Thymidylate synthase pharmacogenetics in colorectal cancer

Novel mass spectrometry-based assay for thymidylate synthase activity

Thymidylate synthase (TS) is an enzyme responsible for the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), with the co-substrate 5,10-methylenetetrahydrofolate (5,10-CH2-THF) as the methyl donor. TS is the only enzyme capable of de novo biosynthesis of dTMP in humans, a nucleotide crucial for DNA synthesis and therefore cell proliferation and survival. As such, TS is a major drug target in chemotherapy by compounds such as 5-fluorouracil. Due to the clinical and physiological importance of TS, the ability to accurately assay its activity is crucial. Several assays have been developed for this purpose, relying on spectrophotometry or radioisotope labeling methods. In this study, we have developed a liquid chromatography - mass spectrometry-based method for assessing TS activity by direct and specific measurement of the reaction product, dTMP. The assay was tested on mouse liver homogenates. We noted that excessive 5,10-CH2-THF concentration (400 µM) led to substrate inhibition and therefore 200 µM was used. The activity assayed at 1 µM dUMP was linear with protein content and time (up to 60 min) and was 0.56 ± 0.12 pmol/mg protein/min, in line with previously reported values. Additionally, by using a high mass resolution Orbitrap instrument side reactions were monitored, revealing major changes in folate pools and nucleotide metabolism. These findings highlight the value of the developed TS assay for routine TS activity monitoring in complex matrixes such as clinical material.

Clinical outcomes of chemotherapy in cancer patients with different ethnicities

BACKGROUND

Choosing the most effective chemotherapeutic agent with safest side effect profile is a common challenge in cancer treatment. Although there are standardized chemotherapy protocols in place, protocol changes made after extensive clinical trials demonstrate significant improvement in the efficacy and tolerability of certain drugs. The pharmacokinetics, pharmacodynamics, and tolerance of anti-cancer medications are all highly individualized. A driving force behind these differences lies within a person's genetic makeup.

RECENT FINDINGS

Pharmacogenomics, the study of how an individual's genes impact the processing and action of a drug, can optimize drug responsiveness and reduce toxicities by creating a customized medication regimen. However, these differences are rarely considered in the initial determination of standardized chemotherapeutic protocols and treatment algorithms. Because pharmacoethnicity is influenced by both genetic and nongenetic variables, clinical data highlighting disparities in the frequency of polymorphisms between different ethnicities is steadily growing.  Recent data suggests that ethnic variations in the expression of allelic variants may result in different pharmacokinetic properties of the anti-cancer medication. In this article, the clinical outcomes of various chemotherapy classes in patients of different ethnicities were reviewed.

CONCLUSION

Genetic and nongenetic variables contribute to the interindividual variability in response to chemotherapeutic drugs. Considering pharmacoethnicity in the initial determination of standard chemotherapeutic protocols and treatment algorithms can lead to better clinical outcomes of patients of different ethnicities.

Regulations of expressions of rat/human sulfotransferases by anticancer drug, nolatrexed, and micronutrients

Cancer is related to the cellular proliferative state. Increase in cell-cycle regulatory function augments cellular folate pool. This pathway is therapeutically targeted. A number of drugs influences this metabolism, that is, folic acid, folinic acid, nolatrexed, and methotrexate. Our previous study showed methotrexate influences on rat/human sulfotransferases. Present study explains the effect of nolatrexed (widely used in different cancers) and some micronutrients on the expressions of rat/human sulfotransferases. Female Sprague-Dawley rats were treated with nolatrexed (01-100 mg/kg) and rats of both sexes were treated to folic acid (100, 200, or 400 mg/kg) for 2-weeks and their aryl sulfotransferase-IV (AST-IV; β-napthol sulfation) and sulfotransferase (STa; DHEA sulfation) activities, protein expression (western blot) and mRNA expression (RT-PCR) were tested. In human-cultured hepatocarcinoma (HepG2) cells nolatrexed (1 nM-1.2 mM) or folinic acid (10 nM-10 μM) were applied for 10 days. Folic acid (0-10 μM) was treated to HepG2 cells. PPST (phenol catalyzing), MPST (dopamine and monoamine), DHEAST (dehydroepiandrosterone and DHEA), and EST (estradiol sulfating) protein expressions (western-blot) were tested in HepG2 cells. Present results suggest that nolatrexed significantly increased sulfotransferases expressions in rat (protein, STa, F = 4.87, P < 0.05/mRNA, AST-IV, F = 6.702, P < 0.014; Student's t test, P < 0.01-0.05) and HepG2 cells. Folic acid increased sulfotransferases activity/protein in gender-dependant manner. Both folic and folinic acid increased several human sulfotransferases isoforms with varied level of significance (least or no increase at highest dose) in HepG2 cells pointing its dose-dependent multiphasic responses. The clinical importance of this study may be furthered in the verification of sulfation metabolism of several exogenous/endogenous molecules, drug-drug interaction and their influences on cancer pathophysiological processes. Further studies are necessary.

Molecular determinants of response to 5-fluorouracil-based chemotherapy in colorectal cancer: The undisputable role of micro-ribonucleic acids

5-flurouracil (5-FU)-based chemotherapy is the main pharmacological therapy for advanced colorectal cancer (CRC). Despite significant progress in the treatment of CRC during the last decades, 5-FU drug resistance remains the most important cause of failure in CRC therapy. Resistance to 5-FU is a complex and multistep process. Different mechanisms including microsatellite instability, increased expression level of key enzyme thymidylate synthase and its polymorphism, increased level of 5-FU-activating enzymes and mutation of TP53 are proposed as the main determinants of resistance to 5-FU in CRC cells. Recently, micro-ribonucleic acids (miRNA) and their alterations were found to have a crucial role in 5-FU resistance. In this regard, the miRNA-mediated mechanisms of 5-FU drug resistance reside among the new fields of pharmacogenetics of CRC drug response that has not been completely discovered. Identification of the biological markers that are related to response to 5-FU-based chemotherapy is an emerging field of precision medicine. This approach will have an important role in defining those patients who are most likely to benefit from 5-FU-based chemotherapy in the future. Thereby, the identification of 5-FU drug resistance mechanisms is an essential step to predict and eventually overcome resistance. In the present comprehensive review, we will summarize the latest knowledge regarding the molecular determinants of response to 5-FU-based chemotherapy in CRC by emphasizing the role of miRNAs.

Current and New Predictors for Treatment Response in Metastatic Colorectal Cancer. The Role of Circulating miRNAs as Biomarkers

Colorectal cancer (CRC) is the third most frequently diagnosed cancer in the world. More than half of all CRC patients will eventually develop metastases and require treatment accordingly, but few validated predictive factors for response to systemic treatments exist. In order to ascertain which patients benefit from specific treatments, there is a strong need for new and reliable biomarkers. We conducted a comprehensive search using the PUBMED database, up to December 2019, in order to identify relevant studies on predictive biomarkers for treatment response in metastatic CRC. We will herein present the currently used and potential biomarkers for treatment response and bring up-to-date knowledge on the role of circulating microRNAs, associated with chemotherapy and targeted therapy regimens used in metastatic CRC treatment. Molecular, tumor-related, disease-related, clinical, and laboratory predictive markers for treatment response were identified, mostly proposed, with few validated. Several circulating microRNAs have already proven their role of prediction for treatment response in CRC, but future clinical studies are needed to confirm their role as biomarkers across large cohorts of patients.

