Pharmacogenetics as a molecular basis for individualized drug therapy: the thiopurine S-methyltransferase paradigm

The cost-effectiveness of alectinib in anaplastic lymphoma kinase-positive (ALK+) advanced NSCLC previously treated with crizotinib

Introduction Anaplastic lymphoma kinase (ALK) targeting drugs provide an important option for advanced non-small cell lung cancer patients with this distinct tumor type; however, there is considerable uncertainty as to which drug provides the optimal value after crizotinib treatment. This study estimated the cost-utility of alectinib vs ceritinib from a US payer perspective. Methods A cost-utility model was developed using partition survival methods and three health states: progression-free (PF), post-progression (PP), and death. Survival data were derived from the key clinical trials (alectinib: NP28761 & NP28673, ceritinib: ASCEND I and II). Costs included drugs, adverse events, and supportive care. Utilities were based on trial data and the literature. One-way and probabilistic sensitivity analyses (PSA) were performed to assess parameter uncertainty. Results Treatment with alectinib vs ceritinib resulted in increases of 2.55 months in the PF state, 0.44 quality adjusted life-years (QALYs), and $13,868, yielding a mean cost/QALY of $31,180. In the PSA, alectinib had a 96% probability of being cost-effective at a willingness-to-pay of $100,000/QALY. Drivers of model results were drug costs and utilities in the PF health state. The ICER ranged from $10,600-$65,000 per QALY in scenario analyses, including a sub-group analysis limited to patients with prior chemotherapy and crizotinib treatment. Conclusions Treatment with alectinib in ALK + crizotinib-treated patients increased time progression-free and QALYs vs ceritinib. The marginal cost increase was driven by longer treatment durations with alectinib. This model demonstrates that alectinib may be considered a cost-effective treatment after progression on crizotinib.

New and cost effective cell-based assay for Dialyzed Leukocyte Extract (DLE)-induced Jurkat cells proliferation under azathioprine treatment

The human Dialyzed Leukocyte Extract (DLE) is a heterogeneous mix of oligopeptides of <10kDa, extracted from leukocytes of healthy donors. There is significant clinical evidence of improvement using DLE during treatment of allergies, cancer,immunodeficiencies, and in mycotic and viral infections. Nevertheless, the DLE exact nature and mechanism of action have been elusive for more than 50 years. DLE biological activity testing is necessary in DLE production and quality control. Both in vitro and in vivo assays exist: E-rosette test, induction of delayed type hypersensitivity in mice, leukocyte migration and IFN-γ secretion. The animal-origin materials and in vivo assays convey a considerable logistic, ethic and economic burden, meanwhile the available in vitro assays have been reported with limited reproducibility and sometimes contradictory results. Here we are reporting a new DLE biological activity cell-based assay. The A20 and Jurkat cell lines were treated with (+Aza) or without (-Aza) azathioprine, DLE (+DLE) or both (+Aza/+DLE). After 72h, the cell proliferation was analyzed by the MTT or BrdU incorporation assays. In +Aza/+DLE treated cells, we observed a significant higher proliferation, when compared with +Aza/-DLE. In the absence of Aza, cells did not present any proliferation difference between -DLE or +DLE treatments. Both assays, MTT and BrdU showed similar results, being the MTT test more cost effective and we select it for validation as DLE biological assay using Jurkat cells only. We tested three different lyophilized DLE batches and we found consistent results with acceptable assay reproducibility and linearity. The DLE capacity for rescuing Jurkat cell proliferation during +Aza treatment was consistent using different liquid and lyophilized DLE batches, presenting also consistent chromatographic profiles. Finally, DLE treatment in Jurkat cells did not result into significant IL-2 of IFN-γ secretion, and known lymphocyte proliferative drugs failed to rescue Jurkat cells viability in presence of +Aza, as +DLE treatment did in our MTT assay. In conclusion, our new cell-based MTT assay has excellent DLE biological activity consistency, robustness and is cost effective, presenting important advantages over previous DLE activity in vitro and in vivo assays.

Physiologically based pharmacokinetic model for 6-mercpatopurine: exploring the role of genetic polymorphism in TPMT enzyme activity

AIMS

To extend the physiologically based pharmacokinetic (PBPK) model developed for 6-mercaptopurine to account for intracellular metabolism and to explore the role of genetic polymorphism in the TPMT enzyme on the pharmacokinetics of 6-mercaptopurine.

METHODS

The developed PBPK model was extended for 6-mercaptopurine to account for intracellular metabolism and genetic polymorphism in TPMT activity. System and drug specific parameters were obtained from the literature or estimated using plasma or intracellular red blood cell concentrations of 6-mercaptopurine and its metabolites. Age-dependent changes in parameters were implemented for scaling, and variability was also introduced for simulation. The model was validated using published data.

RESULTS

The model was extended successfully. Parameter estimation and model predictions were satisfactory. Prediction of intracellular red blood cell concentrations of 6-thioguanine nucleotide for different TPMT phenotypes (in a clinical study that compared conventional and individualized dosing) showed results that were consistent with observed values and reported incidence of haematopoietic toxicity. Following conventional dosing, the predicted mean concentrations for homozygous and heterozygous variants, respectively, were about 10 times and two times the levels for wild-type. However, following individualized dosing, the mean concentration was around the same level for the three phenotypes despite different doses.

CONCLUSIONS

The developed PBPK model has been extended for 6-mercaptopurine and can be used to predict plasma 6-mercaptopurine and tissue concentration of 6-mercaptopurine, 6-thioguanine nucleotide and 6-methylmercaptopurine ribonucleotide in adults and children. Predictions of reported data from clinical studies showed satisfactory results. The model may help to improve 6-mercaptopurine dosing, achieve better clinical outcome and reduce toxicity.

