CYP2C19 genetic variants affect nelfinavir pharmacokinetics and virologic response in HIV-1-infected children receiving highly active antiretroviral therapy

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

Pharmacogenomics in Papua New Guineans: unique profiles and implications for enhancing drug efficacy while improving drug safety

Papua New Guinea (PNG) can be roughly divided into highland, coastal and island peoples with significant mitochondrial DNA differentiation reflecting early and recent distinct migrations from Africa and East Asia, respectively. Infectious diseases such as tuberculosis, malaria and HIV severely impact on the health of its peoples for which drug therapy is the major treatment and pharmacogenetics has clinical relevance for many of these drugs. Although there is generally little information about known single nucleotide polymorphisms in the population, in some instances, their frequencies have been shown to be higher than anywhere worldwide. For example, CYP2B6*6 is over 50%, and CYP2C19*2 and *3 are over 40 and 25%, respectively. Conversely, CYP2A6*9, 2B6*2, *3, *4 and *18, and 2C8*3 appear to be much lower than in Whites. CYP2D6 known variants are unclear, and for phase II enzymes, only UGT2B7 and UGT1A9 data are available, with variant frequencies either slightly lower than or similar to Whites. Although almost all PNG people tested are rapid acetylators, but which variant(s) define this phenotype is not known. For HLA-B*13:01, HLA-B*35:05 and HLA-C*04:01, the frequencies show some regioselectivity, but the clinical implications with respect to adverse drug reactions are not known. There are minimal phenotype data for the CYPs and nothing is known about drug transporter or receptor genetics. Determination of genetic variants that are rare in Whites or Asians but common in PNG people is a topic of both scientific and clinical importance, and further research needs to be carried out. Optimizing the safety and efficacy of infectious disease drug therapy through pharmacogenetic studies that have translation potential is a priority.

Pharmacogenetic Associations with ADME Variants and Virologic Response to an Initial HAART Regimen in HIV-Infected Women

BACKGROUND

Clinical response to highly active antiretroviral therapy (HAART) varies among different populations. A portion of this variability may be due to variation in genes involved in the absorption, distribution, metabolism, and excretion (ADME) of HAART.

DESIGN

To identify genetic factors involved in virologic responses to HAART, 13 genes in ADME pathways were analyzed in a cohort of HIV-infected women on HAART. A total of 569 HIV-positive participants from the Women's Interagency HIV Study who initiated HAART from 1994-2012 and had genotype data were included in these analyses.

METHODS

Admixture maximum likelihood burden testing was used to evaluate gene-level associations between common genetic variation and virologic response (achieving <80 viral copies/mL) to HAART overall and with specific drug classes. Results: Six statistically significant (P<0.05) gene-level burden tests were observed with response to specific regimen types. CYP2B6, CYP2C19 and CYP2C9 were significantly associated with response to protease inhibitor (PI)-based regimens. CYP2C9, ADH1A and UGT1A1 were significantly associated with response to triple nucleoside reverse transcriptase inhibitor (NRTI) treatment.

CONCLUSIONS

Although no genome-wide associations with virologic response to HAART overall were detected in this cohort of HIV-infected women, more statistically significant gene-level burden tests were observed than would be expected by chance (two and a half expected, six observed). It is likely that variation in one of the significant genes is associated with virologic response to certain HAART regimens. Further characterization of the genes associated with response to PI-based treatment is warranted.

Neurotoxicity in the Post-HAART Era: Caution for the Antiretroviral Therapeutics

Despite the advent of highly active antiretroviral therapy (HAART), HIV-associated neurological disorders (HAND) remain a major challenge in human immunodeficiency virus (HIV) treatment. The early implementation of HAART in the infected individuals helps suppress the viral replication in the plasma and other compartments. Several studies also report the beneficial effect of drugs that successfully penetrate central nervous system (CNS). However, recent data in both clinical setup and in in vitro studies indicate CNS toxicity of the antiretrovirals (ARVs). Although the evidence is limited, correlation between prolonged use of ARVs and neurotoxicity strongly suggests that it is essential to study the underlying mechanisms responsible for such toxicity. Furthermore, closer attention toward clinical outcomes is required to screen various ARV regimens for their association with HAND and other comorbidities. A growing body of literature also indicates a possible role of accelerated aging in the antiretroviral therapy-associated neurotoxicity. Lastly, owing to high pill burden, multiple drugs in the HIV treatment also invite a possible role of drug-drug interaction via various cytochrome P450 enzymes. The particular emphasis of this review is to highlight the need to identify alternative approaches in reducing the CNS toxicity of the ARV drugs in HIV-infected individuals.