Emerging targets in cancer drug resistance

Drug resistance is a complex phenomenon that frequently develops as a failure to chemotherapy during cancer treatment. Malignant cells increasingly generate resistance to various chemotherapeutic drugs through distinct mechanisms and pathways. Understanding the molecular mechanisms involved in drug resistance remains an important area of research for identification of precise targets and drug discovery to improve therapeutic outcomes. This review highlights the role of some recent emerging targets and pathways which play critical role in driving drug resistance.

Thymidylate synthase genotype-directed chemotherapy for patients with gastric and gastroesophageal junction cancers

BACKGROUND

Retrospective studies indicate associations between TSER (thymidylate synthase enhancer region) genotypes and clinical outcomes in patients receiving 5-FU based chemotherapy, but well-controlled prospective validation has been lacking.

METHODS

In this phase II study (NCT00515216 registered through ClinicalTrials.gov, http://clinicaltrials.gov/show/NCT00515216), patients with "good risk" TSER genotypes (at least one TSER*2 allele) were treated with FOLFOX chemotherapy to determine whether prospective patient selection can improve overall response rates (ORR) in patients with gastric and gastroesophageal junction (GEJ) cancers, compared with historical outcomes in unselected patients (estimated 43%).

RESULTS

The ORR in genotype-selected patients was 39.1% (9 partial responses out of 23 evaluable patients, 95% CI, 22.2 to 59.2), not achieving the primary objective of improving ORR. An encouraging disease control rate (DCR, consisting of partial responses and stable diseases) of 95.7% was noted and patients with homozygous TSER*2 genotype showed better tumor response.

CONCLUSIONS

In this first prospective, multi-institutional study in patients with gastric or GEJ cancers, selecting patients with at least one TSER*2 allele did not improve the ORR but led to an encouraging DCR. Further studies are needed to investigate the utility of selecting patients homozygous for the TSER*2 allele and additional genomic markers in improving clinical outcomes for patients with gastric and GEJ cancers.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00515216.

Apoptosis and molecular targeting therapy in cancer

Apoptosis is the programmed cell death which maintains the healthy survival/death balance in metazoan cells. Defect in apoptosis can cause cancer or autoimmunity, while enhanced apoptosis may cause degenerative diseases. The apoptotic signals contribute into safeguarding the genomic integrity while defective apoptosis may promote carcinogenesis. The apoptotic signals are complicated and they are regulated at several levels. The signals of carcinogenesis modulate the central control points of the apoptotic pathways, including inhibitor of apoptosis (IAP) proteins and FLICE-inhibitory protein (c-FLIP). The tumor cells may use some of several molecular mechanisms to suppress apoptosis and acquire resistance to apoptotic agents, for example, by the expression of antiapoptotic proteins such as Bcl-2 or by the downregulation or mutation of proapoptotic proteins such as BAX. In this review, we provide the main regulatory molecules that govern the main basic mechanisms, extrinsic and intrinsic, of apoptosis in normal cells. We discuss how carcinogenesis could be developed via defective apoptotic pathways or their convergence. We listed some molecules which could be targeted to stimulate apoptosis in different cancers. Together, we briefly discuss the development of some promising cancer treatment strategies which target apoptotic inhibitors including Bcl-2 family proteins, IAPs, and c-FLIP for apoptosis induction.

Molecular biomarkers of colorectal cancer: prognostic and predictive tools for clinical practice

Colorectal cancer remains one of the most common types of cancer and leading causes of cancer death worldwide. Although we have made steady progress in chemotherapy and targeted therapy, evidence suggests that the majority of patients undergoing drug therapy experience severe, debilitating, and even lethal adverse drug events which considerably outweigh the benefits. The identification of suitable biomarkers will allow clinicians to deliver the most appropriate drugs to specific patients and spare them ineffective and expensive treatments. Prognostic and predictive biomarkers have been the subjects of many published papers, but few have been widely incorporated into clinical practice. Here, we want to review recent biomarker data related to colorectal cancer, which may have been ready for clinical use.

Genetic variability in the multidrug resistance associated protein-1 (ABCC1/MRP1) predicts hematological toxicity in breast cancer patients receiving (neo-)adjuvant chemotherapy with 5-fluorouracil, epirubicin and cyclophosphamide (FEC)

BACKGROUND

To assess the impact of single-nucleotide polymorphisms (SNPs) on predefined severe adverse events in breast cancer (BC) patients receiving (neo-)adjuvant 5-fluorouracil (FU), epirubicin and cyclophosphamide (FEC) chemotherapy.

PATIENTS AND METHODS

Twenty-six SNPs in 16 genes of interest, including the drug transporter gene ABCC1/MRP1, were selected based on a literature survey. An additional 33 SNPs were selected in these genes, as well as in 12 other genes known to be involved in the metabolism of the studied chemotherapeutics. One thousand and twelve female patients treated between 2000 and 2010 with 3-6 cycles of (neo-)adjuvant FEC were genotyped for these SNPs using Sequenom MassARRAY. Severe adverse events were evaluated through an electronic chart review for febrile neutropenia (FN, primary end point), FN first cycle, prolonged grade 4 or deep (<100/µl) neutropenia, anemia grade 3-4, thrombocytopenia grade 3-4 and non-hematological grade 3-4 events (secondary end points).

RESULTS

Carriers of the rs4148350 variant T-allele in ABCC1/MRP1 were associated with FN relative to homozygous carriers of the G-allele [P = 0.0006; false discovery rate (FDR) = 0.026]. Strong correlations with secondary end points such as prolonged grade 4 neutropenia (P = 0.002, FDR = 0.046) were also observed. Additionally, two other SNPs in ABCC1/MRP1 (rs45511401 and rs246221) correlated with FN (P = 0.007 and P = 0.01, respectively; FDR = 0.16 and 0.19), as well as two SNPs in UGT2B7 and FGFR4 (P = 0.024 and P = 0.04; FDR = 0.28 and 0.38).

CONCLUSION

Genetic variability in ABCC1/MRP1 was associated with severe hematological toxicity of FEC.

Impact of Pharmacogenomics on Clinical Practice in Oncology

Multiple drug strategies for many cancer types are now readily available and there is a clear need for tools to inform decision making on therapy selection. Although there is still a long way to go before pharmacogenomics achieves the goal of individualized selection of cancer treatment, promising progress is being made. Genetic testing for thiopurine methyltransferase (TPMT) variant alleles in patients prior to mercaptopurine administration, and for UGT1A1*28 in patients prior to administration of irinotecan therapy, along with the instigation of genotype-guided clinical trials (e.g. TYMS) are important advances in cancer pharmacogenomics. Markers for the toxicity and efficacy of many oncology drugs remain unknown; however, the examples highlighted here suggest progress is being made towards the incorporation of pharmacogenomics into clinical practice in oncology.

