Influence of CYP2B6 516G>T polymorphism and interoccasion variability (IOV) on the population pharmacokinetics of efavirenz in HIV-infected South African children

Genetic and non-genetic factors influencing efavirenz population pharmacokinetics among human immunodeficiency virus-1-infected children in Ethiopia

Despite the potential for efavirenz (EFV) to be an effective alternative antiretroviral agent, its sources of wide inter- and intra-individual pharmacokinetic (PK) variability are not well-characterized in children. We investigated the effects of genetic and non-genetic factors, including demographic, treatment duration, baseline clinical, and biochemical characteristics, on the PKs of EFV through population-PK modeling. Antiretroviral therapy (ART) naïve HIV infected children, 3-16 years (n = 100), were enrolled in Ethiopia and received EFV-based combination ART. EFV concentrations after the first dose and at steady-state collected over a span of 1 year were modeled using population-based methods. A one-compartment model with first-order absorption kinetics described the observed EFV data adequately. The CYP2B6*6 and ABCB1c.4036A>G genotypes were identified as major factors influencing EFV clearance. The typical estimates of oral clearance, volume of distribution, and absorption rate constant for typical 22 kg children with CYP2B6 *1/*1 and ABCB1c.4036G/G genotypes were 4.3 L/h, 124 L, and 0.776/h, respectively. Clearance was reduced by 28% and 72% in CYP2B6*1/*6 and CYP2B6*6/*6 genotypes, respectively. Compared to week 1, clearance was higher from weeks 8 and 12 in CYP2B6*1/*6 and CYP2B6*1/*1 genotypes, respectively. Simulations indicated that EFV 12-h concentrations were comparable across weight bands, but more than 80% of subjects with CYP2B6*6/*6 had EFV concentrations greater than 4 μg/mL. EFV PK variability among children is partly explained by body weight, treatment duration, CYP2B6*6, and ABCB1 rs3842 genotypes. Therefore, in addition to body weight, pediatric dosing of EFV should consider pharmacogenetic variability, duration of therapy, and individual treatment outcomes.

Predictors of Efavirenz Plasma Exposure, Auto-Induction Profile, and Effect of Pharmacogenetic Variations among HIV-Infected Children in Ethiopia: A Prospective Cohort Study

(1) Background: Efavirenz plasma concentration displays wide between-patient variability partly due to pharmacogenetic variation and autoinduction. Pediatric data on efavirenz pharmacokinetics and the relevance of pharmacogenetic variation are scarce, particularly from sub-Saharan Africa, where >90% of HIV-infected children live and population genetic diversity is extensive. We prospectively investigated the short- and long-term effects of efavirenz auto-induction on plasma drug exposure and the influence of pharmacogenetics among HIV-infected Ethiopian children. (2) Method: Treatment-naïve HIV-infected children aged 3-16 years old (n = 111) were enrolled prospectively to initiate efavirenz-based combination antiretroviral therapy (cART). Plasma efavirenz concentrations were quantified at 4, 8, 12, 24, and 48 weeks of cART. Genotyping for CYP2B6, CYP3A5, UGT2B7, ABCB1, and SLCO1B1 common functional variant alleles was performed. (3) Results: The efavirenz plasma concentration reached a peak at two months, declined by the 3rd month, and stabilized thereafter, with no significant difference in geometric mean over time. On average, one-fourth of the children had plasma efavirenz concentrations ≥4 µg/mL. On multivariate analysis, CYP2B6*6 and ABCB1c.3435 C > T genotypes and low pre-treatment low-density lipoprotein (LDL) were significantly associated with higher plasma efavirenz concentration regardless of treatment duration. Duration of cART, sex, age, nutritional status, weight, and SLCO1B, CYP3A5, UGT2B7, and ABCB1 rs3842 genotypes were not significant predictors of efavirenz plasma exposure. (4) Conclusion: Pre-treatment LDL cholesterol and CYP2B6*6 and ABCB1c.3435 C > T genotypes predict efavirenz plasma exposure among HIV-infected children, but treatment-duration-dependent changes in plasma efavirenz exposure due to auto-induction are not statistically significant.

Associations between efavirenz concentrations, pharmacogenetics and neurocognitive performance in people living with HIV in Nigeria

OBJECTIVE

Efavirenz (EFV) use is associated with neuropsychiatric side effects, which may include poor neurocognitive performance. We evaluated single nucleotide polymorphisms in genes that contribute to EFV pharmacokinetics and examined them in association with EFV concentrations in plasma and hair, as well as neurocognitive performance.

