Dihydropyrimidine dehydrogenase pharmacogenetics in Caucasian subjects

DPYD genotyping and predicting fluoropyrimidine toxicity: where do we stand?

Fluoropyrimidines (FPs) are antineoplastic drugs widely used in the treatment of various solid tumors. Nearly 30% of patients treated with FP chemotherapy experience severe FP-related toxicity, and in some cases, toxicity can be fatal. Patients with reduced activity of DPD, the main enzyme responsible for the breakdown of FP, are at an increased risk of experiencing severe FP-related toxicity. While European regulatory agencies and clinical societies recommend pre-treatment DPD deficiency screening for patients starting treatment with FPs, this is not the case with American ones. Pharmacogenomic guidelines issued by several pharmacogenetic organizations worldwide recommend testing four DPD gene (DPYD) risk variants, but these can predict only a proportion of toxicity cases. New evidence on additional common DPYD polymorphisms, as well as identification and functional characterization of rare DPYD variants, could partially address the missing heritability of DPD deficiency and FP-related toxicity.

Predictive value of clinical toxicities of chemotherapy with fluoropyrimidines and oxaliplatin in colorectal cancer by DPYD and GSTP1 gene polymorphisms

BACKGROUND

Fluoropyrimidines and platinum are still widely used for colorectal cancer (CRC) management. Several studies have reported that mutations of dihydropyrimidine dehydrogenase (DPYD) and glutathione S-transferase pi-1 (GSTP1) polymorphisms are related to chemotherapy-related adverse events. In the present study, we purposed to assess the impact of DPYD and GSTP1 variants on the toxicity of adjuvant chemotherapy risk among the Hakka population, minimize adverse events, and to maximize therapy outcome for individualized treatment.

METHODS

Genotyping was examined in 104 patients diagnosed with CRC cases and receiving fluoropyrimidine and platinum drug-based chemotherapy regimen by direct sequencing of DPYD and GSTP1 polymorphisms. Three DPYD variants including *2A, *5A, *9A, and GSTP1 c.313A>G were analyzed and clinical outcomes were assessed.

RESULTS

The data suggest that the incidence of DPYD*5A, DPYD*9A, and GSTP1 c.313A>G variants were 38.4%, 24%, and 32.7%, respectively. DPYD*2A variant was not found. A total of 23 patients (22.1%) suffered severe vomiting and 19 patients (18.3%) suffered severe anemia. DPYD*5A polymorphism was found significantly associated with grade 3/4 ulceration (p = 0.001). GSTP1 was determined to be an independent risk factor for severe vomiting and skin ulceration (p = 0.042 and p = 0.018, respectively). Patients with GSTP1 c. 313A>G mutant type contributed to a higher risk for grade severe toxicity compared with wild genotype (p = 0.027). Nevertheless, no significant difference was found between patients with DPYD*2A, *5A, and *9A for chemotherapeutic toxicity.

CONCLUSIONS

The results demonstrated that GSTP1 polymorphisms were useful predictors of severe events. Screening of single-nucleotide polymorphisms of GSTP1 in colorectal cancer patients before chemotherapy may help to realize personalized therapy.

DPYD and Fluorouracil-Based Chemotherapy: Mini Review and Case Report

5-Fluorouracil remains a foundational component of chemotherapy for solid tumour malignancies. While considered a generally safe and effective chemotherapeutic, 5-fluorouracil has demonstrated severe adverse event rates of up to 30%. Understanding the pharmacokinetics of 5-fluorouracil can improve the precision medicine approaches to this therapy. A single enzyme, dihydropyrimidine dehydrogenase (DPD), mediates 80% of 5-fluorouracil elimination, through hepatic metabolism. Importantly, it has been known for over 30-years that adverse events during 5-fluorouracil therapy are linked to high systemic exposure, and to those patients who exhibit DPD deficiency. To date, pre-treatment screening for DPD deficiency in patients with planned 5-fluorouracil-based therapy is not a standard of care. Here we provide a focused review of 5-fluorouracil metabolism, and the efforts to improve predictive dosing through screening for DPD deficiency. We also outline the history of key discoveries relating to DPD deficiency and include relevant information on the potential benefit of therapeutic drug monitoring of 5-fluorouracil. Finally, we present a brief case report that highlights a limitation of pharmacogenetics, where we carried out therapeutic drug monitoring of 5-fluorouracil in an orthotopic liver transplant recipient. This case supports the development of robust multimodality precision medicine services, capable of accommodating complex clinical dilemmas.

Pharmacogenetics of toxicity of 5-fluorouracil, doxorubicin and cyclophosphamide chemotherapy in breast cancer patients

The differences in patients' response to the same medication, toxicity included, are one of the major problems in breast cancer treatment. Chemotherapy toxicity makes a significant clinical problem due to decreased quality of life, prolongation of treatment and reinforcement of negative emotions associated with therapy. In this study we evaluated the genetic and clinical risk factors of FAC chemotherapy-related toxicities in the group of 324 breast cancer patients. Selected genes and their polymorphisms were involved in FAC drugs transport (ABCB1, ABCC2, ABCG2,SLC22A16), metabolism (ALDH3A1, CBR1, CYP1B1, CYP2C19, DPYD, GSTM1, GSTP1, GSTT1, MTHFR,TYMS), DNA damage recognition, repair and cell cycle control (ATM, ERCC1, ERCC2, TP53, XRCC1). The multifactorial risk models that combine genetic risk modifiers and clinical characteristics were constructed for 12 toxic symptoms. The majority of toxicities was dependent on the modifications in components of more than one pathway of FAC drugs, while the impact level of clinical factors was comparable to the genetic ones. For the carriers of multiple high risk factors the chance of developing given symptom was significantly elevated which proved the factor-dosage effect. We found the strongest associations between concurrent presence of clinical factors - overall and recurrent anemia, nephrotoxicity and early nausea and genetic polymorphisms in genes responsible for DNA repair, drugs metabolism and transport pathways. These results indicate the possibility of selection of the patients with expected high tolerance to FAC treatment and consequently with high chance of chemotherapy completion without the dose reduction, treatment delays and decline in the quality of life.

