Safety and pharmacokinetics of abacavir (1592U89) following oral administration of escalating single doses in human immunodeficiency virus type 1-infected adults

Pharmacokinetics of Antiretroviral Drugs in Older People Living with HIV, Part II: Drugs Licensed Before 2005

BACKGROUND AND OBJECTIVE

Advances in antiretroviral therapy led to an increase in life expectancy among people living with human immunodeficiency virus (HIV). As aging is characterized by several physiological changes that can influence pharmacokinetics (PK), this systematic review aims to describe the impact of aging on the PK of antiretrovirals (ARV) approved by the Food and Drug Administration (FDA) before 2005.

METHODS

Searches were performed in BVS, EMBASE, and PubMed databases for publications until June 2024. Peer-reviewed published studies were included if they met the following criteria: adults (≥ 18 years) living with HIV; reporting at least one PK parameter or plasma concentration of any ARV approved by the US FDA before 2005 and still used in the clinic: lamivudine (3TC), emtricitabine (FTC), tenofovir disoproxil fumarate (TDF), abacavir (ABC), zidovudine (ZDV), efavirenz (EFV), nevirapine (NVP), atazanavir (ATV), lopinavir (LPV), ritonavir (RTV), tipranavir (TPV), and fosamprenavir (FPV); PK parameters stratified per age group as young (aged 18-49 years) or older (age ≥ 50 years) adults; and manuscripts published in English, Portuguese, or Spanish. All studies were evaluated for risk of bias. The review protocol was registered in the PROSPERO database (registration no. CRD42023463092).

RESULTS

Among 106 studies included, only 22 evaluated the PK of participants aged 50 years or older and only 5 studies compared the PK between young and older adults for ATV, RTV, EFV, and 3TC. Our analysis revealed an increase in minimal concentration (Cmin) values for LPV, RTV, and ATV in older adults. While increased values of the area under the curve (AUC) and maximum concentration (Cmax) were observed in older adults using ATV, 3TC, and FTC, no differences in PK were apparent between young and older adults for ABC and EFV, with no estimation possible for ZDV.

CONCLUSION

Exposure to 3TC, TDF, FTC, ATV, LPV, and RTV increases with age, while exposure to ABC and EFV appears to be unaffected. Despite the large quantity of data on PK in young adults, there is still a gap in knowledge about the effects of aging on the PK of these ARVs.

Effect of Food on the Pediatric Dispersible Tablet Formulations of TRIUMEQ and DOVATO in Healthy Adult Participants

This randomized food effect study in healthy adult participants examined dispersible tablet formulations of fixed-dose combinations of dolutegravir/abacavir/lamivudine (TRIUMEQ) and dolutegravir/lamivudine (DOVATO). While adult tablet formulations of these combinations are currently approved for the treatment of human immunodeficiency virus, alternate formulations for children are urgently needed to facilitate appropriate pediatric dosing for patients who may have difficulty swallowing a conventional tablet. This study compared the effect of a high-fat, high-calorie meal on the pharmacokinetics, safety, and tolerability of dispersible tablet (DT) formulations of the two-drug and three-drug regimens, with administration under fasting conditions. Both the two-drug and three-drug dispersible tablet formulations, administered under fasting conditions and following a high-fat, high-calorie meal, were well tolerated in healthy participants. There were no clinically relevant differences in drug exposure for either regimen when administered with a high-fat meal as compared to under fasting conditions. Safety observations were similar for both treatments, either in the fed or fasted state. Both TRIUMEQ DT and DOVATO DT formulations can be administer with or without food.

The Prevalence of HLA-B*57 Serotype Associated with Hypersensitivity Reactions in the Treatment of HIV İnfection in the Turkish Population

OBJECTIVES

The aim of our study is to reveal the prevalence of HLA-B*57 in the Turkish population and to provide new perspectives to physicians starting abacavir therapy in HIV patients.

BACKGROUND

Abacavir, one of the drugs used to treat HIV infection, can cause hypersensitivity reactions in some patients. These hypersensitivity reactions have been shown to be associated with the HLA-B*57:01 allele. High-resolution HLA-B*57:01 scanning has a time and cost disadvantage compared with low-resolution HLA-B*57 scanning. Before starting abacavir treatment, we will discuss whether high-resolution scanning is more beneficial in individuals who are positive on HLAB* 57 screening. This is the study with the largest cohort to investigate the prevalence of HLA-B*57 in Turkey.

METHODS

The results of 25 thousand 318 people who applied to Bursa Uludağ University Faculty of Medicine, Department of Immunology for HLA-B* typing were scanned.

RESULTS

In our study, the HLA-B*57 serotype was detected in 827 (3.3%) individuals.

CONCLUSION

Considering these results, it can be assumed that the prevalence of HLA-B*57:01 in Turkey is lower than 3.3%. Instead of a high-resolution HLA-B*57:01 scan in all patients starting abacavir therapy, a high-resolution HLA-B*57:01 scan might be of greater benefit in patients who are positive on a low-resolution HLA-B*57 scan.

Abacavir adverse reactions related with HLA-B*57: 01 haplotype in a large cohort of patients infected with HIV

OBJECTIVE

Carriage of human leukocyte antigen (HLA)-B*57:01 allele increases the risk of abacavir hypersensitivity reaction. Therefore, since 2008 HIV treatment guidelines recommend HLA-B*57:01 screening before abacavir administration, greatly reducing hypersensitivity reaction rate. However, clinically suspected abacavir-related hypersensitivity reactions are described in allele non-carriers. Major aim of this study was to evaluate the relationship between HLA-B*57:01 pattern and abacavir-related hypersensitivity reaction, focusing on hypersensitivity reaction prevalence in allele non-carriers.

METHODS

We included all outpatients aged >18 years old with HIV infection and known HLA-B*57:01 pattern, followed at our Department from January 2000 until December 2017. Patients were divided according to HLA-B*57:01 pattern and first antiretroviral treatment prescribed (containing or not abacavir) as follows: HLA-B*57:01 allele carriers treated with abacavir and HLA-B*57:01 allele non-carriers treated with abacavir. We considered all adverse events reported during first abacavir administration, differentiating between confirmed hypersensitivity reactions and non-hypersensitivity reactions, according to abacavir hypersensitivity reaction definition included in the abacavir EU Summary of Product Characteristics and the US Prescribing Information.

RESULTS

A total of 3144 patients had a known HLA-B*57:01 pattern. About 5.4% of them showed allele polymorphism; Caucasian ethnicity was the most represented. In this cohort, 1801 patients were treated with a first abacavir-containing regimen (98.2% of them was represented by allele non-carriers). 191 out of 1801 patients discontinued abacavir because of toxicity/intolerance; among them 107 described adverse events fulfilled the criteria of confirmed abacavir hypersensitivity reaction (22/32 allele-positive patients and 85/1769 allele-negative patients). After having experienced a confirmed abacavir hypersensitivity reaction, abacavir was re-administered to eight HLA-B*57:01 negative patients. Seven of them re-experienced a syndrome consistent with hypersensitivity reaction, finally leading to drug discontinuation. Overall, no fatal reactions were described.

CONCLUSION

Not all abacavir-related side effects occur as a result of classic HLA-B*57:01-mediated hypersensitivity reaction, as they can develop irrespective of HLA-B*57:01 status. Clinical vigilance must be an essential part of the management of individuals starting abacavir, at any time during treatment. In a 'real-life' setting, clinical diagnosis of suspected abacavir hypersensitivity reaction in allele non-carriers remains crucial for further clinical decision making.

The crosstalk between antiretrovirals pharmacology and HIV drug resistance

INTRODUCTION

The clinical development of antiretroviral drugs has been followed by a rapid and concomitant development of HIV drug resistance. The development and spread of HIV drug resistance is due on the one hand to the within-host intrinsic HIV evolutionary rate and on the other to the wide use of low genetic barrier antiretrovirals.

AREAS COVERED

We searched PubMed and Embase on 31 January 2020, for studies reporting antiretroviral resistance and pharmacology. In this review, we assessed the molecular target and mechanism of drug resistance development of the different antiretroviral classes focusing on the currently approved antiretroviral drugs. Then, we assessed the main pharmacokinetic/pharmacodynamic of the antiretrovirals. Finally, we retraced the history of antiretroviral treatment and its interconnection with antiretroviral worldwide resistance development both in , and middle-income countries in the perspective of 90-90-90 World Health Organization target.