DESIGN

Cross-sectional study in which adults with HIV receiving 600-mg EFV for at least 2 months were recruited and paired hair and dried blood spots (DBS) samples collected.

METHODS

Participants (N = 93, 70.3% female) were genotyped for seven single nucleotide polymorphisms in CYP2B6, NRII3 and ABCB1 using DBS. EFV was quantified in DBS and hair using validated liquid-chromatography-tandem-mass-spectrometry methods, with plasma EFV concentrations derived from DBS levels. Participants were also administered a neurocognitive battery of 10 tests (seven domains) that assessed total neurocognitive functioning.

RESULTS

Strong correlation (r = 0.66, P < 0.001) was observed between plasma and hair EFV concentrations. The median (interquartile range) hair EFV concentration was 6.85 ng/mg (4.56-10.93). CYP2B6 516G>T, (P < 0.001) and CYP2B6 983T>C (P = 0.001) were each associated with hair EFV concentrations. Similarly, 516G>T (P < 0.001) and 983T>C (P = 0.009) were significantly associated with plasma EFV concentration. No other genetic associations were observed. Contrary to other studies, total neurocognitive performance was significantly associated with plasma EFV concentrations (r = 0.23, P = 0.043) and 983T>C genotype (r = 0.38, P < 0.0005).

CONCLUSION

This study demonstrated approximately three-fold and two-fold higher EFV plasma and hair concentrations, respectively, among CYP2B6 516TT compared with 516GG. Higher EFV concentrations were associated with better neurocognitive performance, requiring further study to elucidate the relationships between adherence, adverse effects and outcomes.

Potential challenges to sustained viral load suppression in the HIV treatment programme in South Africa: a narrative overview

Background

South Africa, with one of the highest HIV prevalences in the world, introduced the universal test and treat (UTT) programme in September 2016. Barriers to sustained viral suppression may include drug resistance in the pre-treated population, non-adherence, acquired resistance; pharmacokinetics and pharmacodynamics, and concurrent use of alternative treatments.

Objective

The purpose of this review is to highlight potential challenges to achieving sustained viral load suppression in South Africa (SA), a major expectation of the UTT initiative.

Methodology

Through the PRISMA approach, published articles from South Africa on transmitted drug resistance; adherence to ARV; host genetic factors in drug pharmacokinetics and pharmacodynamics, and interactions between ARV and herbal medicine were searched and reviewed.

Results

The level of drug resistance in the pre-treated population in South Africa has increased over the years, although it is heterogeneous across and within Provinces. At least one study has documented a pre-treated population with moderate (> 5%) or high (> 15%) levels of drug resistance in eight of the nine Provinces. The concurrent use of ARV and medicinal herbal preparation is fairly common in SA, and may be impacting negatively on adherence to ARV. Only few studies have investigated the association between the genetically diverse South African population and pharmacokinetics and pharmacodynamics of ARVs.

Conclusion

The increasing levels of drug resistant viruses in the pre-treated population poses a threat to viral load suppression and the sustainability of first line regimens. Drug resistance surveillance systems to track the emergence of resistant viruses, study the burden of prior exposure to ARV and the parallel use of alternative medicines, with the goal of minimizing resistance development and virologic failure are proposed for all the Provinces of South Africa. Optimal management of the different drivers of drug resistance in the pre-treated population, non-adherence, and acquired drug resistance will be beneficial in ensuring sustained viral suppression in at least 90% of those on treatment, a key component of the 90-90-90 strategy.

Population pharmacokinetics of efavirenz in HIV and TB/HIV coinfected children: the significance of genotype-guided dosing

OBJECTIVES

The current WHO weight-based dosing recommendations for efavirenz result in a wide variability of drug exposure in children. Our objectives are to characterize the effects of rifampicin- and isoniazid-containing anti-TB therapy and CYP2B6 activity on efavirenz concentrations in children, using non-linear mixed-effects modelling.

METHODS

This is a pharmacokinetic (PK) substudy of a prospective study that examined the interactions between anti-TB therapy and efavirenz in HIV-infected children with and without TB. PK samples were obtained 4 weeks after starting efavirenz (PK1) and repeated 4 weeks after completing TB therapy (PK2) in TB/HIV coinfected patients. Drug concentrations were measured using LC-MS/MS. Composite CYP2B6 516/983/15582 genotype was determined. Population PK modelling was performed in Monolix. Simulations were performed to obtain the predicted mid-dose concentrations (C12).

RESULTS

One hundred and five HIV-infected Ghanaian children (46 with TB/HIV) were included. The median age and weight were 7 years and 19 kg. The efavirenz concentrations over time were adequately described using a one-compartment model. Weight, composite CYP2B6 genotype and PK visit had a significant influence on the PK parameters, while TB therapy had no significant effect. Simulations showed adequate C12 for intermediate composite CYP2B6 metabolizers only.