Role of Genetic Variations in Determining Treatment Outcome in Head and Neck Cancer

Worldwide, head and neck squamous cell carcinoma (HNSCC) is responsible for >550,000 diagnoses and 380,000 deaths annually. It originates in the upper aerodigestive tract and has a multifactorial origin involving both genetic and lifestyle risk factors. The clinical management of HNSCC involves surgery, radiotherapy, and chemotherapy. Several studies point to the role of genetic variations in predicting drug efficacy and toxicity. Cancer pharmacogenomics has fast emerged as a new and promising field for the early identification of genetic markers that can predict drug response or toxicity, with the number of studies of genetic polymorphisms as prognostic factors of HNSCC treatment outcomes growing. The number of studies evaluating the association of candidate polymorphisms in drug-metabolising Phase I and II enzymes with treatment outcome far exceed the studies involving other candidate genes, such as those involved in drug metabolism, DNA repair, and cell cycle regulation. This review focusses on the relevance of genetic variations in genes, where the corresponding gene products play an important role in drug metabolism (TPMT, DPD), DNA repair (X-ray repair cross complementing 1), cell cycle (tumour protein P53), and carcinogenesis (matrix metalloproteinase 3 and 7), thereby contributing to the treatment outcome for HNSCC. This could greatly help clinicians in identifying genetic markers useful for the selection of optimal drugs, dose, and treatment duration on an individual basis, resulting in improved drug efficacy and decreased toxicity. However, further studies are needed in well characterised and larger HNSCC populations with proper validation of pharmacogenetic markers in experimental settings before application in clinical routine diagnostics.

Hydrogel microarray for detection of polymorphisms in the UGT1A1, DPYD, GSTP1 and ABCB1 genes

BACKGROUND

Improving the efficacy of anticancer therapy remains an urgent and very important task. Screening of the individual genetic metabolism of cancer patients allows for prescribing adequate medication in the correct dose as well as for decreasing side effects associated with drug toxicity.

OBJECTIVE

Estimation of a microarray-based method for genotyping of the UGT1A1, DPYD, GSTP1, and ABCB1 metabolic regulation genes to evaluate for an increased risk of toxicity of anticancer drugs.

METHODS

The microarray was used to conduct genotyping of specimens taken from 115 cancer patients and 31 healthy donors.

RESULTS

A microarray-based method for identification of the rs8175347, rs3918290, rs1695, and rs1045642 polymorphisms in the corresponding UGT1A1, DPYD, GSTP1, and ABCB1 genes has been developed for genotyping. The results obtained were in full concordance with those obtained using control sequencing. The frequencies of the rs8175347, rs3918290, rs1695, and rs1045642 genetic variations were 0.38, 0, 0.35, and 0.56, respectively.

CONCLUSION

The implementation of this biochip-based method in diagnostic practice should increase the overall survival and quality of life of cancer patients, decrease the length of their hospital stay, and reduce treatment costs.

Endogenous plasma and salivary uracil to dihydrouracil ratios and DPYD genotyping as predictors of severe fluoropyrimidine toxicity in patients with gastrointestinal malignancies

OBJECTIVE

The aim of this study was to evaluate the use of plasma and saliva uracil (U) to dihydrouracil (UH₂) metabolic ratio and DPYD genotyping, as a means to identify patients with dihydropyrimidine dehydrogenase (DPD) deficiency and fluoropyrimidine toxicity.

METHODS

Paired plasma and saliva samples were obtained from 60 patients with gastrointestinal cancer, before fluoropyrimidine treatment. U and UH₂ concentrations were measured by LC-MS/MS. DPYD was genotyped for alleles *7, *2A, *13 and Y186C. Data on toxicity included grade 1 to 4 neutropenia, mucositis, diarrhea, nausea/vomiting and cutaneous rash.

RESULTS

35% of the patients had severe toxicity. There was no variant allele carrier for DPYD. The [UH₂]/[U] metabolic ratios were 0.09-26.73 in plasma and 0.08-24.0 in saliva, with higher correlation with toxicity grade in saliva compared to plasma (r_s=-0.515 vs r_s=-0.282). Median metabolic ratios were lower in patients with severe toxicity as compared to those with absence of toxicity (0.59 vs 2.83 saliva; 1.62 vs 6.75 plasma, P<0.01). A cut-off of 1.16 for salivary ratio was set (AUC 0.842), with 86% sensitivity and 77% specificity for the identification of patients with severe toxicity. Similarly, a plasma cut-off of 4.0 (AUC 0.746), revealed a 71% sensitivity and 76% specificity.

CONCLUSIONS

DPYD genotyping for alleles 7, *2A, *13 and Y186C was not helpful in the identification of patients with severe DPD deficiency in this series of patients. The [UH₂]/[U] metabolic ratios, however, proved to be a promising functional test to identify the majority of cases of severe DPD activity, with saliva performing better than plasma.

Influence of metastatic disease on the usefulness of uracil pharmacokinetics as a screening tool for DPD activity in colorectal cancer patients

PURPOSE

Dihydropyrimidine dehydrogenase (DPD) deficiency can lead to severe toxicity in patients treated with a standard dose of a fluoropyrimidine such as 5-fluorouracil or capecitabine (CAP). Administration of oral uracil and subsequent measurement of uracil and dihydrouracil (DHU) plasma concentrations has been used to identify patients with DPD deficiency. Liver metastasis might influence systemic DPD activity. The aim of the study was to investigate the effect of metastatic disease on the pharmacokinetics of uracil and DHU after oral administration of uracil.

METHODS

500 mg/m(2) uracil was administered orally to 12 subjects with stages II-III colorectal cancer (CRC) who were treated in the adjuvant setting and to 12 subjects with stage IV metastasized CRC, all treated with CAP containing therapy. All subjects had a normal DPD activity defined as >6 nmol/mg/h determined in peripheral blood mononuclear cells.

RESULTS

The mean uracil clearance [CL 51.7 (SD 6.4) vs. 46.7 (SD 13.0) l/h], area under the curve [AUC0-220min 20.6 (SD 6.4) vs. 21.0 (SD 5.7) h mg/l], elimination half-life [t 1/2 21 (SD 7) vs. 21 (SD 8) min], maximum concentration time [T max 27 (SD 9) vs. 25 (SD 9) min], volume of distribution [V 26.58 (SD 10.11) vs. 21.10 (SD 8.48) l] and the elimination constant [k el 2.01 (SD 0.56) vs. 2.41 (SD 0.72) h(-1)] did not differ significantly (p > 0.05) non-metastatic CRD versus metastatic CRC.

