Weight related differences in the pharmacokinetics of abacavir in HIV-infected patients

Intentional overdose of dolutegravir/abacavir/lamivudine (Triumeq) in a 26-year-old man

Triumeq is a single-tablet regimen for patients with HIV infection comprising dolutegravir, abacavir and lamivudine. Overdoses with Triumeq have not been reported previously. We present a case of a 26-year-old man who presented to our hospital after intentionally ingesting 30 tablets of Triumeq. An intoxication with Triumeq can lead to several side effects. An overdose of abacavir and lamivudine can cause mitochondrial toxicity and lactic acidosis. An intoxication with dolutegravir appears to be relatively harmless. As Triumeq will be used on a regular basis as treatment for patients with HIV-1 infection, these intoxications are expected to be encountered more often.

Population pharmacokinetics of abacavir in pregnant women

For the first time, a population approach was used to describe abacavir (ABC) pharmacokinetics in HIV-infected pregnant and nonpregnant women. A total of 266 samples from 150 women were obtained. No covariate effect (from age, body weight, pregnancy, or gestational age) on ABC pharmacokinetics was found. Thus, it seems unnecessary to adapt the ABC dosing regimen during pregnancy.

Clinical pharmacokinetics of antiretroviral drugs in older persons

INTRODUCTION

Combination antiretroviral therapy has enabled HIV-infected persons to reach older ages in high numbers. Hepatic and renal changes that normally occur with advancing age occur earlier and with higher incidence in HIV-infected individuals. A limited number of prospective controlled studies have demonstrated small reductions (17 to 41%) in lopinavir, atazanavir and lamivudine clearance in older versus younger adults. A much larger number of retrospective studies in adults (age range ∼ 20 to 60 years), including all antiretroviral drugs, have evaluated age as a covariate for pharmacokinetics. Most studies did not detect substantial associations between drug exposures and age.

AREAS COVERED

This review summarizes antiretroviral drug pharmacokinetics in older persons. The authors review articles from PubMed (search terms: elderly, antiretroviral, pharmacokinetics) in addition to the bibliographies of those selected.

EXPERT OPINION

The evidence to date does not support major pharmacokinetic changes in adults between ∼ 20 and 60 years of age. However, additional prospective, well-controlled studies are needed in more persons > 60 years, including those with frailty and comorbidities, with assessment of unbound drug clearance, and incorporation of adherence, pharmacogenetics and concomitant medications. Until then, guidelines for drug-drug interactions and dosing in renal and hepatic impairment should be followed in older HIV-infected individuals.

Population pharmacokinetics and maximum a posteriori probability Bayesian estimator of abacavir: application of individualized therapy in HIV-infected infants and toddlers

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Abacavir is used to treat HIV infection in both adults and children. The recommended paediatric dose is 8 mg kg(-1) twice daily up to a maximum of 300 mg twice daily. Weight was identified as the central covariate influencing pharmacokinetics of abacavir in children.

WHAT THIS STUDY ADDS

A population pharmacokinetic model was developed to describe both once and twice daily pharmacokinetic profiles of abacavir in infants and toddlers. Standard dosage regimen is associated with large interindividual variability in abacavir concentrations. A maximum a posteriori probability Bayesian estimator of AUC(0-) (t) based on three time points (0, 1 or 2, and 3 h) is proposed to support area under the concentration-time curve (AUC) targeted individualized therapy in infants and toddlers.

AIMS

To develop a population pharmacokinetic model for abacavir in HIV-infected infants and toddlers, which will be used to describe both once and twice daily pharmacokinetic profiles, identify covariates that explain variability and propose optimal time points to optimize the area under the concentration-time curve (AUC) targeted dosage and individualize therapy.

METHODS

The pharmacokinetics of abacavir was described with plasma concentrations from 23 patients using nonlinear mixed-effects modelling (NONMEM) software. A two-compartment model with first-order absorption and elimination was developed. The final model was validated using bootstrap, visual predictive check and normalized prediction distribution errors. The Bayesian estimator was validated using the cross-validation and simulation-estimation method.

