Pharmacokinetic optimisation of antiretroviral therapy in patients with HIV infection

Nucleoside and nucleotide reverse transcriptase inhibitors in children

By the end of 2006, approximately 2.3 million children worldwide were living with HIV infection, representing about 15% of all HIV-infected individuals but only 5-7% of the total population of treated patients worldwide. Despite a general increase in the use of antiretroviral therapy (ART) in resource-limited settings, appropriate care and ART remain inaccessible for most of the world's HIV-infected children. ART of children is challenging because of a general lack of paediatric formulations (including tablets in paediatric strengths), limited options of drugs available for children (some have been approved only for use in adults), different viral and immunological responses, dependency on caregivers for administration of the therapy, and specific issues of toxicity in long-term therapy related to maturation and development. As in adults, nucleoside reverse transcriptase inhibitors (NRTIs) are a key component of any ART schedule in children, being the recommended 'backbone' treatment in US, European and WHO guidelines, and, indeed, NRTIs have been extensively studied in children. NRTIs are the class of antiretroviral drugs that have more drugs licensed for paediatric use and more paediatric formulations.Generally, the dual NRTI backbone treatment of combination with a non-NRTI (NNRTI) or protease inhibitor (PI) should comprise a cytidine analogue (lamivudine, emtricitabine) and a thymidine analogue (stavudine, zidovudine), guanosine analogue (i.e. abacavir), or nucleotide RTI (NtRTI; i.e. tenofovir). European and US guidelines recommend the use of triple NRTI therapy (abacavir/lamivudine/zidovudine) in children with anticipated poor adherence to other treatment regimens because of tablet burden. In conclusion, while use of ART in children needs to be dramatically increased, selecting and administering the best drug combination for children is still limited by a lack of paediatric formulations and knowledge of drug metabolism, safety and efficacy in children. NRTIs are already a key component of paediatric ART, but fixed-dose combinations and specific research in children are needed to optimise their use. In this article we review the available information to facilitate selection of the best NRTI for backbone treatment in combination ART for HIV-infected children.

Population pharmacokinetic analysis of lamivudine, stavudine and zidovudine in controlled HIV-infected patients on HAART

OBJECTIVE

This work aimed at building a population pharmacokinetic (PK) model for lamivudine (LMV), stavudine (STV) and zidovudine (ZDV), estimating their inter and intraindividual PK variability and investigating the influence of different covariates.

METHODS

Population PK of LMV, STV and ZDV was separately evaluated from plasma concentrations obtained in 54, 39 and 27 HIV1-infected patients, respectively, enrolled in the COPHAR1-ANRS102 trial. The primary objective of this trial was to study the pharmacokinetics of indinavir (IDV) and nelfinavir (NFV) in treated patients with a sustained virological response. Concentrations of nucleoside analogs (NA) were measured in plasma as a secondary objective. A one-compartment model with first-order elimination was used, with zero-order absorption for LMV and first-order absorption for STV and ZDV.

RESULTS

Mean parameters [interpatient variability in coefficient of variation (CV%)] of LMV, STV and ZDV were: oral volume of distribution (V/F) 145 l (52%), 24 l (81%) and 248 l (80%), oral clearance (Cl/F) 32 l/h, 16 l/h (74%) and 124 l/h (51%), respectively. For LMV, absorption duration (Ta) was 1.46 h (64%). For STV and ZDV, ka was 0.46 h(-1) and 2.9 h(-1), respectively. We found a systematic effect of combination with NFV vs. IDV. We found that intrapatient variability was greater than interpatient variability (except for STV) and greater than 55% for the three drugs.

CONCLUSION

This trial enabled the estimation of the population PK parameters of three NA in patients with a sustained virological response, and the median curves could be used as references for concentration-controlled strategies. We observed, as for the protease inhibitors, a great variability of PK parameters.