CONCLUSIONS

Metastasis does not alter uracil pharmacokinetics and is similar in CRC patients with and without metastasis. Therefore, the uracil test dose could be used as a DPD phenotype test in both adjuvantly treated and metastatic CRC patients using similar cutoff criteria to identify patients with DPD deficiency.

Germline oncopharmacogenetics, a promising field in cancer therapy

Pharmacogenetics (PGx) is the study of the relationship between inter-individual genetic variation and drug responses. Germline variants of genes involved in drug metabolism, drug transport, and drug targets can affect individual response to medications. Cancer therapies are characterized by an intrinsically high toxicity; therefore, the application of pharmacogenetics to cancer patients is a particularly promising method for avoiding the use of inefficacious drugs and preventing the associated adverse effects. However, despite continuing efforts in this field, very few labels include information about germline genetic variants associated with drug responses. DPYD, TPMT, UGT1A1, G6PD, CYP2D6, and HLA are the sole loci for which the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA) report specific information. This review highlights the germline PGx variants that have been approved to date for anticancer treatments, and also provides some insights about other germline variants with potential clinical applications. The continuous and rapid evolution of next-generation sequencing applications, together with the development of computational methods, should help to refine the implementation of personalized medicine. One day, clinicians may be able to prescribe the best treatment and the correct drug dosage based on each patient's genotype. This approach would improve treatment efficacy, reduce toxicity, and predict non-responders, thereby decreasing chemotherapy-associated morbidity and improving health benefits.

DPYD variants as predictors of 5-fluorouracil toxicity in adjuvant colon cancer treatment (NCCTG N0147)

BACKGROUND

Previous studies have suggested the potential importance of three DPYD variants (DPYD*2A, D949V, and I560S) with increased 5-FU toxicity. Their individual associations, however, in 5-FU-based combination therapies, remain controversial and require further systematic study in a large patient population receiving comparable treatment regimens with uniform clinical data.

METHODS

We genotyped 2886 stage III colon cancer patients treated adjuvantly in a randomized phase III trial with FOLFOX or FOLFIRI, alone or combined with cetuximab, and tested the individual associations between functionally deleterious DPYD variants and toxicity. Logistic regressions were used to assess univariate and multivariable associations. All statistical tests were two-sided.

RESULTS

In 2594 patients with complete adverse event (AE) data, the incidence of grade 3 or greater 5FU-AEs in DPYD*2A, I560S, and D949V carriers were 22/25 (88.0%), 2/4 (50.0%), and 22/27 (81.5%), respectively. Statistically significant associations were identified between grade 3 or greater 5FU-AEs and both DPYD*2A (odds ratio [OR] = 15.21, 95% confidence interval [CI] = 4.54 to 50.96, P < .001) and D949V (OR = 9.10, 95% CI = 3.43 to 24.10, P < .001) variants. Statistical significance remained after adjusting for multiple variables. The DPYD*2A variant statistically significantly associated with the specific AEs nausea/vomiting (P = .007) and neutropenia (P < .001), whereas D949V statistically significantly associated with dehydration (P = .02), diarrhea (P = .003), leukopenia (P = .002), neutropenia (P < .001), and thrombocytopenia (P < .001). Although two patients with I560S had grade≥3 5FU-AEs; a statistically significant association could not be demonstrated because of its low frequency (P = .48).

CONCLUSION

In the largest study to date, statistically significant associations were found between DPYD variants (DPYD*2A and D949V) and increased incidence of grade 3 or greater 5FU-AEs in patients treated with adjuvant 5-FU-based combination chemotherapy.

Molecular markers of chemotherapeutic response and toxicity in colorectal cancer

Outcomes in colorectal cancer have improved over the last 15 years; this is in part due to the optimization of 5-fluorouracil schedules and the introduction of new and effective chemotherapeutic agents, such as irinotecan and oxaliplatin. However, not all patients respond to these agents and a proportion may suffer severe side effects from particular chemotherapy drugs. These observations have resulted in a concerted research effort to identify markers of chemotherapy efficacy and toxicity. Here we review the evidence for using molecular markers to individualize chemotherapy treatment in colorectal cancer.

Dihydropyrimidine dehydrogenase gene as a major predictor of severe 5-fluorouracil toxicity

The importance of polymorphisms in the dihydropyrimidine dehydrogenase (DPD) gene (DPYD) for the prediction of severe toxicity in 5-fluorouracil (5-FU) based chemotherapy has been controversially debated. As a key enzyme in the catabolism of 5-FU, DPD is the top candidate for pharmacogenetic studies on 5-FU toxicity, since a reduced DPD activity is thought to result in an increased half-life of the drug, and thus, an increased risk of toxicity. Here, we review the current knowledge on well-known and frequently studied DPYD variants such as the c.1905+1G>A splice site variant, as well as the recent discoveries of important functional variation in the noncoding regions of DPYD. We also outline future directions that are needed to further improve the risk assessment of 5-FU toxicity, in particular with respect to metabolic profiling and in the context of different combination therapeutic regimens, in which 5-FU is used today.

Phase II study of pharmacogenetic-tailored therapy in elderly colorectal cancer patients

BACKGROUND

Retrospective analyses suggested that a pharmacogenetic approach may allow a tailored selection of chemotherapy for metastatic colorectal cancer.

AIM

We conducted a phase II study of pharmacogenetic-selected first-line chemotherapy in elderly patients with advanced colorectal cancer, with the aim to improve efficacy and to reduce toxicity in this group of patients.

METHODS

24 patients were enrolled in this study. Chemotherapy regimen was prospectively assigned based on TS, DPD, ERCC-1 and UGT1A1 genotyping results. Twelve patients (50%) were treated with modified FOLFIRI, 11 patients (46%) with modified FOLFOX6 and 1 (4%) with De Gramont regimen.

RESULTS

A partial remission was obtained in 4 cases (17%), stable disease in 8 cases (33%) and progressive disease in 12 cases (50%). Grade 3-4 neutropenia was observed in 7 patients (29%) and diarrhoea in 3 cases (12%). The trial was then interrupted according to study design requiring 13 partial remissions out of the first 24 patients enrolled as the necessary response rate level in order to continue.

CONCLUSION

Prospective selection of chemotherapy based on TS, DPD, ERCC-1 and UGT1A1 expression in elderly advanced colorectal cancer patients failed to confirm previous results. A more accurate validation of retrospective findings is warranted before these molecular markers can be used for treatment selection in the clinical practice.