Pharmacogenomics and personalized medicine: a review focused on their application in the Chinese population

The field of pharmacogenomics was initiated in the 1950s and began to thrive after the completion of the human genome project 10 years ago. Thus far, more than 100 drug labels and clinical guidelines referring to pharmacogenomic biomarkers have been published, and several key pharmacogenomic markers for either drug safety or efficacy have been identified and subsequently adopted in clinical practice as pre-treatment genetic tests. However, a tremendous variation of genetic backgrounds exists between different ethnic groups. The application of pharmacogenomics in the Chinese population is still a long way off, since the published guidelines issued by the organizations such as US Food and Drug Administration require further confirmation in the Chinese population. This review highlights important pharmacogenomic discoveries in the Chinese population and compares the Chinese population with other nations regarding the pharmacogenomics of five most commonly used drugs, ie, tacrolimus, cyclosporine A, warfarin, cyclophosphamide and azathioprine.

The effect of breed and sex on sulfamethazine, enrofloxacin, fenbendazole and flunixin meglumine pharmacokinetic parameters in swine

Drug use in livestock has received increased attention due to welfare concerns and food safety. Characterizing heterogeneity in the way swine populations respond to drugs could allow for group-specific dose or drug recommendations. Our objective was to determine whether drug clearance differs across genetic backgrounds and sex for sulfamethazine, enrofloxacin, fenbendazole and flunixin meglumine. Two sires from each of four breeds were mated to a common sow population. The nursery pigs generated (n = 114) were utilized in a random crossover design. Drugs were administered intravenously and blood collected a minimum of 10 times over 48 h. A non-compartmental analysis of drug and metabolite plasma concentration vs. time profiles was performed. Within-drug and metabolite analysis of pharmacokinetic parameters included fixed effects of drug administration date, sex and breed of sire. Breed differences existed for flunixin meglumine (P-value<0.05; Cl, Vdss ) and oxfendazole (P-value<0.05, AUC0→∞ ). Sex differences existed for oxfendazole (P-value < 0.05; Tmax ) and sulfamethazine (P-value < 0.05, Cl). Differences in drug clearance were seen, and future work will determine the degree of additive genetic variation utilizing a larger population.

Cancer pharmacogenomics in children: research initiatives and progress to date

Over the last few decades, cure rates for pediatric cancer have increased dramatically, and now over 80 % of children with cancer are cured of their disease. This improvement in cure has come with a significant cost, with many children suffering irreversible, life-threatening, or long-lasting toxicities due to the medications required during their treatment. In the last 2 decades, major technological advances in genomics and the mapping of the human genome have made it possible to identify genetic differences between children in order to investigate differing responses to cancer therapy and to help explain why children treated with the same medications can have different outcomes. The emerging field of pharmacogenomics has had many important findings in pediatric cancer. The focus of this review is drug toxicity in pediatric cancer and the use of pharmacogenomics to reduce these adverse drug reactions, with a specific focus on thiopurines, methotrexate, cisplatin, vincristine and anthracyclines. Future areas of research and the need for international collaboration are discussed.

Part 3: Pharmacogenetic variability in phase II anticancer drug metabolism

Equivalent drug doses may lead to wide interpatient variability in drug response to anticancer therapy. Known determinants that may affect the pharmacological response to a drug are, among others, nongenetic factors, including age, gender, use of comedication, and liver and renal function. Nonetheless, these covariates do not explain all the observed interpatient variability. Differences in genetic constitution among patients have been identified to be important factors that contribute to differences in drug response. Because genetic polymorphism may affect the expression and activity of proteins encoded, it is a key covariate that is responsible for variability in drug metabolism, drug transport, and pharmacodynamic drug effects. We present a series of four reviews about pharmacogenetic variability. This third part in the series of reviews is focused on genetic variability in phase II drug-metabolizing enzymes (glutathione S-transferases, uridine diphosphoglucuronosyl transferases, methyltransferases, sulfotransferases, and N-acetyltransferases) and discusses the effects of genetic polymorphism within the genes encoding these enzymes on anticancer drug therapy outcome. Based on the literature reviewed, opportunities for patient-tailored anticancer therapy are proposed.

The cutaneous and systemic manifestations of azathioprine hypersensitivity syndrome

BACKGROUND

Azathioprine (AZA) hypersensitivity syndrome is a rare side effect that typically occurs early in the initiation of therapy and may include a cutaneous eruption. It is often under-recognized because it mimics infection or disease exacerbation. Until recently, the cutaneous findings associated with AZA hypersensitivity have been reported using nonspecific, descriptive terms without a supportive diagnostic biopsy.

OBJECTIVE

To characterize the cutaneous and histologic findings associated with AZA hypersensitivity syndrome.

METHODS

We conducted a retrospective analysis of two cases of AZA hypersensitivity syndrome and describe the cutaneous manifestations and histological findings of each case. A review of the English literature for cases of AZA hypersensitivity or allergic or adverse reactions associated with AZA was performed.

RESULTS

Sixty-seven cases of AZA hypersensitivity were reviewed; 49% (33/67) had cutaneous manifestations. Of those cases presenting with cutaneous findings, 76% (25/33) had biopsy results or clinical features consistent with a neutrophilic dermatosis, whereas the other 24% (8/33) were reported as a nonspecific cutaneous eruption.

LIMITATIONS

Only case reports in which the skin findings could be classified were reviewed.

CONCLUSIONS

The predominant cutaneous reaction reported in the literature and observed in the present case series is a neutrophilic dermatosis. Hypersensitivity to AZA can manifest along a wide clinical spectrum from local neutrophilic disease to a systemic syndrome. Skin findings may be an important early clue to the diagnosis of AZA hypersensitivity and aid in prompt recognition and treatment of this potentially life-threatening adverse drug effect.

Pharmacogenomics in thrombosis

Inherited or acquired genetic abnormalities play a major role in thromboembolic complications. The goal of pharmacogenomics is to tailor medications to an individual's genetic makeup in order to improve the benefit-to-risk ratio. Significant findings have been documented showing the effect of certain genetic variations (e.g., in CYP2C9 and VKORC1) on the dose response to warfarin. Pharmacogenomic and genetic information is crucial to improving the efficacy and safety of pharmacotherapy and for the optimal management of thromboembolic disorders.