HIV vaccine trial preparedness among Spanish-speaking Latinos in the US

Latinos are under-represented in HIV/AIDS medical research in the US. Although they are disproportionately impacted by HIV/AIDS, Latinos may be reluctant to participate in HIV vaccine trials. Three focus groups were conducted with 32 Spanish-speaking Latinos recruited from two community-based healthcare organizations in Los Angeles, California. A qualitative focus group interview guide was developed to explore concerns, motivators and intentions in regard to participation in HIV vaccine trials. Mistrust and fear of government emerged as important themes related to reluctance to participate in an HIV vaccine trial. Specific concerns regarding trial participation included: (1) fear of vaccine-induced HIV infection, (2) physical side effects, (3) stigma and (4) false-induced HIV-positive test results and their social repercussions. Motivators for enrolling in an HIV vaccine trial included: (1) incentives, (2) convenience of participating in a study, (3) sufficient and appropriate study information, (4) personal benefits and (5) altruism. Interventions to facilitate participation by Latinos in HIV vaccine trials should address mistrust and fear of government-sponsored HIV/AIDS medical research, increase access to and convenience of clinical trials, address fear of vaccine-induced infection, combat HIV/AIDS stigma and raise awareness of the relevance of HIV/AIDS to Latino communities.

Pharmacokinetics of antiretroviral therapy in HIV-1-infected children

The initiation of antiretroviral therapy has resulted in an impressive reduction in the rate of disease progression in AIDS and HIV-1-related deaths in children; however, there are still several major challenges to be faced in order to improve therapy. A major topic that needs to be dealt with is the establishment of the optimal dosage of antiretroviral therapy for children. This review presents the currently available peer-reviewed data on the pharmacokinetics of nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), protease inhibitors (PIs) and fusion inhibitors (FIs) in children. In addition, the data are discussed in relation to the currently available European and US guidelines and the US FDA-approved drug labels. High intra- and interpatient variability in pharmacokinetics are often observed for all antiretroviral drugs. The number of children included in the pharmacokinetic studies is often small and children are often divided into divergent groups using different dosage levels and/or drug formulations. For a substantial number of antiretroviral drugs, dosage recommendations, especially for young children, are still absent in the European and US guidelines. The recommended drug dosages in the guidelines are often different from that in the officially approved drug product label. In addition, the recommended drug dosages may deviate between the European and US guidelines. Thus, while practioners aim to meet the recommendations in the official guidelines, patients may receive highly divergent dosages of medication. The high intra- and interpatient variability in pharmacokinetics of antiretroviral drugs in children hampers the application of fixed dosages of antiretroviral drugs. For PIs and NNRTIs, plasma drug levels correlate with viral suppression and drug toxicity. NRTIs are prodrugs that are intracellularly converted to their active triphosphate form and, therefore, plasma NRTI levels correlate poorly with viral suppression. Therapeutic drug monitoring of PIs and NNRTIs should be considered to optimise HIV therapy in children.

Pharmacodynamic Effects of Zidovudine 600 mg Once/Day versus 300 mg Twice/Day in Therapy-Naïve Patients Infected with Human Immunodeficiency Virus

STUDY OBJECTIVE

To compare the virologic activity of zidovudine monotherapy administered as 600 mg once/day versus 300 mg twice/day.

DESIGN

Phase II, randomized (1:1), open-label study.

SETTING

Thirteen medical centers in the United States.

PATIENTS

Thirty-two antiretroviral-naive patients infected with human immunodeficiency virus (HIV).

INTERVENTION

Patients were administered either zidovudine 600 mg every 24 hours (16 patients) or 300 mg every 12 hours (16 patients) for 13 days.

MEASUREMENTS AND MAIN RESULTS

Plasma HIV-1 RNA concentration was measured daily. Study end points were between-group differences in change from baseline of log10-transformed HIV-1 RNA and in rates of viral load decline measured by the slope of HIV-1 RNA over time. At baseline, mean HIV-1 RNA was similar in the once/day and twice/day groups (4.33 and 4.40 log10 copies/ml, respectively). At day 14, a trend toward lower mean reduction in HIV-1 RNA from baseline was observed in the once/day group (-0.585, 95% confidence interval [CI] -0.728 to -0.442 log10 copies/ml) compared with the twice/day group (-0.849, 95% CI -1.067 to -0.630 log10 copies/ml, p=0.056). Viral load reduction also tended to be slower in the once/day group, as indicated by the smaller slope of viral load decline in the once/day group than in the twice/day group during days 1-14 (-0.045 vs -0.065 logic copies/ml/day, p=0.065). Both zidovudine regimens were similarly well tolerated.