Impact of Genetic Polymorphisms of ABCB1 (MDR1, P-Glycoprotein) on Drug Disposition and Potential Clinical Implications: Update of the Literature

ATP-binding cassette transporter B1 (ABCB1; P-glycoprotein; multidrug resistance protein 1) is an adenosine triphosphate (ATP)-dependent efflux transporter located in the plasma membrane of many different cell types. Numerous structurally unrelated compounds, including drugs and environmental toxins, have been identified as substrates. ABCB1 limits the absorption of xenobiotics from the gut lumen, protects sensitive tissues (e.g. the brain, fetus and testes) from xenobiotics and is involved in biliary and renal secretion of its substrates. In recent years, a large number of polymorphisms of the ABCB1 [ATP-binding cassette, sub-family B (MDR/TAP), member 1] gene have been described. The variants 1236C>T (rs1128503, p.G412G), 2677G>T/A (rs2032582, p.A893S/T) and 3435C>T (rs1045642, p.I1145I) occur at high allele frequencies and create a common haplotype; therefore, they have been most widely studied. This review provides an overview of clinical studies published between 2002 and March 2015. In summary, the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance. Future research activities are warranted, considering a deep-sequencing approach, as well as well-designed clinical studies with appropriate sample sizes to elucidate the impact of rare ABCB1 variants and their potential consequences for effect sizes.

Impact of CYP2C19 polymorphism on the pharmacokinetics of nelfinavir in patients with pancreatic cancer

AIM

This study evaluated the influence of CYP2C19 polymorphisms on the pharmacokinetics of nelfinavir and its metabolite M8 in patients with pancreatic cancer.

METHODS

Nelfinavir was administered orally to patients for over 10 days. The plasma concentrations of nelfinavir and M8 were measured by HPLC. The genotypes of CYP2C19*1, CYP2C19*2 and CYP2C19*3 were determined by the polymerase chain reaction-restriction fragment length polymorphism method.

RESULTS

Pharmacokinetic profiles of nelfinavir and M8 were characterized by wide interindividual variability. The mean Cmax of nelfinavir in CYP2C19*1/*1 patients was 3.89 ± 0.40 (n = 3) and 5.12 ± 0.41 (n = 30) µg ml(-1) , while that of CYP2C19*1/*2 patients was 3.60 (n = 1) and 6.14 ± 0.31 (n = 5) µg ml(-1) at the doses of 625 and 1250 mg nelfinavir twice daily, respectively. For the M8 metabolite, the mean Cmax of CYP2C19*1/*1 patients was 1.06 ± 0.06 (n = 3) and 1.58 ± 0.27 (n = 30) µg ml(-1) , while those of CYP2C19*1/*2 patients were 1.01 (n = 1) and 1.23 ± 0.15 (n = 5) µg ml(-1) at the doses of 625 and 1250 mg nelfinavir twice daily, respectively. The area under the plasma concentration-time curve (AUC(0,12 h)) values of nelfinavir for CYP2C19*1/*1 patients were 28.90 ± 1.27 and 38.90 ± 4.99 µg ml(-1) ·h and for CYP2C19*1/*2 patients, AUC(0,12 h) was 28.20 (n = 1) and 40.22 ± 3.17 (n = 5) µg ml(-1) ·h at the doses of 625 and 1250 mg nelfinavir twice daily, respectively. The Cmax of nelfinavir was significantly higher (P <0.05) in CYP2C19*1/*2 patients but there was no statistical difference in AUC(0,12 h).

CONCLUSION

CYP2C19*1/*2 genotype modestly affected the pharmacokinetic profiles of nelfinavir and M8 in patients with locally advanced pancreatic cancer.

Polymorphisms of the drug transporters ABCB1, ABCG2, ABCC2 and ABCC3 and their impact on drug bioavailability and clinical relevance

INTRODUCTION

Human ATP-binding cassette (ABC) transporters act as translocators of numerous substrates across extracellular and intracellular membranes, thereby contributing to bioavailability and consequently therapy response. Genetic polymorphisms are considered as critical determinants of expression level or activity and subsequently response to selected drugs.

AREAS COVERED

Here the influence of polymorphisms of the prominent ABC transporters P-glycoprotein (MDR1, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and the multidrug resistance-associated protein (MRP) 2 (ABCC2) as well as MRP3 (ABCC3) on the pharmacokinetic of drugs and associated consequences on therapy response and clinical outcome is discussed.