EXPERT OPINION

Drug resistance development is an invariably evolutionary driven phenomenon, which challenge the 90-90-90 target. In high-income countries, the antiretroviral drug resistance seems to be stable since the last decade. On the contrary, multi-intervention strategies comprehensive of broad availability of high genetic barrier regimens should be implemented in resource-limited setting to curb the rise of drug resistance.

Repriming by PrimPol is critical for DNA replication restart downstream of lesions and chain-terminating nucleosides

PrimPol is a DNA damage tolerance enzyme possessing both translesion synthesis (TLS) and primase activities. To uncover its potential role in TLS-mediated IgVλ hypermutation and define its interplay with other TLS polymerases, PrimPol(-/-) and PrimPol(-/-)/Polη(-/-)/Polζ (-/-) gene knockouts were generated in avian cells. Loss of PrimPol had no significant impact on the rate of hypermutation or the mutation spectrum of IgVλ. However, PrimPol(-/-) cells were sensitive to methylmethane sulfonate, suggesting that it may bypass abasic sites at the IgVλ segment by repriming DNA synthesis downstream of these sites. PrimPol(-/-) cells were also sensitive to cisplatin and hydroxyurea, indicating that it assists in maintaining / restarting replication at a variety of lesions. To accurately measure the relative contribution of the TLS and primase activities, we examined DNA damage sensitivity in PrimPol(-/-) cells complemented with polymerase or primase-deficient PrimPol. Polymerase-defective, but not primase-deficient, PrimPol suppresses the hypersensitivity of PrimPol(-/-) cells. This indicates that its primase, rather than TLS activity, is pivotal for DNA damage tolerance. Loss of TLS polymerases, Polη and Polζ has an additive effect on the sensitivity of PrimPol(-/-) cells. Moreover, we found that PrimPol and Polη-Polζ redundantly prevented cell death and facilitated unperturbed cell cycle progression. PrimPol(-/-) cells also exhibited increased sensitivity to a wide variety of chain-terminating nucleoside analogs (CTNAs). PrimPol could perform close-coupled repriming downstream of CTNAs and oxidative damage in vitro. Together, these results indicate that PrimPol's repriming activity plays a central role in reinitiating replication downstream from CTNAs and other specific DNA lesions.

Abacavir, an anti-HIV-1 drug, targets TDP1-deficient adult T cell leukemia

Adult T cell leukemia (ATL) is an aggressive T cell malignancy caused by human T cell leukemia virus type 1 (HTLV-1) and has a poor prognosis. We analyzed the cytotoxic effects of various nucleoside analog reverse transcriptase inhibitors (NRTIs) for HIV-1 on ATL cells and found that abacavir potently and selectively kills ATL cells. Although NRTIs have minimal genotoxicities on host cells, the therapeutic concentration of abacavir induced numerous DNA double-strand breaks (DSBs) in the chromosomal DNA of ATL cells. DSBs persisted over time in ATL cells but not in other cell lines, suggesting impaired DNA repair. We found that the reduced expression of tyrosyl-DNA phosphodiesterase 1 (TDP1), a repair enzyme, is attributable to the cytotoxic effect of abacavir on ATL cells. We also showed that TDP1 removes abacavir from DNA ends in vitro. These results suggest a model in which ATL cells with reduced TDP1 expression are unable to excise abacavir incorporated into genomic DNA, leading to irreparable DSBs. On the basis of the above mechanism, we propose abacavir as a promising chemotherapeutic agent for ATL.

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.

Inhibition of telomerase activity by human immunodeficiency virus (HIV) nucleos(t)ide reverse transcriptase inhibitors: a potential factor contributing to HIV-associated accelerated aging

BACKGROUND

Human immunodeficiency virus (HIV)-infected patients on combination active antiretroviral therapy (cART) are at increased risk of age-related complications. We hypothesized that nucleos(t)ide reverse transcriptase inhibitors (NRTI) may contribute to accelerated aging in HIV-infected individuals on cART via inhibition of telomerase activity.

METHODS

Telomerase activity and telomere length (TL) were measured by quantitative polymerase chain reaction in vitro in activated peripheral blood mononuclear cells (PBMCs) cultured with NRTI and ex vivo in PBMCs from uninfected patients exposed to NRTI and from HIV-infected patients on NRTI-containing cART.

RESULTS

Lamivudine, abacavir, zidovudine, emtricitabine, and tenofovir significantly inhibited telomerase activity in activated PBMCs in vitro. Tenofovir was the most potent inhibitor of telomerase activity and caused greatest shortening of TL in vitro at the therapeutic concentration of 0.3 μM. PBMCs from HIV-infected patients receiving NRTI-containing cART (n = 39) had significantly lower telomerase activity than HIV-uninfected patients (n = 47; P = .011) and HIV-infected patients receiving non-NRTI-containing cART (n = 11; P < .001). TL was significantly inversely associated with age (P = .009) and the total duration on any NRTI (P = .01).

CONCLUSIONS

NRTIs and, specifically tenofovir at therapeutic concentrations, inhibit telomerase activity leading to accelerated shortening of TL in activated PBMCs. The relationship between NRTI, reduced telomerase activity, and accelerated aging requires further investigation in HIV-infected individuals on cART.

Therapeutic levels of lopinavir in late pregnancy and abacavir passage into breast milk in the Mma Bana Study, Botswana

BACKGROUND

Pharmacokinetic data for lopinavir in late pregnancy and in breastfeeding are limited, and no data for abacavir in breast milk are available.

METHODS

Women in the Mma Bana Study initiated HAART from 18 to 34 weeks of gestation. We determined trough plasma and whole breast milk concentrations of lopinavir (LPV), abacavir (ABC), nevirapine (NVP), lamivudine (3TC) and zidovudine (ZDV) among separate subsets of pregnant and breastfeeding women, and in plasma of exposed infants. Lopinavir was measured 1 month after starting HAART or 1 month postpartum, and other drugs were measured 1 month postpartum.

RESULTS

Sampling occurred a median of 14 h (range 11-17) from last maternal drug ingestion. Although 50% higher median LPV levels were seen in postpartum than antepartum plasma (8.29 μg/ml versus 5.51 μg/ml; P = 0.02), antepartum levels with standard LPV dosing were therapeutic for all women (> 1.0 μg/ml). Very low LPV levels (< 0.25 μg/ml) were detected in breast milk. Median ABC levels in breast milk were 85% of those in plasma (0.057 μg/ml versus 0.067 μg/ml). Breast milk concentrations of NVP and 3TC were 27% and 74% of plasma levels, respectively. At these trough maternal time points, only NVP was detectable in potentially inhibitory levels in breastfeeding infants, and most infants had non-detectable levels of LPV, ABC, ZDV and 3TC via maternal breast milk.

CONCLUSIONS

Standard LPV dosing achieved therapeutic levels in pregnancy and no appreciable concentrations in breast milk. ABC is detectable in breast milk at similar concentrations to plasma, but does not result in appreciable infant exposure.

Timing of the components of the HIV life cycle in productively infected CD4+ T cells in a population of HIV-infected individuals

We estimate the time required for HIV to complete separate stages of its infection cycle in productively infected CD4+ T cells in vivo by comparing initial delays after administration of single antiretroviral drugs until HIV RNA reduction in peripheral blood. Data were obtained from monotherapy studies of eight antiretroviral drugs from all currently licensed HIV drug classes: CCR5 blockers (maraviroc), fusion inhibitors (enfuvirtide), nucleoside and nonnucleoside reverse transcriptase inhibitors (abacavir, tenofovir, and rilpivirine), integrase inhibitors (raltegravir), and protease inhibitors (ritonavir and nelfinavir). We find that HIV requires an average of 52 h between export of virions in one generation to export in the next, with most of this (33 h) taken up by reverse transcription. Reverse transcription in vivo was three times longer than in vitro and began soon after virion fusion, as we determined no difference in mean times for commencement of reverse transcription and virion fusion as calculated by timing of the effects for tenofovir and maraviroc. Approximately 7 h is required between HIV integration and virion production. First-phase HIV RNA decay (half-life of 17 h over all drugs) seemed to slow as the stage being inhibited by the drug was further from viral production. The mean estimated half-life of plasma virions was 5 min, significantly shorter than previous estimates.

