Pharmacogenetics of human carboxylesterase 2, an enzyme involved in the activation of irinotecan into SN-38

Carboxylesterase isoform 2 mRNA expression in peripheral blood mononuclear cells is a predictive marker of the irinotecan to SN38 activation step in colorectal cancer patients

PURPOSE

Irinotecan (CPT11) is a prodrug activated in humans mainly by carboxylesterase 2 (CES2) generating the SN38 metabolite responsible for the drug efficacy and toxicity. The interpatients variability in CPT11 activation step could cause unpredictable toxicity. To identify a predictive molecular marker for CPT11 activation in cancer patients, we investigated the CES2 mRNA expression in peripheral blood mononuclear cells (PBMC) and correlated it to CPT11 activation rate, toxic effects, and response.

EXPERIMENTAL DESIGN

Forty-five colorectal cancer patients were treated with a CPT11-including regimen (FOLFIRI). CES2 mRNA expression in PBMC was quantified by reverse transcription-PCR in real time. Plasma concentrations of CPT11, SN38, and SN38-glucuronide were determined by high-performance liquid chromatography and the pharmacokinetic variables calculated adopting the noncompartmental model. Toxicity was evaluated by the National Cancer Institute Common Toxicity Criteria scale and response by the WHO criteria.

RESULTS

A high interindividual variability in CES2 mRNA relative expression was observed (median, 1.45; range, 0.01-28.21). CES2 mRNA expression level was significantly associated with CPT11 activation ratio [(AUC(SN38) + AUC(SN38G))/AUC(CPT11)]. Patients with CES2 mRNA expression above the median cutoff value presented an activation ratio higher (median, 0.25; range, 0.15-0.42) than those with CES2 mRNA below the median (median, 0.20; range, 0.10-0.40; P = 0.013). This was associated with a nonsignificant trend of 1.34-fold increase of SN38 AUC in the group of patients with high CES2 mRNA expression (mean, 1.03 +/- 0.62 versus 0.77 +/- 0.32 micromol/L hour). Eight of 23 high CES2 mRNA-expressing patients (34.8%) developed grade 3 to 4 neutropenia or diarrhea compared with 2 of 22 (9.1%) in the low CES2-expressing group (P = 0.071).

CONCLUSION

Our data support a predictive power of CES2 mRNA expression in PBMC for the activation rate of CPT11.

Characterization of pyrethroid hydrolysis by the human liver carboxylesterases hCE-1 and hCE-2

Carboxylesterases hydrolyze a large array of endogenous and exogenous ester-containing compounds, including pyrethroid insecticides. Herein, we report the specific activities and kinetic parameters of human carboxylesterase (hCE)-1 and hCE-2 using authentic pyrethroids and pyrethroid-like, fluorescent surrogates. Both hCE-1 and hCE-2 hydrolyzed type I and II pyrethroids with strong stereoselectivity. For example, the trans-isomers of permethrin and cypermethrin were hydrolyzed much faster than corresponding cis-counterparts by both enzymes. Kinetic values of hCE-1 and hCE-2 were determined using cypermethrin and 11 stereoisomers of the pyrethroid-like, fluorescent surrogates. K(m) values for the authentic pyrethroids and fluorescent surrogates were in general lower than those for other ester-containing substrates of hCEs. The pyrethroid-like, fluorescent surrogates were hydrolyzed at rates similar to the authentic pyrethroids by both enzymes, suggesting the potential of these compounds as tools for high throughput screening of esterases that hydrolyze pyrethroids.

Pharmacogenetics of irinotecan metabolism and transport: an update

The anticancer agent irinotecan (CPT-11) is converted to SN-38, which is approximately 100 to 1,000-fold more cytotoxic than the parent drug. The pharmacokinetics of irinotecan are extremely complex and have been the subject of intensive investigation in recent years. Irinotecan is subject to extensive metabolism by various polymorphic enzymes, including CES2 to form SN-38, members of the UGT1A subfamily, and CYP3A4 and CYP3A5, which form several pharmacologically inactive oxidation products. Elimination of irinotecan is also dependent on drug-transporting proteins, notably ABCB1 (P-glycoprotein), ABCC2 (cMOAT) and ABCG2 (BCRP), present on the bile canalicular membrane. The various processes mediating drug elimination, either through metabolic breakdown or excretion, likely impact substantially on interindividual variability in drug handling. This report provides an update on current strategies to individualize irinotecan chemotherapy based on each patient's genetic constitution, which may ultimately lead to more selective use of this agent.

