Pharmacokinetic modeling and simulations of interaction of amprenavir and ritonavir

Inhibition and induction of CYP enzymes in humans: an update

The cytochrome P450 (CYP) enzyme family is the most important enzyme system catalyzing the phase 1 metabolism of pharmaceuticals and other xenobiotics such as herbal remedies and toxic compounds in the environment. The inhibition and induction of CYPs are major mechanisms causing pharmacokinetic drug–drug interactions. This review presents a comprehensive update on the inhibitors and inducers of the specific CYP enzymes in humans. The focus is on the more recent human in vitro and in vivo findings since the publication of our previous review on this topic in 2008. In addition to the general presentation of inhibitory drugs and inducers of human CYP enzymes by drugs, herbal remedies, and toxic compounds, an in-depth view on tyrosine-kinase inhibitors and antiretroviral HIV medications as victims and perpetrators of drug–drug interactions is provided as examples of the current trends in the field. Also, a concise overview of the mechanisms of CYP induction is presented to aid the understanding of the induction phenomena.

Population pharmacokinetic modelling of the changes in atazanavir plasma clearance caused by ritonavir plasma concentrations in HIV-1 infected patients

AIMS

The aim of the present study was to develop a simultaneous population pharmacokinetic model for atazanavir (ATV) incorporating the effect of ritonavir (RTV) on clearance to predict ATV concentrations under different dosing regimens in HIV-1-infected patients.

METHODS

A Cross-sectional study was carried out in 83 HIV-1-infected adults taking ATV 400 mg or ATV 300 mg/RTV 100 mg once daily. Demographic and clinical characteristics were registered and blood samples collected to measure drug concentrations. A population pharmacokinetic model was constructed using nonlinear mixed-effects modelling and used to simulate six dosing scenarios.

RESULTS

The selected one-compartmental model described the pharmacokinetics of RTV and ATV simultaneously, showing exponential, direct inhibition of ATV clearance according to the RTV plasma concentration, which explained 17.5% of the variability. A mean RTV plasma concentration of 0.63 mg l-1 predicted an 18% decrease in ATV clearance. The percentages of patients with an end-of-dose-interval concentration of ATV below or above the minimum and maximum target concentrations of 0.15 mg l-1 and 0.85 mg l-1 favoured the selection of the simulated ATV/RTV once-daily regimens (ATV 400 mg, ATV 300 mg/RTV 100 mg, ATV 300 mg/RTV 50 mg, ATV 200/RTV 100 mg) over the unboosted twice-daily regimens (ATV 300 mg, ATV 200 mg).

CONCLUSIONS

A one-compartment simultaneous model can describe the pharmacokinetics of RTV and ATV, including the effect of RTV plasma concentrations on ATV clearance. This model is promising for predicting individuals' ATV concentrations in clinical scenarios, and supports further clinical trials of once-daily doses of ATV 300 mg/RTV 50 mg or ATV 200 mg/RTV 100 mg to confirm efficacy and safety.

Salvage Therapy with Ritonavir-Boosted Amprenavir/Fosamprenavir: Virological and Immunological Response in Two Years Follow-up

OBJECTIVE

To evaluate the efficacy of salvage regimens containing ritonavir-boosted amprenavir (APV/r) or fosamprenavir (FPV/r) in heavily pretreated protease inhibitor (PI)-experienced HIV-1 patients.

METHOD

Evaluation of APV/r- or FPV/r-containing antiretroviral regimens in PI-experienced HIV-1 patients with 2 or more antiretroviral failures. Follow-up continued to 96 weeks with prospective collection of data.

RESULTS

54 episodes (48 on APV/r and 6 on FPV/r) were considered in 45 patients who had received a median of 5 prior antiretroviral regimens (range, 2-13) including a median of 3 PIs (range, 2-4). Median time of treatment at analysis was 72 weeks (range, 12-210). At baseline, plasma viral load (pVL) and CD4 cell count was 67,000 copies/mL and 167 cell/mm(3), respectively. At week 96, the median pVL was < 50 copies/mL and CD4 cell count was 519 cells/mm(3). Proportion of patients with pVL below detection was 62% at week 48 and 61% at week 96. Fifteen patients stopped treatment because of virologic failure; one presented a full resistance profile to APV/r, based on the ANRS 2003 resistance algorithm. Median trough APV plasma concentration 4 weeks after treatment initiation was 1406 ng/mL (range, 452-4321); dose adaptation was required in only 7 patients.

