Predisposition to abacavir hypersensitivity conferred by HLA-B*5701 and a haplotypic Hsp70-Hom variant

T cell-mediated hypersensitivity reactions to drugs

The immunological mechanisms driving delayed hypersensitivity reactions (HSRs) to drugs mediated by drug-reactive T lymphocytes are exemplified by several key examples and their human leukocyte antigen (HLA) associations: abacavir and HLA-B*57:01, carbamazepine and HLA-B*15:02, allo-purinol and HLA-B*58:01, and both amoxicillin-clavulanate and nevirapine with multiple class I and II alleles. For HLA-restricted drug HSRs, specific class I and/or II HLA alleles are necessary but not sufficient for tissue specificity and the clinical syndrome. Several models have been proposed to explain the immunopathogenesis of severe T cell-mediated drug HSRs, and our increased understanding of the risk factors and mechanisms involved in the development of these reactions will further the development of sensitive and specific strategies for preclinical screening that will lead to safer and more cost-effective drug design.

Hypersensitivity Reaction Associated with Abacavir Therapy in an Indian HIV Patient - A Case Report

The most important and unique adverse effect of abacavir (ABC) is fatal hypersensitivity reaction (HSR). The objective of this report is to describe a case of ABC induced HSR that occurred in an Indian HIV patient during treatment. Although this adverse effect is not uncommon, it is perhaps underreported or has never been reported so far in an Indian case scenario. A 44-year-old known case of HIV-1 was admitted in view of his worsening condition and very low CD4 cell counts 3 cells/μL. He was on anti-retroviral therapy since three years but not regular. On the basis of treatment failure, non-compliance and progressive low CD4 counts, the anti HIV regime was switched over to abacavir 600 mg+ atazanavir/ ritonavir 300mg/100mg Two weeks after ABC therapy he presented with maculopapular rash, headache and signs of hepatic damage (serum AST, ALP and ALT increased to 3-4 fold) suggestive of hypersensitivity reaction. As we know discontinuation of the drug is the ultimate litmus test to confirm diagnosis of drug induced adverse reaction. We did confirm ABC induced HSR by de-challenge wherein, rash disappeared within 2-3 days and LFT came back to normal within 5 days. However, no rechallenge was done. HSR was more in favour of ABC because atazanavir failed to produce any similar reaction after re-challenge.

Frequencies of immune hypersensitivity reaction-associated HLA class I alleles in healthy South African Indian and mixed ancestry populations determined by a novel real-time PCR assay

We have determined the frequencies of human leucocyte antigen (HLA)-B*57:01, HLA-B*35:05, HLA-C*04 and HLA-C*08 in healthy individuals of South African Indian (SAI) ethnicity (n = 50) and South African mixed (SAM) ancestry (n = 50) using real-time allele-specific polymerase chain reaction (AS-PCR) assay. HLA-B*57:01 associates with immune hypersensitivity reaction (IHR) in individuals exposed to abacavir (ABC), while nevirapine (NVP) IHR associates with HLA-B*35:05, HLA-C*04 and HLA-C*08. Real-time AS-PCR assays typically use less DNA, are more cost-effective and rapid compared with conventional genotyping methods, such as sequence-based typing (SBT). The assay was developed using samples of known HLA class I genotype and subsequently applied to the SAI and SAM samples. HLA-B*57:01 was detected in SAM and SAI populations at frequencies of 8.0% and 12.0%, respectively, while HLA-B*35:05 was not found in SAI individuals, but was present in 6.0% of SAM individuals. HLA-C*04 was detected in 22.0% and 24.0% of SAM and SAI individuals, respectively, while 10.0% and 8.0% of SAM and SAI individuals, respectively, were HLA-C*08 positive. This study reports the development of a novel real-time AS-PCR assay to identify HLA class I alleles associated with ABC and NVP IHR and has established the frequencies of these alleles present in healthy SAI and SAM populations. Using South African demographic data, our hypothetical analysis suggests that a substantial number of individuals would benefit from the assay.

CYP2B6 c.983T>C polymorphism is associated with nevirapine hypersensitivity in Malawian and Ugandan HIV populations

Background

Nevirapine, an NNRTI used in HIV treatment, can cause hypersensitivity reactions in 6%–10% of patients. In the most serious cases (1.3%) this can manifest as Stevens–Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN).

Methods

DNA samples were obtained and analysed from a total of 209 adult patients with nevirapine hypersensitivity (57 from a prospective cohort and 152 routine clinic patients) and compared with 463 control patients on nevirapine without any hypersensitivity. The case group included 70 patients with SJS/TEN. All individuals were genotyped for two SNPs in the CYP2B6 gene [c.516G>T (CYP2B6*9) and c.983T>C (CYP2B6*18)] using the TaqMan real-time genotyping platform. The replication cohort comprised 29 controls and 55 nevirapine hypersensitive patients, including 8 SJS/TEN cases.