Systems pharmacology assessment of the 5-fluorouracil pathway

AIM

To assess the impact of the 5-fluorouracil (5-FU) drug-pathway genes on cytotoxicity, and determine whether loss-of-function analyses coupled with functional assays can help prioritize pharmacogenomic candidate genes.

MATERIALS & METHODS

Dose-response experiments were used to quantify the phenotype of sensitivity to 5-FU following the specific knockdown of genes selected from the 5-FU PharmGKB drug pathway in three human colorectal cell lines. Changes in sensitivity were considered significant if the IC(50) for shRNA-exposed cells were three standard deviations outside the mean IC(50) for control-treated cells.

RESULTS

Of the 24 genes analyzed, 13 produced significant changes on the phenotype of sensitivity to 5-FU (DHFR, DPYS, DTYMK, DUT, FPGS, GGH, NME1, NT5C, RRM1, TYMS, UCK2, UNG and UMPS).

CONCLUSION

The RNAi screening strategy enabled prioritization of the genes from the 5-FU drug pathway. Further validation of the genes credentialed in this study should include gene activity or expression and mutation analyses of clinical samples.

Routine dihydropyrimidine dehydrogenase testing for anticipating 5-fluorouracil-related severe toxicities: hype or hope?

5-Fluorouracil (5-FU) is a mainstay for treating colorectal cancer, alone or more frequently as part of combination therapies. However, its efficacy/toxicity balance is often limited by the occurrence of severe toxicities, showing in about 15%-20% of patients. Several clinical reports have shown the deleterious effect of dihydropyrimidine dehydrogenase (DPD) genetic polymorphism, a condition that reduces the liver detoxification step of standard dosages of 5-FU, in patients undergoing fluoropyrimidine-based therapy. Admittedly, DPD deficiency accounts for 50%-75% of the severe and sometimes life-threatening toxicities associated with 5-FU (or oral 5-FU). However, technical consensus on the best way to identify patients with DPD deficiency before administrating 5-FU is far from being achieved. Consequently, no regulatory step has been undertaken yet to recommend DPD testing as part of routine clinical practice for securing the administration of 5-FU. This review covers the limits and achievements of the various strategies proposed so far for determining DPD status in patients scheduled for 5-FU therapy.

The Value of Dihydrouracil/Uracil Plasma Ratios in Predicting 5-Fluorouracil-Related Toxicity in Colorectal Cancer Patients

This study investigated the relationship between the dihydrouracil/uracil (UH2/U) plasma ratio, a surrogate marker of dihydropyrimidine dehydrogenase (DPD) activity, and 5-fluorouracil (5-FU)-related early toxicity. Plasma UH2/U ratios were determined in 68 colorectal cancer patients and 100 healthy controls. A cutoff value indicative of DPD deficiency was calculated using receiver operator characteristics. Patients experiencing toxicity were screened for the DPD G-to-A point mutation within the 5′-splicing donor site of intron 14 (IVS14+1G>A). Overall, 24/68 patients (35%) experienced toxicity (all grades) and abnormal UH2/U ratios were demonstrated in 21/24 (87.5%) patients. Drug concentrations up to 130 times the recommended level were found in 13/24 (54%) patients experiencing toxicity. One patient experiencing toxicity was a heterozygous carrier of the IVS14+1G>A mutation. A low UH2/U plasma ratio had a sensitivity of 0.87 and specificity of 0.93 for predicting 5-FU-induced toxicity. Systematic detection of DPD-deficient patients using the UH2/U ratio could optimize 5-FU-based chemotherapy and minimize life-threatening toxicity.

Associations of various gene polymorphisms with toxicity in colorectal cancer patients receiving oral uracil and tegafur plus leucovorin: a prospective study

BACKGROUND

To assess the predictive value of polymorphism in nine genes, primarily thymidylate synthase (TS) and orotate phosphoribosyltransferase (OPRT), which relates to 5-fluorouracil (5-FU) metabolism, for toxicity in patients treated with oral uracil/tegafur (UFT) plus leucovorin (LV).

PATIENTS AND METHODS

We treated 99 patients with stage II or III colorectal carcinoma with oral UFT + LV. Germline DNA from patients was genotyped for 5-FU and folate metabolism-relating genes. CYP2A6, tegafur-activating enzyme, and uridine diphosphate-glucuronosyltransferase 1A1 genetic variation were also assessed. Toxicity was graded by the National Cancer Institute Common Toxicity Criteria, version 2.0.

RESULTS

The multivariate logistic regression revealed that OPRT 638G>C polymorphism was associated with grade 3 diarrhea [odds ratio (OR) 19.84 for patients with the C/C homozygous type compared with patients with wild type, P = 0.014] and polymorphisms of UGT1A1 were associated with hyperbilirubinemia (OR 38.76 for homozygotes and double heterozygotes of *6 or *28 compared with wild type, P = 0.0008). No relationships were observed between TS polymorphisms and any toxicity.

CONCLUSIONS

OPRT polymorphism predicts toxicity, especially grade 3 or greater diarrhea to oral UFT + LV adjuvant chemotherapy, whereas TS does not, in our study cohort. UGT1A1 polymorphism seems to be a risk factor for hyperbilirubinemia due to UFT+LV.

Variants in the Dihydropyrimidine Dehydrogenase, Methylenetetrahydrofolate Reductase and Thymidylate Synthase Genes Predict Early Toxicity of 5-Fluorouracil in Colorectal Cancer Patients

Adverse drug reactions to 5-fluorouracil (5-FU)-based chemotherapy have been reported to be due, in part, to genetic variants of the genes for the drug-related enzymes thymidylate synthase (TS; TYMS gene), methylenetetrahydrofolate reductase ( MTHFR gene) and dihydropyrimidine dehydrogenase (DPD; DPYD gene). This study investigated whether selected genetic variants of the TYMS, MTHFR and DPYD genes predict 5-FU-related early toxicity. The prevalence of the genetic variants was determined in 122 colorectal cancer patients and in a reference population of 320 blood donors. Subgroup analysis of 68 of the colorectal cancer patients was carried out to determine the relationship between selected gene variants detected in peripheral mononuclear cells and tolerability during the first or second cycle of 5-FU based treatment. Toxicity was linked to the TYMS 2R/2R variant (relative risk [RR] 1.66; sensitivity 0.37; specificity 0.77) and to the MTHFR c1298 C/C genetic variant (RR 1.77; sensitivity 0.17; specificity 0.91). Patients with the genetic variant IVS14+1 G/A or c1896 C/T in the DPYD gene had a statistically significant increased risk of experiencing toxicity (RR 2 and 6, respectively), both having a high specificity (0.97 and 0.98, respectively) and low sensitivity (0.04 and 0.13, respectively). It is concluded that pre-treatment detection of genetic variants can help to predict early toxicity experienced by patients receiving 5-FU-based chemotherapy.