CONCLUSION

This study provides long-term follow-up of APV/r and FPV/r in the setting of salvage therapy, showing a high and sustained rate of virologic and immunologic response.

Pharmacokinetics, safety and antiviral activity of fosamprenavir/ritonavir-containing regimens in HIV-infected children aged 4 weeks to 2 years-48-week study data

BACKGROUND

Pharmacokinetics, safety and antiviral activity of fosamprenavir (FPV) with ritonavir (RTV) twice daily were evaluated in HIV-1-infected infants and children 4 weeks to <2 years over 48 weeks.

METHODS

Results from intensive pharmacokinetic sampling of subjects enrolled in single dose visits was used to determine individualized dosing for the first 6-10 subjects in each of 2 cohorts (4 weeks to <6 months, 6 months to <2 years); steady state pharmacokinetic data were then used to select the dosage regimen for the remaining subjects recruited to the cohort. Intensive pharmacokinetic sampling was performed at week 2 or 8; predose samples were collected every 4-12 weeks thereafter. Safety and plasma HIV-1 RNA were monitored every 4-12 weeks.

RESULTS

Fifty-nine subjects received study medication. FPV 45 mg/kg boosted with RTV 7 to 10 mg/kg BID achieved average plasma amprenavir area under curve(0-τ) values 26% to 28% lower and Cmax similar to historical adult data for FPV/RTV 700/100 mg BID; amprenavir Cτ values were lower in the subjects <6 months of age. At week 48, 35 of 54 (65%) subjects had achieved plasma HIV-1 RNA <400 copies/mL and 33 of 54 (61%) had plasma HIV-1 RNA values <50 copies/mL. The most common adverse events were diarrhea, upper respiratory tract infection, gastroenteritis and otitis media.

CONCLUSIONS

Final FPV/RTV dosing regimens achieved plasma amprenavir exposures comparable with those from regimens approved in adults, with the exception of trough exposures in the <6-month-old infants. The FPV/RTV regimens led to viral suppression in 61% of patients and were generally well tolerated.

Population pharmacokinetic modeling and simulation of amprenavir following fosamprenavir/ritonavir administration for dose optimization in HIV infected pediatric patients

Fosamprenavir (FPV) is the phosphate ester prodrug of the HIV-1 protease inhibitor amprenavir (APV). A pediatric population pharmacokinetic model for APV was developed and simulation was used to identify dosing regimens for pediatric patients receiving FPV in combination with ritonavir (RTV) which resulted in concentrations similar to those in adults receiving FPV/RTV 700/100 mg BID. Pharmacokinetic data was obtained from HIV infected subjects aged 2 months to 18 years receiving either FPV or FPV/RTV. A two-compartment model with first order absorption and elimination was an appropriate structural model. Significant covariates in the model included RTV coadministration on clearance, fed status on bioavailability for the oral suspension, body weight on clearance and volume terms, black race on clearance, and age on clearance. The following FPV/RTV twice daily dosing regimens in pediatric patients delivered plasma APV exposure similar to adults: 45/7 mg/kg in patients weighing <11 kg, 30/3 mg/kg in patients weighing 11 to <15 kg, 23/3 mg/kg in patients weighing 15 to <20 kg, and 18/3 mg/kg in patients weighting ≥20 kg. Additionally children weighing ≥39 kg can receive the adult regimen.

NIMROD: a program for inference via a normal approximation of the posterior in models with random effects based on ordinary differential equations

Models based on ordinary differential equations (ODE) are widespread tools for describing dynamical systems. In biomedical sciences, data from each subject can be sparse making difficult to precisely estimate individual parameters by standard non-linear regression but information can often be gained from between-subjects variability. This makes natural the use of mixed-effects models to estimate population parameters. Although the maximum likelihood approach is a valuable option, identifiability issues favour Bayesian approaches which can incorporate prior knowledge in a flexible way. However, the combination of difficulties coming from the ODE system and from the presence of random effects raises a major numerical challenge. Computations can be simplified by making a normal approximation of the posterior to find the maximum of the posterior distribution (MAP). Here we present the NIMROD program (normal approximation inference in models with random effects based on ordinary differential equations) devoted to the MAP estimation in ODE models. We describe the specific implemented features such as convergence criteria and an approximation of the leave-one-out cross-validation to assess the model quality of fit. In pharmacokinetics models, first, we evaluate the properties of this algorithm and compare it with FOCE and MCMC algorithms in simulations. Then, we illustrate NIMROD use on Amprenavir pharmacokinetics data from the PUZZLE clinical trial in HIV infected patients.