CONCLUSIONS

Our model showed that rifampicin- and isoniazid-containing anti-TB therapy does not influence efavirenz PK parameters. On the other hand, it describes the effect of efavirenz autoinduction after completing TB treatment. In addition, dosing efavirenz in children based only on weight results in a large variability in drug exposure. We propose dose adjustments for slow and extensive composite CYP2B6 metabolizers.

The Pharmacogenetics of Efavirenz Metabolism in Children: The Potential Genetic and Medical Contributions to Child Development in the Context of Long-Term ARV Treatment

Efavirenz (EFV) is a well-known, effective anti-retroviral drug long used in first-line treatment for children and adults with HIV and HIV/AIDS. Due to its narrow window of effective concentrations, between 1 and 4 μg/mL, and neurological side effects at supratherapeutic levels, several investigations into the pharmacokinetics of the drug and its genetic underpinnings have been carried out, primarily with adult samples. A number of studies, however, have examined the genetic influences on the metabolism of EFV in children. Their primary goal has been to shed light on issues of appropriate pediatric dosing, as well as the manifestation of neurotoxic effects of EFV in some children. Although EFV is currently being phased out of use for the treatment of both adults and children, we share this line of research to highlight an important aspect of medical treatment that is relevant to understanding the development of children diagnosed with HIV.

Pharmacogenetics and pharmacokinetics of CNS penetration of efavirenz and its metabolites

BACKGROUND

There are limited data on the pharmacogenetics and pharmacokinetics of the CNS penetration of efavirenz.

OBJECTIVES

We investigated genetic polymorphisms associated with CSF concentrations of efavirenz and its metabolites and explored the relationships with neurocognitive performance.

METHODS

We included 47 HIV-infected South African black adults with and without HIV-associated neurocognitive disorder on efavirenz/tenofovir/emtricitabine and collected paired plasma-CSF samples. We considered 2049 SNPs, including SNPs known to affect plasma efavirenz exposure, from potentially relevant genes (ABCC5, ABCG2, ABCB1, SLCO2B1, SCLO1A2, ABCC4, CYP2B6 and CYP2A6) and 880 met a linkage disequilibrium (LD)-pruning threshold.

RESULTS

We identified 9 slow, 21 intermediate and 17 extensive metabolizers. The CYP2B6 983 genotype in multivariate analyses predicted log10-transformed concentrations of plasma efavirenz (β = 0.38, P = 2.7 × 10-03), plasma 7-hydroxy-efavirenz (β = 0.59, P = 3.7 × 10-03), plasma 8-hydroxy-efavirenz:efavirenz ratio (β = -0.31, P = 1.8 × 10-04) and CSF efavirenz (β = 0.36, P = 0.01). Lower plasma 7-hydroxy-efavirenz concentrations were independently associated with CYP2A6 rs10853742 (β = -0.55, P = 3.5 × 10-05), ABCB1 rs115780656 (β = -0.65, P = 4.1 × 10-05) and CYP2A6 -48A→C (β = -0.59, P = 0.01). CYP2A6 -48A→C was independently associated with higher CSF 8-hydroxy-efavirenz:efavirenz ratio (β = 0.54, P = 0.048). CYP2B6 rs2279345 polymorphism was associated with lower plasma 7-hydroxy-efavirenz:efavirenz ratio in multivariate analyses (P < 0.05). No polymorphisms were associated with CSF:plasma ratios of efavirenz, plasma or CSF concentrations of 8-hydroxy-efavirenz or neurocognitive performance.

CONCLUSIONS

We identified novel genetic associations with plasma efavirenz, plasma 7-hydroxy-efavirenz, plasma 7-hydroxy-efavirenz:efavirenz ratio, plasma 8-hydroxy-efavirenz:efavirenz ratio, CSF efavirenz and CSF 8-hydroxy-efavirenz:efavirenz ratio.

Pharmacogenetic associations with cytochrome P450 in antiretroviral therapy: what does the future hold?

INTRODUCTION

Several antiretroviral drugs used to treat infection with the human immunodeficiency virus (HIV) are substrates of enzymes belonging to the cytochrome P450 (CYP) superfamily, which are polymorphically expressed. It may therefore be useful to take into account the genetic variation in these enzymes to predict the likelihood of anti-HIV treatment success, toxicity and the potential for drug-drug interactions. Areas covered: In this manuscript, the authors discuss the current state of knowledge regarding pharmacogenetic associations between CYP and all major antiretrovirals, as well as the importance of these associations. Expert opinion: While many pharmacogenetic associations for CYP have been described in the literature, replication studies are sometimes lacking. The implementation of this knowledge in clinical practice also remains difficult. Further efforts are required both to expand this field of knowledge and to enable its use in everyday clinical practice.