Results

An association between the CYP2B6 c.983T>C polymorphism and nevirapine-induced SJS/TEN was observed. In the SJS/TEN group, 30% of individuals possessed at least one c.983T>C versus 16% in the tolerant group [P = 0.006; OR (95% CI) 2.24 (1.27–3.94)]. This association was not significant in the replication cohort [P = 0.075; OR (95% CI) 4.33 (0.80–23.57)]. Combined analysis resulted in an OR of 2.52 (95% CI 1.48–4.20; P = 0.0005) for the association of c.983T>C with SJS/TEN. No association was observed for c.983T>C with other hypersensitivity phenotypes and for CYP2B6 c.516G>T with any hypersensitivity phenotypes.

Conclusions

Our data show an association between the c.983T>C polymorphism and nevirapine-induced SJS/TEN. CYP2B6 c.983T>C has a frequency of 5%–10% in a variety of African populations, but is not observed in Caucasians, thus representing an ethnic-specific predisposing factor.

Virological efficacy of abacavir: systematic review and meta-analysis

OBJECTIVES

The efficacy of abacavir/lamivudine has been reported to be inferior to tenofovir/emtricitabine. Several randomized clinical trials (RCTs) investigated the effectiveness and safety of abacavir/lamivudine and tenofovir/emtricitabine combined antiretroviral treatment (cART) and we have reviewed the available evidence.

DESIGN

Systematic review and meta-analysis of RCTs using standard Cochrane Collaboration methodologies.

METHODS

We calculated risk ratios (RRs) with 95% CIs. The primary outcome was the rate of patients with viral load (VL) below the pre-defined cut-off at 48 weeks and/or at 96 weeks. Where available, results were analysed according to VL screening levels (<100,000 or >100,000 copies/mL) with conventional meta-analytical pooling by subgroups and meta-regression.

RESULTS

Meta-analytical pooling of RCTs with a direct comparison of abacavir/lamivudine and tenofovir/emtricitabine according to baseline VL at 48 weeks (six trials, 4118 patients) showed that the proportions of subjects with VL <50 copies/mL were similar in the overall comparison (RR 0.98; 95% CI 0.94-1.03), in the low baseline VL strata (RR 1.01; 95% CI 0.99-1.03) and in the high baseline VL strata (RR 0.96; 95% CI 0.90-1.03). Meta-regression analysis at 48 weeks confirms the results of subgroup analysis. Similar virological results were found at 96 weeks (four trials, 2003 patients). Differences in the occurrence of adverse events requiring discontinuation of treatment favoured tenofovir recipients (RR 1.26; 95% CI 0.99-1.61), but this difference, mostly related to suspected abacavir hypersensitivity reaction, was not statistically significant.

CONCLUSIONS

Our cumulative, cross-sectional data suggest a similar virological efficacy of abacavir/lamivudine and tenofovir/emtricitabine regardless of the baseline VL.

Specific HLA types are associated with antiepileptic drug-induced Stevens-Johnson syndrome and toxic epidermal necrolysis in Japanese subjects

AIM

This preliminary study investigated genomic biomarkers for Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), related to three antiepileptic drugs, zonisamide, phenobarbital and phenytoin.

PATIENTS & METHODS

HLA class I and HLA-DRB1 loci were genotyped for Japanese patients with zonisamide-, phenobarbital- or phenytoin-induced SJS/TEN (n = 12, 8 and 9, respectively) and for healthy Japanese volunteers (n = 2878).

RESULTS

Carrier frequencies of HLA-A*02:07 in patients with zonisamide-induced SJS/TEN and in the general Japanese population were 41.7 and 6.81%, respectively. Carrier frequencies of HLA-B*51:01 in patients with phenobarbital- and phenytoin-induced SJS/TEN and in controls were 75.0, 55.6 and 15.2%, respectively. HLA-A*02:07 and HLA-B*51:01, in a dominant model, were significantly associated with zonisamide- and phenobarbital-induced SJS/TEN, respectively (Pc = 0.0176 and 0.0042, respectively).

CONCLUSION

Our data suggest that HLA-A*02:07 and HLA-B*51:01 are potential biomarkers for zonisamide- and phenobarbital-induced SJS/TEN, respectively, in Japanese individuals.

Development of multiplex pyrosequencing for HLA-B*57:01 screening using single nucleotide polymorphism haplotype

WHAT IS KNOWN AND OBJECTIVE

Abacavir (ABC) is a commonly used nucleoside reverse-transcriptase inhibitor with potent antiviral activity against HIV-1. The US Food and Drug Administration and international HIV treatment guidelines recommend HLA-B*57:01 screening before initiating treatment with ABC. The current standard method for HLA-B*57:01 screening is limited by its high-cost, time-consuming and labour-intensive procedure with the requirement of a specialized laboratory. Our study aims to develop a more reliable screening test by selecting rs3093726 as an additional single nucleotide polymorphism (SNP) to combine with rs2395029 for multiplex pyrosequencing development. It offers high-accuracy, cost-effective and rapid detection.

METHODS

Multiplex pyrosequencing was developed for HLA-B*57:01 screening using rs2395029 and rs3093726 as a surrogate marker and tested in 130 Thai subjects in parallel with singleplex pyrosequencing of each SNP and the standard sequence-based method.