Identification of prognostic genes for recurrent risk prediction in triple negative breast cancer patients in Taiwan

Discrepancies in the prognosis of triple negative breast cancer exist between Caucasian and Asian populations. Yet, the gene signature of triple negative breast cancer specifically for Asians has not become available. Therefore, the purpose of this study is to construct a prediction model for recurrence of triple negative breast cancer in Taiwanese patients. Whole genome expression profiling of breast cancers from 185 patients in Taiwan from 1995 to 2008 was performed, and the results were compared to the previously published literature to detect differences between Asian and Western patients. Pathway analysis and Cox proportional hazard models were applied to construct a prediction model for the recurrence of triple negative breast cancer. Hierarchical cluster analysis showed that triple negative breast cancers from different races were in separate sub-clusters but grouped in a bigger cluster. Two pathways, cAMP-mediated signaling and ephrin receptor signaling, were significantly associated with the recurrence of triple negative breast cancer. After using stepwise model selection from the combination of the initial filtered genes, we developed a prediction model based on the genes SLC22A23, PRKAG3, DPEP3, MORC2, GRB7, and FAM43A. The model had 91.7% accuracy, 81.8% sensitivity, and 94.6% specificity under leave-one-out support vector regression. In this study, we identified pathways related to triple negative breast cancer and developed a model to predict its recurrence. These results could be used for assisting with clinical prognosis and warrant further investigation into the possibility of targeted therapy of triple negative breast cancer in Taiwanese patients.

Germline Genetic Testing to Predict Drug Response and Toxicity in Oncology— Reality or Fiction?

In addition to 6-mercaptopurine, 5-fluorouracil and irinotecan, the United States Food and Drug Administration (US FDA) has recently recommended label change for tamoxifen, to include pharmacogenetic information on treatment outcome. With the increasing availability of pharmacogenetic testing, on germline as well as somatic mutations, oncologists are now able to identify individuals at risk of severe treatment toxicity or poor treatment response. However, there are still knowledge gaps to fill before rationalised therapy based on pharmacogenetics can be fully integrated into clinical practice. This review provides an overview on the application of pharmacogenetic testing for germ line mutations in oncology to predict response and toxicity. Key words: Pharmacogenetics, Response, Toxicity

CoFactor: Folate Requirement for Optimization of 5-Fluouracil Activity in Anticancer Chemotherapy

Intracellular reduced folate exists as a "pool" of more than 6 interconvertable forms. One of these forms, 5,10 methylenetetrahydrofolic acid (CH(2)THF), is the key one-carbon donor and reduced folate substrate for thymidylate synthase (TS). This pathway has been an important target for chemotherapy as it provides one of the necessary nucleotide substrates for DNA synthesis. The fluoropyrimidine 5-fluorouracil (5-FU) exerts its main cytotoxic activity through TS inhibition. Leucovorin (5-formyltetrahydrofolate; LV) has been used to increase the intracellular reduced folate pools and enhance TS inhibition. However, it must be metabolized within the cell through multiple intracellular enzymatic steps to form CH2THF. CoFactor (USAN fotrexorin calcium, (dl)-5,10,-methylenepteroyl-monoglutamate calcium salt) is a reduced folate that potentiates 5-FU cytotoxicity. According to early clinical trials, when 5-FU is modulated by CoFactor instead of LV, there is greater anti-tumor activity and less toxicity. This review presents the emerging role of CoFactor in colorectal and nongastrointestinal malignancies.

Pharmacogenetic predictors of adverse events and response to chemotherapy in metastatic colorectal cancer: results from North American Gastrointestinal Intergroup Trial N9741

PURPOSE

With three available chemotherapy drugs for advanced colorectal cancer (CRC), response rate (RR) and survival outcomes have improved with associated morbidity, accentuating the need for tools to select optimal individualized treatment. Pharmacogenetics identifies the likelihood of adverse events or response based on variants in genes involved in drug transport, metabolism, and cellular targets.

PATIENTS AND METHODS

Germline DNA was extracted from 520 patients on the North American Gastrointestinal Intergroup N9741 study. Three study arms were evaluated: IFL (fluorouracil [FU] + irinotecan [IRN]), FOLFOX (FU + oxaliplatin), and IROX (IRN + oxaliplatin). Information on adverse events, response, and disease-free survival was available. Thirty-four variants in 15 candidate genes for analysis based on previous associations with adverse events or outcome were assessed. Genotyping was performed using pyrosequencing.

RESULTS

All variants were polymorphic. The homozygous UGT1A1*28 allele observed in 9% of patients was associated with risk of grade 4 neutropenia in patients on IROX (55% v 15%; P = .002). Deletion in GSTM1 was associated with grade 4 neutropenia after FOLFOX (28% v 16%; P = .02). Patients with a homozygous variant genotype for GSTP1 were more likely to discontinue FOLFOX because of neurotoxicity (24% v 10%; P = .01). The presence of a CYP3A5 variant was significantly associated with RR on IFL (29% v 60%; P = .0074). Most previously published genotype-toxicity or -efficacy relationships were not validated in this study.

CONCLUSION

This study provides a platform to evaluate pharmacogenetic predictors of response or severe adverse events in advanced CRC. Pharmacogenetic studies can be conducted in multicenter trials, and our findings demonstrate that with continued research, clinical application is practical.

Cancer pharmacoethnicity: ethnic differences in susceptibility to the effects of chemotherapy

A long-term goal of pharmacogenomics research is the design of individualized therapy based on the genomic sequence of the patient, in order to maximize response and minimize adverse drug reactions. Pharmacoethnicity, or ethnic diversity in drug response or toxicity, is becoming increasingly recognized as an important factor accounting for interindividual variation in anticancer drug responsiveness. Although pharmacoethnicity is determined by genetic and nongenetic factors, there is rapidly accumulating clinical evidence about ethnic differences in the frequencies of polymorphisms within many of the important cancer drug-related genes. This article reviews the current clinical evidence for ethnic differences in anticancer drug disposition and sensitivity while highlighting the challenges, and potential solutions, to acquiring such knowledge. The discovery of "ethnic-specific genetic signatures," representing unique sets of drug susceptibility-governing polymorphisms, may be the outcome of such work. Ultimately, such understanding will further the lofty goal of individualization of chemotherapy based on a person's unique genetic make-up to improve the tolerability and effectiveness of chemotherapy for all patients.

Pharmacogenetics of fluoropyrimidine and cisplatin. A future application to gastric cancer treatment

Chemotherapy plays an important role in the treatment of gastric cancer both in adjuvant or advanced settings. Recent randomized trials in Japan have proved that S-1, a novel fluoropyrimidine derivative, and cisplatin are the most promising agents. However, both the efficacy and toxicity of a given regimen vary widely among patients due to the inherited variability of genes that involve drug anabolism and catabolism. A narrow therapeutic index of antitumor agents, i.e. a given regimen being too toxic and/or less effective to some segment of patients, prevents the overall improvement of treatment outcomes. Pharmacogenetics, a research field elucidating genetic polymorphism in drug metabolizing enzymes, may contribute to identifying patients who benefit from chemotherapy or who will experience life-threatening toxicity. There are several crucial enzymes identified involving anabolism and the catabolism of fluoropyrimidine and cisplatin, including dihydropyrimidine dehydrogenase, thymidylate synthase, orotate phosphoribosyl transferase, glutathione S transferase, and excision repair cross complementary group. Various polymorphisms and ethnic variabilities of these genes have been elucidated. This review highlights variations within biological functions, detection systems, and possible clinical applications of these enzymatic polymorphisms. This knowledge provides a tool to determine an optimum regimen according to the patient's drug metabolizing characteristics. This stance will contribute to establishing individualized therapies for gastric cancer, which offers superior efficacy with a minimal chance of severe toxicity.