CYP2B6 Haplotype Predicts Efavirenz Plasma Concentration in Black South African HIV-1-Infected Children: A Longitudinal Pediatric Pharmacogenomic Study

South Africa has the highest burden of the human immunodeficiency virus (HIV) infection globally. Efavirenz (EFV), a frequently used drug against HIV infection, displays a relationship between drug concentration and pharmacodynamics effects clinically. However, haplotype-based genetic variation in drug metabolism in a pediatric sample has been little considered in a longitudinal long-term context. CYP2B6 plays a key role in variation of EFV plasma concentration through altered drug metabolism. We report here on a prospective clinical pharmacogenomics/pharmacokinetic study of Bantu-speaking children, importantly, over a period of 24 months post-initiation of EFV-based treatment in South Africa. We characterized the HIV-1-infected children (n = 60) for the CYP2B6 c.516G>T, c.785A>G, c.983T>C, and c.1459C>T single nucleotide polymorphisms (SNPs). These SNPs were determined using polymerase chain reaction/restricted fragment length polymorphism and SNaPshot genotyping. Longitudinal mid-dose EFV plasma concentrations were determined by LC-MS/MS and association analyses with genotypes and haplotypes at 1, 3, and 24 months were performed. The CYP2B6 c.516T/T genotype showed significantly higher EFV plasma concentrations (p < 0.001) compared to non 516T-allele carriers at all three time points. The minor allele frequencies (MAF) for CYP2B6 c.516T, c.785G, c.983C, and c.1459T were 0.410, 0.408, 0.110, and 0.000 respectively. Haplotypes were constructed using CYP2B6 c.516G>T,-c.785A>G and c.983T>C. The haplotype T-G-T presented with significantly increased EFV plasma concentrations compared to the reference G-A-T haplotype at 1, 3, and 24 months (p = 0.009; p = 0.003; p = 0.001), suggesting that the T-G-T haplotype predisposes a risk of EFV plasma concentrations >4 μg/mL. The clinical implications of these pharmacogenomics observations for EFV toxicity and treatment resistance warrant further future research.

The impact of CYP2B6 polymorphisms on the interactions of efavirenz with lumefantrine: Implications for paediatric antimalarial therapy

Lumefantrine is a widely used antimalarial in children in sub-Saharan Africa and is predominantly metabolised by CYP3A4. The concomitant use of lumefantrine with the antiretroviral efavirenz, which is metabolised by CYP2B6 and is an inducer of CYP3A4, increases the risk of lumefantrine failure and can result in an increased recrudescence rate in HIV-infected children. This is further confounded by CYP2B6 being highly polymorphic resulting in a 2-3 fold higher efavirenz plasma concentration in polymorphic subjects, which enhances the potential for an efavirenz-lumefantrine drug-drug interaction (DDI). This study developed a population-based PBPK model capable of predicting the impact of efavirenz-mediated DDIs on lumefantrine pharmacokinetics in African paediatric population groups, which also considered the polymorphic nature of CYP2B6. The validated model demonstrated a significant difference in lumefantrine target day 7 concentrations (C_d7) in the presence and absence of efavirenz and confirmed the capability of efavirenz to initiate this DDI. This was more apparent in the *6/*6 compared to *1/*1 population group and resulted in a significantly lower (P < 0.001) lumefantrine C_d7. A prospective change in dosing schedule from 3-days to 7-days resulted in a greater number of *6/*6 subjects (28-57%) attaining the target C_d7 across age bands (0.25-13 years), with the greatest increase evident in the 1-4 year old group (3-day: 1%; 7-day: 28%).

Genetic Polymorphisms Affecting the Pharmacokinetics of Antiretroviral Drugs

BACKGROUND

Antiretroviral treatment is highly effective in enhancing HIV-positive patients' survival and quality of life. Despite an increased tolerability in recent years, a substantial amount of patients experience side effects. Antiretrovirals' efficacy and tolerability have been associated with plasma concentrations and single nucleotide polymorphisms in selected genes involved in drug disposition.

OBJECTIVE

Our aim was to review the current knowledge in genetic polymorphisms affecting plasma, intracellular or compartmental concentrations of antiretrovirals.