The contribution of deleterious DPYD gene sequence variants to fluoropyrimidine toxicity in British cancer patients

PURPOSE

The fluoropyrimidines have been extensively used for almost five decade worldwide for the treatment of solid cancers. However, severe toxicity is a major clinical problem and has been reported in association with deleterious sequence variants in dihydropyrimidine dehydrogenase (DPD) coding-gene (DPYD), causing DPD deficiency. Genetic DPD deficiency has previously been considered to be insignificant in the British population. The study aim was to assess the contribution of deleterious DPYD sequence variants to fluoropyrimidine toxicity amongst British cancer patients.

METHODS

Sequencing of the coding region of DPYD was undertaken in 47 patients (27 female, mean age 61 years), mainly with GI malignancy, experiencing grade 3 or 4 toxicity on fluoropyrimidines according to CTCAE criteria.

RESULTS

Myelotoxicity (37.5%) and diarrhoea (37.5%) were the most frequent toxicities followed by mucositis (19.6%), hand-foot syndrome (3.6%) and neurotoxicity (1.8%). 4 of 47 (8.5%) patients carried the 1905+1G>A splice site variant. All 4 cases were female and 3 of 4 suffered severe diarrhoea. A further five cases carried other sequence variants (2846A>T n = 4, 1679T>G n = 1). In total, 9 (19%) patients carried deficiency associated DPYD sequence variants.

CONCLUSIONS

Contrary to previous estimates for a UK population, genetic DPD deficiency accounts for around 19% of cases of severe fluoropyrimidine toxicity. The influence of DPD deficiency is such that toxicity can be avoided by prior testing and appropriate 5-FU dose/regimen alteration.

Pharmacogenomics of Colorectal Cancer Prevention and Treatment

Pharmacogenomic tools are beginning to emerge that will provide guidance in the treatment and prevention of colorectal cancer. Significant individual genetic variation exists in drug metabolism of 5FU, capecitabine, irinotecan, and oxaliplatin that influences both the toxicity and efficacy of these agents. Recent FDA approval of genetic testing for mutations in the UGT1A1 gene that predict adverse reactions to irinotecan is ushering in a new era that will increasingly rely on genotyping to individualize treatment decisions for patients with cancer as well as for patients at high risk who may be candidates for chemoprevention agents. This review focuses on current knowledge regarding key mutations and polymorphisms which affect outcomes for colorectal cancer patients, as well as the pharmacogenetics of chemoprevention trials.

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.

Pharmacogenetics in chemotherapy of colorectal cancer

Although in recent years, chemotherapeutic options for colorectal carcinoma have expanded, overall response rates are still too low, with high rates of toxicity. Pharmacogenetics aim at predicting both treatment response and adverse effects in individual patients. This review describes the current knowledge of pharmacogenetic markers in the systemic treatment of colorectal cancer. UGT1A1*28 leads to reduced conjugation of SN-38, the active metabolite of irinotecan, resulting in an increased rate of adverse effects, especially neutropenia. To a lesser extent, increased 5-FU toxicity is predicted by DPYD*2A. A variable number of tandem repeats polymorphism in the thymidylate synthase enhancer region, in combination with a single nucleotide polymorphism C>G, may predict poorer response to 5-FU. Efficacy of oxaliplatin is influenced by polymorphisms in components of DNA repair systems, such as ERCC1 and XRCC1. Polymorphic changes in the endothelial growth factor receptor probably predict cetuximab efficacy. Furthermore, the antibody-depended cell-mediated cytotoxic effect of cetuximab may be reduced by polymorphisms in the immunoglobin G fragment C receptors. Bevacizumab efficacy is suspected to be influenced by polymorphisms in the VEGF gene and the hypoxia inducible factor 1alpha gene. Although the interpretation of pharmacogenetic studies is complicated, results imply a promising way of pretreatment prediction of chemotherapy efficacy and toxicity.

Strong association of a common dihydropyrimidine dehydrogenase gene polymorphism with fluoropyrimidine-related toxicity in cancer patients

Background

Cancer patients carrying mutations in the dihydropyrimidine dehydrogenase gene (DPYD) have a high risk to experience severe drug-adverse effects following chemotherapy with fluoropyrimidine drugs such as 5-fluorouracil (5-FU) or capecitabine. The pretreatment detection of this impairment of pyrimidine catabolism could prevent serious, potentially lethal side effects. As known deleterious mutations explain only a limited proportion of the drug-adverse events, we systematically searched for additional DPYD variations associated with enhanced drug toxicity.

Methodology/Principal Findings

We performed a whole gene approach covering the entire coding region and compared DPYD genotype frequencies between cancer patients with good (n = 89) and with poor (n = 39) tolerance of a fluoropyrimidine-based chemotherapy regimen. Applying logistic regression analysis and sliding window approaches we identified the strongest association with fluoropyrimidine-related grade III and IV toxicity for the non-synonymous polymorphism c.496A>G (p.Met166Val). We then confirmed our initial results using an independent sample of 53 individuals suffering from drug-adverse-effects. The combined odds ratio calculated for 92 toxicity cases was 4.42 [95% CI 2.12–9.23]; p (trend)<0.001; p (corrected) = 0.001; the attributable risk was 56.9%. Comparing tumor-type matched sets of samples, correlation of c.496A>G with toxicity was particularly present in patients with gastroesophageal and breast cancer, but did not reach significance in patients with colorectal malignancies.

Conclusion

Our results show compelling evidence that, at least in distinct tumor types, a common DPYD polymorphism strongly contributes to the occurrence of fluoropyrimidine-related drug adverse effects. Carriers of this variant could benefit from individual dose adjustment of the fluoropyrimidine drug or alternate therapies.

