Pharmacogenetics: a tool for individualizing antineoplastic therapy

Pharmacogenetics of Drugs Used in the Treatment of Cancers

Pharmacogenomics is based on the understanding of the individual differences in drug use, the response to drug therapy (efficacy and toxicity), and the mechanisms underlying variable drug responses. The identification of DNA variants which markedly contribute to inter-individual variations in drug responses would improve the efficacy of treatments and decrease the rate of the adverse side effects of drugs. This review focuses only on the impact of polymorphisms within drug-metabolizing enzymes on drug responses. Anticancer drugs usually have a very narrow therapeutic index; therefore, it is very important to use appropriate doses in order to achieve the maximum benefits without putting the patient at risk of life-threatening toxicities. However, the adjustment of the appropriate dose is not so easy, due to the inheritance of specific polymorphisms in the genes encoding the target proteins and drug-metabolizing enzymes. This review presents just a few examples of such polymorphisms and their impact on the response to therapy.

Assessing Home Health Care Pharmacists' Knowledge of Cytochrome P450 Pharmacogenetics

Objective:

Genetic code governs cytochrome P450 activity as a consequence, it may influence an individual's response to medications metabolized by these enzymes. Pharmacists have a prominent role in providing education concerning adverse drug reactions and variability in drug response. This investigation assessed the knowledge of a group of pharmacists regarding cytochrome P450 pharmacogenetics.

Methods:

This observational, cross-sectional study presented a Web–based questionnaire available for completion by pharmacists contacted via E-mail. Fifty-two pharmacists involved with a nationwide home-care facility participated in the study on a volunteer basis. The main outcome measure was percentage score on a 10-question examination.

Results:

Fifty-two of 171 contacted pharmacists participated in the study, yielding a response rate of 30%. The mean percentage questionnaire score ± SD was 17.5 ± 15.1. Scores were slightly higher for pharmacists with 2 or more pharmacokinetic classes in their formal education (p < 0.02).

Conclusions:

Understanding the principles of pharmacogenetics will allow pharmacists to appreciate that patients respond differently to certain medications as influenced by genetic variations encoding drug-metabolizing enzyme activity. As a consequence, some patients will have a normal therapeutic response, whereas others may experience drug toxicity or therapeutic inefficacy when given the same dose of a drug. This study determined, albeit with many limitations, that there is likely a need for improved availability of pharmacogenetic continuing education programs.

Evaluation of the germline single nucleotide polymorphism rs583522 in the TNFAIP3 gene as a prognostic marker in esophageal cancer

Most esophageal cancer patients die because of disease relapse, hence an accurate prognosis of disease relapse and survival is essential. Genetic variations in cancer patients may serve as important indicators. Three genotypes (GG, AG, and AA) are displayed by the single nucleotide polymorphism (SNP) rs583522, which maps to the TNFAIP3 gene on chromosome 6. Evaluation of the potential prognostic value of the TNFAIP3-SNP in esophageal cancer (EC) was the aim of this study. A total of 158 patients underwent complete surgical resection of the esophagus for EC. None of them received any neoadjuvant or adjuvant treatment. Peripheral blood was sampled, and genomic DNA was extracted from leukocytes before each operation. Clinicopathologic parameters, tumor cell dissemination in bone marrow, and clinical outcome were correlated with the TNFAIP3-SNP. A-allele carriers showed advanced tumor stages compared with those of homozygous G-allele carriers (P<0.001). Patients with an A-allele genotype (AA or AG) were significantly more likely to experience a relapse (P=0.003). Survival analysis (log-rank test) revealed a significant difference in overall survival between the three groups (P=0.039); however, none of the genotypes was identified as a disease stage-independent prognostic marker. In conclusion, TNFAIP3-SNP stratifies patients into different risk groups; however, it could not be identified as an independent prognostic marker.