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.

Long-term efficacy and safety of once-daily fosamprenavir 1400 mg boosted by ritonavir 100 mg: the BOLD100 study

In a retrospective database study at two HIV treatment centres, medical records were accessed to evaluate long-term efficacy and safety parameters in all HIV-infected adults who had achieved HIV-1 RNA <50 copies/mL following the initiation of fosamprenavir (FPV)/ritonavir (RTV) 1400 mg/100 mg once-daily (QD)-containing regimens between January 2004 and January 2006. Data were available for 20 antiretroviral (ARV)-naïve patients (baseline median HIV-1 RNA 5.0 log(10) copies/mL; CD4+ cell count 307 cells/mm(3)), 30 protease inhibitor (PI)-naïve, ARV-experienced patients (HIV-1 RNA 3.6 log(10) copies/mL; CD4+ count 348 cells/mm(3)) and 25 PI-experienced patients switching to FPV/RTV100 for reasons other than virological failure (HIV-1 RNA 2.7 log(10) copies/mL; CD4+ count 328 cells/mm(3)). HIV-1 RNA <50 copies/mL was achieved in 100% of the ARV-naïve cohort (median monitoring period, 2.4 years; range, 1.4-3.2 years), 87% of the PI-naïve cohort (2.4 years; range, 1.2-3.4 years) and 88% of the PI-experienced cohort (2.2 years; range, 1.0-3.2 years). Virological failure occurred in 0%, 7% and 8% of the cohorts, respectively, and median CD4+ count increased above baseline by 224, 155 and 115 cells/mm(3), respectively. Change from baseline in median fasting lipids was: total cholesterol +12, -6, -2 mg/dL; low-density lipoprotein-cholesterol 0, -5, +12 mg/dL; high-density lipoprotein-cholesterol +4, +2, +7 mg/dL; triglycerides +9, -21, -65 mg/dL, respectively. In conclusion, FPV/RTV 1400/100 mg QD-containing regimens remained effective long-term in all ARV-naïve and most PI-naïve and PI-experienced HIV-infected patients.

Viral response in stable patients switching to fosamprenavir/ritonavir monotherapy (the FONT Study)

OBJECTIVE

The aim of the study was to evaluate the efficacy of fosamprenavir/ritonavir (FPV/r) monotherapy in plasma and reservoirs in virologically suppressed patients.

METHODS

A 48-week, prospective, single-arm pilot trial was carried out (trial registration: ISRCTN78584791). Patients receiving triple therapy [FPV/r plus two nucleoside reverse transcriptase inhibitors (NRTIs) for at least the previous month], with viral load (VL) <40 HIV-1 RNA copies/mL and no previous virological failure (VF) on protease inhibitors (PIs), were included in the trial and received FPV/r monotherapy (700/100 mg/12 h). VL and FPV/r levels [by liquid chromatography-tandem mass spectrometry (LC/MS/MS); limit of detection (LOD) 0.5 ng/mL] in cerebrospinal fluid (CSF) were determined at week 24. VF was defined as VL >40 copies/mL in three consecutive samples or >500 copies/mL in two samples.