CONCLUSION

Zidovudine 600 mg once/day has antiviral activity, although less pronounced and more slowly achieved than that seen with zidovudine 300 mg twice/day. No differences were observed between the two treatment groups with respect to safety profile or tolerability

Dose-dependent pharmacokinetics of 1-(2-Deoxy-?-D- ribofuranosyl)-2,4-difluoro-5-iodobenzene: A potential mimic of 5-iodo-2?-deoxyuridine

The dose-range pharmacokinetics of l-(2-deoxy-beta-D-ribofuranosyl)-2,4-difluoro-5-iodobenzene (5-IDFPdR), a C-aryl nucleoside mimic of IUdR, were studied in male Sprague-Dawley rats following single intravenous (i.v.) and oral doses. After i.v. administration, the blood clearance decreased from approximately 32 ml/min/kg at a dose of 15 mg/kg, to approximately 19 ml/min/kg when dosed at 54 mg/kg, and the elimination half-life increased from 8.4 min to 21.5 min, for the respective doses. While the dose-normalized area under the concentration-time curve (AUCnorm) remained practically unchanged (0.132 kg min ml(-1)) upon increasing the i.v. dose from 5 to 15 mg/kg, it increased by about 44% ( approximately 0.19 kg min ml(-1)) when the i.v. dose was increased from 15 to 54 mg/kg. Similarly, there was a dose-dependent increase in AUCnorm with increasing oral doses: AUCnorm increased by 49% as the oral dose increased from 20 to 40 mg/kg, and further by 55% as the oral dose was increased from 40 mg/kg to 54 mg/kg. For the respective oral doses, the elimination half-life increased from 24.5 min to 176 min, while blood clearance was reduced from approximately 37 ml/min/kg to approximately 17 ml/min/kg. The urinary recoveries of unchanged 5-IDFPdR and its glucuronides (as percent of the dose) were somewhat increased at higher doses. This increase was more pronounced following the highest oral dose. The total biliary recovery of 5-IDFPdR (as percent of the dose) was, however, decreased with increasing doses. The overall kinetic profile of 5-IDFPdR based on these data is suggestive of dose-dependent pharmacokinetics. Decreased elimination of 5-IDFPdR with increasing dose, as supported by longer elimination half-lives at higher doses, is one likely mechanism contributing to the dose-dependent behaviour of this compound. Saturable non-renal metabolism might explain the reduced total body clearance of 5-IDFPdR at higher doses, despite the unchanged or increased urinary clearance. For drugs exhibiting nonlinear kinetics, the dosage regimens may need to be carefully designed to avoid potential unpredictable toxicity and/or lack of pharmacological response associated with the disproportional changes in steady state drug concentrations on changing dose. Manifestation in the rat of nonlinear kinetics at doses of 5-IDFPdR, which may be of therapeutic relevance, warrants extended dose-range evaluations of this compound in future preclinical and clinical studies, to establish safe and efficacious dosage regimens.

Relationship between exposure to zidovudine and decrease of P24 antigenemia in HIV-infected patients in monotherapy

The link between virological response and exposure to zidovudine was studied in 40 HIV-infected patients of the protocol ANRS 01. During this 45-day trial, the patients received only oral zidovudine in six treatment groups. Our objectives were: to analyze and model the pharmacokinetics of zidovudine and the decrease of P24 antigenemia; to study the links between exposure and efficacy. For the pharmacokinetic study, 12 blood samples were collected from 0.16 to 24 h after the first dose and a compartmental model was used. For the pharmacodynamic study of P24 antigenemia, blood samples were collected before treatment and every 3 days until day 45; an exponantial decay model was used. The pharmacokinetic and pharmacodynamic parameters were estimated for each patient by nonlinear regression. The correlations between efficacy parameters and exposure parameters, were then studied in the 40 patients. The mean (+/- SD) apparent volume of distribution and clearance were 151 L (+/- 94) and 184 L/h (+/- 72), respectively. The mean initial antigen level was 472 pg/mL (+/- 409), the coefficient of reduction of antigenemia was 0.27 (+/- 0.21) and the rate of decrease was 0.27/day (+/- 0.16). The coefficient of P24 reduction was found to be significantly correlated to the daily area under the curve (P < 0.0014). This relationship was adequately described by an Imax model and the daily area under the curve, leading to 50% of antigenemia decrease, was estimated to be 2.32 mg x h/L (+/- 0.33). In conclusion, a significant relationship between exposure to zidovudine at day 1, and decrease of P24 antigenemia was found. It was estimated that the average steady-state concentration, which corresponds to 70% of maximal efficacy, was 0.22 mg/L. Together with the large interpatient variability of zidovudine pharmacokinetics, these findings confirmed that zidovudine should be monitored and a clinical target concentration was defined.