METHODS

A search of the PubMed database was conducted to identify relevant articles, using the following terms: 'pharmacogenetics' or 'pharmacogenomics' or 'single nucleotide polymorphisms' or 'genetic/allelic variants' and 'pharmacokinetics' or 'concentrations' and 'HIV' or 'antiretroviral'. Abstracts from the main HIV conferences during 2015 and 2016 were also searched using the same keywords. Abstracts were manually checked and, if relevant, full papers were obtained. Only articles published in English were selected.

RESULTS

Several genetic polymorphisms in genes coding enzymes involved in drug metabolism (cytochrome P450 isoenzymes and uridine diphosphate glucuronosyltransferases) and transport (P-glycoprotein, anionic and cationic transporters, other transporters), as well as nuclear receptors (pregnane X receptor and the constitutive androstane receptor), have been associated with concentrations of antiretrovirals. The extent of such influence, the conflicting data, and the potential clinical relevance are discussed in the main section of this article.

CONCLUSION

Genetic polymorphisms may affect antiretroviral disposition, as well as both efficacy and toxicity. Despite a large amount of data, such precious knowledge has seldom been applied in patients. Studies on the clinical relevance and cost effectiveness of tailoring antiretroviral regimens to patients' genetic assets are lacking, but their importance may grow with the increasing age and complexity of persons living with HIV/AIDS.

Genetic variants in CYP2B6 and CYP2A6 explain interindividual variation in efavirenz plasma concentrations of HIV-infected children with diverse ethnic origin

Background

Approximately 2.6 million children live with HIV globally, and efavirenz (EFV) is one of the most widely used antiretroviral agents for HIV treatment in children and adults. There are concerns about the appropriateness of current EFV dosing and it has been discussed whether EFV dosing should be adapted according to genotype in children as suggested for adults.

Aim

To investigate if pediatric EFV dosing should be guided by genetic variation in drug metabolizing enzymes rather than by body weight.

Method

EFV plasma concentrations measured for clinical purposes from all children less than 18 years old at Karolinska University Hospital, Stockholm, Sweden, treated with EFV were collected retrospectively. They were genotyped for eleven polymorphisms in genes coding for drug-metabolizing enzymes and P-glycoprotein, of potential importance for EFV disposition. Data on country of origin, sex, age, weight, HIV RNA, viral resistance patterns, CD4 cells, adherence to treatment, subjective health status and adverse events were collected from their medical records.

Results

Thirty-six patients and 182 (mean 5 samples/patient) EFV plasma concentration measurements from children of African, Asian and Latin American origin were included. EFV plasma concentration varied 21-fold between measurements (n = 182) (0.85–19.3 mg/L) and 9-fold measured as mean EFV plasma concentration across the subjects (1.55–13.4 mg/L). A multivariate mixed-effects restricted maximum likelihood regression model, including multiple gene polymorphisms, identified CYP2B6*6 T/T (p < 0.0005), CYP2B6*11 G/G (p < 0.0005), CYP2A6*9 A/C (p = 0.001) genotypes, age at treatment initiation (p = 0.002) and time from treatment initiation (p < 0.0005) as independent factors significantly related to log_e concentration/(dose/weight). The contribution of the model to the intra- and interindividual variation were 6 and 75%, respectively (Bryk/Raudenbush R-squared level).

Conclusion

Genetic polymorphisms in CYP2B6 and CYP2A6 explained a significant proportion of variability in EFV plasma concentration in HIV-infected children in a multi-ethnic outpatient clinic. Knowledge about individual variants in key drug metabolizing enzyme genes could improve clinical safety and genotype directed dosing could achieve more predictable EFV plasma concentrations in HIV-infected children.

Pharmacokinetics of Efavirenz at a High Dose of 25 Milligrams per Kilogram per Day in Children 2 to 3 Years Old