RESULTS

Enrolment was prematurely stopped because of a high percentage of VF. Twenty patients (45% men; median age 43.5 years) were included in the trial. Nine patients (45%) presented therapeutic failure [seven (35%) had VF, and two discontinued therapy]. Resistance testing was available in five patients. One patient presented major PI mutations (54L, 32I and 47V) in addition to one minor mutation (13V), whereas two patients had minor PI mutations (10V+36I and 71T, respectively). The patient with major PI mutations switched from FPV/r to darunavir/r and VL was re-suppressed. In the other six patients with VF, VL was re-suppressed after the reintroduction of NRTIs. VL was <40 copies/mL in all CSF samples (n=10). Median amprenavir plasma levels were 2.5 μg/mL (range 0.7-8.6 μg/mL) at week 24 and 2.5 μg/mL (range 0.4-3.8 μg/mL) at VF. The CSF amprenavir concentration was 28.1 ng/mL (range 6.39-83.6 ng/mL), exceeding the reported 50% inhibitory concentration (IC(50) ) range for CSF in nine of 11 patients.

CONCLUSIONS

The high percentage of patients with VF in our study suggests that the use of FPV/r in a simplification monotherapy strategy should be discouraged. Adequate amprenavir levels and undetectable VL in CSF were documented in all samples evaluated.

Steady-state amprenavir and tenofovir pharmacokinetics after coadministration of unboosted or ritonavir-boosted fosamprenavir with tenofovir disoproxil fumarate in healthy volunteers

Objective An open-label, three-period pharmacokinetic study was conducted to investigate the drug interaction potential between fosamprenavir (FPV) and tenofovir disoproxil fumarate (TDF). Methods Thirty-six healthy subjects received TDF 300 mg once daily (qd) for 7 days (period 1), and then were randomized to 14 days of either FPV 1400 mg twice daily (bid) or FPV/ritonavir (RTV) 700/100 mg bid alone or with TDF (period 2). Subjects continued their randomized dose of FPV for 14 more days, adding or removing TDF based upon its receipt in period 2 (period 3). Twenty-four-hour pharmacokinetic sampling was carried out on day 7 of period 1 and on day 14 of periods 2 and 3. Steady-state plasma amprenavir (APV) and tenofovir (TFV) pharmacokinetics were assessed by noncompartmental analysis and parameter values observed with each regimen were compared using geometric mean ratios with 90% confidence intervals. Results After TDF coadministration, APV geometric mean minimum concentration (C(min)), maximum concentration (C(max)), and area under the plasma concentration-time curve (AUC) increased by 31, 3 and 7% above values observed with unboosted FPV alone; they also increased by 31, 4 and 16% above values observed with FPV/RTV alone. TFV C(min), C(max) and AUC decreased by 12, 25 and 15% after FPV coadministration and by 9, 18 and 7% after FPV/RTV coadministration. No significant changes in RTV pharmacokinetics were observed. No differences were noted in adverse events among dosing periods. Conclusions In this evaluation of the interaction between FPV and TDF, increases in APV exposures and modest decreases in TFV exposures were observed. These were unlikely to be clinically significant.

Comparison of once-daily fosamprenavir boosted with either 100 or 200 mg of ritonavir, in combination with abacavir/lamivudine: 96-week results from COL100758

The long-term efficacy of once-daily (qd) fosamprenavir (FPV) 1400 mg boosted by ritonavir 100 mg (FPV/r100) has not been evaluated previously. A 96-week open-label, randomized, multicenter study compared the efficacy/safety of FPV/r100 with FPV 1400 mg boosted by ritonavir 200 mg qd (FPV/r200), plus abacavir/lamivudine 600 mg/300 mg qd, in antiretroviral-naive, HIV-infected patients with viral load (VL)> or =1000 copies/ml. Primary endpoints were proportion of patients achieving VL <400 copies/ml or discontinuing for drug-related reasons. In the intent-to-treat:exposed (ITT-E) population, missing = failure (M = F), and observed approaches were used to assess between-arm differences in VL responses by Cochran-Mantel-Haenszel test and CD4(+) count by Wilcoxon rank-sum test. One hundred and fifteen (115) patients enrolled, with 58 on FPV/r100 (median VL 4.7 log(10) copies/ml; CD4(+) count 259 cells/mm(3)) and 57 on FPV/r200 (median VL 4.9 log(10) copies/ml; CD4(+) count 179 cells/mm(3)). Fewer FPV/r100-treated patients discontinued treatment prematurely (12 vs. 24) and experienced virologic failure (5 vs. 8, none developing major protease inhibitor resistance mutations). At week 96, more FPV/r100-treated patients had VL <400 copies/ml [ITT-E,M = F: 78% (45/58) vs. 53% (30/57), p = 0.006; observed: 98% (45/46) vs. 94% (30/32)] and VL<50 copies/ml [ITT-E,M = F: 66% (38/58) vs. 53% (30/57); observed: 83% (38/46) vs. 94% (30/32)]. The FPV/r100 and FPV/r200 arms were similar at week 96 regarding median change from baseline in CD4(+) count (+265 vs. +260 cells/mm(3)) and total cholesterol (+33 vs. +35 mg/dl), and in total-cholesterol:HDL-cholesterol ratio (4.0 vs. 4.1) and type/frequency of treatment-related grade 2-4 adverse events, although FPV/r100 was associated with a lower elevation in triglycerides (+27 vs. +48 mg/dl). In conclusion, through 96 weeks, FPV/r100 was more effective and prompted less elevation in triglycerides than FPV/r200.