Diagnostic microarrays in hematologic oncology: applications of high- and low-density arrays

Microarrays have become important tools for high-throughput analysis of gene expression, chromosome aberrations, and gene mutations in cancer cells. In addition to high-density experimental microarrays, low-density, gel-based biochip technology represents a versatile platform for translation of research into clinical practice. Gel-based microarrays (biochips) consist of nanoliter gel drops on a hydrophobic surface with different immobilized biopolymers (primarily nucleic acids and proteins). Because of the high immobilization capacity of the gel, such biochips have a high probe concentration and high levels of fluorescence signals after hybridization, which allow the use of simple, portable detection systems. The notable accuracy of the analysis is reached as a result of the high level of discrimination between positive and negative gel-bound probes. Different applications of biochips in the field of hematologic oncology include analysis of chromosomal translocations in leukemias, diagnostics of T-cell lymphomas, and pharmacogenetics.

Integrative genomics and drug development

The pharmaceutical industry faces unprecedented pressures based largely on the inability to bring sufficient new medicines to market. The high failure rate of drug candidates in clinical development highlights a need for new approaches to the study of disease mechanisms and drug discovery. We advocate an integrated approach based on the study of the entire organism leveraging the power of detailed phenotyping, high-throughput genomic technologies and mathematical modeling. Key to this paradigm is the realization that the systematic genetic perturbations that exist in populations provide an ideal structure for uncovering the interactions that define molecular networks or states. By linking molecular states to physiological states and in turn understanding how molecular states drive disease processes, the promise of truly rational drug-design with a high probability of success in clinical development can be realized.

Thiopurine S-methyltransferase (TPMT) gene polymorphism in Brazilian children with acute lymphoblastic leukemia: association with clinical and laboratory data

The frequency of allele variants of gene TPMT*2, *3A, *3B, and *3C was estimated in a population of 116 Brazilian children with acute lymphoblastic leukemia. The association between genotype and clinical and laboratory data obtained during chemotherapy maintenance phase and the correlation of intraerythrocyte concentration of 6-mercaptopurine metabolites (6-tioguanine nucleotide nucleotides and methylmercaptopurine) were analyzed. A multiplex amplification refractory mutation system-polymerase chain reaction (ARMS-PCR) was used in DNA amplification. Twelve patients presented TPMT gene mutation, all in heterozygous form. The most frequent allele variation was TPMT*3A (3.9%), followed by *3C (0.9%), *2 (0.4%), and *3B (0%). There was no significant association between clinical and laboratory data and the presence of mutation in TPMT gene. Of the 36 patients who were monitored for 6-mercaptopurine metabolite levels, only 1 had the mutation. In this patient, high 6-tioguanine nucleotide and low methylmercaptopurine concentrations were found. Event-free survival (EFS) for the whole group was 73.4%. There was no significant difference in event-free survival in the comparison between the groups with and without mutation (P = 0.06).

TPMT genetic variations in populations of the Russian Federation

BACKGROUND

Polymorphisms that reduce the activity of thiopurine S-methyltransferase (TPMT) cause adverse reactions to conventional doses of thiopurines, routinely used for antileukemic and immunosuppressive treatment. There are more than 20 variant alleles of TPMT that cause decreased enzymatic activity. We studied the most common variant alleles of TPMT and their frequency distribution in a large cohort of multiracial residents in the Russian Federation and compared their frequencies in children with and without malignancy to determine whether TPMT gene abnormality is associated with hematologic malignancy.

PROCEDURE

The TPMT biochip was used to detect 6 TPMT single nucleotide polymorphisms (SNPs) corresponding to 7 TPMT-deficiency alleles (TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C, TPMT*3D, TPMT*7, and TPMT*8). We analyzed allele frequencies in the whole cohort, the childhood cancer group, and the non-cancer group. We also characterized disease features and outcome according to the presence of TPMT SNPs in children with acute lymphoblastic leukemia (ALL).

RESULTS

Fifty-five (5.5%) study participants overall had heterozygous TPMT genotypes (1 variant and 1 wild-type allele): TPMT*1/*3A (n = 45; 4.5%), TPMT*1/*3C (n = 8; 0.8%), and TPMT*1/*2 (n = 2; 0.2%). TPMT SNPs were more frequent in children with hematologic malignancy than in other participants (7.5% vs. 4.0%, P = 0.02). We found no significant association between TPMT SNPs and ALL treatment outcome (median follow-up, 31.3 months).

CONCLUSIONS

TPMT*3A is the most prevalent variant allele in the Russian Federation. The estimated frequency of variant alleles in the study cohort (5.5%) was similar to that observed in the White populations in the U.S. and Eastern Europe.

The role of biotransformation and bioactivation in toxicity

Biotransformation is essential to convert lipophilic chemicals to water-soluble and readily excretable metabolites. Formally, biotransformation reactions are classified into phase I and phase II reactions. Phase I reactions represent the introduction of functional groups, whereas phase II reactions are conjugations of such functional groups with endogenous, polar products. Biotransformation also plays an essential role in the toxicity of many chemicals due to the metabolic formation of toxic metabolites. These may be classified as stable but toxic products, reactive electrophiles, radicals, and reactive oxygen metabolites. The interaction of toxic products formed by biotransformation reactions with cellular macromolecules initiates the sequences resulting in cellular damage, cell death and toxicity.

The potential clinical and economic outcomes of pharmacogenomic approaches to EGFR-tyrosine kinase inhibitor therapy in non-small-cell lung cancer

OBJECTIVES

Pharmacogenomic applications in oncology offer significant promise, but the clinical and economic implications remain unclear. The objective of this study was to evaluate the potential cost-utility of implementing epidermal growth factor receptor (EGFR) testing before initiating second-line therapy for advanced refractory non-small-cell lung cancer (NSCLC).

METHODS

We developed a decision analytic model to evaluate the cost-utility of EGFR protein expression or gene copy number testing compared to standard care with erlotinib in refractory advanced NSCLC patients. Costs and utilities were obtained from publicly available sources. We performed sensitivity analyses to evaluate uncertainty in the results.

RESULTS

The quality-adjusted life expectancies for erlotinib, EGFR protein expression testing, and gene copy number testing were: 0.44, 0.48, and 0.50 quality-adjusted life years (QALYs); and the costs were: $57,238, $63,512, and $66,447, respectively. The most cost-effective testing option, EGFR gene copy number testing, produced an incremental cost-effectiveness ratio of $162,018/QALY compared to no testing (erlotinib). The results were most sensitive to the survival estimates, health state utilities, and cost of disease progression. In the probabilistic sensitivity analyses, erlotinib without testing was the optimal treatment strategy until the $150,000/QALY willingness-to-pay threshold, after which gene copy testing was optimal. The discounted expected value of perfect information at a $100,000/QALY threshold in the USA over 5 years was $31.4 million.