An A/C germline single-nucleotide polymorphism in the TNFAIP3 gene is associated with advanced disease stage and survival in only surgically treated esophageal cancer

Prognostication of disease relapse and survival is essential for cancer patients and genetic variations in cancer patients may serve as important indicators. A single-nucleotide polymorphism (SNP) mapping to the tumor necrosis factor, alpha-induced protein 3 (TNFAIP3) gene at position 138241110 displays three genotypes (AA, AC and CC). The aim of this study was to evaluate the potential prognostic value of the TNFAIP3-SNP in esophageal cancer (EC). Genomic DNA was extracted from peripheral blood leukocytes of 173 patients who underwent complete surgical resection for EC and did not receive any neoadjuvant or adjuvant therapy. For SNP detection, a 260- bp fragment was PCR amplified, purified and sequenced with tested primers. The product was analyzed by automatic DNA sequencer.The TNFAIP3 genotypes were correlated with clinico-pathological parameters, tumor cell dissemination in bone marrow and clinical outcome. The C-allele carrier presented with higher disease stage (P<0.001). This was predominantly because of the presence of lymph node metastasis (P<0.001). The recurrence rate was higher in C-allele carriers (AC and CC genotype; P=0.004). Kaplan-Meier plots for disease-free (P=0.017) and overall survival (P<0.001) displayed a gene dosage-associated outcome with AA genotype patients presenting the longest and CC genotype patients the poorest survival. In disease stage-adjusted multivariate analysis the TNFAIP3-SNP was identified as an independent prognostic factor for survival (hazard ratio 1.9; P=0.008). The TNFAIP3-SNP allows risk stratification of EC patients and may be a useful tool to identify patients eligible for multimodal therapy concepts.

The T393C polymorphism of GNAS1 is a predictor for relapse and survival in resectable non-small cell lung cancer

INTRODUCTION

The GNAS1 T393C single nucleotide polymorphism (T393C-SNP) correlates with Gαs mRNA stability and protein expression and augmented apoptosis. Genetic germ line variations as stable and reproducible markers potentially serve as prognostic marker in oncology. The aim of this study was to evaluate the potential prognostic value of T393C-SNP in complete resected non-small cell lung cancer (NSCLC).

PATIENTS AND METHODS

In total 163 Caucasian patients, who had been surgically treated for NSCLC between 1998 and 2010, were included in this study. Genotyping of peripheral blood cells was performed by polymerase chain reaction and digestion using the restriction enzyme FokI. The T393C-SNP was correlated with clinic-pathological parameters and survival. Chi-square test, Kaplan-Meier estimator and cox regression hazard model were used to assess the prognostic value of the T393C-SNP.

RESULTS

C-allele carriers had a higher recurrence rate (p=0.018) and a shorter disease-free survival compared to homozygous T-allele carriers (12.26 months vs. 44.65 months, p=0.009). The overall survival in homozygous C allele carriers was shorter (19.10 months vs. 53.95 months, p=0.019). Multivariate Cox regression identified the CC genotype as a negative independent prognostic factor for recurrence (hazard ratio 2.36, p=0.007) and survival (hazard ratio 2.51, p=0.008).

CONCLUSION

Determination of T393C-SNP preoperatively potentially allows allocation of NSCLC patients into different risk profiles and may influence the therapeutic strategy.

Heme oxygenase-1 germ line GTn promoter polymorphism is an independent prognosticator of tumor recurrence and survival in pancreatic cancer

BACKGROUND

Heme oxygenase-1 (HO-1) correlates with aggressive tumor behavior and chemotherapy resistance in pancreatic cancer (PC). We evaluated the prognostic value of the basal transcription controlling germ line GTn repeat polymorphism (GTn) in the promoter region of the HO-1 gene in PC.

PATIENTS AND METHODS

We determined the GTn in 100 controls and 150 PC patients. DNA was extracted from blood leukocytes and GTn determined by PCR, electrophoresis, and sequencing. Clinicopathological parameters, disease-free, and overall survival (DFS, OS) were correlated with GTn.