The MONOD ANRS 12206 trial was designated to assess simplification of a successful lopinavir (LPV)-based antiretroviral treatment in HIV-infected children younger than 3 years of age using efavirenz (EFV; 25 mg/kg of body weight/day) to preserve the class of protease inhibitors for children in that age group. In this substudy, EFV concentrations were measured to check the consistency of an EFV dose of 25 mg/kg and to compare it with the 2016 FDA recommended dose. Fifty-two children underwent blood sampling for pharmacokinetic study at 6 months and 12 months after switching to EFV. We applied a Bayesian approach to derive EFV pharmacokinetic parameters using the nonlinear mixed-effect modeling (NONMEM) program. The proportion of midinterval concentrations 12 h after drug intake (C_{12 h}) corresponding to the EFV therapeutic pharmacokinetic thresholds (1 to 4 mg/liter) was assessed according to different dose regimens (25 mg/kg in the MONOD study versus the 2016 FDA recommended dose). With both the 25 mg/kg/day dose and the 2016 FDA recommended EFV dose, simulations showed that the majority of C_{12 h} values were within the therapeutic range (62.6% versus 62.8%). However, there were more children underexposed with the 2016 FDA recommended dose (11.6% versus 1.2%). Conversely, there were more concentrations above the threshold of toxicity with the 25 mg/kg dose (36.2% versus 25.6%), with C_{12 h} values of up to 15 mg/liter. Only 1 of 52 children was switched back to LPV because of persistent sleeping disorders, but his C_{12 h} value was within therapeutic ranges. A high EFV dose of 25 mg/kg per day in children under 3 years old achieved satisfactory therapeutic effective levels. However, the 2016 FDA recommended EFV dose appeared to provide more acceptable safe therapeutic profiles. (This study has been registered at ClinicalTrials.gov under identifier NCT01127204.).

Plasma Efavirenz Exposure, Sex, and Age Predict Virological Response in HIV-Infected African Children

BACKGROUND

Owing to insufficient evidence in children, target plasma concentrations of efavirenz are based on studies in adults. Our analysis aimed to evaluate the pediatric therapeutic thresholds and characterize the determinants of virological suppression in African children.

METHODS

We analyzed data from 128 African children (aged 1.7-13.5 years) treated with efavirenz, lamivudine, and one among abacavir, stavudine, or zidovudine, and followed up to 36 months. Individual pharmacokinetic (PK) measures [plasma concentration 12 hours after dose (C12h), plasma concentration 24 hours after dose (C24h), and area under the curve (AUC0-24)] were estimated using population PK modeling. Cox multiple failure regression and multivariable fractional polynomials were used to investigate the risks of unsuppressed viral load associated with efavirenz exposure and other factors among 106 initially treatment-naive children, and likelihood profiling was used to identify the most predictive PK thresholds.

RESULTS

The risk of viral load >100 copies per milliliter decreased by 42% for every 2-fold increase in efavirenz mid-dose concentration [95% confidence interval (CI): 23% to 57%; P < 0.001]. The most predictive PK thresholds for increased risk of unsuppressed viral load were C12h 1.12 mg/L [hazard ratio (HR): 6.14; 95% CI: 2.64 to 14.27], C24h 0.65 mg/L (HR: 6.57; 95% CI: 2.86 to 15.10), and AUC0-24 28 mg·h/L (HR: 5.77; 95% CI: 2.28 to 14.58). Children older than 8 years had a more than 10-fold increased risk of virological nonsuppression (P = 0.005); among children younger than 8 years, boys had a 5.31 times higher risk than girls (P = 0.007). Central nervous system adverse events were infrequently reported.

CONCLUSIONS

Our analysis suggests that the minimum target C24h and AUC0-24 could be lowered in children. Our findings should be confirmed in a prospective pediatric trial.

The impact of genetic polymorphisms on the pharmacokinetics of efavirenz in African children

Aims

Using a model‐based approach, the efavirenz steady‐state pharmacokinetics in African children is characterized, quantifying demographic and genotypic effects on the drug's disposition. Simulations are also conducted allowing prediction of optimized doses of efavirenz in this population.

Methods

We modelled the steady‐state population pharmacokinetics of efavirenz in Ugandan and Zambian children using nonlinear mixed‐effects modelling. Individual mid‐dose efavirenz concentrations were derived and simulations explored genotype‐based dose optimization strategies.

Results

A two‐compartment model with absorption through transit compartments well described 2086 concentration‐time points in 169 children. The combined effect of single nucleotide polymorphisms (SNPs) 516G>T and 983T>C explained 44.5% and 14.7% of the variability in efavirenz clearance and bioavailability, respectively. The detected frequencies of composite CYP2B6 genotype were 0.33 for 516GG|983TT, 0.35 for 516GT|983TT, 0.06 for 516GG|983TC, 0.18 for 516TT|983TT, 0.07 516GT|983TC and 0.01 for 516GG|983CC. The corresponding estimated clearance rates were 6.94, 4.90, 3.93, 1.92, 1.36, and 0.74 l h⁻¹ for a 15.4 kg child and median (95% CI) observed mid‐dose concentrations 1.55 (0.51–2.94), 2.20 (0.97–4.40), 2.03 (1.19–4.53), 7.55 (2.40–14.74), 7.79 (3.66–24.59) and 18.22 (11.84–22.76) mg l⁻¹, respectively. Simulations showed that wild‐type individuals had exposures at the bottom of therapeutic range, while slower metabolizers were overexposed.