Medication persistence in the treatment of HIV infection: a review of the literature and implications for future clinical care and research

Persistence, continuous treatment with a prescribed medication or intervention, is an important, but underrecognized aspect of medication treatment, especially for HIV. In contrast to adherence, which measures the percentage of patient behavior to a prescribed therapy, persistence measures the duration during which a patient remains on a prescribed therapy. Decreased persistence for HIV treatment, or shorter duration on therapy, is associated with increased rates of virological failure, development of antiretroviral resistance, and increased morbidity and mortality. Additionally, frequency and duration of nonpersistent episodes rather than adherence may be a better predictor of clinical outcomes in HIV-infected patients on certain regimens. In this review, we codify the constructs of persistence and adherence, and further define persistence as either patient or regimen persistence. Furthermore, current literature on the clinical consequences of and factors associated with suboptimal persistence is summarized. Finally, methods to measure persistence as well as interventions that may improve persistence and clinical outcomes are suggested.

Successful implementation of a national HLA-B*5701 genetic testing service in Canada

Abacavir is a nucleoside reverse transcriptase inhibitor (NRTI) that is used in combination antiretroviral therapy in HIV-infected patients. It is currently recommended as a preferred or an alternative NRTI in antiretroviral-naïve patients. The major toxicity of abacavir is a hypersensitivity reaction (HSR), which occurs in approximately 5% of treated patients. There is a strong association between the human leukocyte antigen (HLA)-B*5701 allele and abacavir HSR, which has allowed for rapid acceptance of genetic screening for HLA-B*5701 in clinical use. Canadian clinicians working in hospital centers with HLA typing capacity opted to launch a pilot project in 2006 to offer the screening test as standard of care to HIV-infected patients. Currently, more than 11,000 HLA-B*5701 tests have been performed, among which 6.3% are positive. Continued efforts have been made to ensure that testing is available to all HIV-infected patients to widen the patients' therapeutic options. HLA-B*5701 screening shows clinical use and preliminary data suggest cost-effectiveness.

Synthesis of methoxypoly(ethylene glycol) carbonate prodrugs of zidovudine and penetration through human skin in vitro

OBJECTIVES

The aim of this study was to synthesise a series of novel methoxypoly(ethylene glycol) carbonate prodrugs of the antiretroviral drug zidovudine (azidothymidine, AZT) in an attempt to enhance the physicochemical properties for transdermal delivery, which may reduce the severe side-effects and toxicity associated with high oral doses of AZT.

METHODS

Methoxypoly(ethylene glycol) carbonates of AZT were synthesised in two steps: activation of the relevant methoxypoly(ethylene glycol) with p-nitrophenyl chloroformate, followed by reaction with AZT. Analysis of the hydrolytic stability in phosphate buffer at pH 5.0 and 7.4 revealed that all the carbonates were markedly more stable at pH 5.0 than at pH 7.4 (0.01 M), with half-lives ranging from 15 to 44 days at pH 5.0 and from 6 to 24 days at pH 7.4. The potential of the series to penetrate the skin was evaluated in vitro by measuring diffusion through excised abdominal female human skin at pH 5.0.

KEY FINDINGS

Prodrugs with 1-3 or 8 oxyethylene units in the methoxypoly(ethylene glycol) moiety were found to permeate the skin whereas those with 12 or 17 units did not. The prodrug with eight oxyethylene units was the most effective penetrant, permeating the skin with a mean flux of 53.3 +/- 46.5 nmol/cm2 per h, which is 2.4-10.1 times that of AZT (8.55 +/- 5.3 nmol/cm2 per h).

CONCLUSIONS

The bioreversible conjugation of the methoxypoly(ethylene glycol) promoiety to AZT appears to be a promising strategy for the transdermal delivery of AZT at a therapeutic dose.

Abacavir and metabolite pharmacokinetics in HIV-1-infected children and adolescents

OBJECTIVES

Abacavir (ABC) oral clearance, adjusted for body size, is approximately 2 times higher for children than adults with a corresponding difference in dose regimens. However, there are limited data available in the adolescent population. The pharmacokinetics (PKs) of ABC and primary metabolites were determined in HIV-1-infected children and adolescents to evaluate age and patient characteristics as a basis for adjusting ABC dose regimens and to assess the influence of metabolite formation on PK parameters.

METHODS

Pediatric subjects 9-18 years of age receiving antiretroviral therapy for HIV-1 infection were stratified by Tanner stage and given a single 8 mg/kg dose of ABC oral solution. Blood samples (n = 10) were obtained over 8 hours and measured for ABC, glucuronide, and carboxylate metabolites using high-performance liquid chromatography. PK parameters for children (Tanner stages 1-2; TS1) and adolescents (Tanner stages 3-5; TS2) were compared.

RESULTS

Twenty-five subjects were enrolled. ABC mean (range) maximum concentration (Cmax; microg/mL), area under the curve (microg.hr/mL), half-life (hours), and apparent clearance (CL/F; mL/min per kg) for TS1 and TS2 were 3.5 (1.2-5.6) vs 3.4 (1.8-5.9), 8.0 (2.1-18.6) vs 8.9 (3.1-17.2), 1.3 (0.7-2.5) vs 1.4 (0.9-1.9), and 22.1 (7.0-59.2) vs 18.4 (7.7-42.9) and not significantly different. Age, Tanner stage, and sex were not correlated with ABC clearance by univariate analysis. The ratios of metabolites to ABC area under the curve were correlated with ABC clearance as were the ratios of metabolites to ABC concentrations at the 6-hour time point.

CONCLUSIONS

ABC oral clearance in HIV-1-infected pediatric patients does not change during puberty, is similar to younger children, and is higher than previously published in adults. Therefore, dosing adolescents as adults should be reexamined. Intersubject PK variability is substantial and is not correlated with body size or age but more likely due to differences in metabolite formation that may be genetic in origin.

Abacavir pharmacokinetics during chronic therapy in HIV-1-infected adolescents and young adults

The pharmacokinetics of abacavir and its metabolites were investigated in 30 human immunodeficiency virus (HIV)-infected adolescents and young adults 13-25 years of age, equally divided into two groups: <18 years of age and >or=18 years of age. All the subjects received the recommended adult dose of 300 mg twice daily. The area under the plasma concentration-time curve (AUC) and half-life of abacavir did not differ significantly between the age groups or by gender or race, and there were only modest associations of age with apparent abacavir clearance and with volume of distribution. There were no significant correlations of carboxylate or glucuronide metabolite levels with age or gender, although glucuronide AUC was higher in Hispanic subjects than in African-American subjects. Zidovudine and lamivudine concentration profiles were also similar in the two age groups. A novel aspect of the study included an assessment of intracellular carbovir, zidovudine, and lamivudine triphosphate levels, and these were found to be similar in the two age-based groups. Overall, these findings suggest that current recommendations relating to adult dosages are appropriate for adolescents and young adults.

Early virologic failure and rescue therapy of tenofovir, abacavir, and lamivudine for initial treatment of HIV-1 infection: TONUS study

BACKGROUND

To assess the efficacy and safety of the triple NRTI combination of abacavir (ABC), lamivudine (3TC), and tenofovir (TDF) in a once-daily regimen.

METHOD

38 HIV-naive patients (pts) were treated in a prospective open-arm study over 48 weeks (W48). Virological failure was defined as never achieving plasma HIV-1 RNA < 400 copies/mL or rebound of > or = 0.7 log10.

RESULTS

12/36 (33%) pts had virologic failure at W24 and 10 additional pts had HIV RNA > 50 copies/mL at W12 or W24. There was a significant association between baseline viral load (VL) and virologic failure in 0%, 29%, and 64% pts with baseline VL levels < 4, 4-5, and > 5 log10 copies/mL, respectively (p = .014). 76% of pts developed K65R and M184V/I mutations by W24, and 19% developed M184V/I alone. At W4, 86% of pts had adequate plasma Cmin for the 3 drugs. 14 pts with K65R and M184V/I were given a rescue therapy with a successful outcome (< 50 copies/mL; median follow-up 48 weeks).

CONCLUSION

Convergent genetic pathway to resistance, in conjunction with lower antiretroviral potency, may explain the high rate of selection K65R and M184V mutations. These mutations did not appear to have a negative effect on rescue therapy with a variety of regimens.