RESULTS

Three genotypes were defined based on short (S) <25 and long (L) ≥25 GTn repeat alleles. In PC patients, a steadily increasing risk was evident between LL, SL, and SS genotype patients for larger tumor size, presence of lymph node metastasis, poor tumor differentiation and higher recurrence rate (P < 0.001 each). The SS genotype displayed the most aggressive tumor biology. The LL genotype had the best and the SS genotype the worst DFS and OS (P < 0.001 each). The GTn genotype was the strongest prognostic factor for recurrence and survival (P < 0.001 each).

CONCLUSION

The GTn repeat polymorphism is a strong prognostic marker for recurrence and survival in PC patients.

The GNAS1 T393C single nucleotide polymorphism predicts the natural postoperative course of complete resected esophageal cancer

BACKGROUND

Genetic variations in cancer patients may serve as important prognostic indicators of clinical outcome. The GNAS1 T393C single nucleotide polymorphism (SNP) diversely correlates with the clinical outcome in cancer. The aim of this study was to evaluate the potential prognostic value of T393C-SNP in complete resected only surgically treated esophageal cancer (EC).

METHODS

Genomic DNA was extracted from peripheral blood leucocytes of 190 patients who underwent only complete surgical resection for EC. T393C-SNP was correlated with clinic-pathological parameters, tumor cell dissemination in bone marrow (DTC) and clinical outcome.

RESULTS

T-allele carriers had more advanced disease due to presence of lymph node metastasis (P < 0.0001) and DTC (P = 0.01) and higher recurrence rate (P = 0.01) compared to CC genotype. The disease-free (P < 0.001) and overall survival (P < 0.001) was better in CC compared to TT and TC patients. In the multivariate Cox regression disease-stage adjusted analysis the T393C-SNP was identified as a strong independent prognostic factor for recurrence (hazard ratio 1.8, P = 0.01) and survival (hazard ratio 2.5, P < 0.001) in EC patients.

CONCLUSION

Determination of T393C-SNP preoperatively will allow allocation of EC patients into different risk profiles which may help to stratify patients eligible for neoadjuvant and or adjuvant therapy.

Analysis of the DPYD gene implicated in 5-fluorouracil catabolism in Chinese cancer patients

BACKGROUND AND OBJECTIVE

5-fluorouracil (5-FU) is still a widely used anticancer drug. More than 85% of the 5-FU administered is catabolized by dihydropyrimidine dehydrogenase (DPD) in the liver. However, mutations in the DPD gene have been found to be associated with low DPD activity causing severe complications. The purpose of this study was to determine the mutation frequency of four exons in Chinese cancer patients and the relationship between genotype and DPD activity.

METHODS

Samples from 142 cancer patients were investigated in this study. The DPD activity was determined by reversed-phase HPLC. Exons 2, 13, 14 and 18 were amplified by polymerase chain reaction (PCR), sequenced and analysed from both sense and antisense directions. Nonparametric one-sample Kolmogorov-Smirnov test was used for distribution analysis; two independent samples t-test and one-way anova was performed for two groups and three groups analyses, respectively.

RESULTS AND DISCUSSION

Plasma-DPD activities in the 142 cancer patients followed a Gaussian distribution. The mean plasma-DPD activity in women was lower than that in men (P = 0.006). Four mutations, 85T>C(DPYD*9A), 1627A>G(DPYD*5), 1896T>C and 2194G>A(DPYD*6), were found in the 142 cancer patients. The following mutations reported by others were not detected: 61C>T, 62G>A, 74A>G, 1601G>A(DPYD*4), 1679T>G(DPYD*13), 1714C>G, 1897delC(DPYD*3) and IVS 14 + 1G>A. No significant correlation was found between three mutations [85T>C(DPYD*9A), 1627A>G (DPYD*5) and 1896T>C], and DPD activity was found.

CONCLUSION

No clear correlation between the mutations studied and DPD activity could be established in this study. However, larger-scale prospective studies are needed to better assess the reported genotype-phenotype correlations.