Conclusions

Dosage guidelines for African children should take into consideration the combined effect of SNPs CYP2B6 516G>T and 983T>C.

Pharmacogenetics of plasma efavirenz exposure in HIV-infected adults and children in South Africa

AIMS

Genetic factors, notably CYP2B6 516G→T [rs3745274] and 983T→C [rs28399499], explain much of the interindividual variability in efavirenz pharmacokinetics, but data from Africa are limited. We characterized relationships between genetic polymorphisms and plasma efavirenz concentrations in HIV-infected Black South African adults and children.

METHODS

Steady-state mid-dosing interval efavirenz concentrations were measured. We genotyped 241 polymorphisms in genes potentially relevant to efavirenz metabolism and transport, including ABCB1, CYP2A6, CYP2B6, CYP3A4, CYP3A5, NR1I2 and NR1I3.

RESULTS

Among 113 participants (59 adults and 54 children), minor allele frequencies for CYP2B6 516G→T, 983T→C, and 15582C→T [rs4803419] were 0.36, 0.07, and 0.09, respectively. Based on composite CYP2B6 15582/516/983 genotype, there were 33 extensive metabolizer, 62 intermediate metabolizer and 18 slow metabolizer genotypes. Median (IQR) mid-dose efavirenz concentrations were 1.44 (1.21-1.93) µg ml(-1), 2.08 (1.68-2.94) µg ml(-1) and 7.26 (4.82-8.34) µg ml(-1) for extensive, intermediate and slow metabolizers, respectively. In univariate analyses, a model that included composite genotype best predicted efavirenz concentrations (β = 0.28, 95% CI 0.21, 0.35, P = 2.4 × 10(-11)). Among individual CYP2B6 polymorphisms, 516G→T best predicted efavirenz concentrations (β = 0.22, 95% CI 0.13, 0.30, P = 1.27 × 10(-6)). There was also associations with 983T→C (β = 0.27, 95% CI 0.10, 0.44, P = 0.002) and 15582C→T (β = 0.11, 95% CI 0.01, 0.22, P = 0.04). Associations were consistent in adults and children. No other polymorphisms were independently associated with efavirenz concentrations.

CONCLUSIONS

Composite CYP2B6 genotype based on CYP2B6 516G→T, 983T→C, and 15582C→T best described efavirenz exposure in HIV-infected Black South African adults and children.

Population pharmacometrics in support of analgesics studies

Population pharmacometric modeling is used to explain both population trends as well as the sources and magnitude of variability in pharmacokinetic and pharmacodynamics data; the later, in part, by taking into account patient characteristics such as weight, age, renal function and genetics. The approach is best known for its ability to analyze sparse data, i.e. when only a few measurements have been collected from each subject, but other benefits include its flexibility and the potential to construct more detailed models than those used in the traditional individual curve fitting approach. This review presents the basic concepts of population pharmacokinetic and pharmacodynamic modeling and includes several analgesic drug examples. In addition, the use of these models to design and optimize future studies is discussed. In this context, finding the best design factors, such as the sampling times or the dose, for future studies within pre-defined criteria using a previously constructed population pharmacokinetic model can help researchers acquire clinically meaningful data without wasting resources and unnecessarily exposing vulnerable patient groups to study drugs and additional blood sampling.

High predictive value of CYP2B6 SNPs for steady-state plasma efavirenz levels in South African HIV/AIDS patients

INTRODUCTION

Efavirenz is primarily metabolized by CYP2B6, with a minor contribution from CYP1A2, CYP2A6, CYP3A4 and CYP3A5. Genetic variability in these genes contributes towards differences in plasma efavirenz concentration, which ultimately leads to either development of adverse drug events or emergence of virus resistance. However, the clinical utility or validity of introducing genotype-assisted dosing is not known. The aim of this study was therefore to evaluate the effects of 14 single-nucleotide polymorphisms (SNPs) in five drug-metabolizing enzyme genes on steady-state plasma efavirenz levels in South African HIV/AIDS patients as well as their clinical validity.

METHODS

HIV/AIDS patients were recruited from Themba Lethu Clinic, at Helen Joseph Hospital, Johannesburg. Blood samples for plasma drug levels and DNA extraction were obtained from each participant. PCR/RFLP and SNaPshot genotyping were used for SNPs in CYP1A2, CYP2A6, CYP2B6, CYP3A4 and CYP3A5 among 464 Bantu-speaking South Africans. Plasma efavirenz concentrations were measured using LC/MS/MS. Genotypes and plasma efavirenz levels were used to calculate predictive values. Multivariate analysis was used to select the minimal set of SNPs with significant clinical validity.