Thymidylate synthase gene variations: predictive and prognostic markers

Since its introduction more than 50 years ago by Heidelberger et al., the fluoropyrimidine 5-fluorouracil (5-FU) has remained the mainstay of therapeutic regimens used in the treatment of colorectal cancer and other human malignancies, with single-agent response rates of 20% to 25% in advanced disease stage. Pharmacogenomics has emerged as a useful tool to address interindividual gene variations by analyzing the interplay of host and tumor genotype and drug efficacy and toxicity. Having a reliable panel of prognostic and predictive markers will be critical in selecting an individualized and tailored chemotherapy regimen based on the particular tumor and host genotype. Although conflicting results have been reported, higher thymidylate synthase (TS) protein and mRNA expression levels in tumors have generally been associated with poor clinical outcome in patients treated with 5-FU-based chemotherapy regimens. However, the cause of the variability in TS expression still remains not fully understood, although several germ-line polymorphisms seem to affect the expression of TS, some of which have been found to have an effect on prognosis and the probability of response to 5-FU-based chemotherapy. This review will provide an update on pharmacogenomic studies of TS that were aimed at elucidating their role as prognostic and predictive markers.

Mechanisms of substrate selectivity for Bacillus anthracis thymidylate kinase

Bacillus anthracis is well known in connection with biological warfare. The search for new drug targets and antibiotics is highly motivated because of upcoming multiresistant strains. Thymidylate kinase is an ideal target since this enzyme is at the junction of the de novo and salvage synthesis of dTTP, an essential precursor for DNA synthesis. Here the expression and characterization of thymidylate kinase from B. anthracis (Ba-TMPK) is presented. The enzyme phosphorylated deoxythymidine-5'-monophosphate (dTMP) efficiently with K (m) and V (max) values of 33 microM and 48 micromol mg(-1) min(-1), respectively. The efficiency of deoxyuridine-5'-monophosphate phosphorylation was approximately 10% of that of dTMP. Several dTMP analogs were tested, and D-FMAUMP (2'-fluoroarabinosyl-5-methyldeoxyuridine-5'-monophosphate) was selectively phosphorylated with an efficiency of 172% of that of D-dTMP, but L-FMAUMP was a poor substrate as were 5-fluorodeoxyuridine-5'-monophosphate (5FdUMP) and 2',3'-dideoxy-2',3'-didehydrothymidine-5'-monophosphate (d4TMP). No activity could be detected with 3'-azidothymidine-5'-monophosphate (AZTMP). The corresponding nucleosides known as efficient anticancer and antiviral compounds were also tested, and d-FMAU was a strong inhibitor with an IC(50) value of 10 microM, while other nucleosides--L-FMAU, dThd, 5-FdUrd, d4T, and AZT, and 2'-arabinosylthymidine--were poor inhibitors. A structure model was built for Ba-TMPK based on the Staphylococcus aureus TMPK structure. Docking with various substrates suggested mechanisms explaining the differences in substrate selectivity of the human and the bacterial TMPKs. These results may serve as a start point for development of new antibacterial agents.

UGT1A1*28 polymorphism predicts irinotecan-induced severe toxicities without affecting treatment outcome and survival in patients with metastatic colorectal carcinoma

BACKGROUND

It is known that the uridine-diphosphoglucuronosyl transferase 1A1 (UGT1A1)*28 polymorphism reduces UGT1A1 enzyme activity, which may lead to severe toxicities in patients who receive irinotecan. This study was conducted to assess the influence of this polymorphism on the efficacy and toxicity of irinotecan treatment in Chinese patients with metastatic colorectal carcinoma (CRC).

METHODS

In total, 128 patients with metastatic CRC who had received previous treatment with irinotecan plus 5-fluorouracil/leucovorin were analyzed retrospectively. Genomic DNA samples were obtained from patients' leukocytes, and genotypes were determined by analyzing the sequence of TATA boxes in the UGT1A1 gene. The influence of the UGT1A1*28 polymorphism on toxicity and treatment outcome was analyzed.

RESULTS

Approximately 20% of patients were identified with the UGT1A1*28 polymorphism, including 15.6% (n = 20 patients) with the thymine-adenine (TA)6/TA7 genotype and 4.7% (n = 6 patients) with the TA7/TA7 genotype. The remaining 79.7% of patients (n = 102) had wild type TA6/TA6. Marked increases in grade 3 or 4 neutropenia (53.8% vs 4.9%; P < .01), neutropenic fever (38.5% vs 3.9%; P < .01), diarrhea (26.9% vs 5.9%; P < .01), and pretreatment bilirubin level (23.1% vs 8.8%; P = .04) were observed in patients who had the TA6/TA7 or TA7/TA7 genotypes. Patients' pretreatment bilirubin levels correlated well with irinotecan-induced neutropenia (P < .01). It was noted that, although the requirement for irinotecan dose reduction was significantly greater in patients who had this genetic variant (42.3% vs 12.7%; P < .01), it did not affect the response rate to irinotecan-based chemotherapy (42.3% vs 45.1%; P = .80), and it did not significantly affect progression-free survival (10 months vs 11 months; P = .94) or overall survival (19 months vs 18 months; P = .84).

CONCLUSIONS

The current data suggested that the UGT1A1*28 polymorphism may be a key determinant for predicting irinotecan-induced severe toxicities without affecting treatment outcome for patients with metastatic CRC. Further prospective studies are warranted for using this polymorphism to optimize irinotecan-based chemotherapy.

G>C SNP of thymidylate synthase with respect to colorectal cancer

Several studies indicate that low thymidylate synthase (TS) protein levels in tumor and normal tissues of colorectal cancer patients are associated with better clinical response to fluorouracil-based chemotherapy and higher risk of toxicity. However, no correlation or even reverse correlation has also been reported. These conflicting results may be partly due to the methodological limitations of the immunohistochemical techniques generally used to quantify thymidylate synthase expression. In this sense, a genetic approach aiming at determining the influence of the TS gene polymorphisms on clinical outcome seems more appealing. So far three polymorphisms have been identified and studied in the TYMS gene: the variable number of 28-bp tandem repeats (2R or 3R) in the 5 UTR; the G>C substitution at the 12th nucleotide in the second repeat of the 3R allele (3RG>3RC) and the 6-bp deletion in the 3 UTR (+6bp/-6bp 3 UTR). In vitro studies indicate that each of these polymorphisms can influence thymidylate synthase expression. In particular, the G>C SNP, which alters the E-box sequence binding an upstream stimulatory factor (USF-1), seems more important than the variable number of tandem repeats in determining TS gene expression in that the 3RC allele has a reduced translational activity compared with the 3RG allele, while showing the same activity as the 2R allele. In contrast with the in vitro findings, the clinical studies in colorectal patients failed to find a consistent relationship between the G>C polymorphism and clinical outcome measures (response, survival or toxicity). This discrepancy may be due to methodological heterogeneities amongst the studies, including genotyping in normal or tumor tissues, loss of heterozygosity in tumor cells not evaluated, variable doses and schedules of fluorouracil-based therapy, and variable tumor stage. The complexity of TYMS gene regulation, and the possibility that other polymorphisms may contribute to fluorouracil response, call for further studies before TYMS genotyping can be used in clinical practice to select colorectal cancer patients who are most likely to benefit from chemotherapy.