RESULTS

The typical population pharmacokinetic parameters and relative standard errors (RSE) were apparent systemic clearance (CL) 13.4 () h−1 (RSE 6.3%), apparent central volume of distribution 4.94 () (RSE 28.7%), apparent peripheral volume of distribution 8.12 () (RSE14.2%), apparent intercompartment clearance 1.25 () h−1 (RSE 16.9%) and absorption rate constant 0.758 h−1 (RSE 5.8%). The covariate analysis identified weight as the individual factor influencing the apparent oral clearance: CL = 13.4 × (weight/12)1.14. The maximum a posteriori probability Bayesian estimator, based on three concentrations measured at 0, 1 or 2, and 3 h after drug intake allowed predicting individual AUC0–t.

CONCLUSIONS

The population pharmacokinetic model developed for abacavir in HIV-infected infants and toddlers accurately described both once and twice daily pharmacokinetic profiles. The maximum a posteriori probability Bayesian estimator of AUC(0-) (t) was developed from the final model and can be used routinely to optimize individual dosing.

Pharmacokinetic study of once-daily versus twice-daily abacavir and lamivudine in HIV type-1-infected children aged 3-<36 months

BACKGROUND

Once-daily dosing of abacavir and lamivudine has been approved for adults, but paediatric data are insufficient. We conducted a pharmacokinetic study of once-daily and twice-daily abacavir and lamivudine in children aged 3-<36 months.

METHODS

Children with stable HIV type-1 (HIV-1) RNA levels after 12 weeks treatment with twice-daily abacavir (8 mg/kg) with or without lamivudine (4 mg/kg) underwent plasma pharmacokinetic sampling. Children then switched to once-daily abacavir (16 mg/kg) with or without lamivudine (8 mg/kg), and sampling was repeated 4 weeks later. The area under the plasma concentration-time curve over 24 h (AUC(0-24)) and the maximum concentration (C(max)) were compared using geometric mean ratios (GMRs); 90% confidence intervals (CIs) within the range of 0.80-1.25 were considered bioequivalent.

RESULTS

A total of 18 children (4, 6 and 8 in the 3-<12, 12-<24 and 24-<36 month age ranges, respectively) provided pharmacokinetic data for abacavir (17 for lamivudine). The GMR of AUC(0-24), once-daily versus twice-daily, was 1.07 (90% CI 0.92-1.23) for abacavir and 0.91 (90% CI 0.79-1.06) for lamivudine. C(max) almost doubled on once-daily versus twice-daily dosing: abacavir and lamivudine GMRs were 2.04 (90% CI 1.73-2.42) and 1.78 (90% CI 1.52-2.09), respectively. At baseline, 12, 24 and 48 weeks, 89%, 94%, 100% and 89% of children had HIV-1 RNA<400 copies/ml, respectively.

CONCLUSIONS

Bioequivalence was demonstrated on AUC(0-24) between twice-daily and once-daily abacavir; very similar AUC(0-24) values were seen for twice-daily and once-daily lamivudine. Given that viral load suppression rates were maintained, these data suggest that once-daily abacavir and lamivudine might be an option for children aged 3-<36 months.

Steady-state pharmacokinetics of abacavir in plasma and intracellular carbovir triphosphate following administration of abacavir at 600 milligrams once daily and 300 milligrams twice daily in human immunodeficiency virus-infected subjects