CONCLUSIONS

The study results suggest that EGFR pharmacogenomic testing has the potential to improve quality-adjusted life expectancy in the treatment of refractory NSCLC by a clinically meaningful margin at a value commensurate with the approved therapies in this setting. Additional research in this area is warranted.

An Interethnic Comparison of Polymorphisms of the Genes Encoding Drug-Metabolizing Enzymes and Drug Transporters: Experience in Singapore

Much of the interindividual variability in drug response is attributable to the presence of single nucleotide polymorphisms (SNPs) in genes encoding drug-metabolizing enzymes and drug transporters. In recent years, we have investigated the polymorphisms in a number of genes encoding phase I and II drug-metabolizing enzymes including CYPIA1, CYP3A4, CYP3A5, GSTM1, NAT2, UGT1A1, and TPMT and drug transporter (MDR1) in three distinct Asian populations in Singapore, namely the Chinese, Malays, and Indians. Significant differences in the frequencies of common alleles encoding these proteins have been observed among these three ethnic groups. For example, the frequency of the variant A2455G polymorphism of CYP1A1 was 28% in Chinese and 31% in Malays, but only 18% in Indians. CYP3A4*4 was detected in two of 110 Chinese subjects, but absent in Indians and Malays. Many Chinese and Malays (61-63%) were homozygous for the GSTM1*0 null genotype compared with 33% of Indians. The frequency of the UGTIA1*28 allele was highest in the Indian population (35%) compared to similar frequencies that were found in the Chinese (16%) and Malay (19%) populations. More importantly, our experience over the years has shown that the pharmacogenetics of these drug-metabolizing enzymes and MDR1 in the Asian populations are different from these in the Caucasian and African populations. For example, the CYP3A4*1B allele, which contains an A-290G substitution in the promoter region of CYP3A4, is absent in all three Asian populations of Singapore studied, but occurs in more than 54% of Africans and 5% of Caucasians. There were no difference in genotype and allelic variant frequencies in exon 12 of MDR1 between the Chinese, Malay, and Indian populations. When compared with other ethnic groups, the distribution of the wild-type C allele in exon 12 in the Malays (34.2%) and Indians (32.8%) was relatively high and similar to the Japanese (38.55%) and Caucasians (41%) but different from African-Americans (15%). The frequency of wild-type TT genotype in Asians (43.5% to 52.1%) and Japanese (61.5%) was much higher than those found in Caucasians (13.3%). All the proteins we studied represent the primary hepatic or extrahepatic enzymes, and their polymorphic expression may be implicated in disease risk and the disposition of drugs or endogenous substances. As such, dose requirements of certain drugs may not be optimal for Asian populations, and a second look at the factors responsible for this difference is necessary.

Clinical pharmacology and pharmacogenetics of thiopurines

The thiopurine drugs-azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine-are widely used to treat malignancies, rheumatic diseases, dermatologic conditions, inflammatory bowel disease, and solid organ transplant rejection. However, thiopurine drugs have a relatively narrow therapeutic index and are capable of causing life-threatening toxicity, most often myelosuppression. Thiopurine S-methyltransferase (TPMT; EC 2.1.1.67), an enzyme that catalyzes S-methylation of these drugs, exhibits a genetic polymorphism in 10% of Caucasians, with 1/300 individuals having complete deficiency. Patients with intermediate or deficient TPMT activity are at risk for excessive toxicity after receiving standard doses of thiopurine medications. This report reviews the recent advances in the knowledge of the mechanism of action as well as the molecular basis and interethnic variations of TPMT and inosine triphosphate pyrophosphatase (ITPase; EC 3.6.1.19), another enzyme implicated in thiopurine toxicity. In addition, an update on pharmacokinetics, metabolism, drug-drug interactions, safety, and tolerability of thiopurine drugs is provided.

Four human thiopurine s-methyltransferase alleles severely affect protein structure and dynamics

Thiopurine S-methyltransferase (TPMT) metabolizes cytotoxic thiopurine drugs used in the treatment of leukemia and inflammatory bowel disease. TPMT is a major pharmacogenomic target with 23 alleles identified to date. Several of these alleles cause rapid protein degradation and/or aggregation, making it extremely difficult to study the structural impact of the TPMT polymorphisms experimentally. We, therefore, have performed multiple molecular dynamics simulations of the four most common alleles [TPMT*2 (A80P), *3A (A154T/Y240C), *3B (A154T) and *3C (Y240C)] to investigate the molecular mechanism of TPMT inactivation at an atomic level. The A80P polymorphism in TPMT*2 disrupts helix alpha3 bordering the active site, which breaks several salt-bridge interactions and opens up a large cleft in the protein. The A154T polymorphism is located within the co-substrate binding site. The larger threonine alters the packing of substrate-binding residues (P68, L69, Y166), increasing the solvent exposure of the polymorphic site. This packing rearrangement may account for the complete lack of activity in the A154T mutant. The Y240C polymorphism is located in beta-strand 9, distant from the active site. Side-chain contacts between residue 240 and helix alpha8 are lost in TPMT*3C. Residues 154 and 240 in TPMT*3A are connected through a hydrogen-bonding network. The dual polymorphisms result in a flattened, slightly distorted protein structure and an increase in the thiopurine-binding site solvent accessibility. The two variants that undergo the most rapid degradation in vivo, TPMT*2 and *3A, are also the most deformed in the simulations.