Simultaneous population pharmacokinetic model for lopinavir and ritonavir in HIV-infected adults

BACKGROUND

Lopinavir is a protease inhibitor indicated for the treatment of HIV infection. It is coformulated with low doses of ritonavir in order to enhance its pharmacokinetic profile. After oral administration, plasma concentrations of lopinavir can vary widely between different HIV-infected patients.

OBJECTIVE

To develop and validate a population pharmacokinetic model for lopinavir and ritonavir administered simultaneously in a population of HIV-infected adults. The model sought was to incorporate patient characteristics influencing variability in the drug concentration and the interaction between the two compounds.

METHODS

HIV-infected adults on stable therapy with oral lopinavir/ritonavir in routine clinical practice for at least 4 weeks were included. A concentration-time profile was obtained for each patient, and blood samples were collected immediately before and 1, 2, 4, 6, 8, 10 and 12 hours after a morning lopinavir/ritonavir dose. Lopinavir and ritonavir concentrations in plasma were determined by high-performance liquid chromatography. First, a population pharmacokinetic model was developed for lopinavir and for ritonavir separately. The pharmacokinetic parameters, interindividual variability and residual error were estimated, and the influence of different patient characteristics on the pharmacokinetics of lopinavir and ritonavir was explored. Then, a simultaneous model estimating the pharmacokinetics of both drugs together and incorporating the influence of ritonavir exposure on oral clearance (CL/F) of lopinavir was developed. Population analysis was performed using nonlinear mixed-effects modelling (NONMEM version V software). The bias and precision of the final model were assessed through Monte Carlo simulations and data-splitting techniques.

RESULTS

A total of 53 and 25 Caucasian patients were included in two datasets for model building and model validation, respectively. Lopinavir and ritonavir pharmacokinetics were described by one-compartment models with first-order absorption and elimination. The presence of advanced liver fibrosis decreased CL/F of ritonavir by nearly half. The volume of distribution after oral administration (Vd/F) and CL/F of lopinavir were reduced as alpha1-acid glycoprotein (AAG) concentrations increased. CL/F of lopinavir was inhibited by ritonavir concentrations following a maximum-effect model (maximum inhibition [Imax] = 1, concentration producing 50% of the I(max) [IC50] = 0.36 mg/L). The final model appropriately predicted plasma concentrations in the model-validation dataset with no systematic bias and adequate precision.

CONCLUSION

A population model to simultaneously describe the pharmacokinetics of lopinavir and ritonavir was developed and validated in HIV-infected patients. Bayesian estimates of the individual parameters of ritonavir and lopinavir could be useful to predict lopinavir exposure based on the presence of advanced liver fibrosis and the AAG concentration in an individual manner, with the aim of maximizing the chances of treatment success.

Fosamprenavir or atazanavir once daily boosted with ritonavir 100 mg, plus tenofovir/emtricitabine, for the initial treatment of HIV infection: 48-week results of ALERT

BACKGROUND

Once-daily (QD) ritonavir 100 mg-boosted fosamprenavir 1400 mg (FPV/r100) or atazanavir 300 mg (ATV/r100), plus tenofovir/emtricitabine (TDF/FTC) 300 mg/200 mg, have not been compared as initial antiretroviral treatment. To address this data gap, we conducted an open-label, multicenter 48-week study (ALERT) in 106 antiretroviral-naïve, HIV-infected patients (median HIV-1 RNA 4.9 log10 copies/mL; CD4+ count 191 cells/mm3) randomly assigned to the FPV/r100 or ATV/r100 regimens.