ERCC2 2251A>C genetic polymorphism was highly correlated with early relapse in high-risk stage II and stage III colorectal cancer patients: a preliminary study

Background

Early relapse in colorectal cancer (CRC) patients is attributed mainly to the higher malignant entity (such as an unfavorable genotype, deeper tumor invasion, lymph node metastasis and advance cancer stage) and poor response to chemotherapy. Several investigations have demonstrated that genetic polymorphisms in drug-targeted genes, metabolizing enzymes, and DNA-repairing enzymes are all strongly correlated with inter-individual differences in the efficacy and toxicity of many treatment regimens. This preliminary study attempts to identify the correlation between genetic polymorphisms and clinicopathological features of CRC, and evaluates the relationship between genetic polymorphisms and chemotherapeutic susceptibility of Taiwanese CRC patients. To our knowledge, this study discusses, for the first time, early cancer relapse and its indication by multiple genes.

Methods

Six gene polymorphisms functional in drug-metabolism – GSTP1 Ile105Val, ABCB1 Ile1145Ile, MTHFR Ala222Val, TYMS double (2R) or triple (3R) tandem repeat – and DNA-repair genes – ERCC2 Lys751Gln and XRCC1 Arg399Gln – were assessed in 201 CRC patients using a polymerase chain reaction-restriction fragment-length polymorphism (PCR-RFLP) technique and DNA sequencing. Patients were diagnosed as either high-risk stage II (T2 and 3 N0 M0) or III (any T N1 and 2 M0) and were administered adjuvant chemotherapy regimens that included 5-fluorouracil (5FU) and leucovorin (LV). The correlations between genetic polymorphisms and patient clinicopathological features and relapses were investigated.

Results

In this study, the distributions of GSTP1 (P = 0.003), ABCB1 (P = 0.001), TYMS (P < 0.0001), ERCC2 (P < 0.0001) and XRCC1 (P = 0.006) genotypes in the Asian population, with the exception of MTHFR (P = 0.081), differed significantly from their distributions in a Caucasian population. However, the unfavorable genotype ERCC2 2251A>C (P = 0.006), tumor invasion depth (P = 0.025), lymph node metastasis (P = 0.011) and cancer stage (P = 0.008) were significantly correlated with early relapse. Patients carrying the ERCC2 2251AC or2251CC genotypes had a significantly increased risk of early relapse (OR = 3.294, 95% CI, 1.272–8.532).

Conclusion

We suggest that ERCC2 2251A>C alleles may be genetic predictors of early CRC relapse.

Integrating pharmacogenomics into oncology clinical practice

Oncology pharmacogenomics has seen a great deal of progress in the past 10 years. The release of the Human Genome Project data and the availability of fast, affordable genotyping platforms has allowed the field to expand and has provided invaluable data for pharmacogenomics research. The introduction of US FDA-approved targeted therapy (trastuzumab), package insert changes (irinotecan and tamoxifen) and the initiation of a genotype-guided clinical trial for cancer therapy (TYMS TSER in rectal cancer), along with panels of DNA and expression markers (Roche AmpliChip(®) and Oncotype Dx™ panel) are paving the way towards the integration of pharmacogenomics into clinical practice.

The multidisciplinary management of gastrointestinal cancer. The use of molecular markers in the diagnosis and treatment of colorectal cancer

Rapidly growing insights into the molecular biology of colorectal cancer led to high hopes for the identification of molecular markers to be used in optimised and tailored treatment regimens for this disorder. However, no molecular marker has yet made it into daily practice. In this review we will discuss some of the potential molecular markers, focus on the lessons learnt from marker development and identify strategies for the future.

Epidermal growth factor receptor R497K polymorphism is a favorable prognostic factor for patients with colorectal carcinoma

PURPOSE

It has been shown that the R497K polymorphism of the epidermal growth factor receptor (EGFR) has attenuated functions in ligand binding, tyrosine kinase activation, and growth stimulation. Because the activation of EGFR results in an unfavorable prognosis of patients with colorectal carcinoma, a pilot study was conducted to assess the influence of this polymorphism on colorectal carcinoma patients.

EXPERIMENTAL DESIGN

We retrospectively analyzed the effect of the R497K polymorphism of EGFR on clinicopathologic features in 209 colorectal carcinoma patients, including 100 with stage II/III colorectal carcinoma receiving curative surgery and the other 109 with metastatic diseases.

RESULTS

An excellent correlation in codon 497 statuses examined by patients' WBCs and tumor tissues was found but no significant between-group difference in patients with or without colorectal carcinoma (P = 0.97). A marked decrease on EGFR phosphorylation (P < 0.01) and c-Myc activation (P = 0.02) was observed in patients with R497K polymorphism, which is associated with decreased invasion (P = 0.01), lower nodal involvement (P = 0.02), reduced subsequent metastasis (P < 0.01), and longer disease-free (P < 0.01) as well as overall (P < 0.01) survival in stage II/III colorectal carcinoma patients who had received curative surgery. For patients with metastatic colorectal carcinoma, this polymorphism was associated with a higher response to 5-fluorouracil/oxaliplatin treatment (P = 0.02) and a longer survival (P < 0.01). By multivariate analysis, this polymorphism was also identified as an independent prognostic factor (P = 0.03).

CONCLUSIONS

These data suggest that the R497K polymorphism of the EGFR, by reducing its activation and a consequential down-regulation of its target genes, could be a key determinant for reduced tumor recurrence of stage II/III colorectal carcinoma patients receiving curative surgery and a longer survival of patients with stage II/III as well as metastatic colorectal carcinoma.

The role of pharmacogenetics in cancer therapeutics

The variability in treatment responses and narrow therapeutic index of anticancer drugs are some of the key challenges oncologists face. The knowledge of pharmacogenetics can potentially aid in the discovery, development and ultimately individualization of anticancer drugs. The identification of genetic variations that predict for drug response is the first step towards the translation of pharmacogenetics into clinical practice. This review provides an update on the results of studies assessing the effects of germline polymorphisms and somatic mutations on therapeutic outcomes and highlights the potential applications and future challenges in pharmacogenetic research pertaining to cancer therapeutics.