The dihydrouracil/uracil ratio in plasma, clinical and genetic analysis for screening of dihydropyrimidine dehydrogenase deficiency in colorectal cancer patients treated with 5-fluorouracil

A rapid and cost-effective reversed phase high performance liquid chromatography (HPLC) method for quantification of dihydrouracil to uracil ratio (UH2/U) in plasma has been developed and used to screen for dihydropyrimidine dehydrogenase (DPD) deficiency in nine patients treated with 5-fluorouracil (5-FU). This HPLC method is based on the use of a simultaneous UV detection at 205 and 268nm during the analysis run of the plasma extract and taking into account the particularity that UH2 shows no absorbance response at 268nm. The plasma UH2/U ratio values evaluated by the use of our HPLC assay were found to be highly correlated with the plasma 5-FU-half-life values and were significantly associated with the toxic side effects, whereas, data set provided from genetic analysis of the coding sequences of the DPD gene (DPYD) were found to be insufficient to explain all the cases of the 5-FU-related toxicity pattern. The proposed HPLC assay could be available for routine clinical use for DPD deficiency assessment in patients prior to 5-FU administration.

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.

CYP2A6 and the plasma level of 5-chloro-2, 4-dihydroxypyridine are determinants of the pharmacokinetic variability of tegafur and 5-fluorouracil, respectively, in Japanese patients with cancer given S-1

S-1 is an oral anticancer agent composed of tegafur (FT), 5-chloro-2,4-dihydroxypyridine (CDHP), and potassium oxonate. CDHP is added to prevent degradation of 5-fluorouracil (5-FU) by inhibiting dihydropyrimidine dehydrogenase. CYP2A6 is involved in the biotransformation of FT to 5-FU. Thus, we prospectively analyzed the effects of the CYP2A6 genotype, plasma level of CDHP, and patient characteristics on the pharmacokinetic (PK) variability of FT and 5-FU. Fifty-four Japanese patients with metastatic or recurrent cancers who received S-1 were enrolled. The CYP2A6 polymorphisms (*4A, *7, and *9) with deficient or reduced activity were analyzed. All subjects were classified into three groups according to their CYP2A6 genotype: wild type (*1/*1), one-variant allele (*1/any), or two-variant alleles (combination other than *1). The PK of FT, 5-FU, and CDHP were measured on day 1 of treatment. Multivariate regression analysis revealed that oral clearance of FT was associated with the CYP2A6 genotype (analysis of variance [ANOVA], P = 0.000838). The oral clearance of FT seen in patients with the two-variant alleles was significantly lower than those in wild type and the one-variant allele (95% confidence intervals 0.75-2.41 and 0.41-1.82, respectively; Tukey-Kramer test). The area under the time-concentration curve (AUC) of 5-FU was significantly correlated with the AUC of CDHP (ANOVA, P = 0.00126). The AUC of 5-FU and CDHP were inversely correlated with creatinine clearance (ANOVA, P = 0.0164 and P = 0.000762, respectively). Although the CYP2A6 variants are the cause of the PK variability of FT, the AUC of CDHP affected by renal function is the key determinant of the variability in the PK of 5-FU.

Pharmacogenomics of drug-metabolizing enzymes and drug transporters in chemotherapy

There is wide variability in the response of individuals to standard doses of drug therapy. This is an important problem in clinical practice, where it can lead to therapeutic failures or adverse drug events. Polymorphisms in genes coding for metabolizing enzymes and drug transporters can affect drug efficacy and toxicity. Pharmacogenomics aims to identify individuals predisposed to high risk of toxicity and low response from standard doses of anticancer drugs. This chapter focuses on the clinical significance of polymorphisms in drug-metabolizing enzymes and drug transporters in influencing efficacy and toxicity of anticancer therapy. The most important examples to demonstrate the influence of pharmacogenomics on anticancer therapy are thiopurine methyltransferase (TPMT), UGT (uridine diphosphate glucuronosyltransferase) 1A1*28, and DPD (dihydropyrimidine dehydrogenase) *2A, respectively, for 6-mercaptopurine, irinotecan, and 5-fluorouracil therapy. However, in most other anticancer therapies no clear association has been found for polymorphisms in drug-metabolizing enzymes and drug transporters and pharmacokinetics or pharmacodynamics of anticancer drugs. Evaluation of different regimens and tumor types showed that polymorphisms can have different, sometimes even contradictory, pharmacokinetic and pharmacodynamic effects in different tumors in response to different drugs. The clinical application of pharmacogenomics in cancer treatment therefore requires more detailed information regarding the different polymorphisms in drug-metabolizing enzymes and drug transporters. A greater understanding of complexities in pharmacogenomics is needed before individualized therapy can be applied on a routine basis.

Pharmacogenomics in gastrointestinal disorders

It is anticipated that unraveling the human genome will have a direct impact on the management of specific diseases. Variations or mutations in genes involved in drug metabolism or disease pathophysiology in gastroenterology and hepatology are expected to have effect on response to therapy. The spectrum of diseases is vast. Thus, we focus this review on clinical pharmacogenetics of inflammatory bowel disease, Helicobacter pylori infections, gastroesophageal reflux disease, irritable bowel syndrome, liver transplantation, and colon cancer. Although only a few genotyping tests are used regularly in clinical practice, we anticipate that in the future there will be more routine use of many of the tests described in this review.

Mutational spectrum of dihydropyrimidine dehydrogenase gene (DPYD) in the Tunisian population

Dihydropyrimidine dehydrogenase enzyme (DPD) deficiency is a pharmacogenetic syndrome leading to severe side-effects in patients receiving therapies containing the anticancer drug 5-fluorouracil (5-FU). The aim of this population study is to evaluate gene variations in the coding region of the dihydropyrimidine dehydrogenase gene (DPYD) in the Tunisian population. One hundred and six unrelated healthy Tunisian volunteers were genotyped by denaturing HPLC (DHPLC). Twelve variants in the coding region of the DPYD were detected. Allele frequencies of DPYD*5 (A1627G), DPYD*6 (G2194A), DPYD*9A (T85C), A496G, and G1218A were 12.7%, 7.1%, 13.7%, 5.7%, and 0.5%, respectively. The DPYD alleles DPYD*2A (IVS 14+1g>1), DPYD*3 (1897 del C) and DPYD*4 (G1601A) associated with DPD deficiency were absent from the examined subjects. We describe for the first time a new intronic polymorphism IVS 6-29 g>t, found in an allelic frequency of 4.7% in the Tunisian population. Comparing our data with that obtained in Caucasian, Egyptian, Japanese and African-American populations, we found that the Tunisian population resembles Egyptian and Caucasian populations with regard to their allelic frequencies of DPYD polymorphisms. This study describes for the first time the spectrum of DPYD sequence variations in the Tunisian population.

