Tamoxifen (TAM) is a prodrug with a complex metabolic pathway involving several metabolic enzymes. Although TAM is mainly metabolised by cytochrome P450 (CYP) 2D6 and CYP3A4/5 enzymes, other CYP enzymes such as CYP1A2, CYP2B6, CYP2C9, and CYP2C19, also catalysed the metabolism of TAM to N-desmethyltamoxifen (NDM), 4-hydroxytamoxifen (4OHT) and endoxifen (END) with 4OHT and END being the active metabolites of TAM. The aim of this study was to investigate the impact of CYP2C19 polymorphisms on the plasma concentrations and metabolic ratios of TAM and its three metabolites, NDM, 4OHT and END. The exons and intron-boundaries of CYP2C19 gene as well as the upstream and downstream regions (about 14 kilobases) were sequenced in 240 Asian healthy subjects (Chinese, Malay, Indians, N=80 each) to identify the polymorphisms present in Asian population. Linkage disequilibrium between identified polymorphisms was examined via Haploview (version 4.0) and tag-SNPs were identified via TAGGER. These tag-SNPs were subsequently genotyped in 164 Asian breast cancer patients using direct sequencing. Plasma levels of TAM and its metabolites were determined at steady state using HPLC with fluorescence detection. Genotypic-phenotypic associations were performed using non-parametric Kruskal-Wallis test and Mann-Whitney U-test. A moderate linkage pattern was observed across the CYP2C19 polymorphisms in the three Asian healthy populations. A total of 13 tag-SNPs and one reported functional SNP were analyzed in Asian breast cancer patients. Patients carrying the AC and CC genotypes of the exonic polymorphism 1251A>C (rs17886522) was associated with 2.3-fold reduction in the median (range) MREND-NDM compared to patients with the reference genotype [AA vs AC+CC: 5.14 (0.92 − 27.81) vs 2.25 (1.26 − 10.72), P = 0.026]. In contrast, median (range) MREND-4-OHT was found to be 1.3-fold higher in patients carrying one or two copies of the −3219T>G variant allele compared to patients carrying the two copies of wild-type allele [TT vs TG + GG: 6.57 (1.99 − 13.18) vs 8.76 (3.78 − 12.80), P = 0.001]. Modest increases in the plasma concentrations of TAM and NDM were associated with 1251A>C (rs17886522). In addition to CYP2D6 polymorphisms, polymorphic variants present in the 5α upstream region of CYP2C19 were found to influence the metabolic ratios of TAM and its metabolites but not the plasma concentrations of the analytes in this exploratory study. The combinative effect of genetic variants in various phase I pharmacogenes on plasma levels of tamoxifen warrants further study. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2668. doi:1538-7445.AM2012-2668

2588 Background: The aims were to characterize the genetic profiles of UGT1A4, UGT2B7 and UGT2B15in Asian healthy populations (Chinese, Malay, Indians, N=80 each) and investigate the effects of these SNPs on the disposition of TAM in Asian breast cancer patients (N=202). Methods: Healthy Asian subjects and Asian breast cancer patients were genotyped for UGT1A4, UGT2B7 and UGT2B15 SNPs. Plasma levels of tamoxifen and its metabolites in Asian patients were determined via LC-MS/MS analysis. Genotypic-phenotypic differences were examined using non-parametric tests. Haplotypic effects were examined using haplotype-specific generalized linear model. Results: Screening of the 5′ upstream, exonic, exonic-intronic junctions and 3′ downstream regions identified 61, 77 and 47 SNPs in UGT1A4, UGT2B7 and UGT2B15respectively. A total of 16, 14 and 20 tag-SNPs were identified in these genes respectively and genotyped in patients. Haplotypic analysis revealed associations between LD block 1 (-1548A>G, -1531C>T, -419G>A, -219C>T, -163G>A, 142T>G, 448T>C, 804G>A, IVS1+196T>C, IVS1+346T>G, IVS1+414A>G, IVS2+307A>G) and higher plasma Tam-N-Gluc level and MRTAM-N-Gluc/TAM after adjustment for significant covariates. The median (range) MRTAM-N-Gluc/TAM was 2.1- and 1.9-fold higher among patients carrying one and two copies of H2 (GCATAGCACTGG), respectively, compared to patients who carried two copies of H1 haplotype (ACGCGTTGTTAA) [H1/H1 vs H1/H2 vs H2/H2: 0.35 (0.11 - 2.09) vs 0.74 (0.19 - 3.60) vs 0.66 (0.52 - 1.66), P < 0.0001]. Similar association was observed between H2 and Tam-N-Gluc level [H1/H1 vs H1/H2 vs H2/H2: 0.75 (0.15 - 4.45) vs 1.29 (0.19 - 9.20) vs 1.66 (0.62 - 2.83) ng/ml, P= 0.004]. UGT2B15 LD block 3 (1568A>C, *168C>T, *186A>T, *+630C>G and *+888A>G) was associated with modest decrease and increase in (E)-END and MRZ-NDM-4-O-GLUC/E-END, respectively. UGT2B7haplotypes were not associated with O-glucuronidations of 4-OHT and END. Conclusions: This study highlights the involvement of UGT1A4 haplotypes in the variability in TAM N-glucuronidation while UGT2B7 and UGT2B15 variants played limited role in the variabilities of O-glucuronidations of 4-OHT and END in Asian breast cancer patients. Clinical trial information: 0822570P.