[Monitoring of thiopurine methyltransferase and thiopurine metabolites to optimize azathioprine therapy in inflammatory bowel disease]

Determination of the activity of thiopurine methyltransferase (TPMT) and of thiopurine metabolites (6-thioguanine and 6-methylmercaptopurine nucleotides) could be useful for individualized monitoring of azathioprine (AZA) and 6-mercaptopurine (6-MP) doses. TPMT activity in the general population follows a trimodal distribution, in which approximately 0.3% of the population is homozygotic for the low-activity allele. A notable correlation has been observed between the low TPMP activity genotype or phenotype and the risk of myelotoxicity. Patients with a high TPMT activity genotype or homozygous phenotype should receive immunosuppressive doses that have clearly been demonstrated to be effective. In contrast, in patients with a low TPMT activity genotype or homozygous phenotype, the use of AZA/6-MP should be contraindicated or only very small doses should be administered. Importantly, TPMP deficiency explains only some cases of myelotoxicity and consequently periodic laboratory testing should be performed in patients receiving AZA/6-MP, even though TPMP function may be normal. Currently, the utility of routine thiopurine metabolite determinations in patients undergoing AZA/6-MP therapy has not been established and this practice should be limited to specific situations such as lack of response to thiopurine therapy or the occurrence of thiopurine-related adverse effects. Randomized trials comparing the routine strategy of AZA/6-MP dosing (based exclusively on the patient's weight) versus individualized monitoring (based on quantification of TPMP activity and/or thiopurine metabolites) are required before definitive conclusions on the most effective alternative can be drawn.

The impact of thiopurine s-methyltransferase polymorphism on azathioprine-induced myelotoxicity in renal transplant recipients

Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. TPMT activity exhibits an interindividual variability, mainly as a result of genetic polymorphism. Patients with intermediate or deficient TMPT activity are at risk for toxicity after receiving standard doses of thiopurine drugs. It has previously been reported that 3 variant alleles: TPMT*2, *3A, and *3C are responsible for over 95% cases of low enzyme activity. The purpose of this study was to explore the association between these polymorphisms and the occurrence of azathioprine adverse effects in 112 renal transplant recipients undergoing triple immunosuppressive therapy including azathioprine, cyclosporine, and prednisone. TPMT genetic polymorphism was determined using PCR-RFLP and allele-specific PCR methods. Azathioprine dose, leukocyte, erythrocyte, and platelet counts, graft rejection episodes, as well as cyclosporine levels were analyzed throughout the first year after organ transplantation. We found the frequency of leukopenia episodes (WBC < 4.0 x 10(9)/L) significantly higher in heterozygous patients (53.8%) compared with those with TPMT wild-type genotype (23.5%). One patient, who was a compound homozygote (3A/*3C), experienced severe azathioprine-related myelotoxicity each time after receiving the standard drug dose. Our results suggest that polymorphisms in TPMT gene may be responsible for approximately 12.5% of all leukopenia episodes in renal transplant recipients treated with azathioprine. Genotyping for the major TPMT variant alleles may be a valuable tool in preventing AZA toxicity and optimization of immunosuppressive therapy.

The thiopurines: An update

The thiopurine drugs, 6-mercaptopurine (6-MP), 6-thioguanine (6-TG) are commonly used cytotoxic agents. A derivative of 6-MP, azathioprine, is commonly used as an immunosuppressant. A prominent route for the metabolism of these agents is mediated by the enzyme thiopurine methyltransferase (TPMT). This enzyme exhibits considerable inter-individual variation in activity, partly due to the presence of common genetic polymorphisms, which influence cytotoxicity of the thiopurine drugs. Variations in the number of tandem repeats in the 5' promoter region have also been shown to influence TPMT expression in vitro. In this article, we review the impact of variations in TPMT activity on sensitivity to the thiopurine drugs in vitro and also in vivo in terms of their clinical efficacy and toxicity. A possible relationship between TPMT and secondary malignancies is also reviewed.

Determination of intra-ethnic differences in the polymorphisms of thiopurine S-methyltransferase in Chinese

BACKGROUND

Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs. Several mutations in the TPMT gene have been identified which correlate with a low activity phenotype. The molecular basis for TPMT deficiency is not well defined in minority Chinese. We investigated differences in the activity of TPMT and the frequencies of mutant TPMT alleles in 4 ethnic groups of Chinese.

METHOD

The frequency of 4 common TPMT mutant alleles, TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C, were determined in healthy subjects from Han (n=312), Jing (n=103), Yao (n=126) and Uygur Chinese (n=160) by allele-specific PCR and PCR-restriction RFLP analysis. TPMT activity in erythrocytes was determined by HPLC.

RESULTS

There was no significant difference in the mean TPMT activity between all ethnic groups studied and no subject with TPMT deficiency was found in all populations studied. TPMT*3C was found in 2.2% of Han and 1.9% of Jing Chinese. TPMT*2, TPMT*3B and TPMT*3A alleles were not detected in any of the Han or Jing Chinese tested. In contrast, 3.7% of Uygur Chinese had TPMT*3C and TPMT*3A alleles. Neither allele was detected in Yao Chinese. The overall frequencies of variant TPMT allele in Uygur were higher than in Han or Jing Chinese. However, neither the overall frequency of mutant TPMT alleles nor the genotype frequencies were significantly different between Han, Jing, Yao and Uygur Chinese.

CONCLUSIONS

The TPMT*3C was the most prevalent allele in Han, Jing and Uygur Chinese, while TPMT*3A is a rare allele in Uygur Chinese who belong to Caucasian. Ethnicity may be an important factor affecting the variability in response to thiopurine chemotherapy.

An association study of 43 SNPs in 16 candidate genes with atorvastatin response

Variation in individual response to statin therapy has been widely studied for a potential genetic component. Multiple genes have been identified as potential modulators of statin response, but few study findings have replicated. To further examine these associations, 2735 individuals on statin therapy, half on atorvastatin and the other half divided among fluvastatin, lovastatin, pravastatin and simvastatin were genotyped for 43 SNPs in 16 genes that have been implicated in statin response. Associations with low-density lipoprotein cholesterol (LDL-C) lowering, total cholesterol lowering, HDL-C elevation and triglyceride lowering were examined. The only significant associations with LDL-C lowering were found with apoE2 in which carriers of the rare allele who took atorvastatin lowered their LDL-C by 3.5% more than those homozygous for the common allele and with rs2032582 (S893A in ABCB1) in which the two groups of homozygotes differed by 3% in LDL-C lowering. These genetic effects were smaller than those observed with the demographic variables of age and gender. The magnitude of all the differences found is sufficiently small that genetic data from these genes should not influence clinical decisions on statin administration.