RESULTS AND DISCUSSION

Multiplex pyrosequencing showed 100% concordance when compared with both singleplex pyrosequencing and standard sequence-based method. This method showed 100% of negative predictive value (NPV), positive predictive value (PPV), specificity and sensitivity.

WHAT IS NEW AND CONCLUSION

Multiplex pyrosequencing is a powerful tool for HLA-B*57:01 screening using the rs2395029 and rs3093726 haplotype genotyping as surrogate marker for this HLA-B. The assay provides accurate, cost-effective and rapid detection of this haplotype. It can be applied for ABC hypersensitivity screening of the Thai population before initiating treatment with ABC.

Digging up the human genome: current progress in deciphering adverse drug reactions

Adverse drug reactions (ADRs) are a major clinical problem. In addition to their clinical impact on human health, there is an enormous cost associated with ADRs in health care and pharmaceutical industry. Increasing studies revealed that genetic variants can determine the susceptibility of individuals to ADRs. The development of modern genomic technologies has led to a tremendous advancement of improving the drug safety and efficacy and minimizing the ADRs. This review will discuss the pharmacogenomic techniques used to unveil the determinants of ADRs and summarize the current progresses concerning the identification of biomarkers for ADRs, with a focus on genetic variants for genes encoding drug-metabolizing enzymes, drug-transporter proteins, and human leukocyte antigen (HLA). The knowledge gained from these cutting-edge findings will form the basis for better prediction and management for ADRs, ultimately making the medicine personalized.

Genotyping for severe drug hypersensitivity

Over the past decade, there have been significant advances in our understanding of the immunopathogenesis and pharmacogenomics of severe immunologically-mediated adverse drug reactions. Such T-cell-mediated adverse drug reactions such as Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN), drug-induced liver disease (DILI) and other drug hypersensitivity syndromes have more recently been shown to be mediated through interactions with various class I and II HLA alleles. Key examples have included the associations of HLA-B*15:02 and carbamazepine induced SJS/TEN in Southeast Asian populations and HLA-B*57:01 and abacavir hypersensitivity. HLA-B*57:01 screening to prevent abacavir hypersensitivity exemplifies a successful translational roadmap from pharmacogenomic discovery through to widespread clinical implementation. Ultimately, our increased understanding of the interaction between drugs and the MHC could be used to inform drug design and drive pre-clinical toxicity programs to improve drug safety.

Safety analysis of Epzicom® (lamivudine/abacavir sulfate) in post-marketing surveillance in Japan

PURPOSE

To obtain safety and effectiveness data on a combined anti-HIV drug, Epzicom (abacavir 600 mg/lamivudine 300 mg), a post-marketing surveillance on Epzicom that was required by the Japanese regulatory authority was conducted between January 2005 and December 2010.

METHODS

A joint survey (HIV-related drug [HRD] survey) has been conducted involving manufacturers of drugs for treatment of HIV infection in Japan. Safety and effectiveness data from total 624 cases (1107.3 person-years) registered to the HRD surveys and received Epzicom were obtained. Adverse drug reactions (ADRs) were defined as adverse events (AE) of which association with Epzicom could not be 'ruled out'.

RESULTS

It was found that the incidence of ADR was 32.4% (202/624 cases) on the case basis. In addition, the frequently reported ADR included hyperlipidaemia (59 cases), hypertriglyceridaemia (21 cases), blood bilirubin increased (19 cases), gamma-glutamyltransferase increase (14 cases), blood triglyceride increase (14 cases) and rash (14 cases). Serious AEs were seen in 19 patients (30 events), including one death (no evident association with Epzicom). There were four cases (0.6%) of survey-defined 'hypersensitivity', and the incidence was 0.9% (4/445) among abacavir naïve patients; none of which was reported as serious. No case of myocardial infarction was reported. One pregnant case who delivered a normal baby by caesarean section was reported to have experienced aggravation of anaemia and nausea.

CONCLUSIONS

The post-marketing surveillance indicated that the incidence of both ischaemic heart disease and hypersensitivity associated with Epzicom was considerably low, suggesting that this drug can be safely used in the Japanese population.

Contributions of pharmacogenetics and transcriptomics to the understanding of the hypersensitivity drug reactions

Hypersensitivity drug reactions (HDRs) represent a large and important health problem, affecting many patients and leading to a variety of clinical entities, some of which can be life-threatening. The culprit drugs include commonly used medications including antibiotics and NSAIDs. Nontherapeutical agents, such as contrast media, are also involved. Because the pathophysiological mechanisms are not well known and the current diagnostic procedures are somewhat insufficient, new approaches are needed for understanding the complexity of HDRs. Histochemical and molecular biology studies have enabled us to classify these reactions more precisely. Pharmacogenetics has led to the identification of several genes, involved mainly in T-cell-dependent responses, with a number of markers being replicated in different studies. These markers are now being considered as potential targets for reducing the number of HDRs. Transcriptomic approaches have also been used to investigate HDRs by identifying genes that show different patterns of expression in a number of clinical entities. This information can be of value for further elucidation of the mechanisms involved. Although first studies were performed using RT-PCR analysis to monitor the acute phase of the reaction, nowadays high-density expression platforms represent a more integrative way for providing a complete view of gene expression. By combining a detailed and precise clinical description with information obtained by these approaches, we will obtain a better understanding and management of patients with HDRs.

