Abstract 2668: The impact of CYP2C19 polymorphisms on plasma concentrations and metabolic ratios of tamoxifen and its metabolites in Asian breast cancer patients
Authors: Lim Joanne Siok-Liu , Ramasamy Saminathan , Koilan Subramaniyan , Singh Onkar , Chen Xiangai , Yap Yoon-Sim , Ng Raymond Chee-Hui , Sandanaraj Edwin , Chowbay Balram 
Published: August 2012

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&gt;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&gt;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&gt;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