Freshwater selenium-methylating bacterial thiopurine methyltransferases: diversity and molecular phylogeny

The diversity of bacterial thiopurine methyltransferases (bTPMT) among five natural Se-methylating freshwaters was investigated by polymerase chain reaction (PCR) screenings and sequencings. DNA sequence analyses confirmed the cloned products' identity and revealed a broad diversity of freshwater TPMTs. Neighbour-joining (NJ) phylogenetic analyses combining these sequences, all GenBank entries closely related to these sequences and deduced TPMTs obtained in this work from selected gamma-proteobacteria showed TPMTs to form a distinct radiation, closely related to UbiG methyltransferases. Inside the TPMT phylogenetic cluster, eukaryote sequences diverged early from the bacterial ones, and all the bacterial database entries belonged to a subgroup of gamma-proteobacteria, with an apparent lateral transfer of a particular allele to beta-proteobacteria of Bordetella. The NJ phylogenetic tree revealed 22 bTPMT lineages, 10 of which harboured freshwater sequences. All lineages showed deep and long branches indicative of major genetic drifts outside regions encoding highly conserved domains. Selected residues among these highly variable domains could reflect adaptations for particular ecological niches. PCR lineage-specific primers differentiated Se-methylating freshwaters according to their 'tpm lineage' signatures. Most freshwater tpm alleles were found to be distinct from those available in the databases, but a group of tpm was found encoding TPMTs identical to an Aeromonas veronii TPMT characterized in this work.

Pharmacogenetics and Pediatric Cancer

The great advances in therapeutic success for childhood cancers have provided the impetus for strategies to avoid serious systemic toxicities from chemotherapy. This review describes the impact of genetic mutations in drug metabolism pathways on the toxicity of anticancer agents. Although many polymorphisms have been related to toxicity in adults, these associations are less well defined in children. The role of genetic polymorphisms in MTHFR, TYMS, TPMT, and UGT1A1 in influencing drug toxicity is reviewed. Better understanding of the pharmacogenetic determinants of drug metabolism or pharmacologic cofactors may allow for prospective identification of potential patients who are at increased risk for toxicity, allowing for dose optimization and resulting in a decrease in toxic risk while maximizing efficacy.

The clinical impact of thiopurine methyltransferase polymorphisms on thiopurine treatment

Acute lymphoblastic leukaemia (ALL) is the most common malignancy of childhood. Although current treatment results in long term survival in over 70% of cases there is evidence that as many as 50% could have been cured using a less complex regimen with a lower incidence of long term side effects. In previous studies it has been found that thiopurines given as part of continuing therapy are key agents in preventing relapse. However, optimal administration during continuing therapy is often not achieved. Variation in the level of thiopurine methyltransferase (TPMT) activity appears to be a major molecular determinant of the extent of thiopurine metabolism. TPMT activity shows a trimodal distribution pattern. A lack of activity is found in approximately one in 300 Caucasians; approximately 11% have intermediate activity and the remaining 89% high activity. Congenital loss of activity is associated with grossly elevated levels of active drug and profound myelosuppression on exposure to thiopurines. This loss of activity has been attributed to single nucleotide polymorphisms (SNPs) within the TPMT gene. The frequency of SNPs is related to ethnicity, with the most common in Caucasians being TPMT*3A which is characterized by a G to A transition at position 460 with a substitution of alanine for tyrosine at amino acid 154 (A154Y) and a transition of A to G at nucleotide 719 resulting in a change of tyrosine to cysteine at position 240 (Y240C). Polymorphisms have also been identified within the 5' flanking promoter region of the TPMT gene due to a variable number of tandem repeats (VNTR*3-*8). An overview of the polymorphisms identified to date, their implication on the metabolism of the thiopurine drugs and therapeutic importance will be discussed.

Genetic Testing in Crohn Disease

Inflammatory bowel disease (IBD), with its two subforms of Crohn disease and ulcerative colitis, is a polygenic disease that manifests due to environmental trigger factors on the background of a complex genetic predisposition. The first risk gene underlying susceptibility to Crohn disease has been identified as CARD15 (located on chromosome 16q12, encoding NOD2). Three single nucleotide polymorphisms in the leucine rich region (LRR) of this gene are strongly and independently associated with Crohn disease susceptibility and explain up to 20% of the total genetic predisposition for Crohn disease. These variants have been consistently replicated as associated with a particular sub-phenotype characterized by small bowel (ileum) involvement and early age at onset. Presently, genetic testing for the CARD15 variants has only a modest relevance in clinical practice. The most attractive use of genetic testing is for the prediction of response to therapy. Most therapies only show efficacy in subgroups of patients and no clinical parameters are available to distinguish, prior to therapy, whether the patients will be responders or non-responders, or if the patients will experience adverse effects. The pharmacogenetic basis of toxicity is well known for azathioprine: several thiopurine methyltransferase (TPMT) polymorphisms that are associated with reduced activity of this thiopurine drug metabolizing enzyme result in cytotoxic and immunosuppressive adverse effects of azathioprine. Genetic screening, which has found its way into routine clinical diagnostics, allows the identification of the patients who will not tolerate a standard dose of the drug. The extensive search for genetic predictors of response to the anti-tumor necrosis factor treatment with infliximab, which results in a remission rate of 30-40%, has, however, failed to identify a variation associated with a differential response.

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.

Guidelines for prescribing azathioprine in dermatology

Azathioprine has been available as an immunosuppressive agent for over 40 years, and current routine usage in dermatology is not restricted to licensed indications. Advances in understanding the metabolic fate of azathioprine have led to significant changes in prescribing practice and toxicity monitoring by U.K. dermatologists. The current state of knowledge concerning the use of azathioprine in dermatology is summarized, with identification of strength of evidence. Clinical indications and contraindications for azathioprine usage in dermatology are identified. Evidence-based recommendations are made for routine safety monitoring of patients treated with azathioprine, including pretreatment assessment of red blood cell thiopurine methyltransferase activity.