Abacavir (ABC) is administered either at 600 mg once daily (ABC 600 mg QD) or 300 mg twice daily (ABC 300 mg BID) in anti-human immunodeficiency virus (anti-HIV) combination therapy. Although ABC plasma pharmacokinetics following each regimen has been well defined, no study has directly compared the regimens with respect to pharmacokinetics of ABC's active intracellular anabolite, carbovir-triphosphate (CBV-TP). In an open-label, two-period, crossover study, 34 HIV-infected male and female subjects stabilized on antiretroviral regimens containing either ABC 600 mg QD or ABC 300 mg BID received their usual doses on days -1 and 1 and then switched regimens for days 2 to 11. Serial blood samples collected on days 1 and 11 were assayed for plasma ABC and intracellular CBV-TP concentrations using validated high-performance liquid chromatography-tandem mass spectrometry methods. Pharmacokinetic parameters were calculated using noncompartmental methods. Analysis of variance with a mixed-effect model was performed for treatment and gender comparisons. In 27 evaluable subjects, the regimens provided bioequivalent ABC daily areas under the concentration-time curve from 0 to 24 h (AUC(0-24)) and comparable CBV-TP concentrations at the end of the dosing interval (C(tau)). As expected, ABC QD resulted in 109% higher ABC maximum concentrations of drug in plasma (C(max)) than did ABC BID. ABC QD also resulted in 32% higher CBV-TP AUC(0-24) and 99% higher CBV-TP C(max) than did ABC BID. Females had a 38% higher weight-adjusted ABC AUC(0-24) and 81% higher weight-adjusted CBV-TP AUC(0-24) than did males. Virologic suppression was maintained during regimen switch, and no tolerability differences between regimens were observed. In conclusion, this study showed that ABC 600 mg QD and ABC 300 mg BID regimens led to similar intracellular CBV-TP C(tau) values, thus providing pharmacokinetic support for the interchangeability of these two regimens. Women had higher intracellular CBV-TP exposure than did men.

Abacavir Plasma Pharmacokinetics in the Absence and Presence of Atazanavir/Ritonavir or Lopinavir/Ritonavir and Vice Versa in HIV-Infected Patients

Background

Significant interactions between abacavir and other antiretrovirals have not been reported. This study investigated the steady-state plasma pharmacokinetics of abacavir when co-administered with atazanavir/ritonavir or lopinavir/ritonavir in HIV-infected individuals.

Methods

HIV-infected subjects on abacavir (600 mg once daily) plus two nucleoside reverse transcriptase inhibitors (NRTIs) (excluding tenofovir) underwent a 24 h pharmacokinetic assessment for plasma abacavir concentrations. Atazanavir/ritonavir (300/100 mg once daily; arm 1) or lopinavir/ritonavir (400/100 mg twice daily; arm 2) were then added and the 24 h pharmacokinetic assessment repeated. Arm 3 included subjects stable on atazanavir/ritonavir or lopinavir/ritonavir and two NRTIs (excluding tenofovir or abacavir). These patients underwent a pharmacokinetic assessment for atazanavir/ritonavir or lopinavir/ritonavir concentrations on day 1, abacavir (600 mg once daily) was then added to the regimen and the pharmacokinetic assessment repeated. Within-subject changes in drug exposure were evaluated by geometric mean (GM) ratios and 95% confidence intervals (CI).

Results

Twenty-four patients completed the study. GM (95% CI) abacavir area under the curve (AUC) was 18,621 (15,900–21,807) and 15,136 (13,339–17,174) ng.h/ml without and with atazanavir/ritonavir and 15,136 (12,298–18,628) and 10,471 (9,270–11,828) ng.h/ml without and with lopinavir/ritonavir. GM (95% CI) atazanavir AUC without and with abacavir was 26,915 (13,252–54,666) and 28,840 (19,213–43,291) ng.h/ml; lopinavir AUC without and with abacavir was 60,253 (48,084–75,509) and 63,096 (48,128–82,718) ng.h/ml.

Conclusions

No changes in atazanavir or lopinavir exposures were observed following the addition of abacavir; however, decreases in abacavir plasma exposure of 17% and 32% were observed following the addition of atazanavir/ritonavir or lopinavir/ritonavir, respectively.

ADME pathway approach for pharmacogenetic studies of anti-HIV therapy

Pharmacogenetics holds promise in HIV treatment because of the complexity and potential toxicity of multidrug therapies that are prescribed for long periods. However, there has been limited success with the current approach where one or few candidate genes are examined for a limited number of allelic variants. A change in paradigm emerges from the availability of the HapMap, the wealth of data on less common genetic polymorphisms, and new genotyping technology. We present a comprehensive review of functional and putative functional variants in genes encoding absorption, distribution, metabolism and excretion (ADME) proteins relevant to HIV therapy. We propose an analytical array based on our review of the literature, web resources and use of bioinformatic analysis. We identified 126 genes with proven or potential role in HIV therapy. Variation in these genes can be characterized by 2428 SNPs (in Caucasians). On average, a gene is covered by 20 SNPs. This review compiles information for future analysis of the role of specific genes/variants in the exposure and response to antiretroviral therapy to generate a ranked list of new genetic variants for future studies.