EXPERT OPINION

ABC transporter genetic variants were assumed to affect interindividual differences in pharmacokinetics and subsequently clinical response. However, decades of medical research have not yielded in distinct and unconfined reproducible outcomes. Despite some unique results, the majority were inconsistent and dependent on the analyzed cohort or study design. Therefore, variability of bioavailability and drug response may be attributed only by a small amount to polymorphisms in transporter genes, whereas transcriptional regulation or post-transcriptional modification seems to be more critical. In our opinion, currently identified genetic variants of ABC efflux transporters can give some hints on the role of transporters at interfaces but are less suitable as biomarkers to predict therapeutic outcome.

The dual role of pharmacogenetics in HIV treatment: mutations and polymorphisms regulating antiretroviral drug resistance and disposition

Significant intra- and interindividual variability has been observed in response to use of pharmacological agents in treatment of HIV infection. Treatment of HIV infection is limited by high rates of adverse drug reactions and development of resistance in a significant proportion of patients as a result of suboptimal drug concentrations. The efficacy of antiretroviral therapy is challenged by the emergence of resistant HIV-1 mutants with reduced susceptibility to antiretroviral drugs. Moreover, pharmacotherapy of patients infected with HIV is challenging because a great number of comorbidities increase polypharmacy and the risk for drug-drug interactions. Drug-metabolizing enzymes and drug transporters regulate drug access to the systemic circulation, target cells, and sanctuary sites. These factors, which determine drug exposure, along with the emergence of mutations conferring resistance to HIV medications, could explain variability in efficacy and adverse drug reactions associated with antiretroviral drugs. In this review, the major factors affecting the disposition of antiretroviral drugs, including key drug-metabolizing enzymes and membrane drug transporters, are outlined. Genetic polymorphisms affecting the activity and/or the expression of cytochromes P450 or UGT isozymes and membrane drug transport proteins are highlighted and include such examples as the association of neurotoxicity with efavirenz, nephrotoxicity with tenofovir, hepatotoxicity with nevirapine, and hyperbilirubinemia with indinavir and atazanavir. Mechanisms of drug resistance conferred by specific viral mutations are also reviewed, with particular attention to replicative viral fitness and transmitted HIV drug resistance with the objectives of providing a better understanding of mechanisms involved in HIV drug resistance and helping health care providers to better manage interpatient variability in drug efficacy and toxicity.

Role of cytochrome P450 genotype in the steps toward personalized drug therapy

Genetic polymorphism for cytochrome 450 (P450) enzymes leads to interindividual variability in the plasma concentrations of many drugs. In some cases, P450 genotype results in decreased enzyme activity and an increased risk for adverse drug effects. For example, individuals with the CYP2D6 loss-of-function genotype are at increased risk for ventricular arrhythmia if treated with usual does of thioridazine. In other cases, P450 genotype may influence the dose of a drug required to achieve a desired effect. This is the case with warfarin, with lower doses often necessary in carriers of a variant CYP2C9*2 or *3 allele to avoid supratherapeutic anticoagulation. When a prodrug, such as clopidogrel or codeine, must undergo hepatic biotransformation to its active form, a loss-of-function P450 genotype leads to reduced concentrations of the active drug and decreased drug efficacy. In contrast, patients with multiple CYP2D6 gene copies are at risk for opioid-related toxicity if treated with usual doses of codeine-containing analgesics. At least 25 drugs contain information in their US Food and Drug Administration-approved labeling regarding P450 genotype. The CYP2C9, CYP2C19, and CYP2D6 genes are the P450 genes most often cited. To date, integration of P450 genetic information into clinical decision making is limited. However, some institutions are beginning to embrace routine P450 genotyping to assist in the treatment of their patients. Genotyping for P450 variants may carry less risk for discrimination compared with genotyping for disease-associated variants. As such, P450 genotyping is likely to lead the way in the clinical implementation of pharmacogenomics. This review discusses variability in the CYP2C9, CYP2C19, and CYP2D6 genes and the implications of this for drug efficacy and safety.

P-glycoprotein: tissue distribution, substrates, and functional consequences of genetic variations

P-glycoprotein (ABCB1, MDR1) belongs to the ABC transporter family transporting a wide range of drugs and xenobiotics from intra- to extracellular at many biological interfaces such as the intestine, liver, blood-brain barrier, and kidney. The ABCB1 gene is highly polymorphic. Starting with the observation of lower duodenal protein expression and elevated digoxin bioavailability in relation to the 3435C>T single nucleotide polymorphism, hundreds of pharmacokinetic and outcome studies have been performed, mostly genotyping 1236C>T, 2677G>T/A, and 3435C>T. Though some studies pointed out that intracellular concentrations of anticancer drugs, for example, within lymphocytes, might be affected by ABCB1 variants resulting in differential outcome, current knowledge of the functional significance genetic variants of ABC membrane transporters does not allow selection of a particular SNP to predict an individual's pharmacokinetics.