Pharmacogenetics in clinical practice: considerations for testing

Pharmacogenetics is a rapidly evolving field that will undoubtedly lead to the development of pharmacogenetic tests. Such tests will need to be assimilated into healthcare systems, but represent a further call on scarce healthcare resources. Therefore, in order for a pharmacogenetic test to fulfill its potential beyond the laboratory and into the clinical environment, it must prove itself on a wide range of multifaceted criteria. The test must have proven and reproducible analytical and clinical validity, and stand up to critical appraisal of clinical utility and cost-effectiveness. Pharmacogenetic testing can be considered to be a form of screening, and the experience that has been gained to date in evaluating other forms of screening tests may prove beneficial in evaluating pharmacogenetic technology. It is essential that the goals of pharmacogenetic testing are defined as early as possible to ensure that appropriate studies can be designed to provide the evidence base, and thereby enable appropriate evaluation of the technology by clinicians and healthcare administrators for incorporation into clinical practice. This review focuses on issues that will need to be considered in the scientific assessment of pharmacogenetic testing.

Flow cytometry analysis with a new FITC-conjugated monoclonal antibody-3E12 for HLA-B*57:01 rapid screening in prevention of abacavir hypersensitivity in HIV-1-infected patients

BACKGROUND

Rapid screening for the detection of HLA-B*57:01 in the prevention of abacavir hypersensitivity in HIV-1-infected patients is a hallmark for clinical services.

OBJECTIVE

The aim of this work was to analyze the utility of flow cytometry with a new FITC-conjugated B-17 monoclonal antibody (mAb3E12) for HLA-B*57:01 screening in a Spanish cohort of 577 HIV-1+ individuals.

METHODS

Cryopreserved peripheral blood mononuclear cell samples from HIV-1+ individuals were analyzed by flow cytometry with the mAb 3E12 that recognizes both HLA-B*57 and HLA-B*58 alleles (members of the group specificity, HLA-B17). Patients' DNA samples had been previously typed for HLA-B*57:01 with PCR-SSO or PCR-SSP and additional DNA sequencing (EPI Study). The results obtained by flow cytometry were compared with the results obtained by the DNA-PCR techniques.

RESULTS

By flow cytometry, 46 samples (7.97%) were positive for HLA-B17, 530 (91.86%) were negative, and 1 (0.17%) was undetermined. All samples found negative by flow cytometry were negative for HLA-B*57:01 by DNA-PCR. Of the HLA-B17 positive samples, 31 (67.4%) were positive for HLA-B*57:01, 2 (3.25%) were positive for HLA-B*57:03, 11 (26.1%) were positive for HLA-B*58, and 2 (3.25%) were negative for both HLA-B*57 and HLA-B*58 antigens. The undetermined sample was negative for HLA-B*57 and HLA-B*58 alleles by DNA-PCR.

CONCLUSIONS

This study shows that flow cytometry with mAb3E12 is a highly sensitive method (no false negatives) to implement prior to DNA-PCR analysis for rapid screening of HLA-B*57:01. Additional confirmation by molecular HLA typing method would be required in less than 10% of the cohort of HIV-1-infected individuals.

A genome-wide association meta-analysis of self-reported allergy identifies shared and allergy-specific susceptibility loci

Allergic disease is very common and carries substantial public-health burdens. We conducted a meta-analysis of genome-wide associations with self-reported cat, dust-mite and pollen allergies in 53,862 individuals. We used generalized estimating equations to model shared and allergy-specific genetic effects. We identified 16 shared susceptibility loci with association P<5×10(-8), including 8 loci previously associated with asthma, as well as 4p14 near TLR1, TLR6 and TLR10 (rs2101521, P=5.3×10(-21)); 6p21.33 near HLA-C and MICA (rs9266772, P=3.2×10(-12)); 5p13.1 near PTGER4 (rs7720838, P=8.2×10(-11)); 2q33.1 in PLCL1 (rs10497813, P=6.1×10(-10)), 3q28 in LPP (rs9860547, P=1.2×10(-9)); 20q13.2 in NFATC2 (rs6021270, P=6.9×10(-9)), 4q27 in ADAD1 (rs17388568, P=3.9×10(-8)); and 14q21.1 near FOXA1 and TTC6 (rs1998359, P=4.8×10(-8)). We identified one locus with substantial evidence of differences in effects across allergies at 6p21.32 in the class II human leukocyte antigen (HLA) region (rs17533090, P=1.7×10(-12)), which was strongly associated with cat allergy. Our study sheds new light on the shared etiology of immune and autoimmune disease.