Impact of pregnancy on abacavir pharmacokinetics

OBJECTIVE

To describe abacavir pharmacokinetics during pregnancy and postpartum; physiological changes during pregnancy are known to affect antiretroviral drug disposition.

DESIGN

The Pediatric AIDS Clinical Trials Group P1026s study is an on-going, prospective, non-blinded pharmacokinetic study of pregnant women receiving one or more antiretroviral drugs for routine clinical care, including a cohort receiving abacavir 300 mg twice daily.

METHODS

Serial plasma samples (predose, 1, 2, 4, and 6 h postdose) obtained antepartum (30-36 weeks of gestation) and again postpartum (6-12 weeks after delivery) were assayed for abacavir concentration by reversed-phase high-performance liquid chromatography.

RESULTS

Antepartum evaluations were available for 25 women [mean age, 28.6 years (SD, 6); mean third-trimester weight 92 kg (SD, 35.4); and race/ethnicity 52% black, 28% Hispanic, 16% white, 4% Asian], with geometric mean abacavir area under the concentration-time curve (AUC) of 5.9 mg.h/l [90% confidence interval (CI), 5.2-6.8] and maximum plasma concentration (Cmax) of 1.9 mg/l (90% CI, 1.6-2.2). Seventeen women completed postpartum sampling, and the ratios of antepartum to postpartum AUC and Cmax were 1.04 (90% CI, 0.91-1.18) and 0.79 (90% CI, 0.65-0.98), respectively.

CONCLUSIONS

Abacavir AUC during pregnancy was similar to that at 6-12 weeks postpartum and to that for non-pregnant historical controls (5.8 mg.h/l). Consequently, pregnancy does not appear to affect overall abacavir exposure significantly or to necessitate dose adjustments.

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

AIM

To study the possible influence of patient characteristics on abacavir pharmacokinetics.

METHODS

A population pharmacokinetic model for abacavir was developed using data from 188 adult patients by the use of a nonlinear mixed effects modelling method performed with NONMEM.

RESULTS

Abacavir pharmacokinetics was well described by a two-compartment open model with linear absorption and elimination. Typical population estimates for the absorption rate constant (Ka), the apparent central distribution volume (Vc/F), the apparent peripheral distribution volume (Vp/F), the apparent intercompartmental clearance (Q/F) and the apparent plasma clearance (CL/F) were 1.8 h(-1), 75 l, 23.6 l, 10 l h(-1) and 47.5 l h(-1), respectively. Apparent plasma clearance was positively related to bodyweight. Individual Bayesian estimates of CL/F were used to calculate abacavir AUC. The latter decreased from 10.7 +/- 5.0 to 5.7 +/- 1.6 mgh l(-1) when bodyweight increased from 36 to 102 kg. This drop in abacavir exposure could lead to suboptimal treatment for the heaviest patients, as antiviral efficacy of abacavir is known to be related to its AUC. A 400 mg abacavir dose would be necessary to achieve adequate exposure to abacavir in patients weighing more than 60 kg.

CONCLUSIONS

The apparent plasma clearance of abacavir was positively related to bodyweight. The efficacy of the current recommended abacavir dosage for patients with high bodyweight should be evaluated in further studies.

Plasma Pharmacokinetics of Once- versus Twice-Daily Lamivudine and Abacavir: Simplification of Combination Treatment in HIV-1-Infected Children (Penta-13)

Background

There are few data on plasma and intracellular pharmacokinetics (PK) of once-daily (q24h) nucleoside analogues in HIV-infected children.

Methods

Children aged 2–13 years receiving combination treatment containing lamivudine (3TC) (4 mg/kg) and/or abacavir (ABC) (8 mg/kg) twice daily (q12h) were included in this single-arm, open-label, crossover study. Intensive plasma PK sampling was performed at steady state, after which children switched to q24h dosing and PK sampling was repeated 4 weeks later. Daily area under the curve (AUC0–24) and peak level (Cmax) of q24h and q12h regimens were compared by geometric mean ratios (GMRs) with 90% confidence intervals (CIs). Children were followed for 24 weeks to evaluate safety and virological response.

Results

24 children were enrolled, of whom 20 [median age (range) 5.6 (2.1–12.8) years] had evaluable PK data for 3TC ( n=19) and/or ABC ( n=14). GMRs of 3TC and ABC AUC0–24 and Cmax q24h versus q12h significantly exceeded 1.0. GMRs were not significantly different between children aged 2–6 versus 6–13 years old ( P>0.08). Of note, 3TC Cmax values for both q12h and q24h were significantly lower in children aged 2–6 versus 6–13 years old. No child discontinued due to adverse events. At baseline, 16 out of 20 children had a viral load <100 copies/ml compared with 17 out of 19 at week 24.

Conclusion

AUC0–24 and Cmax of both 3TC and ABC q24h were not inferior to q12h dosing in children. Insufficient results were obtained concerning intracellular levels of the active triphosphate moieties of both agents. Virological data did not indicate a marked difference in antiviral activity between q12h and q24h regimens.

Membrane permeation characteristics of abacavir in human erythrocytes and human T-lymphoblastoid CD4+ CEM cells: comparison with (-)-carbovir

Abacavir, (-)-(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene-1-methanol, is a novel purine carbocyclic nucleoside analogue that has been approved by the FDA for the treatment of HIV (as Ziagen trade mark [abacavir sulfate]). Chemically, abacavir and (-)-carbovir (CBV) differ only at the 6-position of the purine ring; abacavir contains a cyclopropylamino moiety in place of the 6-lactam functionality of CBV. Intracellularly both are ultimately metabolized to CBV triphosphate. We compared the membrane permeation characteristics of these two compounds at 20 degrees C in human erythrocytes and in human T-lymphoblastoid CD4+ CEM cells, using a "papaverine-stop" assay. In erythrocytes, abacavir influx was rapid, nonsaturable (rate constant=200 pmol/s/mM/microl cell water), and unaffected by inhibitors of nucleoside or nucleobase transport. CBV influx was slow, saturable, strongly inhibited by adenine or hypoxanthine, and occurred via both the nucleobase carrier (Vmax=0.67 pmol/s/microl cell water; Km=50 microM) and the nucleoside carrier (Vmax=0.47 pmol/s/microl cell water; Km=440 microM). Similar qualitative results were obtained with CD4+ CEM cells, although CBV influx rates were somewhat higher and abacavir influx rates lower, compared to the corresponding rates in erythrocytes. Equilibrium studies further revealed that both compounds are concentrated intracellularly, but nonmetabolically, in both cell types, apparently due to cytosolic protein binding (absent in erythrocyte ghosts). We conclude that, in both cell types, while CBV influx is slow and carrier-dependent, abacavir influx occurs rapidly by nonfacilitated diffusion. The membrane permeation characteristics of abacavir are consistent with its superior oral bioavailability and its impressive ability to penetrate the central nervous system.

Indinavir, efavirenz, and abacavir pharmacokinetics in human immunodeficiency virus-infected subjects

Adult AIDS Clinical Trials Group (AACTG) Protocol 886 examined the dispositions of indinavir, efavirenz, and abacavir in human immunodeficiency virus-infected subjects who received indinavir at 1,000 mg every 8 h (q8h) and efavirenz at 600 mg q24h or indinavir at 1,200 mg and efavirenz at 300 mg q12h with or without abacavir 300 at mg q12h. Thirty-six subjects participated. The median minimum concentration in plasma (C(min)) for indinavir administered at 1,200 mg q12h was 88.1 nM (interquartile range [IR], 61.7 to 116.5 nM), whereas the median C(min) for indinavir administered at 1,000 mg q8h was 139.3 nM (IR, 68.8 to 308.7 nM) (P = 0.19). Compared to the minimum C(min) range for wild-type virus (80 to 120 ng/ml) estimated by the AACTG Adult Pharmacology Committee, the C(min) for indinavir administered at 1,200 mg q12h (54 ng/ml) is inadequate. The apparent oral clearance (CL/F) (P = 0.28), apparent volume of distribution at steady state (V(ss)/F) (P = 0.25), and half-life (t(1/2)) (P = 0.80) of indinavir did not differ between regimens. The levels of efavirenz exposure were similar between regimens. For efavirenz administered at 600 mg q24h and 300 mg q12h, the median maximum concentrations in plasma (C(max)s) were 8,968 nM (IR, 5,784 to 11,768 nM) and 8,317 nM (6,587 to 10,239 nM), respectively (P = 0.66), and the C(min)s were 4,289 nM (IR, 2,462 to 5,904 nM) and 4,757 nM (IR, 3,088 to 6,644 nM), respectively (P = 0.29). Efavirenz pharmacokinetic parameters such as CL/F (P = 0.62), V(ss)/F (P = 0.33), and t(1/2) (P = 0.37) were similar regardless of the dosing regimen. The median C(max), C(min), CL/F, V(ss)/F, and t(1/2) for abacavir were 6,852 nM (IR, 5,702 to 7,532), 21.0 nM (IR, 21.0 to 87.5), 43.7 liters/h (IR, 37.9 to 55.2), 153.9 liters (IR, 79.6 to 164.4), and 2.0 h (IR, 1.8 to 2.8), respectively. In summary, when indinavir was given with efavirenz, the trough concentration of indinavir after administration of 1,200 mg q12h was inadequate. Abacavir did not influence the pharmacokinetics or exposure parameters of either indinavir or efavirenz. The levels of efavirenz exposure were similar in subjects receiving efavirenz q12h or q24h.

