjuvant chemotherapy was investigated in the present study. The allelic frequency of the ABCB1 c.1236T, c.2677T, c.2677A, and c.3435T variants were 60%, 38%, 7%, and 22%, respectively, and the frequency of the ABCG2 c.421A allele was 23%. Pairwise analysis showed increased exposure levels to doxorubicin in patients harboring at least one ABCB1 c.1236T allele (P = 0.03). Patients homozygous for the CC-GG-CC genotype had significantly lower doxorubicin exposure levels compared to the patients who had CT-GT-CT (P = 0.02) and TT-TT-TT genotypes (P = 0.03). Significantly increased clearance of doxorubicin was also observed in patients harboring CC-GG-CC genotypes when compared to patients harboring the CT-GT-CT genotype (P = 0.01). Patients harboring the CC-GG-CC genotypes had significantly lower peak plasma concentrations of doxorubicinol compared to patients who had TT-TT-TT genotypes (P = 0.03). No significant influences on doxorubicin pharmacokinetic parameters were observed in relation to the ABCG2 c.421C>A polymorphism. In conclusion, the present exploratory study suggests that the three high frequency linked polymorphisms in the ABCB1 gene might be functionally important with regards to the altered pharmacokinetics of doxorubicin in Asian breast cancer patients, resulting in significantly increased exposure levels and reduced clearance. (Cancer Sci 2008; 99: 816-823)

The present study aimed to identify polymorphic genes encoding carbonyl reductases (CBR1, CBR3) and investigate their influence on doxorubicin disposition in Asian breast cancer patients (n = 62). Doxorubicin (60 mg/m 2 ) was administered every 3 weeks for four to six cycles and the pharmacokinetic parameters were estimated using non-compartmental analysis (WinNonlin). The Mann-Whitney U-test was used to assess genotypic-phenotypic correlations. (1) The phase I and II enzymes are expressed abundantly in hepatic tissues and display large interindividual variations in their metabolic capacities toward a wide array of therapeutic agents.The CBR are ubiquitously expressed monomeric NADPHdependent cytosolic enzymes that catalyze the reduction of chemically diverse substrates such as aldehydes, ketones, quinones, and other xenobiotics.(2,3) Apart from the metabolism of endogenous compounds and drug detoxification, CBR are also assumed to participate in cellular processes such as signal transduction, (4) apoptosis,mutagenesis,carcinogenesis,and drug resistance.(8) Four CBR isoforms (CBR1, CBR2, CBR3, and CBR4) that were initially assigned to the aldo-keto reductase (AKR) family are now classified under the family of short-chain dehydrogenases and represent one of the largest protein families identified to date.(9) CBR1 is the major carbonyl reductase and is expressed widely in different tissues. The CBR1 gene is mapped to chromosome 21q22.12, has three exons spanning 3.3 kb, and encodes a 30-kDa monomeric protein comprising 277 amino acids. The CBR1 gene lacks a CAAT and TATA box and contains a GC-rich island extending into the first exon, a structure characteristic of genes having a housekeeping function.(4) The identified substrates of human CBR1 include endogenous compounds (prostaglandins and steroids) and drugs such as loxoprofen, (4) metyrapone, (10) haloperidol, (11) bromoperidol,timirepone, (13) and doxorubicin. (14) CBR2 has low sequence identity with CBR1 and has not been identified in human tissues. The CBR3 gene contains three exons spanning a region of 11.2 kb and has a 72% sequence similarity with CBR1. It is located 62 kb telomeric to the CBR1 gene. Although widely expressed, the relative expression of CBR3 is much lower than CBR1 in most of the tissues analyzed.(15) The CBR4 gene is located on human chromosome 4 (4q32.3) and encodes a protein composed of 237 amino acids, but its enzymatic properties and tissue distribution remain unknown. (15) To date, there are limited studies investigating the influence of genetic polymorphisms in the CBR genes on the pharmacokinetics and pharmacodynamics of drugs. Avramopoulos et al. identified the CBR1 3′-untranslated region (UTR) G>A transition for the linkage mapping of the CBR gene on chromosome 21. (16) However, the effect of the polymorphism on enzyme activity and tissue expression is not known. Gonzalez-Covarrubias et al. reported the V88I (262G>A, rs1143663), L73L (312G>C, rs25678), A209A (720C>T, rs20572), and V231V (786G>A, rs2230192) polymorphisms by scree...