Profiling dihydropyrimidine dehydrogenase deficiency in patients with cancer undergoing 5-fluorouracil/capecitabine therapy

Fluoropyrimidine drugs such as 5-fluorouracil (5-FU) and capecitabine are a mainstay in the treatment of numerous solid tumors, including colorectal cancers, alone or as part of combination therapies. Cytotoxic drugs such as 5-FU and oral capecitabine display narrow therapeutic indexes combined with high interpatient pharmacokinetic variability. As a result, severe toxicities often limit or delay the administration of successive, optimal chemotherapeutic courses, leading to unfavorable clinical outcome in patients with cancer. Catabolism and deactivation of fluoropyrimidine drugs depend on a single and exclusive enzymatic step driven by dihydropyrimidine dehydrogenase (DPD). Dihydropyrimidine dehydrogenase is prone to marked circadian rhythms, drug-drug interactions, and genetic polymorphisms; influence of its erratic activity on 5-FU pharmacokinetics and toxicity profile has been extensively investigated, and it is now well known that DPD deficiency leads to severe toxicities with 5-FU or possibly capecitabine exposure. With the ever-increasing number of patients with cancer likely to be treated with fluoropyrimidines, predicting and preventing the occurrence of such toxicities is now a major issue in clinical oncology. Early determination of DPD status in patients with cancer would allow identification of those at risk and help in subsequent dose adjustment or selection of other treatment modalities. Numerous methods, either genotypic or phenotypic, have been proposed to achieve this goal. This review covers a wide range of techniques available to establish DPD status in patients with cancer.

Intra-ethnic differences in genetic variants of the UGT-glucuronosyltransferase 1A1 gene in Chinese populations

Variants within the human UGT1A1 gene are associated with irinotecan induced severely adverse reactions and hyperbilirubinemia. Intra-ethnic differences in the genetic variation and haplotypes of UGT1A1 gene have been analyzed in the present study. Relationship between the concentrations of total serum bilirubin (T-bil) and haplotype structure of UGT1A1 in healthy people were also evaluated. We genotyped five functional polymorphisms including -3279T>G and -3156G>A in the enhancer region, (TA)6>7 in the TATA box, and 211G>A (G71R), 686C>A (P229Q) in the exon1 region of UGT1A1 in three groups of healthy Chinese ethnic populations, consisting of 264 subjects of She origin, 539 of Han origin and 273 of Dong origin. The distribution of -3279T>G, (TA)6>7, 211G>A of UGT1A1 differed greatly as between the three ethnic groups. All of six haplotypes differed considerably between at least two of the three groups, which highlighted the need to analyze clinically irinotecan toxicity relevant SNPs and haplotypes in a variety of different racial groups within the Chinese population. Total bilirubin concentration in homozygous carriers of the -3279G and (TA)7 allele were significantly higher than those in heterozygous carriers or homozygous carriers of wild-type alleles. Carriers of the variant haplotypes (-3279G; -3156A; (TA)7; 211G; 686C) had higher serum T-Bil concentrations compared with the other groups. Our results indicate that heterogeneity among different ethnic populations is possibly the result of microevolution and is relevant to studies into the effect of tailored drug treatment.

MDR1 genotype-related pharmacokinetics: fact or fiction?

Multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics. The first investigation of the effects of MDR1 genotypes on pharmacotherapy was reported in 2000; a silent single nucleotide polymorphism (SNP), C3435T in exon 26, was found to be associated with the duodenal expression of MDR1, and thereby the plasma concentration of digoxin after oral administration. In the last 5 years, clinical studies have been conducted around the world on the association of MDR1 genotype with MDR1 expression and function in tissues, and with the pharmacokinetics and pharmacodynamics of drugs; however, there are still discrepancies in the results on C3435T. In 1995, a novel concept to predict in vivo oral pharmacokinetic performance from data on in vivo permeability and in vitro solubility has been proposed, and this Biopharmaceutical Classification System strongly suggested that the effects of intestinal MDR1 on the intestinal absorption of substrates is minimal in the case of commercially available oral drugs, and therefore MDR1 genotypes are little associated with the pharmacokinetics after oral administration. This review summarizes the latest reports for the future individualization of pharmacotherapy based on MDR1 genotyping, and attempts to explain discrepancies.