Predicting drug response and toxicity based on gene polymorphisms

The sequencing of the human genome has allowed the identification of thousands of gene polymorphisms, most often single nucleotide polymorphims (SNP), which may play an important role in the expression level and activity of the corresponding proteins. When these polymorphisms occur at the level of drug metabolising enzymes or transporters, the disposition of the drug may be altered and, consequently, its efficacy may be compromised or its toxicity enhanced. Polymorphisms can also occur at the level of proteins directly involved in drug action, either when the protein is the target of the drug or when the protein is involved in the repair of drug-induced lesions. There again, these polymorphisms may lead to alterations in drug efficacy and/or toxicity. The identification of functional polymorphisms in patients undergoing chemotherapy may help the clinician prescribe the optimal drug combination or schedule and predict with more accuracy the response to these prescriptions. We have recorded in this review the polymorphisms that have been identified up till now in genes involved in anticancer drug activity. Some of them appear especially important in predicting drug toxicity and should be determined in routine before drug administration; this is the case of the most common variations of thiopurine methyltransferase for 6-mercaptopurine and of dihydropyrimidine dehydrogenase for fluorouracil. Other appear determinant for drug response, such as the common SNPs found in glutathione S-transferase P1 or xereoderma pigmentosum group D enzyme for the activity of oxaliplatin. However, confusion factors may exist between the role of gene polymorphisms in cancer risk or overall prognosis and their role in drug response.

Pharmacogenetics in Gastrointestinal Tumors

Treatment of gastrointestinal cancers has significantly advanced over the last few years with the introduction of effective chemotherapeutic and targeted drugs. To provide individual treatment with low toxicity on the one hand but maximum benefit on the other hand is still an unsolved problem. Interindividual variation of drug toxicity and efficacy is determined by genetic polymorphisms. The genetic approach based on single-gene (pharmacogenetics) or multi-gene (pharmacogenomics) analyses is developing as a valuable tool to design tailored therapy. This review focuses on clinically significant polymorphisms in genes involved in the metabolism of the major players in the chemotherapy of gastrointestinal cancer: fluoropyrimidines, irinotecan and platinum. In addition, first results of pharmacogenetics in targeted therapy including cetuximab and bevacizumab are discussed. The pharmacogenetic approach to individualize therapy in gastrointestinal cancers is promising, but additional larger and controlled studies are needed to justify changes of treatment strategies.

Genetic diversity and new therapeutic concepts

The differences in medicinal drug responses among individuals had been known for quite some time. Some patients exhibit a life-threatening adverse reaction while others fail to show an expected therapeutic effect. Intermediate responses between the above two extreme cases are also known. In fact, it has been recently reported that approximately 100,000 deaths and more than 2 million hospitalizations annually in the United States are due to properly prescribed medications. This interindividual variability could be due in part to genetically determined characteristics of target genes or drug metabolizing enzymes. This has now been substantiated by a variety of studies. We know that "one size fits all" is not correct. Therefore, the application of pharmacogenetic concepts to clinical practice is an excellent goal in the postgenomic era. The successful completion of the human genome project provided necessary molecular tools, such as high-throughput SNP genotyping, HapMap, and microarray, that can be applied to develop proper therapeutic options for individuals. Recently, there have been considerable scientific, corporate, and policy interest in pharmacotherapy. However, identification of causal variations in a target gene is only a starting point, and the progress in this rapidly developing field is slower than expected. One major drawback could be due to the multigene determinant of drug response that requires a genome-wide screening. Additionally, application of pharmacogenetic knowledge into clinical practice requires a high level of accuracy, precision (risk/benefit ratio), and strict regulations. This is because the pharmacogenetic approach raises several ethical, moral, and legal questions. It is also necessary that both health professionals and the general public must be urgently educated. Despite these limitations, translation of pharmacogenomic data into clinical practice would certainly provide better opportunities to increase the safety and efficacy of medicine in the future.