Advances and challenges in fluoropyrimidine pharmacogenomics and pharmacogenetics

In cancer pharmacogenetics (the study of how variability in a single or set of known genes influences drug response) and pharmacogenomics (the study of variability on a genome-wide scale), one of the most important fields of research focuses on the fluoropyrimdines (FPs) and, in particular, 5-fluorouracil (5-FU). After over 40 years of use, FPs remain one of the most commonly used cancer chemotherapy agents and their application includes a wide spectrum of cancer types. FPs also continue to be the baseline component for many new regimens with novel molecular-targeted agents that are being rapidly introduced. Hence, it would seem appropriate that pharmacogenetic/genomic models for optimizing cancer patient management would involve indicators of FP response. In this article, the current trends in FP pharmacogenetics and pharmacogenomics are reviewed based on the advances made to date and the challenges faced in realizing their full potential.

Pharmacogenetics in the Management of Breast Cancer – Prospects for Individualised Treatment

Increased understanding of how variants in genes encoding metabolising enzymes, transporters and receptors affect drug efficacy and toxicity, in parallel with advances in genotyping technology means that clinical pharmacogenetics is drawing tantalisingly close to reality. This review considers some of the pharmacogenetic variants that have been described that are relevant to the management of women with breast cancer and how these may soon translate into clinical practice.

Thymidylate synthase pharmacogenetics

Thymidylate synthase (TYMS) is an important target for chemotherapy drugs, such as 5-fluorouracil (5FU) and methotrexate. Over-expression of TYMS is linked to resistance to TYMS-targeted chemotherapy drugs. Currently there is no protocol for selecting cancer patients at risk for drug resistance prior to chemotherapy treatment. Three polymorphisms in the 5' and 3' untranslated regions (5'UTR and 3'UTR) of the thymidylate synthase gene have been shown to influence TYMS expression. Preliminary data has suggested a poorer response rate to 5FU or methotrexate is seen in patients with 3 copies of a 28 bp tandem repeat in the 5'UTR enhancer region (TSER polymorphism) and this relationship may be further clarified by the presence of a single nucleotide polymorphism (SNP) with the second repeat of the 3 repeat (TSER(*)3) allele. A 6 bp deletion in the 3'UTR of the TYMS gene has also been shown to affect TYMS RNA expression and has a significant association with poor outcome in 5FU treated patients. Evidence linking all 3 TYMS polymorphisms with TYMS expression and patient response to TYMS-targeted chemotherapy treatment will be highlighted.

Fourteen Novel Genetic Variations and Haplotype Structures of the TYMS Gene Encoding Human Thymidylate Synthase (TS)

Forty genetic variations including 14 novel ones were found in the human TYMS gene, which encodes thymidylate synthase, in 263 Japanese cancer patients who received 5-fluorouracil (FU)-based chemotherapy. Three novel variations were located within the 28-bp tandem repeat sequence in the 5'-untranslated region (UTR) and were designated 5Rc, 3Rc-ins and 4Rc. Allele frequencies were 0.021 for 5Rc, 0.006 for 3Rc-ins and 0.002 for 4Rc. Other novel variations included -133G>C and -125G>C in the 5'-UTR; IVS1-278G>A, IVS2-68C>T, IVS2-23T>C, IVS4+122_+123insATTG, IVS4-141G>A, IVS5-100A>T and IVS6-111G>A in the introns; and 1244(*302)A>G and 1264(*322)G>A in the 3'-UTR. The allele frequencies were 0.34 for IVS4+122_+123insATTG, 0.042 for -133G>C, 0.011 for IVS4-141G>A, 0.006 for -125G>C, 0.004 for IVS1-278G>A, IVS2-68C>T, 1244(*302)A>G and 1264(*322)G>A, and 0.002 for IVS2-23T>C, IVS5-100A>T and IVS6-111G>A. Using the detected polymorphisms, linkage disequilibrium (LD) analysis was performed, which divided the TYMS gene into three LD blocks. The 28-bp tandem repeat sequence in the 5'-UTR was assigned as Block 2 with a total of 7 alleles. In Blocks 1 and 3, 7 and 19 haplotypes were determined/inferred, respectively. Our findings provide fundamental and useful information for genotyping TYMS in the Japanese and probably other Asian populations.

Evaluation of Bacillus anthracis thymidine kinase as a potential target for the development of antibacterial nucleoside analogs

Bacillus anthracis, which causes anthrax, has attracted attention because of its potential use as a biological weapon. The risk of multidrug resistance against B. anthracis increases the need for antibiotics with new molecular targets. Nucleoside analogs are well-known antiviral and anticancer prodrugs, and thymidine kinase catalyzes the rate-limiting step in the activation of pyrimidine nucleoside analogs used in chemotherapy. The thymidine kinase gene from B. anthracis Sterne strain (34F2) (Ba-TK) was cloned and expressed in E. coli, and the product was purified and characterized regarding its substrate specificity. Ba-TK phosphorylated pyrimidine nucleosides and all natural nucleoside triphosphates served as phosphate donors. Size exclusion chromatography indicated a dimeric form of Ba-TK, regardless of the presence of ATP. Thymidine was the most efficient substrate with a low K(m) value (0.6 microM) and a V(max) of 3.3 micromol dTMP mg(-1) min(-1), but deoxyuridine (K(m)=4.2 microM, V(max)=4.1 micromol dUMP mg(-1) min(-1)) was also a good substrate. Several pyrimidine analogs were also tested and analogs with 5-position modifications showed higher activities compared to analogs with 3'- and N3-position modifications. Deoxyuridine analogs were the most potent inhibitors of B. anthracis growth in vitro. These results may be used to guide future development of nucleoside analogs against B. anthracis.

Pharmacogenetics for individualized cancer chemotherapy

The same doses of medication cause considerable heterogeneity in efficacy and toxicity across human populations. Genetic factors are thought to represent important determinants of drug efficacy and toxicity. Pharmacogenetics focuses on the prediction of the response of tumor and normal tissue to standard therapy by genetic profiling and, thereby, to select the most appropriate medication at optimal doses for each individual patient. In the present review, we discuss the relevance of single nucleotide polymorphisms (SNP) in genes, whose gene products act upstream of the actual drug target sites, that is, drug transporters and drug metabolizing phase I and II enzymes, or downstream of them, that is, apoptosis-regulating genes and chemokines. SNPs in relevant genes, which encode for proteins that interact with anticancer drugs, were also considered, that is, enzymes of DNA biosynthesis and metabolism, DNA repair enzymes, and proteins of the mitotic spindle. A significant body of evidence supports the concept of predicting drug efficacy and toxicity by SNP genotyping. As the efficacy of cancer chemotherapy, as well as the drug-related toxicity in normal tissues is multifactorial in nature, sophisticated approaches such as genome-wide linkage analyses and integrate drug pathway profiling may improve the predictive power compared with genotyping of single genes. The implementation of pharmacogenetics into clinical routine diagnostics including genotype-based recommendations for treatment decisions and risk assessment for practitioners represents a challenge for the future.

