Carboxylesterase 1 (CES1) genetic polymorphisms and oseltamivir activation.

Carboxylesterase 1 (CES1) is the major hepatic hydrolytic enzyme responsible for the metabolism of many therapeutic agents, toxins, and endogenous compounds existing as esters, amides, thioesters, and carbamates. CES1 is coded by the CES1 gene in humans, which consists of three isoforms: CES1A1, CES1A2, and CES1A3. Suzaki and associates recently reported the results of a healthy volunteer study investigating the effect of CES1 polymorphisms on the metabolism of the antiviral prodrug oseltamivir [1]. We read this report with great interest and would like the opportunity to comment on their study with the intention of bringing more clarity to their findings and to the field of CES1 pharmacogenetic research in general. First, the “CES1A1 variant” referred to in their report is actually the CES1A2 isoform of the CES1A1 gene. CES1A1 and CES1A2 genes differ only in five nucleotides in exon 1 and yield identical mature protein product [2, 3]. Additionally, the CES1A1 variant shares the same promoter structure as wild-type (WT) CES1A1 [4]. Thus, it is anticipated that the CES1A1 variant has no significant effect on CES1 function. Indeed, both CES1 expression and catalytic activity in human liver microsomes were comparable between the homozygous CES1A1 variant and WT CES1A1 [3]. Suzaki et al.’s report and a previous irinotecan pharmacokinetic (PK) study confirm that the CES1A1 variants exhibit no significant impact on the metabolism of substrates oseltamivir or irinotecan [1, 5]. Therefore, this CES1A1 variant (i.e., CES1A2 isoform) is not a viable pharmacogenetic biomarker for CES1 function. Significantly, a large number (>900) of CES1 variants have been identified and registered in the National Center for Biotechnical Information (NCBI) single nucleotide polymorphism (SNP) database, including 52 nonsynonymous variants and numerous synonymous and intronic variants (NCBI dbSNP, accessed on 10 June 2012). Both in vitro and clinical investigations demonstrate that specific CES1 variants can markedly impair CES1 function, altering both the PK and pharmacodynamics (PD) of medications that are substrates of CES1 [6–12]. Thus, in the development of protocols incorporating candidate variants to screen in formal pharmacogenetic study, much can be gleaned from existing CES1 pharmacogenetic literature. Second, functional CES1 gene number is associated with the genotypes of CES1A2/CES1A3. CES1A2 homozygotes carry four functional CES1 copies, with two of them from the CES1A1 gene. CES1A2 heterozygotes and CES1A3 homozygotes carry three and two functional gene copies, respectively, as CES1A3 is a pseudogene. However, the transcription efficiency of CES1A2 is extremely low (i.e., ∼2 % of the CES1A1 gene) due to significantly lower promoter activity of the CES1A2 gene [3]. Thus, the increase of functional gene number as a result of the presence of the CES1A2 gene is unlikely to lead to a significant increase in CES1 expression. Indeed, the Suzuki study did not reveal any significant differences in oseltamivir metabolism between individuals carrying three functional CES1 genes and those carrying four functional gene copies [1]. Sai and coworkers reported that CES1 activity, as determined by the area under the curve (AUC) ratio of (SN-38+SN-38 G) H.-J. Zhu (*) Department of Pharmacotherapy and Translational Research, University of Florida College of Pharmacy, 1600 SWArcher Road, RM PG-06, Gainesville, FL 32610-0486, USA e-mail: zhu@ufl.edu