Hyperlactataemia syndromes associated with HIV therapy

Hyperlactataemia is seen in 8-18.3% of HIV-infected patients taking nucleoside-analogue reverse transcriptase inhibitors (NRTIs). Recent epidemiological studies suggest that most episodes are transient and subclinical. However, symptomatic and occasionally life-threatening cases accompanied by metabolic acidosis and hepatic steatosis (ie, lactic acidosis syndrome) have also been described. Though yet to be fully elucidated, the proposed mechanism is NRTI-induced inhibition of mitochondrial DNA polymerase culminating in derangements in oxidative phosphorylation and lactate homeostasis. Signs and symptoms range from mild hyperlactataemia accompanied by nausea, abdominal discomfort, and weight loss to severe, intractable lactic acidosis complicated by coma and multi-organ failure. Significant progress has recently been made with regard to the natural history of NRTI-related hyperlactataemia. However, other important aspects of the disorder, such as its pathogenesis, predisposing conditions, and management, remain poorly understood. This article reviews the current published work on these issues, identifies areas of controversy, and addresses directions for future research.

Antiretroviral pharmacokinetics in the paediatric population: a review

Characteristics unique to paediatric pharmacotherapy should be considered when treating children infected with human immunodeficiency virus (HIV). Processes of growth and development in the paediatric patient can significantly affect drug absorption and disposition. Immature renal function, altered hepatic enzyme activity and differences in drug absorption lead to variations in systemic exposure of antiretrovirals among children. Paediatric patients are also subject to unique circumstances that may prevent adherence to antiretroviral regimens. The pharmacokinetics of nucleoside reverse transcriptase inhibitors differ significantly among preterm infants, full-term infants and older children. Decreased hepatic glucuronidation activity in neonates results in pharmacokinetic differences in zidovudine disposition when compared with older children. Didanosine, stavudine and lamivudine are renally eliminated, thus resulting in differences among young children with immature renal function. Pharmacokinetic data for non-nucleoside reverse transcriptase inhibitors in children are limited. Decreased elimination of nevirapine among neonates has been observed, primarily due to decreased enzymatic activity. Pharmacokinetic differences across age groups have been noted for efavirenz, but no formal assessments have been conducted in children weighing less than 10kg. Protease inhibitors are metabolised by the cytochrome P450 enzyme system, which is not fully developed in younger children. Decreased metabolism can result in elevated plasma concentrations, thereby increasing the chance of toxicity. Unfortunately, few studies exist evaluating the pharmacokinetics of antiretrovirals in children. As a result, dosage selection of antiretrovirals in children often occurs without adequate data. As the life expectancy of HIV-infected children increases, use of antiretrovirals to prevent disease progression also increases. If prevention of treatment failure continues to be the goal of antiretroviral therapy, the pharmacokinetics of antiretrovirals in children need to be assessed early in the drug development process.

Antiretroviral-associated liver injury

ART-related hepatotoxicity can manifest in a variety of ways. Although benign, asymptomatic LEEs predominate, liver injury occurring in the context of either hypersensitivity or hyperlactatemia, represents a medical emergency and mandates immediate cessation of ART. Underpinning this broad spectrum of presentations are several, as yet poorly understood, mechanisms of liver damage that reflect contributions by constituents of HAART and host factors. Thus far, the most significant predisposing condition to emerge from clinical studies is chronic viral hepatitis. A more precise understanding, however, of the processes and factors that underlie ART-related hepatotoxicity is critical not only to the management of liver injury from current antiretroviral drugs but also to the design of safer drugs in the future.

Transdermal delivery of zidovudine: effect of vehicles on permeation across rat skin and their mechanism of action

The purpose of this study was to investigate the effects of various solvent systems containing water, ethanol, propylene glycol (PG), and their binary combinations on the ex vivo permeation of zidovudine (AZT) across Sprague-Dawley rat skin using Franz diffusion cells at 37 degrees C. Further, saturation solubility and epidermis/vehicle partition coefficient of AZT in the solvent systems, and their effect on percentage hydration of epidermis using thermogravimetric analysis were determined to understand the mechanisms by which these solvent systems change drug permeability properties. All binary combinations of PG, ethanol and water significantly increased saturation solubility of AZT. Maximum AZT flux was observed with 66.6% ethanol among ethanol-water solvents, with 33.3% PG in PG-water solvents and with 100% ethanol among PG-ethanol combinations. PG-water and PG-ethanol solvents neither reduced the lag time nor increased AZT flux across rat skin. In addition, high concentrations of PG in both water and ethanol reduced steady state flux of AZT. Further, thermogravimetric studies revealed that solvents containing high PG concentrations dehydrate epidermis. Among all the solvent combinations, highest flux and short lag time were achieved with ethanol at 66.6% in water and hence is a suitable vehicle for transdermal delivery of AZT.

Abacavir hypersensitivity reaction

A hypersensitivity reaction occurs in association with initiation of abacavir therapy as part of combination antiretroviral therapy in approximately 3.7% of patients. The reaction is possibly the result of a combination of altered drug metabolism and immune dysfunction, which is poorly understood. White patients appear to be at higher risk and patients of African descent at lower risk of abacavir hypersensitivity. Clinical management involves supportive measures and discontinuation of abacavir therapy. Rechallenge with abacavir in a hypersensitive patient should be avoided because it might precipitate a life-threatening reaction.

Abacavir/lamivudine/zidovudine as a combined formulation tablet: bioequivalence compared with each component administered concurrently and the effect of food on absorption

A single-center, open-label, three-way crossover study was conducted in 24 healthy subjects to assess (1) the bioequivalence of a combined abacavir 300 mg/lamivudine 150 mg/zidovudine 300 mg (A/L/Z) combination tablet relative to the separate brand-name components administered simultaneously and (2) the effect of food on the bioavailability of the drugs from the combination tablet. The subjects were randomly assigned to receive each of the following three treatments, separated by a 2-day washout period: one A/L/Z combination tablet after an overnight fast, one abacavir 300 mg tablet + one lamivudine 150 mg tablet + one zidovudine 300 mg tablet sequentially after an overnight fast, or one A/L/Z combination tablet 5 minutes after completing a standardized high-fat breakfast (67 g fat, 58 g carbohydrate, and 33 g protein). Serial blood samples were collected up to 24 hours postdose for determination of abacavir, lamivudine, and zidovudine serum concentrations. Standard pharmacokinetic parameters were estimated. Treatments were considered bioequivalent if 90% confidence intervals (CI) for geometric least squares (GLS) mean ratios for abacavir, lamivudine, and zidovudine area under the serum concentration-time curve (AUC(infinity)) and maximum observed serum concentration (Cmax) fell entirely within 0.80 to 1.25 for log-transformed parameters. The combined A/L/Z tablet was bioequivalent in the extent (AUC) and rate of absorption (Cmax and time of Cmax [tmax]) to the individual brand-name drug components administered concurrently under fasted conditions. GLS ratios and 90% CI for AUC(infinity) and Cmax were 0.99 (0.96, 1.03) and 1.00 (0.90, 1.11), respectively, for abacavir; 0.95 (0.91, 0.99) and 0.90 (0.84, 0.99), respectively, for lamivudine; and 0.95 (0.89, 1.02) and 0.96 (0.80, 1.15), respectively, for zidovudine. The extent of absorption of abacavir, lamivudine, and zidovudine from the combination tablet was not altered by administration with meals, indicating that this formulation may be administered with or without food. However, food slowed the rate of absorption, delayed the tmax, and reduced the Cmax of abacavir, lamivudine, and zidovudine. These changes, which were consistent with those observed with the individual reference formulations when administered with food, were not considered clinically important. All formulations were well tolerated underfasted and fed conditions.