Genetic factors influencing pyrimidine-antagonist chemotherapy

Pyrimidine antagonists, for example, 5-fluorouracil (5-FU), cytarabine (ara-C) and gemcitabine (dFdC), are widely used in chemotherapy regimes for colorectal, breast, head and neck, non-small-cell lung cancer, pancreatic cancer and leukaemias. Extensive metabolism is a prerequisite for conversion of these pyrimidine prodrugs into active compounds. Interindividual variation in the activity of metabolising enzymes can affect the extent of prodrug activation and, as a result, act on the efficacy of chemotherapy treatment. Genetic factors at least partly explain interindividual variation in antitumour efficacy and toxicity of pyrimidine antagonists. In this review, proteins relevant for the efficacy and toxicity of pyrimidine antagonists will be summarised. In addition, the role of germline polymorphisms, tumour-specific somatic mutations and protein expression levels in the metabolic pathways and clinical pharmacology of these drugs are described. Germline polymorphisms of uridine monophosphate kinase (UMPK), orotate phosphoribosyl transferase (OPRT), thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD) and methylene tetrahydrofolate reductase (MTHFR) and gene expression levels of OPRT, UMPK, TS, DPD, uridine phosphorylase, uridine kinase, thymidine phosphorylase, thymidine kinase, deoxyuridine triphosphate nucleotide hydrolase are discussed in relation to 5-FU efficacy. Cytidine deaminase (CDD) and 5'-nucleotidase (5NT) gene polymorphisms and CDD, 5NT, deoxycytidine kinase and MRP5 gene expression levels and their potential relation to dFdC and ara-C cytotoxicity are reviewed.

Have we made progress in pharmacogenomics? The implementation of molecular markers in colon cancer

For the last 40 years, 5-fluorouracil (5-FU) has remained the treatment of choice in both the adjuvant and advanced treatment of colorectal cancer (CRC). However, 5-FU monotherapy produces response rates of only 10–20% in the advanced setting. 5-FU has been combined with newer agents, such as oxaliplatin and irinotecan, and this has significantly increased response rates to 40–50% in the advanced setting. More recently, novel biological agents, such as the monoclonal antibodies targeting either the epidermal growth factor receptor or vascular endothelial growth factor, have shown to provide additional clinical benefit for patients with metastatic CRC. A number of predictive markers have been identified for CRC to date. However, their usefulness as individual markers of response has led to somewhat inconclusive results. Therefore, there is a need to identify panels of predictive markers of response to therapy for advanced CRC, in order to improve these disappointing response rates. The advent of high-throughput methodologies, such as microarrays, enables tumor samples to be profiled on a global scale. This technology has been utilized to develop predictive markers for a wide range of tumor types to date, and hopefully this technology can be translated into the CRC setting with the hope of predicting the response of each individual tumor to chemotherapy.

Combining several polymorphisms of thymidylate synthase gene for pharmacogenetic analysis

Thymidylate synthase (TS) is an essential enzyme in proliferating cells and an important target for several chemotherapeutics. Several TS gene polymorphisms correlate with variable TS expression: a double (2R) and triple (3R) 28-bp repeat element, a G to C substitution of the 3R allele and a 6 bp variation in 3'UTR. We have previously shown that childhood acute lymphoblastic leukemia (ALL) patients who are homozygous for the 3R allele had reduced event-free survival (EFS) probabilities. Here, we analyzed all three polymorphisms in an extended group of ALL patients (n=259). The effect of the 3R homozygosity on ALL outcome was confirmed (P=0.006), whereas 6 bp polymorphism did not influence EFS when analyzed separately. No significant difference among 3R3R genotype subgroups, as defined by a G to C substitution, was observed. The haplotype analysis revealed the higher frequency of the 3RC/6 bp+ haplotype (P=0.04) and the protective role of the 2R/6b p- (P=0.04). Consequently, homozygosity for the 6 bp- allele appeared to reduce an event-predisposing effect of 3R variant. Although of importance for translation into the clinical practice, these findings need confirmation in larger studies.

Role of Genomic Markers in Colorectal Cancer Treatment

For the last four decades, fluorouracil (FU) has been the main treatment of choice in colorectal cancer (CRC) in both the advanced and adjuvant settings. In the advanced setting, FU monotherapy produces response rates of only 10% to 20%. Furthermore, in resected stage III CRC, FU monotherapy has increased overall survival by only 20%. The combination of FU with newer therapies such as oxaliplatin and irinotecan has significantly improved response rates to 40% to 50%. Despite these improvements, more than half of advanced CRC patients derive no benefit from treatment; this is due to either acquired or inherent drug resistance. This review aims to highlight the current prognostic and predictive markers that have been identified for CRC to date. The limited use of these predictive markers underscores the importance of and need for multiple marker testing in order to improve response rates and decrease toxicity. This review will also focus on high throughput methods to identify panels of predictive markers for CRC, which ultimately aim to tailor treatment according to an individual patient and tumor profile.

Evaluation of sensitivity to 5-FU on the basis of thymidylate synthase (TS)/dihydropyrimidine dehydrogenase (DPD) activity and chromosomal analysis in micro tissue specimens of breast cancer

BACKGROUND

Preoperative assessment of the anticancer drug sensitivity of tumors plays an important role in the selection of therapy. If evaluation of the 5-FU sensitivity of microtissue specimens obtained by techniques such as core needle biopsy could be performed, the addition of fluorouracil to adriamycin and cyclophosphamide may further enhance response rates. In order to evaluate a simple sensitivity test for the anti-tumor agent 5-fluorouracil (5-FU), we examined whether an assay of a small sample could measure mRNA to predict the activities of thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD). In addition, gene abnormalities on chromosomes 1 and 18 corresponding to DPD, TS and the relationships between the gene abnormalities and the amount of mRNA and activity were examined.

METHOD

TS and DPD activity were measured using the fluorodeoxyuridine monophosphate ligand binding assay and radio enzymatic assay, respectively, while mRNA levels were assayed by real-time polymerase chain reaction. Chromosome 1 and 18 aberrations were investigated by fluorescence in situ hybridization (FISH) with centromere probes.

RESULTS

TS mRNA and TS activity showed a positive correlation (r=0.518, p=0.0017). TS activity and TS mRNA were significantly higher in the nuclear grade 3 group than in the other groups (p=0.04, p=0.0072, respectively). TS activity and mRNA in tumor tissue tended to decrease in the progesterone receptor positive groups (p=0.059, p=0.066, respectively). There was no correlation between DPD mRNA and DPD activity in tumor tissue (r=0.139, p=0.4423). DPD mRNA was measured as 282.88+/-170.68 copies/cell in tumor tissue and 635.88+/-310.04 copies/cell in normal tissue, and was thus significantly higher in normal tissue (p<0.001).

CONCLUSIONS

TS mRNA showed a positive correlation with TS activity, suggesting that this method of using small amounts of tissue can replace anti-cancer drug sensitivity tests.