Determination of the high prevalence of Dual/Mixed- or X4-tropism among HIV type 1 CRF01_AE in Hong Kong by genotyping and phenotyping methods

In Hong Kong, the CCR5 antagonist has recently been introduced into salvage therapy for multiclass drug-resistant HIV-1-infected patients. Coreceptor usage must be determined prior to the usage of the CCR5 antagonist, which does not inhibit X4-tropic viruses. This study aimed to determine the tropism prevalence for HIV-1 subtypes B and CRF01_AE in Hong Kong. In addition, a modified promoter-PCR phenotypic assay was used to validate the genotypic tropism prediction on CRF01_AE. One hundred and five subtype B and 98 CRF01_AE antiretroviral-naive patients were recruited for this study. The viral env V3 region isolated from the patients was sequenced and analyzed by Geno2pheno (FPR=5.75% or 10%, Clonal or Clinical), position-specific scoring matrix (WebPSSM, x4r5 subtype B matrix), and the combination of 11/25 and net charge rules. Fifteen concordant and 22 discordant tropism genotyped CRF01_AE samples were further phenotyped by either enhanced sensitivity Trofile assay or an optimized promoter-PCR phenotypic assay. The prevalence of Dual/Mixed- or X4-tropic virus in antiretroviral-naive subtype CRF01_AE was 39.1%, which was significantly higher than subtype B (p<0.05), regardless of the choices of genotypic algorithms. Our phenotypic data proposed that a better genotypic tropism prediction for HIV-1 CRF01_AE would be using both Geno2pheno (FPR=10%, Clonal) and WebPSSM (x4r5 subtype B matrix) algorithms in combination. The sensitivity and specificity for this combination were 88.9% and 89.3%, respectively. The comparatively high prevalence of Dual/Mixed- or X4-tropic virus in CRF01_AE demonstrated the need for special attention to future treatment strategies.

Meta-analysis of genome-wide association studies identifies ten loci influencing allergic sensitization

Allergen-specific immunoglobulin E (present in allergic sensitization) has a central role in the pathogenesis of allergic disease. We performed the first large-scale genome-wide association study (GWAS) of allergic sensitization in 5,789 affected individuals and 10,056 controls and followed up the top SNP at each of 26 loci in 6,114 affected individuals and 9,920 controls. We increased the number of susceptibility loci with genome-wide significant association with allergic sensitization from three to ten, including SNPs in or near TLR6, C11orf30, STAT6, SLC25A46, HLA-DQB1, IL1RL1, LPP, MYC, IL2 and HLA-B. All the top SNPs were associated with allergic symptoms in an independent study. Risk-associated variants at these ten loci were estimated to account for at least 25% of allergic sensitization and allergic rhinitis. Understanding the molecular mechanisms underlying these associations may provide new insights into the etiology of allergic disease.

Phase-defined complete sequencing of the HLA genes by next-generation sequencing

BACKGROUND

The human leukocyte antigen (HLA) region, the 3.8-Mb segment of the human genome at 6p21, has been associated with more than 100 different diseases, mostly autoimmune diseases. Due to the complex nature of HLA genes, there are difficulties in elucidating complete HLA gene sequences especially HLA gene haplotype structures by the conventional sequencing method. We propose a novel, accurate, and cost-effective method for generating phase-defined complete sequencing of HLA genes by using indexed multiplex next generation sequencing.

RESULTS

A total of 33 HLA homozygous samples, 11 HLA heterozygous samples, and 3 parents-child families were subjected to phase-defined HLA gene sequencing. We applied long-range PCR to amplify six HLA genes (HLA-A, -C, -B, DRB1, -DQB1, and -DPB1) followed by transposase-based library construction and multiplex sequencing with the MiSeq sequencer. Paired-end reads (2 × 250 bp) derived from the sequencer were aligned to the six HLA gene segments of UCSC hg19 allowing at most 80 bases mismatch. For HLA homozygous samples, the six amplicons of an individual were pooled and simultaneously sequenced and mapped as an individual-tagging method. The paired-end reads were aligned to corresponding genes of UCSC hg19 and unambiguous, continuous sequences were obtained. For HLA heterozygous samples, each amplicon was separately sequenced and mapped as a gene-tagging method. After alignments, we detected informative paired-end reads harboring SNVs on both forward and reverse reads that are used to separate two chromosomes and to generate two phase-defined sequences in an individual. Consequently, we were able to determine the phase-defined HLA gene sequences from promoter to 3'-UTR and assign up to 8-digit HLA allele numbers, regardless of whether the alleles are rare or novel. Parent-child trio-based sequencing validated our sequencing and phasing methods.

CONCLUSIONS

Our protocol generated phased-defined sequences of the entire HLA genes, resulting in high resolution HLA typing and new allele detection.

Clinical Pharmacogenetics Implementation Consortium guidelines for HLA-B genotype and carbamazepine dosing

Human leukocyte antigen B (HLA-B) is a gene that encodes a cell surface protein involved in presenting antigens to the immune system. The variant allele HLA-B*15:02 is associated with an increased risk of Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) in response to carbamazepine treatment. We summarize evidence from the published literature supporting this association and provide recommendations for the use of carbamazepine based on HLA-B genotype (also available on PharmGKB: http://www.pharmgkb.org). The purpose of this article is to provide information to allow the interpretation of clinical HLA-B*15:02 genotype tests so that the results can be used to guide the use of carbamazepine. The guideline provides recommendations for the use of carbamazepine when HLA-B*15:02 genotype results are available. Detailed guidelines regarding the selection of alternative therapies, the use of phenotypic tests, when to conduct genotype testing, and cost-effectiveness analyses are beyond the scope of this document. Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines are published and updated periodically on the PharmGKB website at (http://www.pharmgkb.org).