Hepatotoxicity of Chemotherapy

The selection of an antineoplastic regimen for an oncology patient is based first on the availability of effective drugs and then on a balancing of potential treatment-related toxicities with the patient's clinical condition and associated comorbidities. Liver function abnormalities are commonly observed in this patient population and identifying their etiology is often difficult. Immunosuppression, paraneoplastic phenomena, infectious diseases, metastases, and poly-pharmacy may cloud the picture. While criteria for standardizing liver injury have been established, dose modifications often rely on empiric clinical judgment. Therefore, a comprehensive understanding of hepatotoxic manifestations for the most common chemotherapeutic agents is essential. We herein review the hepatotoxicity of commonly used antineoplastic agents and regimens.

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.

Role of Cytochrome P450 Activity in the Fate of Anticancer Agents and in Drug Resistance

Although activity of cytochrome P450 isoenzymes (CYPs) plays a major role in the fate of anticancer agents in patients, there are relatively few clinical studies that evaluate drug metabolism with therapeutic outcome. Nevertheless, many clinical reports in various non-oncology fields have shown the dramatic importance of CYP activity in therapeutic efficacy, safety and interindividual variability of drug pharmacokinetics. Moreover, variability of drug metabolism in the liver as well as in cancer cells must also be considered as a potential factor mediating cancer resistance. This review underlines the role of drug metabolism mediated by CYPs in pharmacokinetic variability, drug resistance and safety. As examples, biotransformation pathways of tamoxifen, paclitaxel and imatinib are reviewed. This review emphasises the key role of therapeutic drug monitoring as a complementary tool of investigation to in vitro data. For instance, pharmacokinetic data of anticancer agents have not often been published within subpopulations of patients who show ultra-rapid, extensive or poor metabolism (e.g. due to CYP2D6 and CYP2C19 genotypes). Besides kinetic variability in the systemic circulation, induction of CYP activity may participate in creating drug resistance by speeding up the cancer agent degradation specifically in the target cells. For one cancer agent, various mechanisms of resistance are usually identified within different cell clones. This review also tries to emphasise that drug resistance mediated by CYP activity in cancer cells should be taken into consideration to a greater degree. The unequivocal identification of the metabolising enzymes involved in clinical conditions will eventually allow improvement and individualisation of anticancer agent therapy, i.e. drug dosage and selection. In addition, a more complete understanding of the metabolism of anticancer agents will assist in the prediction of drug-drug interactions, as anticancer agent combinations are becoming more prevalent.

Prevention and management of prostate cancer chemotherapy complications

Prevention and management of the adverse effects of prostate cancer chemotherapy depend on skilled regimen selection, dose adjustment, use of supportive care strategies, and a thorough understanding of the patient- and regimen-related factors that determine the risk for toxicity. Urologists, radiation oncologists, and primary care providers can play an important role before chemotherapy is prescribed by judicious use of treatments that impair bone marrow and other vital organ function. The current role of chemotherapy in prostate cancer is palliative. Successful palliation depends on reducing cancer-related suffering without introducing treatment-related suffering. Thus prevention and management of toxicity is central to the success of chemotherapy in advanced prostate cancer.

Genetic variation and hematology: single-nucleotide polymorphisms, haplotypes, and complex disease

In the course of generating a draft sequence of the human genome, we now recognize the enormous scope of genetic variation among humans, which can be used to probe the genetics of complex diseases such as leukemia or thrombosis. There is already mounting evidence of new susceptibility genes and genes that interact with environmental factors. Genetic variants, especially single-nucleotide polymorphisms (SNPs), also can be utilized to investigate potential modifiers of disease. Genetic variation can be applied to study pharmacogenomics, which could eventually drive the choice of therapeutic and interventional strategies. The genomic revolution ultimately should give insights into key mechanisms in hematological disorders that can be translated into targeted therapies.