Stevens-Johnson syndrome caused by the antiretroviral drug nevirapine

Nevirapine is a non-nucleoside reverse transcriptase inhibitor widely used in combination with other antiretroviral agents for the treatment of HIV infection. Severe rash, including the Stevens-Johnson syndrome (SJS), is the major toxicity of nevirapine and is described in the package labeling with a prominent, boxed warning. Though physicians treating large populations of patients with HIV are well aware of this complication, only one other report of nevirapine-associated SJS has been documented in the dermatology literature. We describe 2 cases of SJS related to nevirapine use and review the literature on this newly recognized association.

Liquid chromatographic assay for simultaneous determination of abacavir and mycophenolic acid in human plasma using dual spectrophotometric detection

A combined bio-analytical assay for abacavir, a reversed transcriptase inhibitor, and mycophenolic acid (MPA), based on reversed-phase liquid chromatography and both ultraviolet (UV) absorption and fluorescence detection, is reported. Both analytes are extracted from plasma with acetonitrile. After centrifugation, evaporation of the supernatant and reconstitution in water, the sample is injected into the chromatograph. Abacavir is detected using UV detection at 285 nm and MPA spectrofluorometrically at 345 and 430 nm for excitation and emission, respectively. The method has been validated in the 80-2000 ng/ml range for abacavir and in the 10-10,000 ng/ml range for MPA for 200-microl plasma samples. The lower limits of quantification are 80 and 10 ng/ml for abacavir and MPA, respectively. Precisions and accuracies are < or = 8% in the valid concentration ranges of both analytes.

Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians

Nucleoside analogs (zidovudine, didanosine, zalcitabine, stavudine, abacavir, lamivudine) have been administered as antiretroviral agents for more than a decade. They undergo anabolic phosphorylation by intracellular kinases to form triphosphates, which inhibit human immunodeficiency virus replication by competitively inhibiting viral reverse transcriptase. Numerous methods are used to elucidate the intracellular metabolic pathways of these agents. Intracellular and extracellular factors affect intracellular phosphorylation. Lack of standardization and complexity of methods used to study phosphorylation in patients limit interpretation of study results and comparability of findings across studies. However, in vitro and in vivo studies give important insights into mechanisms of action, metabolic feedback mechanisms, antiviral effects, and mechanisms of toxicity, and have influenced dosing regimens of nucleoside analogs.

Pharmacokinetic and pharmacodynamic study of the human immunodeficiency virus protease inhibitor amprenavir after multiple oral dosing

In a dose-ranging study of amprenavir (formerly 141W94), an inhibitor of the protease enzyme of human immunodeficiency virus (HIV) type 1, single-dose and steady-state pharmacokinetic parameters were estimated from plasma samples collected on day 1 and during week 3, respectively. Amprenavir was administered on either a twice-daily (b.i.d.) or three-times-daily dosage schedule to 62 HIV-infected adults, 59 of whom had pharmacokinetic data. Log-log regression analysis (the power model) revealed that the steady-state area under the curve (AUC(ss)) and the maximum, minimum, and average concentrations at steady state (C(max,ss), C(min,ss), and C(avg,ss), respectively) increased in a dose-proportional manner over the 300- to 1,200-mg dose range. Steady-state clearance was dose independent. AUC(ss)/AUC(0-->infinity) decreased linearly with dose and correlated significantly with treatment-associated decreases in alpha(1)-acid glycoprotein. After 3 weeks, the dose of 1,200 mg b.i. d. provided a median amprenavir C(min,ss) (0.280 microg/ml) that was higher than the median in vitro 50% inhibitory concentration for clinical HIV isolates (0.023 microg/ml), even after adjustment for protein binding. The median amprenavir C(min,ss) was also greater than the estimated in vivo trough concentration calculated to yield 90% of the maximum antiviral effect (0.228 microg/ml) over 4 weeks. A pharmacodynamic analysis of the relationship between steady-state pharmacokinetic parameters and safety revealed headache and oral numbness to be the only side effects significantly associated with C(max). The pharmacodynamic relationship defined in this study supports the use of 1,200 mg b.i.d. as the approved dose of amprenavir.

Abacavir: a review of its clinical potential in patients with HIV infection

Abacavir is a carbocyclic 2'-deoxyguanosine nucleoside analogue. It is metabolised intracellularly to a 2'-deoxyguanosine nucleoside analogue which competitively inhibits HIV reverse transcriptase and terminates proviral DNA chain extension. In double-blind trials in antiretroviral therapy-experienced or -naive patients, reductions in HIV RNA levels were greater and more prolonged in patients receiving abacavir in combination with other antiretroviral drugs than in those receiving placebo in combination with the same agents. Furthermore, abacavir in combination with lamivudine and zidovudine reduced viral load to below detectable levels in a proportion of patients, and to a similar extent to the protease inhibitor indinavir in combination with lamivudine and zidovudine. Greatest viral load reductions were seen in antiretroviral therapy-naive patients. Preliminary results suggest that the viral suppression achieved with a protease inhibitor plus 2 nucleoside reverse transcriptase inhibitors (NRTIs) can be maintained as effectively with abacavir in combination with 2 NRTIs as it can be by continuing the protease inhibitor-containing treatment regimen. Initial virological data from studies of combination regimens including abacavir and protease inhibitors appear promising but larger controlled trials are required to confirm these observations. Nausea is the most frequently reported adverse event in patients receiving abacavir-containing combination therapy. Adverse events tend to be reported most frequently soon after starting treatment; the majority of events are mild or moderate in intensity and transient. Other adverse events reported in >5% of patients include vomiting, malaise and fatigue, headache, diarrhoea, sleep disorders, cough, anorexia and rash. A major cause of abacavir treatment discontinuation is the development of a hypersensitivity reaction which has been reported in 3 to 5% of patients. The reaction usually occurs within 6 weeks of commencing treatment, shows evidence of multiorgan system involvement and typically includes fever and/or rash. Symptoms resolve rapidly after discontinuation of treatment. Continuing treatment or rechallenge can result in more severe symptoms, life-threatening hypotension and even death.

CONCLUSION

Abacavir used in combination with other antiretroviral drugs effectively reduces viral load in both adults and children with HIV infection. Although these responses are greatest in individuals with little or no previous antiretroviral treatment, useful responses are still sometimes achieved in heavily pretreated individuals. Abacavir in combination with lamivudine and zidovudine provides a simple and convenient dosage regimen which is generally well tolerated, able to produce sustained suppression of viral replication and has the advantage of sparing other classes of antiretroviral drugs for subsequent use. This triple combination represents an alternative antiretroviral regimen for patients intolerant to protease inhibitors or those wishing to retain the option of protease inhibitors for later use. Further clinical studies are needed to define the activity of abacavir in combination with protease inhibitors and non-nucleoside reverse transcriptase inhibitors.