Abacavir pharmacogenetics--from initial reports to standard of care

Abacavir is a nucleoside analogue reverse transcriptase inhibitor indicated for the treatment of human immunodeficiency virus infection as part of a multidrug, highly active antiretroviral therapy regimen. Despite its efficacy, approximately 5% of individuals who receive abacavir develop an immune-mediated hypersensitivity reaction (HSR) that warrants immediate discontinuation of abacavir and switching to an alternative antiretroviral regimen. Abacavir HSR is associated with individuals who carry the *57:01 variant in the human leukocyte antigen B (HLA-B) gene. There is a large volume of evidence to show that those who carry HLA-B*57:01 are at significantly increased risk of developing HSR and should not receive abacavir. Pharmacogenetic screening to ensure individuals who carry HLA-B*57:01 do not receive abacavir can reduce the incidence of HSR and is now considered the standard of care before prescribing abacavir. Genetic testing to prevent abacavir HSR is currently one of the best examples of integrating pharmacogenetic testing into clinical practice.

Pharmacogenomics of severe cutaneous adverse reactions and drug-induced liver injury

Rare but severe adverse drug reactions (ADRs) are an important issue in drug development and in the proper usage of drugs during the post-approval phase. The ability to predict patient susceptibility to severe ADRs would prevent drug administration to high-risk patients. This would save lives and ensure the quality of life for these patients, but occurrence of idiosyncratic severe ADRs had been very difficult to predict for a long time. However, in this decade, genetic markers have been found for several ADRs, especially for severe cutaneous adverse reactions (SCARs) and drug-induced liver injury (DILI). In this review, we summarize recent progress in identifying genetic markers for SCARS and DILI, and discuss issues that remain unresolved. As for SCARs, associations of HLA-B*15:02 or HLA-A*31:01 and HLA-B*58:01 have been revealed for carbamazepine- and allopurinol-related Stevens-Johnson syndrome and toxic epidermal neclolysis, respectively. HLA-B*57:01 is strongly associated with abacavir-induced hypersensitivity syndrome. Several HLA alleles also demonstrate drug-specific associations with DILI, such as HLA-A*33:03 for ticlopidine, HLA-B*57:01 for flucloxacillin and HLA-DQA1*02:01 for lapatinib. Efforts should be continued to find other genetic markers to achieve high predictability for ADRs, with the goal being development of genetic tests for use in clinical settings.

Abacavir Pharmacokinetics in Human Immunodeficiency Virus-Infected Children Ranging in Age From 1 Month to 16 Years: A Population Analysis

Abacavir pharmacokinetics was studied in 105 children by a population approach performed with NONMEM. A 1-compartment open model with linear absorption and elimination adequately described the data. Typical population estimates (percent interindividual variability) of absorption rate constant, apparent distribution volume, and apparent plasma clearance were 1.79 h(-1) (58%), 42.9 L (53%), and 24.3 L/h (30%), respectively. Apparent plasma clearance was positively related to body weight. Individual Bayesian estimates of apparent plasma clearance were used to calculate individual abacavir area under the concentration curve (AUC). For the current weight-based regimen, abacavir exposure was found to be constant throughout the age range of the study, with an overall mean AUC value of 8.5 +/- 2.5 mg x h/L, which is slightly greater than the mean AUC value reported in adults. This study confirms the relevance of the current weight-based abacavir dosage regimen in pediatric patients.

The structural basis of HLA-associated drug hypersensitivity syndromes

Recent data suggest alternative mechanisms that promote human leukocyte antigen (HLA)-associated drug syndromes. Hypersensitive responses have been attributed to drug interactions with HLA molecules, peptides presented by HLA molecules and T-cell antigen receptors. Definition of an increasing number of HLA-associated drug syndromes suggests that polymorphism in the antigen-binding cleft residues influence recognition of specific drugs. Recent data demonstrate that small molecule drugs bind within the antigen-binding cleft of HLA in a manner that alters the repertoire of HLA-bound peptide ligands. This drug recognition mechanism permits presentation of self-peptides to which the host has not been tolerized. This altered repertoire mechanism is analogous to massive polyclonal T-cell responses occurring in mismatched HLA organ transplantation in which the drug in effect creates a novel HLA allele. Alteration of the self-peptide repertoire by HLA-binding small molecules may be the mechanistic basis for a diverse set of deleterious T-cell responses since the antigen-binding cleft has structural features that are compatible with binding drug-like small molecules. Small molecule drugs that bind elements of the trimolecular complex (T-cell receptor, peptide, and HLA) may cause short- and long-term adverse effects by a diverse set of mechanisms.