Incorporation of zidovudine into cord blood DNA of infants and peripheral blood DNA of their HIV-1-positive mothers

The nucleoside analogue 3'-azido-3'-deoxythymidine (AZT) is a weak carcinogen in adult female mice and a moderately strong carcinogen in the offspring of female mice given the drug during gestation. In addition, incorporation of AZT into DNA was observed in multiple organs of transplacentally exposed newborn mice. Here we investigate the incorporation of AZT into peripheral leukocyte DNA of HIV-1-positive adult pregnant women given AZT for variable times during gestation and cord blood of infants exposed to AZT in utero. The length of treatment varied between 10 days and 9 months. High molecular weight DNA was extracted from maternal peripheral blood mononuclear cells (PBMC) and infant cord blood. A specific AZT-DNA radioimmunoassay was used to determine the amount of AZT incorporated into leukocyte DNA. Incorporation of AZT into DNA ranged up to 183.3 and 344.5 molecules of AZT/10(6) nucleotides in the mothers and infants, respectively, and was detected in about 70% of samples. Therefore, AZT-induced mutagenic events are possible in the majority of adults and infants. No correlation was found between level of incorporation and length of AZT treatment, suggesting that the differences observed among the individuals arise from variability in AZT metabolism. These data support previous observations that a high degree of inter-individual variability in AZT phosphorylation occurs in primates.

Incorporation of zidovudine into leukocyte DNA from HIV-1-positive adults and pregnant women, and cord blood from infants exposed in utero

OBJECTIVE

The nucleoside analog 3'-azido-3'-deoxythymidine (ZDV) has widespread clinical use but also is carcinogenic in newborn mice exposed to the drug in utero and becomes incorporated into newborn mouse DNA. This pilot study was designed to determine ZDV incorporation into human blood cell DNA from adults and newborn infants.

DESIGN

In this prospective cohort study, peripheral blood mononuclear cells (PBMC) were obtained from 28 non-pregnant adults and 12 pregnant women given ZDV therapy, six non-pregnant adults with no exposure to ZDV, and six non-pregnant adults who last received ZDV > or = 6 months previously. In addition, cord blood leukocytes were obtained from 22 infants of HIV-1-positive, ZDV-exposed women and from 12 infants unexposed to ZDV. There were 11 mother-infant pairs involving HIV-1 -positive women.

METHODS

DNA was extracted from PBMC obtained from non-pregnant HIV-1-positive adults taking ZDV, pregnant HIV-1-positive women given ZDV during pregnancy, and from adults not taking ZDV. Cord blood leukocytes were examined from infants exposed to ZDV in utero and from unexposed controls. DNA samples were assayed for ZDV incorporation by anti-ZDV radioimmunoassay (RIA).

RESULTS

The majority (76%) of samples from ZDV-exposed individuals, pregnant women (8 of 12), non-pregnant adults (24 of 28), or infants at delivery (15 of 22), had detectable ZDV-DNA levels. The range of positive values for ZDV-treated adults and infants was 25-544 and 22-452 molecules ZDV/10(6) nucleotides, respectively. Analysis of 11 mother-infant pairs showed variable ZDV-DNA incorporation in both, with no correlation by pair or by duration of drug treatment during pregnancy. Two of the 24 samples from individuals designated as controls were positive by anti-ZDV RIA. The 20-fold range for ZDV-DNA values in both adults and infants suggested large interindividual differences in ZDV phosphorylation.

CONCLUSIONS

Incorporation of ZDV into DNA was detected in most of the samples from ZDV-exposed adults and infants. Therefore, the biologic significance of ZDV-DNA damage and potential subsequent events, such as mutagenicity, should be