RESULTS

Qualitative and quantitative differences in allele frequencies were observed when comparing South Africans with African, Caucasian and Asian populations. CYP2B6 516T and 785G (*6) and CYP2B6 983C (*18) alleles were significantly associated with high plasma efavirenz levels. CYP2B6 A-G-A-C-C and A-T-G-T-C haplotypes (with respect to CYP2B6 136A>G; CYP2B6 516G>T; CYP2B6 785A>G; CYP2B6 983T>C; and CYP2B6 1459C>T) were associated with higher levels of efavirenz, whereas G-G-A-T-C and A-G-A-T-C haplotypes showed significantly lower levels of efavirenz. The CYP2B6*1/*6 genotype was significantly associated with an increased risk of loss to follow-up. The sensitivity, specificity and positive predictive values for the CYP2B6*6/*6 genotype in predicting efavirenz levels above 4 µg/ml were 46, 97 and 88%, respectively. However, these values improved to 49, 100 and 100%, respectively, when either the CYP1A2 -163A (*1F) allele or the NR1I3 8784C/C genotype was present.

CONCLUSION

Screening for CYP2B6 516G>T SNP has a high specificity and positive predictive value for efavirenz levels above 4 µg/ml and could be used in deciding on efavirenz dosage among individuals homozygous for this variant, which could lead to better precision medication.

Population pharmacogenetic-based pharmacokinetic modeling of efavirenz, 7-hydroxy- and 8-hydroxyefavirenz

The purpose of this study was to determine the demographic and pharmacogenetic covariates that influence the disposition of efavirenz (EFV) and its major metabolites. A population pharmacokinetic (PK) model was developed from a randomized, cross-over, drug-interaction study in healthy male Korean subjects (n = 17). Plasma concentrations of EFV and its hydroxy-metabolites (0-120 hours) were measured by LC/MS/MS. Genomic DNA was genotyped for variants in the cytochrome P450 (CYP) 2A6, 2B6, 3A5, and MDR1 genes. A PK model was built in a stepwise procedure using nonlinear mixed effect modeling in NONMEM 7. The covariate model was built using the generalized additive modeling and forward selection-backward elimination. Model-based simulations were performed to predict EFV steady-state concentrations following 200, 400, and 600 mg daily oral dose among different CYP2B6 genotypes. The final model included only CYP2B6 genotype as a covariate that predicts EFV clearance through the formation of 8-OH EFV that represented 65% to 80% of EFV clearance. The total clearance of EFV in CYP2B6*6/*6 genotype was ∼30% lower than CYP2B6*1/*1 or CYP2B6*1/*6 alleles (P < .001). Clopidogrel reduced both formation and elimination clearances of 8-OH EFV by 22% and 19%, respectively (P = .033 and .041). Other demographics and genotype of accessory CYP pathways did not predict EFV or metabolites PK. CYP2B6 genotype was the only significant predictor of EFV disposition. The developed model may serve as the foundation for further exploration of pharmacogenetic-based dosing of EFV.

Bioactivation of chlorpyrifos by CYP2B6 variants

Chlorpyrifos (CPF), an organophosphorus (OP) pesticide, is bioactivated by cytochrome P450s (CYPs) to the active metabolite chlorpyrifos oxon (CPF-O). Given that human CYP2B6 has the highest intrinsic clearance (CL(int)) for CPF bioactivation, CYP2B6 polymorphisms may impact human susceptibility to CPF at real world environmental and occupational CPF exposure levels. CYP2B6.4,.5,.7, and .18 were over-expressed in mammalian COS-1 cells to assess the impact of CYP2B6 variants on the K(m) and V(max) for bioactivation of CPF. Cell lysates were incubated with CPF (0-100 μM) and the production of CPF-O was measured via HPLC analysis. CYP2B6 content was determined by western blot. CYP2B6.18 had neither detectable protein nor activity levels. The V(max) value for each remaining variant was significantly higher than wild-type (CYP2B6.1, V(max) 4.13 × 10(4) pmol/min/nmol CYP2B6), with CYP2B6.4,.5, and .7 having V(max) values of 4.52 × 10(5), 1.82 × 10(5), and 9.60 × 10(4) pmol/min/nmol CYP2B6, respectively. The K(m) values for these variants ranged from 0.39 to 1.09 μM and were not significantly different from wild-type. All active variants examined had significantly higher CL(int) than CYP2B6.1. Variants of CYP2B6 have altered capacity to bioactivate CPF and may affect individual susceptibility by altering the V(max) for CPF-O formation.