SSCP screening of the dihydropyrimidine dehydrogenase gene polymorphisms of the Japanese population using a semi-automated electrophoresis unit

The single-strand conformation polymorphism (SSCP) procedure has been applied in routine testing for hereditary diseases. Temperature, running buffer, gel composition, and fragment length can influence its sensitivity. Mutation detection in the clinical setting depends on the development of automated technology, especially for large genes, such as the dihydropyrimidine dehydrogenase (DPYD) gene, which codes the initial, rate-limiting enzyme in the catabolism of 5-fluorouracil (5FU). The authors have optimized the condition of SSCP with an automated system (GenePhor system, GE Healthcare UK Ltd.) to screen genetic polymorphisms in the DPYD gene. The efficiency of the method was evaluated using 21 positive controls (DNA samples with polymorphisms in the DPYD gene, previously characterized) and DNA samples from 35 Japanese. Results showed that the use of three different running buffers (pH 7.4, 8.3, and 9.0) in combination with other optimized conditions (10% polyacrylamide gel, 60-90 min at constant 900 V at 5 degrees C) resulted in a high polymorphism detection rate (95.3%), which was considered appropriate for routine screening. Therefore, this strategy could be useful for pharmacogenetic studies on 5FU.

Genetic variations and haplotype structures of the DPYD gene encoding dihydropyrimidine dehydrogenase in Japanese and their ethnic differences

Dihydropyrimidine dehydrogenase (DPD) is an inactivating and rate-limiting enzyme for 5-fluorouracil (5-FU), and its deficiency is associated with a risk for developing a severe or fatal toxicity to 5-FU. In this study, to search for genetic variations of DPYD encoding DPD in Japanese, the putative promoter region, all exons, and flanking introns of DPYD were sequenced from 341 subjects including cancer patients treated with 5-FU. Fifty-five genetic variations, including 38 novel ones, were found and consisted of 4 in the 5'-flanking region, 21 (5 synonymous and 16 nonsynonymous) in the coding exons, and 30 in the introns. Nine novel nonsynonymous SNPs, 29C>A (Ala10Glu), 325T>A (Tyr109Asn), 451A>G (Asn151Asp), 733A>T (Ile245Phe), 793G>A (Glu265Lys), 1543G>A (Val515Ile), 1572T>G (Phe524Leu), 1666A>C (Ser556Arg), and 2678A>G (Asn893Ser), were found at allele frequencies between 0.15 and 0.88%. Two known nonsynonymous variations reported only in Japanese, 1003G>T (*11, Val335Leu) and 2303C>A (Thr768Lys), were found at allele frequencies of 0.15 and 2.8%, respectively. SNP and haplotype distributions in Japanese were quite different from those reported previously in Caucasians. This study provides fundamental information for pharmacogenetic studies for evaluating the efficacy and toxicity of 5-FU in Japanese and probably East Asians.

Predictive and prognostic markers in colorectal cancer

Despite recent advances in the treatment of both early and advanced colorectal cancer, it remains the second leading cause of cancer deaths in the western world. There is, therefore, a pressing need to optimize the use of the currently available systemic therapies and to identify active new agents for the treatment of this disease. Pharmacogenomic studies have shown that genetically determined variability in key cellular functions can influence toxicity, response to treatment and survival. Numerous examples of these single ‘classical’ markers have been identified for a wide range of agents and each has been studied with regard to its effect on response. However, in any individual or tumor it is likely that a number of complex, interacting factors are involved in determining the likelihood of benefit with a given therapeutic agent. Microarray-based gene-expression profiling has allowed the complex range of molecular changes occurring in the cell and surrounding stroma to be assessed in relation to response and prognosis. Predictive gene sets have been developed and, along with other markers, are being assessed in prospective clinical trials. Treatment may soon be individualized by using this technology to predict which patients will benefit from a particular systemic therapy or which are likely to develop recurrence.

Thymidylate Synthase (TYMS) and Dihydropyrimidine Dehydrogenase (DPYD) Polymorphisms in the Korean Population for Prediction of 5-Fluorouracil-Associated Toxicity

The important cellular proteins for 5-fluorouracil (5-FU) metabolism are the major target enzymes, thymidylate synthase, and the rate-limiting enzyme in the degradation pathway, dihydropyrimidine dehydrogenase. Adverse drug reactions to 5-FU-based chemotherapy have been reported to be in part the result of polymorphisms in the thymidylate synthase (TYMS) and dihydropyrimidine dehydrogenase (DPYD) genes. Therefore, we examined the type and frequency of polymorphisms in the TYMS and DPYD genes in 100 healthy Korean individuals and compared these findings with 21 patients with colorectal cancer who had a grade 3 or greater toxic response to 5-FU treatment. Genotyping analysis of the promotor enhancer region (TSER) and the 3'-untranslated region (3'-UTR) of the TYMS gene as well as haplotype analysis were conducted in all 121 study participants. For the TSER and the 3'-UTR of the TYMS gene, similar genotypes and allele frequencies were observed in control subjects and patients. For the haplotype analysis of the single nucleotide polymorphism G > C at the 12th nucleotide of the second repeat of the 3R allele of the TSER, different haplotype frequencies were noted in comparisons between the two groups; in addition, we found that the 3RC-del 6bp was significantly associated with severe toxicity with 5-FU treatment. Extensive polymorphisms in the DPYD gene were observed; in addition, four polymorphisms were related to the known DPYD allelic variants or to allelic variants that alter protein structure, among which the most common polymorphism was 1627A > G, observed in 20.5% of all alleles. The 496A > G allele and a novel 1774C > T allele were identified in two patients. The DPYD*2A allele, causing exon 14 skipping, was not identified in the study group. The findings, from Korean patients with colon cancer, suggest that polymorphisms of the DPYD gene are not associated with an increased risk for toxic response to 5-FU. These findings suggest that there may be an important relationship between the TYMS haplotypes examined and 5-FU toxicity. The novel variant in the DPYD gene, identified in this study, should be further investigated to confirm its functional significance. A large sample is required before DPYD or TYMS genotyping could be used as markers for individualized treatment of patients with colorectal cancer.

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.

Pharmacokinetic modelling of [2-13C]uracil metabolism in normal and DPD-deficient dogs

A physiologically based pharmacokinetic (PBPK) model to simulate the plasma concentration and 13CO2 exhalation after [2-13C]uracil administration to DPD-suppressed dogs was developed. Simulation using this PBPK model should be useful in clinical situations where DPD-deficient patients at risk are to be detected with [2-13C]uracil as an in vivo probe.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

How may anticancer chemotherapy with fluorouracil be individualised?