Age-related differences in the pharmacokinetics of stavudine in 272 children from birth to 16 years: a population analysis

AIMS

To develop a population pharmacokinetic model for stavudine in children and to investigate the consistency of the currently recommended dose based on adult target concentrations.

METHODS

The pharmacokinetics of stavudine were investigated using a population approach. Individual estimates of CL/F were used to calculate the stavudine dose required to achieve the area under the concentration-time curve reported in adults given recommended doses.

RESULTS

Stavudine pharmacokinetics were well described by a one-compartment model with zero-order absorption. Typical population estimates (% interindividual variability) of the apparent distribution volume (V/F) and plasma clearance (CL/F) were 40.9 l (32%) and 16.5 l h(-1) (38%), respectively. Stavudine V/F and CL/F were similarly related to age. Mean calculated doses (0.61 mg kg(-1) for children less than 2 weeks, 1.23 mg kg(-1) for children more than 2 weeks with bodyweight less than 30 kg, and 31.5 mg for children with a bodyweight between 30 and 60 kg) were in agreement with the current paediatric doses (0.5 mg kg(-1), 1 mg kg(-1), and 30 mg, respectively).

CONCLUSIONS

Our findings support the current recommended paediatric dosage regimens for stavudine, as they result in the same exposure to the drug as in adults.

Increased Body Mass Index Does Not Alter Response to Initial Highly Active Antiretroviral Therapy in HIV-1-Infected Patients

BACKGROUND

Body mass index (BMI) can influence drug metabolism, thus affecting efficacy and risk for toxicities. Hypothesizing that persons with an increased BMI and larger volumes of distribution may experience a suboptimal response to highly active antiretroviral therapy (HAART), we evaluated the effect of BMI on virologic and immunologic response in previously ART-naive patients initiating therapy.

METHODS

Using data from the HIV Outpatient Study, we analyzed the statistical association of BMI and other selected demographic variables with achieving an undetectable viral load and experiencing a CD4 cell count increase of more than 100 cell/microL after 3 to 9 months of therapy among antiretroviral-naive patients initiating HAART.

RESULTS

Among 711 patients included in analysis, 43% had a BMI of more than 25 (overweight-obese). Higher BMI was associated with being female, having black or Hispanic race/ethnicity, being heterosexual, and using injection drugs (all P<0.001). The patients in BMI groups did not differ significantly by baseline CD4 cell count or the duration of the initial HAART regimen. Although median baseline viral loads were significantly lower in obese participants (P=0.008), overweight or obese BMI did not significantly alter the likelihood of achieving an undetectable viral load and a CD4 cell count increase of more than 100 cells/microL compared with normal weight persons.

CONCLUSION

A substantial proportion of HIV-infected outpatients in this cohort were overweight or obese. Increased BMI was not associated with decreased virologic and immunologic responses to initial HAART. Responses were equivalent and within expected ranges between normal weight patients, overweight patients, and obese patients at 3 to 9 months of observation.

Pharmacology of current and promising nucleosides for the treatment of human immunodeficiency viruses

Nucleoside antiretroviral agents are chiral small molecules that have distinct advantages compared to other classes including long intracellular half-lives, low protein binding, sustained antiviral response when a dose is missed, and ease of chemical manufacture. They mimic natural nucleosides and target a unique but complex viral polymerase that is essential for viral replication. They remain the cornerstone of highly active antiretroviral therapy (HAART) and are usually combined with non-nucleoside reverse [corrected] transcriptase and protease inhibitors to provide powerful antiviral responses to prevent or delay the emergence of drug-resistant human immunodeficiency virus (HIV). The pharmacological and virological properties of a selected group of nucleoside analogs are described. Some of the newer nucleoside analogs have a high genetic barrier to resistance development. The lessons learned are that each nucleoside analog should be treated as a unique molecule since any structural modification, including a change in the enantiomeric form, can affect metabolism, pharmacokinetics, efficacy, toxicity and resistance profile.