RESULTS

At baseline, the FPV/r100 or ATV/r100 arms were well-matched for HIV-1 RNA (median, 4.9 log10 copies/mL [both]), CD4+ count (mean, 176 vs 205 cells/mm3). At week 48, intent-to-treat: missing/discontinuation = failure analysis showed similar responses to FPV/r100 and ATV/r100 (HIV-1 RNA < 50 copies/mL: 75% (40/53) vs 83% (44/53), p = 0.34 [Cochran-Mantel-Haenszel test]); mean CD4+ count change-from-baseline: +170 vs +183 cells/mm3, p = 0.398 [Wilcoxon rank sum test]). Fasting total/LDL/HDL-cholesterol changes-from-baseline were also similar, although week 48 median fasting triglycerides were higher with FPV/r100 (150 vs 131 mg/dL). FPV/r100-treated patients experienced fewer treatment-related grade 2-4 adverse events (15% vs 57%), with differences driven by ATV-related hyperbilirubinemia. Three patients discontinued TDF/FTC because their GFR decreased to <50 mL/min.

CONCLUSION

The all-QD regimens of FPV/r100 and ATV/r100, plus TDF/FTC, provided similar virologic, CD4+ response, and fasting total/LDL/HDL-cholesterol changes through 48 weeks. Fewer FPV/r100-treated patients experienced treatment-related grade 2-4 adverse events.

Amprenavir and Lopinavir Pharmacokinetics following Coadministration of Amprenavir or Fosamprenavir with Lopinavir/Ritonavir, with or without Efavirenz

Background

Amprenavir (APV), fosamprenavir (FPV), lopinavir (LPV), ritonavir (RTV) and efavirenz (EFV) are to varying degrees substrates, inducers and inhibitors of CYP3A4. Coadministration of these drugs might result in complex pharmacokinetic drug-drug interactions.

Methods

Two prospective, open-label, non-randomized studies evaluated APV and LPV steady-state pharmacokinetics in HIV-infected patients on APV 750 mg twice daily + LPV/RTV 533/133 mg twice daily with EFV ( n=7) or without EFV ( n=12) + background nucleosides (Study 1) and after switching FPV 1,400 mg twice daily for APV ( n=10) (Study 2).

Results

In Study 1 EFV and non-EFV groups did not differ in APV minimum plasma concentration (Cmin; 1.10 versus 1.06μg/ml, P=0.89), area under the concentration-time curve (AUC; 17.46 versus 24.34μg•h/ml, P=0.22) or maximum concentration (Cmax; 2.61 versus 4.33 μg/ml, P=0.08); for LPV there was no difference in Cmin (median: 3.66 versus 6.18μg/ml, P=0.20), AUC (81.84 versus 93.75 μg•h/ml, P=0.37) or Cmax (10.36 versus 10.93 μg/ml, P=0.61). In Study 2, after switching from APV to FPV, APV Cmin increased by 58% (0.83 versus 1.30 μg/ml, P=0.0001), AUC by 76% (19.41 versus 34.24 μg•h/ml, P=0.0001), and Cmax by 75% (3.50 versus 6.14, P=0.001). Compared with historical controls, LPV and RTV pharmacokinetics were not changed. All treatment regimens were well tolerated. Seven of eight completers (88%) maintained HIV-1 RNA <400 copies/ml 12 weeks after the switch (1 lost to follow up).

Conclusions

EFV did not appear to significantly alter APV and LPV pharmacokinetic parameters in HIV-infected patients taking APV 750 mg twice daily + LPV 533/133 mg twice daily. Switching FPV 1,400 mg twice daily for APV 750 mg twice daily resulted in an increase in APV Cmin, AUC, and Cmax without changing LPV or RTV pharmacokinetics or overall tolerability.

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.