Population pharmacokinetics and pharmacodynamic modeling of abacavir (1592U89) from a dose-ranging, double-blind, randomized monotherapy trial with human immunodeficiency virus-infected subjects

Abacavir (formerly 1592U89) is a carbocyclic nucleoside analog with potent anti-human immunodeficiency virus (anti-HIV) activity when administered alone or in combination with other antiretroviral agents. The population pharmacokinetics and pharmacodynamics of abacavir were investigated in 41 HIV type 1 (HIV-1)-infected, antiretroviral naive adults with baseline CD4(+) cell counts of >/=100/mm(3) and plasma HIV-1 RNA levels of >30,000 copies/ml. Data for analysis were obtained from patients who received randomized, blinded monotherapy with abacavir at 100, 300, or 600 mg twice-daily (BID) for up to 12 weeks. Plasma abacavir concentrations from sparse sampling were analyzed by standard population pharmacokinetic methods, and the effects of dose, combination therapy, gender, weight, and age on parameter estimates were investigated. Bayesian pharmacokinetic parameter estimates were calculated to determine the peak concentration of abacavir in plasma (C(max)) and the area under the concentration-time curve from time zero to infinity (AUC(0-infinity)) for individual subjects. The pharmacokinetics of abacavir were dose proportional over the 100- to 600-mg dose range and were unaffected by any covariates. No significant correlations were observed between the incidence of the five most common adverse events (headache, nausea, diarrhea, vomiting, and malaise or fatigue) and AUC(0-infinity). A significant correlation was observed between C(max) and nausea by categorical analysis (P = 0.019), but this was of borderline significance by logistic regression (odds ratio, 1.45; 95% confidence interval, 0.95 to 2.32). The log(10) time-averaged AUC(0-infinity) minus baseline (AAUCMB) values for HIV-1 RNA and CD4(+) cell count correlated significantly with C(max) and AUC(0-infinity), but with better model fits for AUC(0-infinity). The increase in AAUCMB values for CD4(+) cell count plateaued early for drug exposures that were associated with little change in AAUCMB values for plasma HIV-1 RNA. There was less than a 0.4 log(10) difference over 12 weeks in the HIV-1 RNA levels with the doubling of the abacavir AUC(0-infinity) from 300 to 600 mg BID dosing. In conclusion, pharmacodynamic modeling supports the selection of abacavir 300 mg twice-daily dosing.

Multiple-dose pharmacokinetics and pharmacodynamics of abacavir alone and in combination with zidovudine in human immunodeficiency virus-infected adults

Abacavir (1592U89) is a nucleoside reverse transcriptase inhibitor with potent activity against human immunodeficiency virus type 1 (HIV-1) when used alone or in combination with other antiretroviral agents. The present study was conducted to determine the multiple-dose pharmacokinetics and pharmacodynamics of abacavir in HIV-1-infected subjects following oral administration of daily doses that ranged from 600 to 1,800 mg, with and without zidovudine. Seventy-nine subjects received abacavir monotherapy for 4 weeks (200, 400, or 600 mg every 8 hours [TID] and 300 mg every 12 h [BID]) and thereafter received either zidovudine (200 mg TID or 300 mg BID) or matching placebo with abacavir for 8 additional weeks. Pharmacokinetic parameters were calculated for abacavir after administration of the first dose and at week 4 and for abacavir, zidovudine, and its glucuronide metabolite at week 12. The concentrations of abacavir in cerebrospinal fluid were determined in a subset of subjects. Steady-state plasma abacavir concentrations were achieved by week 4 of monotherapy and persisted to week 12. At steady state, abacavir pharmacokinetic parameters (area under the plasma concentration-time curve for a dosing interval [AUC(tau)] and peak concentration [C(max)]) were generally proportional to dose over the range of a 600- to 1,200-mg total daily dose. Coadministration of zidovudine with abacavir produced a small and inconsistent effect on abacavir pharmacokinetic parameters across the different doses. At the clinical abacavir dose (300 mg BID) zidovudine coadministration had no effect on the abacavir AUC(tau), which is most closely associated with efficacy. Zidovudine pharmacokinetics appeared to be unaffected by abacavir. Statistically significant but weak relationships were found for the change in the log(10) HIV-1 RNA load from the baseline to week 4 versus total daily AUC(tau) and C(tau) (P < 0.05). The incidence of nausea was significantly associated with total daily AUC(tau) and C(max). In conclusion, abacavir has predictable pharmacokinetic characteristics following the administration of multiple doses.

Pharmacokinetic interaction of abacavir (1592U89) and ethanol in human immunodeficiency virus-infected adults

While in vitro results at clinically relevant concentrations do not predict abacavir (1592U89) interactions with drugs highly metabolized by cytochrome P450, the potential does exist for a pharmacokinetic interaction between abacavir and ethanol, as both are metabolized by alcohol dehydrogenase. Twenty-five subjects were enrolled in an open-label, randomized, three-way-crossover, phase I study of human immunodeficiency virus-infected male subjects. The three treatments were administration of (i) 600 mg of abacavir, (ii) 0.7 g of ethanol per kg of body weight, and (iii) 600 mg of abacavir and 0.7 g of ethanol per kg. Twenty-four subjects completed the study with no unexpected adverse events reported. Ethanol pharmacokinetic parameters were unchanged with abacavir coadministration. The geometric least squares mean area under the concentration curve extrapolated to infinite time for abacavir increased 41% (from 11.07 to 15.62 microg. h/ml), and the half-life increased 26% (from 1.42 to 1.79 h) in the presence of ethanol (mean ethanol maximum concentration in plasma of 498 microg/ml). The percentages of abacavir dose recovered in urine as abacavir and its two major metabolites were each altered in the presence of ethanol, but there was no change in the total percentage ( approximately 50%) of administered dose recovered in the 12-h collection interval. In conclusion, while a single 600-mg dose of abacavir does not alter blood ethanol concentration, ethanol does increase plasma abacavir concentrations.

Salvage therapy with abacavir plus a non-nucleoside reverse transcriptase inhibitor and a protease inhibitor in heavily pre-treated HIV-1 infected patients. Swiss HIV Cohort Study

BACKGROUND

Highly active antiretroviral therapy (HAART) may fail, especially in pre-treated patients.

OBJECTIVE

To examine retrospectively whether heavily pre-treated patients not responding to HAART at least once respond to a salvage therapy with abacavir, a nucleoside reverse transcriptase inhibitor (NRTI) plus a non-nucleoside analogue reverse transcriptase inhibitor (NNRTI) and one or two protease inhibitors (PI).

PATIENTS

We retrospectively identified and analysed patients followed in the Swiss HIV Cohort Study with > 1000 HIV RNA copies/ml on HAART, naive to abacavir who were switched to abacavir plus one NNRTI (efavirenz or nevirapine) and one or two PI which had not been used in the previous HAART.

RESULTS

Of 23 identified HIV-infected patients with four (median) therapy changes before salvage, 10 patients (43%) achieved a decrease of plasma HIV RNA > 0.5 log10 at 6 months of therapy. After 6 months only two patients had an HIV-1 RNA < 500 copies/ml, one of them < 50 copies/ml. Seven patients increased their CD4 cell counts by > 30% above baseline. Three patients, all with CD4 cell counts < 100 x 10(6)/l before salvage therapy had a > 30% decline in CD4 cell count. An extended number of resistance-associated mutations was found in almost all patients at baseline. One patient had two new AIDS-defining events. Five patients (22%) discontinued treatment because of side-effects, mainly occurrence of a rash.

CONCLUSION

Salvage therapy in intensively pre-treated patients has a low virological success rate despite usage of abacavir and NNRTI. Nevertheless, this did not correlate with immunological and clinical course. This study emphasizes the difficulty of second-line treatment in HIV-1 infection and stresses the need for new compounds.

The latest progress in the synthesis of carbocyclic nucleosides

This review presents the latest developments in the field of carba-nucleosides (1994-1998). Special attention is paid to the synthesis of key precursors to those carba-nucleosides that possess significant biological activities or have novel structures.

Pharmacokinetics of [(14)C]abacavir, a human immunodeficiency virus type 1 (HIV-1) reverse transcriptase inhibitor, administered in a single oral dose to HIV-1-infected adults: a mass balance study

Abacavir (1592U89) ((-)-(1S, 4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene- 1-m ethanol) is a 2'-deoxyguanosine analogue with potent activity against human immunodeficiency virus (HIV) type 1. To determine the metabolic profile, routes of elimination, and total recovery of abacavir and metabolites in humans, we undertook a phase I mass balance study in which six HIV-infected male volunteers ingested a single 600-mg oral dose of abacavir including 100 microCi of [(14)C]abacavir. The metabolic disposition of the drug was determined through analyses of whole-blood, plasma, urine, and stool samples, collected for a period of up to 10 days postdosing, and of cerebrospinal fluid (CSF), collected up to 6 h postdosing. The radioactivity from abacavir and its two major metabolites, a 5'-carboxylate (2269W93) and a 5'-glucuronide (361W94), accounted for the majority (92%) of radioactivity detected in plasma. Virtually all of the administered dose of radioactivity (99%) was recovered, with 83% eliminated in urine and 16% eliminated in feces. Of the 83% radioactivity dose eliminated in the urine, 36% was identified as 361W94, 30% was identified as 2269W93, and 1.2% was identified as abacavir; the remaining 15.8% was attributed to numerous trace metabolites, of which <1% of the administered radioactivity was 1144U88, a minor metabolite. The peak concentration of abacavir in CSF ranged from 0.6 to 1.4 microg/ml, which is 8 to 20 times the mean 50% inhibitory concentration for HIV clinical isolates in vitro (0.07 microg/ml). In conclusion, the main route of elimination for oral abacavir in humans is metabolism, with <2% of a dose recovered in urine as unchanged drug. The main route of metabolite excretion is renal, with 83% of a dose recovered in urine. Two major metabolites, the 5'-carboxylate and the 5'-glucuronide, were identified in urine and, combined, accounted for 66% of the dose. Abacavir showed significant penetration into CSF.