Thiopurine S-methyltransferase genotype predicts azathioprine-induced myelotoxicity in kidney transplant recipients

Azathioprine (AZA) is an immunosuppressive prodrug that undergoes metabolism by thiopurine S-methyltransferase (TPMT). Eighty to ninety-five percent of low or deficient TPMT enzyme activity is genetically determined by the presence of three nonfunctional mutant alleles: TPMT*2, TPMT*3A and TPMT*3C. Using TPMT as a pharmacogenetic paradigm, we explored the association between these genetic mutations and development of adverse drug effects in an ethnically diverse renal transplant population receiving azathioprine. Biochemical and clinical data were retrospectively evaluated during the first four weeks after kidney transplantation. TPMT nonfunctional mutant alleles were identified by polymerase chain reaction-based methods. Of 89 patients initially consented, 36 met inclusion criteria for this retrospective study. Five patients possessing a single TPMT nonfunctional mutant allele were identified: TPMT*3A: n = 2 Caucasians; TPMT*3B: n = 1 Caucasian; TPMT*3C: n = 2 African-Americans. TPMT nonfunctional mutant alleles were associated with significant reductions in hematological indices and a significant increase in cyclosporine plasma concentrations in the first month post-transplant. TPMT genotype was an independent predictor for hemoglobin, hematocrit and red blood cell changes while mean azathioprine dose (mg/kg/day), azathioprine dose (mg/kg/day) at day 30 and cyclosporinemia at day 30 were not. Prospective application of pharmacogenetic principles may assist in optimization of immunosuppressive drug therapy and minimize drug toxicities.

Therapeutic drug monitoring of thiopurine drugs in patients with inflammatory bowel disease or autoimmune hepatitis

OBJECTIVE

Thiopurine drugs are commonly used immunosuppressants in the treatment of inflammatory bowel disease (IBD) as well as in autoimmune hepatitis (AIH), rheumatic diseases and in transplantation medicine. The relatively narrow therapeutic range requires useful therapy control. Therefore, the purpose of this study was to further investigate the rationale and usefulness of therapeutic drug monitoring in the surveillance of thiopurine drug therapy in Crohn's disease, ulcerative colitis and autoimmune hepatitis.

METHODS

6-Thioguanine nucleotide (TGN) and 6-methylmercaptopurine nucleotide (MMPN) levels were measured in 182 IBD patients and 18 AIH patients using HPLC-UV.

RESULTS

In our cohort of IBD patients, 18% had TGN levels < 235 pmol/8 x 10 red blood cells (RBC) (recommended range, 235-450 pmol/8 x 10 RBC), 41% of these patients were sent for drug failure. Twenty-four per cent of the IBD patients had TGN levels > 450 pmol/8 x 10 RBC, but only 27% of these experienced adverse effects. Fifty-nine per cent of the patients having drug failure had TGN levels in the recommended range and could therefore be classified as non-responders. In the AIH cohort 33% of the patients had TGN levels below the recommended range but showed clinical response to therapy. MMPN levels increased with the duration of treatment and could be useful for controlling compliance. There was 8.8% of IBD patients who were heterozygous for non-functional TPMT alleles.

CONCLUSIONS

TGN monitoring did not identify significant differences between patient groups but allowed the identification of non-responders from non-compliant patients and allowed the differentiation of mild side effects, such as malaise, from genuine toxicity caused by highly increased TGN levels.

Frequency Distribution of Thiopurine S-Methyltransferase Alleles in a Polish Population

Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. TPMT activity exhibits an interindividual variability mainly a result of genetic polymorphism. Patients with intermediate or deficient TPMT activity are at risk for toxicity after receiving standard doses of thiopurine drugs. It has previously been reported that 3 variant alleles:TPMT*2, *3A, and *3C are responsible for over 95% cases of lower enzyme activity. The purpose of this study was to determine the frequency of TPMT variant alleles in a Polish population. DNA samples were obtained from 358 unrelated healthy Polish subjects of white origin, and TPMT genetic polymorphism was determined using PCR-RFLP and allele-specific PCR methods. The results showed that allelic frequencies were 0.4% for TPMT*2, 2.7% for TPMT*3A, and 0.1% for TPMT*3C, respectively. A TPMT*3B allele was not found in the studied population. The general pattern of TPMT allele disposition in the Polish population is similar to those determined for other white populations, but the frequency of total variant alleles is lower than in other European populations studied to date.

Pharmacogenetics of inflammatory bowel disease

The therapeutic efficacy and toxicity of many commonly employed drugs show interindividual variations that relate to several factors, including genetic variability in drug-metabolizing enzymes, transporters or targets. The study of the genetic determinants influencing interindividual variations in drug response is known as pharmacogenetics. The ability to identify, through preliminary genetic screening, the patients most likely to respond positively to a medication should facilitate the best choice of treatment for each patient; drugs likely to exhibit low efficacy or to give negative side-effects can be avoided. Among the medications used for inflammatory bowel disease, the best studied pharmacogenetically is azathioprine. The hematopoietic toxicity of azathioprine is due to single nucleotide polymorphisms in the thiopurine S-methyltransferase enzyme. Additionally, likely gene targets have been investigated to predict the response to glucocorticoids and infliximab, a monoclonal antibody against tumour necrosis factor that induces remission in approximately 30-40% of patients. However, no genetic predictor of response has been identified in either case.

From pharmacokinetics to pharmacogenomics: a new approach to tailor immunosuppressive therapy

One of the main tasks in the management of organ transplantation is the optimization of immunosuppressive therapy, in order to provide therapeutic efficacy limiting drug-related toxicity. In the past years major efforts have been carried out to define therapeutic windows based on blood/plasma levels of each immunosuppressant relating those concentrations to drug dosing and clinical events. Although this traditional approach is able to identify environmental and nongenetic factors that can influence drug exposure during the course of treatment, it presents limitations. Therefore, complementary strategies are advocated. The advent of the genomic era gives birth to pharmacogenomics, a science that studies how the genome as a whole, including single genes as well as gene-to-gene interactions, may affect the action of a drug. This science is of particular importance for drugs characterized by a narrow therapeutic index, such as the immunosuppressants. Preliminary studies focused on polymorphisms of genes encoding for enzymes actively involved in drug metabolism, drug transport and pharmacological target. Pharmacogenomics holds promise for improvement in the ability to individualize immunosuppressive therapy based on the patient's genetic profile, and can be viewed as a support to traditional therapeutic drug monitoring. However, the clinical applicability of this approach is still to be proven.