The p-i Concept: Pharmacological Interaction of Drugs With Immune Receptors

The immune response in drug hypersensitivity is normally explained by the hapten hypothesis. It postulates that drugs with a molecular weight of less than 1000 D are too small to cause an immune response per se. However, if a chemically reactive drug or drug metabolite binds covalently to a protein and thus forms a so-called hapten-carrier complex, this modified protein can induce an immune response. This concept has recently been supplemented by the p-i concept (or pharmacological interaction with immune receptors), which postulates that some drugs that lack hapten characteristics can bind directly and reversibly (noncovalently) to immune receptors and thereby stimulate the cells. For example, a certain drug may bind to a particular T-cell receptor, and this binding suffices to stimulate the T cell to secrete cytokines, to proliferate, and to exert cytotoxicity. The p-i concept has major implications for our understanding of drug interaction with the specific immune system and for drug hypersensitivity reactions. It is based on extensive investigations of T-cell clones reacting with the drug and recently of hybridoma cells transfected with the drug-specific T-cell receptor for antigen (TCR). It is a highly specific interaction dependent on the expression of a TCR into which the drug can bind with sufficient affinity to cause signaling. Small modification of the drug structure may already abrogate reactivity. Stimulation of T cells occurs within minutes as revealed by rapid Ca influx after drug addition to drug-specific T-cell clones or hybridoma cells, thus, before metabolism and processing can occur. As the immune system can only react in an immunologic way, the symptoms arising after drug stimulation of immune receptors imitate an immune response after recognition of a peptide antigen, although it is actually a pharmacological stimulation of some T cells via their TCRs. Clinically, the p-i concept could explain the sometimes rapid appearance of symptoms without previous sensitizations and the sometimes chaotic immune reaction of drug hypersensitivity with participation of different immune mechanisms while normal immune reactions to antigens are highly coordinated. Nevertheless, because the reactions lead to expansion of drug-reactive cells, many features such as skin test reactivity and stronger reactivity upon reexposure are identical to real immune reactions.

Human leukocyte antigen-associated drug hypersensitivity

A growing number of associations between adverse drug reactions and alleles of the human leukocyte antigen (HLA) genes are now known. Although several models have been proposed to explain these associations, an underlying molecular basis has only recently been described. The associations between HLA-B*57:01 and abacavir hypersensitivity syndrome, and HLA-B*15:02 and carbamazepine-induced bullous skin disease have provided new insights into the mechanism associated with hypersensitivity reactions to these drugs. Here we discuss recent evidence that small molecules can interact with specific HLA to distort self-peptide presentation leading to autoimmune-like drug hypersensitivities that potentially provide clues to the mechanisms underlying other immunopathologies.

Human leukocyte antigens (HLA) associated drug hypersensitivity: consequences of drug binding to HLA

Recent publications have shown that certain human leukocyte antigen (HLA) alleles are strongly associated with hypersensitivity to particular drugs. As HLA molecules are a critical element in T-cell stimulation, it is no surprise that particular HLA alleles have a direct functional role in the pathogenesis of drug hypersensitivity. In this context, a direct interaction of the relevant drug with HLA molecules as described by the p-i concept appears to be more relevant than presentation of hapten-modified peptides. In some HLA-associated drug hypersensitivity reactions, the presence of a risk allele is a necessary but incomplete factor for disease development. In carbamazepine and HLA-B*15:02, certain T-cell receptor (TCR) repertoires are required for immune activation. This additional requirement may be one of the 'missing links' in explaining why most individuals carrying this allele can tolerate the drug. In contrast, abacavir generates polyclonal T-cell response by interacting specifically with HLA-B*57:01 molecules. T cell stimulation may be due to presentation of abacavir or of altered peptides. While the presence of HLA-B*58:01 allele substantially increases the risk of allopurinol hypersensitivity, it is not an absolute requirement, suggesting that other factors also play an important role. In summary, drug hypersensitivity is the end result of a drug interaction with certain HLA molecules and TCRs, the sum of which determines whether the ensuing immune response is going to be harmful or not.

The pharmacogenetics of antiretroviral therapy

PURPOSE OF REVIEW

Although the past 20 years have seen many advances in HIV pharmacotherapy, host variability has only been more recently recognized as an important driver of both treatment response and toxicity. This review will focus on the importance of variability in drug metabolizing enzymes and the potential research and clinical applications of these findings in HIV care.

RECENT FINDINGS

The treatment of HIV is complex and involves chronic treatment with multiple drugs and drug classes. Recent research has been important in uncovering genetic differences in drug metabolizing and drug transporter genes, which underpin dose-dependent toxicity and efficacy, but also in genes defining differences in immune response and human leukocyte antigen genes, which restrict processes that are less dependent on the drug dose.

SUMMARY

The advancement of pharmacogenetics will continue to further our knowledge of disease-drug interactions and pathogenesis. Ultimately, the goals of pharmacogenetics and pharmacogenomics in HIV medicine will be the development of the right drugs for the right patient or population. The strong association between HLA-B5701 and abacavir hypersensitivity reaction shows promise for a genetic screening test to be feasibly incorporated into clinical practice. The clinical applicability of many findings demonstrating associations between single nucleotide polymorphisms, particularly in drug metabolism and transporter genes and drug efficacy and toxicity, are currently uncertain.