Abacavir: absolute bioavailability, bioequivalence of three oral formulations, and effect of food

STUDY OBJECTIVES

Study A: to determine the absolute bioavailability of a single 300-mg abacavir hemisulfate tablet. Study B: to determine the bioequivalence of two oral abacavir formulations (300-mg hemisulfate tablet, 100-mg succinate caplet), the effect of food on the bioavailability of the 300-mg hemisulfate tablet, and the bioavailability of the hemisulfate tablet relative to the hemisulfate solution.

DESIGN

Phase I, randomized, open-label, balanced two- (study A) and three- or four-period (study B), crossover studies.

SETTING

Two clinical research centers.

SUBJECTS

Six men infected with the human immunodeficiency virus (HIV), aged 27-39 years (study A), and 18 HIV-infected men and women, aged 21-50 years (study B).

INTERVENTIONS

In study A, all subjects received a single, oral 300-mg tablet of abacavir hemisulfate or a single, intravenous infusion of abacavir hemisulfate 150 mg over 60 minutes. In study B, all subjects received each of three single-dose treatments: three 100-mg abacavir succinate caplets in a fasted state, one 300-mg abacavir hemisulfate tablet in a fasted state, and one 300-mg abacavir hemisulfate tablet with a high-fat breakfast. Twelve subjects in study B also received a fourth treatment of abacavir hemisulfate 300 mg as an oral solution in a fasted state. Plasma samples collected for 24 hours (study A) or 12 hours (study B), and urine samples collected for 12 hours (study A) were analyzed by validated high-performance liquid chromatographic methods.

MEASUREMENTS AND MAIN RESULTS

Abacavir pharmacokinetic parameters were calculated using standard, noncompartmental methods. In study A, the geometric least square (GLS) mean absolute bioavailability of oral abacavir was 83% (range 65-107%). In study B, the hemisulfate tablet was bioequivalent to the succinate caplet, but its time to maximum concentration (Tmax) occurred 30 minutes earlier. Administration of the abacavir hemisulfate tablet with food had no effect on area under the curve from time zero to infinity (AUC0-infinity), decreased maximum concentration (Cmax) by 26%, and delayed Tmax by 38 minutes. The relative bioavailability (GLS mean AUC0-infinity ratio) of the 300-mg abacavir hemisulfate tablet to solution was 101%, Cmax was 11% lower, and Tmax was unchanged. The most common drug-related adverse events associated with abacavir were nausea, vomiting, abdominal pain, and headache, all of which were mild.

CONCLUSION

Based on our results, abacavir is safe and well tolerated and can be administered with or without meals.

Single-dose pharmacokinetics and safety of abacavir (1592U89), zidovudine, and lamivudine administered alone and in combination in adults with human immunodeficiency virus infection

Abacavir (1592U89), a nucleoside reverse transcriptase inhibitor with in vitro activity against human immunodeficiency virus type-1 (HIV-1), has been evaluated for efficacy and safety in combination regimens with other nucleoside analogs, including zidovudine (ZDV) and lamivudine (3TC). To evaluate the potential pharmacokinetic interactions between these agents, 15 HIV-1-infected adults with a median CD4(+) cell count of 347 cells/mm3 (range, 238 to 570 cells/mm3) were enrolled in a randomized, seven-period crossover study. The pharmacokinetics and safety of single doses of abacavir (600 mg), ZDV (300 mg), and 3TC (150 mg) were evaluated when each drug was given alone or when any two or three drugs were given concurrently. The concentrations of all drugs in plasma and the concentrations of ZDV and its 5'-glucuronide metabolite, GZDV, in urine were measured for up to 24 h postdosing, and pharmacokinetic parameter values were calculated by noncompartmental methods. The maximum drug concentration (Cmax), the area under the concentration-time curve from time zero to infinity (AUC0-infinity), time to Cmax (Tmax), and apparent elimination half-life (t1/2) of abacavir in plasma were unaffected by coadministration with ZDV and/or 3TC. Coadministration of abacavir with ZDV (with or without 3TC) decreased the mean Cmax of ZDV by approximately 20% (from 1.5 to 1.2 microg/ml), delayed the median Tmax for ZDV by 0.5 h, increased the mean AUC0-infinity for GZDV by up to 40% (from 11.8 to 16.5 microg. h/ml), and delayed the median Tmax for GZDV by approximately 0.5 h. Coadministration of abacavir with 3TC (with or without ZDV) decreased the mean AUC0-infinity for 3TC by approximately 15% (from 5.1 to 4.3 microg. h/ml), decreased the mean Cmax by approximately 35% (from 1.4 to 0.9 microg/ml), and delayed the median Tmax by approximately 1 h. While these changes were statistically significant, they are similar to the effect of food intake (for ZDV) or affect an inactive metabolite (for GZDV) or are relatively minor (for 3TC) and are therefore not considered to be clinically significant. No significant differences were found in the urinary recoveries of ZDV or GZDV when ZDV was coadministered with abacavir. There was no pharmacokinetic interaction between ZDV and 3TC. Mild to moderate headache, nausea, lymphadenopathy, hematuria, musculoskeletal chest pain, neck stiffness, and fever were the most common adverse events reported by those who received abacavir. Coadministration of ZDV or 3TC with abacavir did not alter this adverse event profile. The three-drug regimen was primarily associated with gastrointestinal events. In conclusion, no clinically significant pharmacokinetic interactions occurred between abacavir, ZDV, and 3TC in HIV-1-infected adults. Coadministration of abacavir with ZDV or 3TC produced mild changes in the absorption and possibly the urinary excretion characteristics of ZDV-GZDV and 3TC that were not considered to be clinically significant. Coadministration of abacavir with ZDV and/or 3TC was generally well tolerated and did not produce unexpected adverse events.

Safety and single-dose pharmacokinetics of abacavir (1592U89) in human immunodeficiency virus type 1-infected children

Abacavir (formerly 1592U89) is a potent 2'-deoxyguanosine analog reverse transcriptase inhibitor that has been demonstrated to have a favorable safety profile in initial clinical trials with adults with human immunodeficiency virus (HIV) type 1 infection. A phase I study was conducted to evaluate the pharmacokinetics and safety of abacavir following the administration of two single oral doses (4 and 8 mg/kg of body weight) to 22 HIV-infected children ages 3 months to 13 years. Plasma was collected for analysis at predose and at 0.5, 1, 1.5, 2, 2.5, 3, 5, and 8 h after the administration of each dose. Plasma abacavir concentrations were determined by high-performance liquid chromatography, and data were analyzed by noncompartmental methods. Abacavir was well tolerated by all subjects. The single abacavir-related adverse event was rash, which occurred in 2 of 22 subjects. After administration of the oral solution, abacavir was rapidly absorbed, with the time to the peak concentration in plasma occurring within 1.5 h postdosing. Pharmacokinetic parameter estimates were comparable among the different age groups for each dose level. The mean maximum concentration in plasma (Cmax) and the mean area under the curve from time zero to infinity (AUC0-infinity) increased by 16 and 45% more than predicted, respectively, as the abacavir dose was doubled from 4 to 8 mg/kg (Cmax increased from 1.69 to 3.94 micrograms/ml, and AUC0-infinity increased from 2.82 to 8.09 micrograms.h/ml). Abacavir was rapidly eliminated, with a mean elimination half-life of 0.98 to 1.13 h. The mean apparent clearance from plasma decreased from 27.35 to 18.88 ml/min/kg as the dose increased. Neither body surface area nor creatinine clearance were correlated with pharmacokinetic estimates at either dose. The extent of exposure to abacavir appears to be slightly lower in children than in adults, with the comparable unit doses being based on body weight. In conclusion, this study showed that abacavir is safe and well tolerated in children when it is administered as a single oral dose of 4 or 8 mg/kg.