Thiopurine methyltransferase (TPMT) genotype distribution in azathioprine-tolerant and -intolerant patients with various disorders. The impact of TPMT genotyping in predicting toxicity

OBJECTIVE

To study the distribution of the thiopurine methyltransferase (TPMT) genotype among azathioprine (Aza)-tolerant and -intolerant patients with various disorders, and to investigate a possible relationship with the Aza metabolite levels.

METHODS

Forty-six Aza-tolerant and six Aza-intolerant patients had the TPMT genotype distribution determined using a polymerase chain reaction (PCR) assay and the forty-six Aza-tolerant patients had the Aza metabolite levels determined using a high-pressure liquid chromatography (HPLC) analysis.

RESULTS

One non-functional TPMT mutant allele was demonstrated in 2 of the 46 Aza-tolerant patients (4.4%) and one or two non-functional mutant alleles in 2 of the 6 Aza-intolerant patients (33.3%). Of the 4 patients, with one or two non-functional mutant alleles 2 (50%) were intolerant to Aza compared with 4 of the 48 patients (8.3%) with no mutations detected. The time to hepatotoxicity did not differ significantly between the 2 patients with one or two non-functional mutant alleles and the remaining 3 patients ( P=0.5). The TPMT genotype distribution differed slightly in the three different categories of disorders ( P=0.05). The median E-6-TGN level among the 2 TPMT heterozygous patients was 275 pmol/8x10(8) RBC (range 240-310), whereas the remaining 44 patients had a median E-6-TGN level of 110 pmol/8x10(8) RBC (range 0-440) ( P=0.07).

CONCLUSION

Although TPMT genotyping cannot be recommended on behalf of the present study, it is to be expected that half of the patients with one or two non-functional TPMT mutant alleles will develop Aza intolerance leading to withdrawal of therapy. Thus, clinicians may anticipate about 5% of the patients to develop intolerance to Aza therapy solely for that reason.

In vitro characterization of four novel non-functional variants of the thiopurine S-methyltransferase

Human thiopurine S-methyltransferase (TPMT) is an enzyme responsible for the detoxification of widely used thiopurine drugs such as azathioprine (Aza). Its activity is inversely related to the risk of developing severe hematopoietic toxicity in certain patients treated with standard doses of thiopurines. DNA samples from four leucopenic patients treated with Aza were screened by PCR-SSCP analysis for mutations in the 10 exons of the TPMT gene. Four missense mutations comprising two novel mutations, A83T (TPMT*13, Glu(28)Val) and C374T (TPMT*12, Ser(125)Leu), and two previously described mutations, G430C (TPMT*10, Gly(144)Arg) and T681G (TPMT*7, His(227)Gln) were identified. Using a recombinant yeast expression system, kinetic parameters (K(m) and V(max)) of 6-thioguanine S-methylation of the four TPMT variants were determined and compared to those obtained with wild-type TPMT. This functional analysis suggests that these rare allelic variants are defective TPMT alleles. The His(227)Gln variant retained only 10% of the intrinsic clearance value (V(max)/K(m) ratio) of the wild-type enzyme. The Ser(125)Leu and Gly(144)Arg variants were associated with a significant decrease in intrinsic clearance values, retaining about 30% of the wild-type enzyme, whereas the Glu(28)Val variant produced a more modest decrease (57% of the wild-type enzyme). The data suggest that the sporadic contribution of the rare Glu(28)Val, Ser(125)Leu, Gly(144)Arg, and His(227)Gln variants may account for the occurrence of altered metabolism of TPMT substrates. These findings improve our knowledge of the genetic basis of interindividual variability in TPMT activity and would enhance the efficiency of genotyping methods to predict patients at risk of inadequate responses to thiopurine therapy.

Genetics. FDA races in wrong direction

Despite recent genetic evidence and the promise of individualized medicine, there is a continuing interest in using self-identified categories of race and ethnicity as variables in scientific and medical research. The U.S. Food and Drug Administration recently proposed a standardized approach for the collection of race and ethnicity data in clinical trials. We believe that this move fails to acknowledge new scientific data and recommend that relevant data from individuals be collected and used rather than broad group statistics. We also encourage that increased funding be committed to this important issue.

The drugs don't work: expectations and the shaping of pharmacogenetics

This article examines one particular set of technologies arising from developments in human genetics, those aimed at improving the targeting, design and use of conventional small molecule drugs-pharmacogenetics. Much of the debate about the applications and consequences of pharmacogenetics has been highly speculative, since little or no working technology is yet on the market. This article provides a novel analysis of the development of pharmacogenetics, and the social and ethical issues it raises, based on the sociology of technological expectations. In particular, it outlines how two alternative visions for the development of the technology are being articulated and embedded in a range of heterogeneous discourses, artefacts, actor strategies and practices, including: competing scientific research agendas, experimental technologies, emerging industrial structures and new ethical discourses. Expectations of how pharmacogenetics might emerge in each of these arenas are actively shaping the trajectory of this nascent technology and its potential socio-economic consequences.

Chemotherapy individualization

The current practice of dosing patients with anticancer drugs based on body size, leads to a large degree of interpatient variation in clinical outcome following standard doses of chemotherapy. Some patients may fail to respond to treatment, whilst others experience unacceptable side effects. Recent studies have identified more rational approaches to drug dosing, based on patient characteristics such as renal function, pharmacogenetic factors, and drug metabolizing activity. These can be used together with therapeutic drug monitoring and adaptive dosing to achieve a targeted systemic drug exposure in each patient, which may lead to more consistent clinical outcomes in patients receiving comparable chemotherapy dosing regimens. The purpose of this review is to present some approaches to chemotherapy individualization, examples of how this might be applied, and speculation as to how recent advances in pharmacogenetics may lead to further dose-optimization. Whilst it is hoped that the design of new agents, targeted to specific genes involved in oncogenesis, will lead to increased success in the treatment of cancer patients, it is essential that the drugs currently available are used to their maximum potential.