Fluorouracil is used clinically against various solid tumours. Both fluorouracil toxicity and pharmacokinetics vary highly within and between individuals. The reasons why doses are not individualised routinely are difficulties in defining, predicting and achieving an optimal fluorouracil exposure or dose because of a narrow therapeutic index, nonlinear pharmacokinetics, variabilities in administration rates and metabolism, and in targets like thymidylate synthase. To individualise fluorouracil administration before the first dose, assessment of the individual dihydropyrimidine dehydrogenase (DPD) activity may be useful, because this genetically highly polymorphic enzyme controls approximately 80% of fluorouracil elimination. A complete or partial loss of DPD activity in 0.1 and 3-5% of Caucasians, respectively, leads to increased fluorouracil exposure and toxicity. Several methods to assess DPD activity in patients have been proposed (genotyping, various phenotyping methods), but each of them has limitations, as has the fluorouracil test dose approach. To adapt exposure towards fluorouracil a priori, a combination of genotyping and phenotyping may yield better prediction of toxicity than one method alone. A prerequisite for dose adaptation is the definition of fluorouracil exposure ranges with sufficient therapeutic activity, but without serious toxicity. While an increased risk of leukopenia, diarrhoea, stomatitis, and hand-foot syndrome during continuous 5-day infusions was related to fluorouracil exposures above an area under the plasma concentration-time curve (AUC) threshold of 25-30 mg.h/L, tumour response was higher when an AUC of approximately 30 mg.h/L was achieved, illustrating the extremely narrow therapeutic window of fluorouracil. Pharmacokinetic target values are less clear for other regimens, including chronomodulated regimens, which yielded a superior clinically efficacy and tolerability in several trials. However, the monitoring of fluorouracil plasma concentrations seems principally useful for individual a posteriori dose adjustment. Whether an adaptation of the fluorouracil starting dose to the results of two DPD activity tests before fluorouracil administration a priori, and the adaptation of doses to fluorouracil exposure a posteriori is a reasonable approach to better prevent toxicity and increase efficacy, remains to be evaluated in randomised clinical studies comparing these strategies to routine clinical safety monitoring.

Pharmacogenetics of cancer chemotherapy

Significant heterogeneity in the efficacy and toxicity of chemotherapeutic agents is observed within cancer populations. Pharmacogenetics (PGx) is the study of inheritance in interindividual variation in drug disposition. The allure of pharmacogenetics, in the treatment of cancer patients, comes from the potential for individualisation of cancer therapy, minimizing toxicity, while maximizing efficacy. In this review we will focus on the current and potential clinical applications of pharmacogenetics in cancer therapy by citing relevant examples and discussing the possible approaches which may be used to establish a reliable, reproducible and cost-effective test for clinically relevant genetic polymorphisms, using easily accessible biological samples (e.g., blood and tumour samples). Ideally, routine management of patients would include analysis of their single nucleotide polymorphism linkage disequilibrium (SNP-LD) profile prior to treatment, allowing stratification of patients into treatment groups, thus individualising their therapy. In order to achieve this ambition, a combination of different approaches (candidate gene, genome-wide and pathway driven) will be required from scientists and clinician scientists, as well as an increased understanding and incorporation of pharmacogenetic aims and endpoints into current and future clinical trials.

Clinical application of pharmacogenetics in gastrointestinal diseases

As knowledge of the human genome grows, there will be a direct impact on the management of specific diseases. Within gastroenterology and hepatology, there has been a change in the understanding of how variations or mutations in genes involved in drug metabolism or disease pathophysiology affect response to therapy. This review discusses the application of clinical pharmacogenetics to the following diseases and disorders: inflammatory bowel disease, Helicobacter pylori infections, gastroesophageal reflux disease, irritable bowel syndrome, functional dyspepsia, liver transplantation and colon cancer. Although only a few genotyping tests are regularly used in clinical practice, it is anticipated that studies will propel the routine use of many of the tests described in this review, in the future.

TPMT, UGT1A1 and DPYD: genotyping to ensure safer cancer therapy?

The Food and Drug Administration (FDA) has approved label changes for two anticancer drugs, 6-mercaptopurine (6-MP) and irinotecan, to include pharmacogenetic testing as a potential means to reduce the rate of severe toxic events. Comprehensive evaluation of the clinical benefit and cost effectiveness of screening strategies with these tests has not been completed. However, the FDA decided that evidence indicates sufficient benefit to warrant informing prescribers, pharmacists and patients of the availability of pharmacogenetic tests and their possible role in the selection and dosing of these anticancer agents. Reviewing the gene-drug-phenotype relationships of 6-MP, irinotecan and 5-fluorouracil reveals properties of these relationships that lead to a clinically useful pharmacogenetic test. Research in the near future should clarify the role of pharmacogenetic testing in reducing the risk of severe toxicity and determine how these same tests might identify a subset of patients who should safely receive higher doses of treatment to derive the same benefit as the rest of the patient population.

FRA1E common fragile site breaks map within a 370kilobase pair region and disrupt the dihydropyrimidine dehydrogenase gene (DPYD)

Common fragile sites represent components of normal chromosome structure that are particularly prone to breakage under replication stress. Although the cytogenetic locations of 88 common fragile sites are listed in the Genome database, the DNA at only 14 of them has been defined and characterized at the molecular level. Here, we identify the precise genomic position of the common fragile site FRA1E, mapped to the chromosomal band 1p21.2, and characterize the genetic complexity of the fragile DNA sequence. We show that FRA1E extends over 370kb within the dihydropyrimidine dehydrogenase (DPYD) gene, which genomically spans approximately 840kb. The 185kb region of the highest fragility, which accounts for 86% of all observed breaks at FRA1E, encompasses the central part of DPYD including exons 13-16. DPYD encodes dihydropyrimidine dehydrogenase (DPD), which is the first and rate-limiting enzyme in a three-step metabolic pathway involved in degradation of the pyrimidine bases uracil and thymine. Deficiency in human DPD is associated with autosomal recessive disease, thymine-uraciluria, and with severe 5-fluorouracil toxicity in cancer patients. To which extent the disruption of the DPYD gene by the fragile site break is only transient, followed by DNA repair to restore the original structure, or occasionally may result in genomic damage associated with human disease remains to be determined.