Pharmacogenetics affects dosing, efficacy, and toxicity of cytochrome P450-metabolized drugs

Drug-metabolizing enzyme activity is one of many factors affecting patient response to medications. The objective of this review is to highlight the potential for genetic variability in cytochrome P450 enzyme activity that can lead to interperson differences in response to drugs. Awareness and application of this knowledge will improve drug use in clinical practice and provide the physician with further appreciation that standard drug dosing may not be appropriate in all patients.

Arylamine N-acetyltransferases and drug response

Arylamine N-acetyltransferases (NATs) play an important role in the interaction of competing metabolic pathways determining the fate of and response to xenobiotics as therapeutic drugs, occupational chemicals and carcinogenic substances. Individual susceptibility for drug response and possible adverse drug reactions are modulated by the genetic predisposition (manifested for example, by polymorphisms) and the phenotype of these enzymes. For all drugs metabolized by NATs, the impact of different in vivo enzyme activities is reviewed with regard to therapeutic use, prevention of side effects and possible indications for risk assessment by phenotyping and/or genotyping. As genes of NATs are susceptibility genes for multifactorial adverse effects and xenobiotic-related diseases, risk prediction can only be made possible by taking the complexity of events into consideration.

Pharmaceutical education in the wake of genomic technologies for drug development and personalized medicine

The development of safe and effective new therapeutics is a long, difficult, and expensive process. Over the last 20-30 years, recombinant DNA (rDNA) technology has provided a multiple of new methods, molecular targets and DNA-based diagnostics to pharmaceutical research that can be utilized in assays for screening and developing potential biopharmaceutical drugs. In parallel, new innovative approaches to drug delivery systems were discovered and reached the market. Pharmaceutical biotechnology, pharmacogenomics, combinatorial chemistry, in close relation to high-throughput screening technologies, and bioinformatics are major advances that give a new direction to pharmaceutical sciences. To meet with the needs of this new dynamic era of pharmaceutical research and health care environment, pharmaceutical education has to set new priorities to keep pace with the challenges related to genomic technologies. The development of new initiative education programs, for both undergraduate and graduate curricula, in pharmacy has to be focused on preparing pharmacists oriented for both pharmacy practice and drug research and development. This can be achieved by providing future pharmacists with knowledge, skills and attitudes to be more competitive in the health care system, pharmacy practice-related fields, pharmaceutical industry and drug research and development areas, or finally in academia. Educators and pharmacy school members have the responsibility of deciding how, to what extent, by which methods, and/or in which way these changes and new directions in the education programs should be developed.

Update on pharmacogenetics in cancer chemotherapy

This review describes how genetic differences among patients may change the therapeutic outcome in cancer chemotherapy. Severe toxicity in genetically predisposed patients is predominantly associated with mutations in drug metabolism enzyme genes, and an update on genetic intolerance to 6-mercaptopurine, 5-fluorouracil, and irinotecan is provided. Moreover, recent findings pointed out that the methylenetetrahydrofolate reductase (MTHFR) C677T mutation might change patient susceptibility to the toxic effects of the cyclophosphamide, methotrexate, 5-fluorouracil (CMF) regimen and raltitrexed. Finally, it is emerging that not only toxicity, but also response to chemotherapy could be influenced by pharmacogenetic determinants, and the clinical relevance of polymorphisms in thymidylate synthase (TS) and glutathione-S-transferase (GST) genes is discussed.

Genetic susceptibility to adverse drug reactions

Adverse drug reactions (ADRs) are a major clinical problem. Genetic factors can determine individual susceptibility to both dose-dependent and dose-independent ADRs. Determinants of susceptibility include kinetic factors, such as gene polymorphisms in cytochrome P450 enzymes, and dynamic factors, such as polymorphisms in drug targets. The relative importance of these factors will depend on the nature of the ADR; however, it is likely that more than one gene will be involved in most instances. In the future, whole genome single nucleotide polymorphism (SNP) profiling might allow an unbiased method of determining genetic predisposing factors for ADRs, but might be limited by the lack of adequate numbers of patient samples. The overall clinical utility of genotyping in preventing ADRs needs to be proven by the use of prospective randomized controlled clinical trials.