Cancer pharmacogenomics: powerful tools in cancer chemotherapy and drug development

Interindividual differences in tumor response and normal tissue toxicities are consistently observed with most chemotherapeutic agents or regimens. While many clinical variables have been associated with drug responses (e.g., age, gender, diet, drug-drug interactions), inherited variations in drug disposition (metabolism and transport) genes and drug target genes also likely contribute to the observed variability in cancer treatment outcome. Pharmacogenomic studies aim to elucidate the genetic bases for interindividual differences and to use such genetic information to predict the safety, toxicity, and/or efficacy of drugs. There exist several clinically relevant examples of the utility of pharmacogenomics that associate specific genetic polymorphisms in drug metabolizing enzymes (e.g., TPMT, UGT1A1, DPD), drug transporters (MDR1), and drug target enzymes (TS) with clinical outcomes in patients treated with commonly prescribed chemotherapy drugs, such as 5-fluorouracil and irinotecan (Camptosar; Pfizer Pharmaceuticals; New York, NY http://www.pfizer.com). Techniques to discover and evaluate the functional significance of these polymorphisms have evolved in recent years and may soon be applied to clinical practice and clinical trials of currently prescribed anticancer drugs as well as new therapeutic agents. This review discusses the current and future applications of pharmacogenomics in clinical cancer therapy and cancer drug development.

Polymorphic tandem repeat sequences of the thymidylate synthase gene correlates with cellular-based sensitivity to fluoropyrimidine antitumor agents

PURPOSE

Thymidylate synthase (TS) is one of the target molecules for the antitumor effects of fluoropyrimidine drugs. The cellular thymidylate synthase level is one of the determining factors for the antitumor activity of fluoropyrimidines. TYMS, which encodes TS, has been reported to possess 28-bp tandem repeat sequences in its 5'-untranslated region, the number of which varies. In addition, single nucleotide polymorphisms have also been shown in a triple repeat sequence. In this study, correlation between the polymorphic tandem repeat sequences of the TYMS gene and the antitumor activities of 5-fluorouracil (5-FU) and 5-fluoro-2'-deoxyuridine (FUdR) were investigated with 30 established human cell lines derived from solid tumors.

METHODS

A reporter assay system was developed in order to compare the ability of the transactivation mediated by the double (2R) and triple (c- or g-type, 3Rc or 3Rg, respectively) repeat sequences using a human colon cancer cell line, DLD-1. The 50% inhibitory concentration (IC(50)) of cell growth by 5-FU and FUdR was measured with 30 different established cell lines of human solid tumors. Genotypes based on the number of the 28-bp TYMS tandem repeat for the above cell lines were determined by electrophoretical analysis of PCR products containing the repeat sequences and nucleotide sequencing.

RESULTS

The reporter activity mediated by the 3Rg sequence was significantly higher than that by the 2R and 3Rc sequences. Activities mediated by the 2R and 3Rc sequences were comparable. According to the reporter assay, 2R and 3Rc were judged as low TS expression alleles and 3Rg as a high TS expression allele. On the basis of IC(50) values, cells possessing the 2R/2R and 2R/3R repeat of TYMS were significantly more sensitive to FUdR than those with the 3R/3R repeat. Cells possessing 3Rg/3Rg (a high TS expression genotype) were significantly less sensitive to FUdR than cells with 2R/2R, 2R/3Rc, and 3Rc/3Rc (low TS expression genotypes).

CONCLUSIONS

Our results of the reporter assays using 2R, 3Rc, and 3Rg repeat sequences prompted us to classify 3Rg as a high TS expression allele, and 2R and 3Rc as low TS expression alleles. The cells with low TS expression alleles were shown to exhibit significantly higher FUdR sensitivity than the cells with high TS expression alleles for the first time. These results were consistent with numerous previous in vitro and in vivo findings that tumors showing high TS expression were less sensitive to fluoropyrimidines. These results support the idea that genotyping the tandem repeat sequences of TYMS in the 5'-untranslated region is useful for individualized therapy involving fluoropyrimidine antitumor drugs.

Simultaneous detection of variable number tandem repeats, single nucleotide polymorphisms, and allelic imbalance in the thymidylate synthase gene enhancer region using denaturing high-performance liquid chromatography

Polymorphisms in the thymidylate synthase enhancer region (TSER) have been reported to be associated with alterations in thymidylate synthase (TS) mRNA protein levels. The TSER is characterized by the presence of variable double (2R) and triple (3R) number tandem repeats (VNTRs). In addition to VNTRs, single nucleotide polymorphisms (SNPs) and allelic imbalance (AI), including loss of heterozygosity (LOH), have recently been associated with response to 5-fluorouracil (5-FU)-based chemotherapy. The aim of the current study was to develop a specific denaturing high-performance liquid chromatography (DHPLC) method for the rapid detection of these variations in the TSER in clinical samples. DHPLC analysis was validated in parallel with agarose gel electrophoresis (AGE), enzyme digestion, and quantitative reverse transcription-polymerase chain reaction (Q-RT-PCR). The optimized DHPLC method resolved 100% of the known TSER variations, differentiated between homozygous and heterozygous genotypes, and allowed the qualitative and quantitative detection of AI, including LOH, in tumor samples. This DHPLC method was developed to permit the rapid, sensitive, and accurate identification of the TSER genotype (VNTRs, SNPs, and AI) in clinical protocols where response to flouropyrimidines may be correlated with TSER polymorphisms.

Predictive factors for response to chemotherapy in colorectal cancer patients

Colorectal cancer represents a major health problem in the western world. A lot of drugs have been employed in treatment of this disease, but only few data are available about predictive factors for response to anticancer treatments in colorectal cancer. Aim of this paper is to review the main data about this investigation field. Using a Medline database search (1966-2003) we reviewed all the relevant papers that investigate clinical and molecular predictors for response to the main drugs used in the treatment of colorectal cancer patients, both in adjuvant and in advanced setting. Moreover we comprehensively reviewed all the data published in abstract form during the most significant international meetings. Our review put in evidence the most important predictive factors for response in colorectal cancer patients treated with anticancer chemotherapy both in adjuvant and in advanced setting. The predictive factors are clustered on the basis of the different anticancer drugs. The results of this review provide the rationale basis for personalizing anticancer treatment in colorectal cancer patients by molecular and clinical features, aiming to improve response rate and reduce toxicities.

Molecular mechanisms of drug resistance

Resistance to chemotherapy limits the effectiveness of anti-cancer drug treatment. Tumours may be intrinsically drug-resistant or develop resistance to chemotherapy during treatment. Acquired resistance is a particular problem, as tumours not only become resistant to the drugs originally used to treat them, but may also become cross-resistant to other drugs with different mechanisms of action. Resistance to chemotherapy is believed to cause treatment failure in over 90% of patients with metastatic cancer, and resistant micrometastic tumour cells may also reduce the effectiveness of chemotherapy in the adjuvant setting. Clearly, if drug resistance could be overcome, the impact on survival would be highly significant. This review focuses on molecular mechanisms of drug resistance that operate to reduce drug sensitivity in cancer cells. Drug resistance can occur at many levels, including increased drug efflux, drug inactivation, alterations in drug target, processing of drug-induced damage, and evasion of apoptosis. Advances in DNA microarray and proteomic technology, and the ongoing development of new targeted therapies have opened up new opportunities to combat drug resistance. We are now able to characterize the signalling pathways involved in regulating tumour cell response to chemotherapy more completely than ever before. This will facilitate the future development of rational combined chemotherapy regimens, in which the newer targeted therapies are used in combination with cytotoxic drugs to enhance chemotherapy activity. The ability to predict response to chemotherapy and to modulate this response with targeted therapies will permit selection of the best treatment for individual patients.