Using companion and coupled diagnostics within strategy to personalize targeted medicines

Regulatory authorities expect the pharmaceutical and biotechnology industries to accelerate their development of companion diagnostics and companion therapeutics towards the goal of personalized medicine, and expect health services to fund, prescribers to adopt and patients to accept these new therapeutic technologies. Expected benefits from more systematic development of combination products (companion diagnostic and its companion therapeutic) are expected to include safer and improved clinical and cost-effective use of medicines, more efficient patient selection for clinical trials, more cost-effective treatment pathways for health services, and a more profitable approach for drug developers. This review discusses challenges to timely development of companion diagnostics and provides case studies of single and multiple protein and genetic biomarkers of clinical response and risk of adverse drug effects.

Practical Considerations For Developing Nucleoside Reverse Transcriptase Inhibitors

Nucleoside reverse transcriptase inhibitors (NRTI) remain a cornerstone of current antiretroviral regimens in combinations usually with a non-nucleoside reverse transcriptase inhibitor (NNRTI), a protease inhibitor (PI), or an integrase inhibitor (INI). The antiretroviral efficacy and relative safety of current NRTI results from a tight and relatively specific binding of their phosphorylated nucleoside triphosphates (NRTI-TP) with the HIV-1 reverse transcriptase which is essential for replication. The intracellular stability of NRTI-TP produces a sustained antiviral response, which makes convenient dosing feasible. Lessons learned regarding NRTI pharmacology screening, development, and use are discussed. NRTI and prodrugs currently under clinical development are outlined.

Immune self-reactivity triggered by drug-modified HLA-peptide repertoire

Human leukocyte antigens (HLAs) are highly polymorphic proteins that initiate immunity by presenting pathogen-derived peptides to T cells. HLA polymorphisms mostly map to the antigen-binding cleft, thereby diversifying the repertoire of self-derived and pathogen-derived peptide antigens selected by different HLA allotypes. A growing number of immunologically based drug reactions, including abacavir hypersensitivity syndrome (AHS) and carbamazepine-induced Stevens-Johnson syndrome (SJS), are associated with specific HLA alleles. However, little is known about the underlying mechanisms of these associations, including AHS, a prototypical HLA-associated drug reaction occurring exclusively in individuals with the common histocompatibility allele HLA-B*57:01, and with a relative risk of more than 1,000 (refs 6, 7). We show that unmodified abacavir binds non-covalently to HLA-B*57:01, lying across the bottom of the antigen-binding cleft and reaching into the F-pocket, where a carboxy-terminal tryptophan typically anchors peptides bound to HLA-B*57:01. Abacavir binds with exquisite specificity to HLA-B*57:01, changing the shape and chemistry of the antigen-binding cleft, thereby altering the repertoire of endogenous peptides that can bind HLA-B*57:01. In this way, abacavir guides the selection of new endogenous peptides, inducing a marked alteration in 'immunological self'. The resultant peptide-centric 'altered self' activates abacavir-specific T-cells, thereby driving polyclonal CD8 T-cell activation and a systemic reaction manifesting as AHS. We also show that carbamazepine, a widely used anti-epileptic drug associated with hypersensitivity reactions in HLA-B*15:02 individuals, binds to this allotype, producing alterations in the repertoire of presented self peptides. Our findings simultaneously highlight the importance of HLA polymorphism in the evolution of pharmacogenomics and provide a general mechanism for some of the growing number of HLA-linked hypersensitivities that involve small-molecule drugs.

Recent progress of elucidating the mechanisms of drug hypersensitivity

Recent technical approaches to investigating drug hypersensitivity have provided a great deal of information to solve the mechanisms that remain poorly understood. First, immunological investigations and in silico analysis have revealed that a novel interaction between T cells and antigen-presenting cells, namely the pharmacological interaction concept, is involved in drug recognition and the hapten theory. Second, progress in immunology has provided a new concept of CD4+ T cell subsets. Th17 cells have proven to be a critical player in acute generalized exanthematous pustulosis. Our recent findings suggest that this subset might contribute to the pathogenesis of Stevens-Johnson syndrome/toxic epidermal necrolysis. Third, alarmins, molecules associated with innate immunity, are also associated with exaggeration and the persistence of severe drug hypersensitivity. The latest innovative techniques are providing a new landscape to examine drug hypersensitivity.

Clinical pharmacogenetics implementation consortium guidelines for HLA-B genotype and abacavir dosing

Human leukocyte antigen B (HLA-B) is responsible for presenting peptides to immune cells and plays a critical role in normal immune recognition of pathogens. A variant allele, HLA-B*57:01, is associated with increased risk of a hypersensitivity reaction to the anti-HIV drug abacavir. In the absence of genetic prescreening, hypersensitivity affects ~6% of patients and can be life-threatening with repeated dosing. We provide recommendations (updated periodically at http://www.pharmkgb.org) for the use of abacavir based on HLA-B genotype.

Tag SNPs of the ancestral haplotype 57.1 do not substitute HLA-B*57:01 typing for eligibility to abacavir treatment in the Italian population

A letter in response to: Sanchez-Giron F, Villegas-Torres B, Jaramillo-Villafuerte K et al. Association of the genetic marker for abacavir hypersensitivity HLA-B*5701 with HCP5 rs2395029 in Mexican Mestizos. Pharmacogenomics 12(6), 809–814 (2011).