Interpretation of genotype and pharmacokinetics for resistance to fosamprenavir-ritonavir-based regimens in antiretroviral-experienced patients

In this study, named the Zephir study (Telzir-pharmacokinetics), 121 antiretroviral-experienced human immunodeficiency virus (HIV) patients failing on highly active antiretroviral therapy (HAART) were included in a prospective cohort and received a fosamprenavir-ritonavir (700 mg/100 mg twice a day)-based regimen. The impact of baseline HIV type 1 (HIV-1) mutations, pharmacokinetic (PK) parameters, and genotype inhibitory quotient (GIQ) on the virological response at week 12 (W12) was assessed. HIV reverse transcriptase and protease were sequenced at W0. The response at W12 was defined as<2.3 log10 HIV-1 RNA copies/ml or a virus load decrease of>or=1 log10 copies/ml. W4 amprenavir PK were determined by high-performance liquid chromatography. Patients had a median of nine previous treatments over 8 years. Median W0 values were as follows: 295 CD4+/microl, 4.4 log10 HIV-1 RNA copies/ml, and 6 protease- and 5 nucleotide reverse transcription inhibitor-related mutations. Respective values for minimum concentration of drug in serum (Cmin) and area under the concentration-time curve (AUC) from 0 to 24 h were 1,400 ng/ml and 35 mg.h/ml. At W12, 52% of the patients were successes, with a median decrease of -0.7 log10 HIV-1 RNA copies/ml. The Zephir mutation score included 12 IAS protease mutations associated with poorer virological response: L10I/F/R/V, L33F, M36I, M46I/L, I54L/M/T/V, I62V, L63P, A71I/L/V/T, G73A/C/F/T, V82A/F/S/T, I84V, L90M, and polymorphism mutations I13V, L19I, K55R, and L89M. Comparing<4 versus>or=4 mutations, HIV-1 RNA decreases were -2.3 log10 copies/ml versus -0.1 log10 copies/ml (P<10(-4)) with 93% versus 19% successes (P<10(-4)), respectively. This score predicted W12 failure with 94% sensitivity, versus 31% for the ANRS 2005 algorithm. Cmin (<1,600 ng/ml), AUC (<40 mg.h/ml), and GIQ (<300) values were associated with failure (all P values were <10(-4)). The need to test genotype-based algorithms using different patient databases before their implementation in clinical practice is highlighted. Specific mutations, PK and GIQ, provide relevant information for monitoring fosamprenavir-ritonavir-based HAART.

Are population pharmacokinetic and/or pharmacodynamic models adequately evaluated? A survey of the literature from 2002 to 2004

Model evaluation is an important issue in population analyses. We aimed to perform a systematic review of all population pharmacokinetic and/or pharmacodynamic analyses published between 2002 and 2004 to survey the current methods used to evaluate models and to assess whether those models were adequately evaluated. We selected 324 articles in MEDLINE using defined key words and built a data abstraction form composed of a checklist of items to extract the relevant information from these articles with respect to model evaluation. In the data abstraction form, evaluation methods were divided into three subsections: basic internal methods (goodness-of-fit [GOF] plots, uncertainty in parameter estimates and model sensitivity), advanced internal methods (data splitting, resampling techniques and Monte Carlo simulations) and external model evaluation. Basic internal evaluation was the most frequently described method in the reports: 65% of the models involved GOF evaluation. Standard errors or confidence intervals were reported for 50% of fixed effects but only for 22% of random effects. Advanced internal methods were used in approximately 25% of models: data splitting was more often used than bootstrap and cross-validation; simulations were used in 6% of models to evaluate models by a visual predictive check or by a posterior predictive check. External evaluation was performed in only 7% of models. Using the subjective synthesis of model evaluation for each article, we judged the models to be adequately evaluated in 28% of pharmacokinetic models and 26% of pharmacodynamic models. Basic internal evaluation was preferred to more advanced methods, probably because the former is performed easily with most software. We also noticed that when the aim of modelling was predictive, advanced internal methods or more stringent methods were more often used.

Recent advances in pharmacokinetic modeling

A major part of the science of pharmacokinetics is the modeling of the underlying processes that contribute to drug disposition. The purpose of pharmacokinetic models is to summarize the knowledge gained in preclinical and clinical studies at various stages in drug development and to rationally guide future studies with the use of adequately predictive models. This review highlights a variety of recent advances in mechanistic pharmacokinetic modeling. It is aimed at a broad audience, and hence, an attempt was made to maintain a balance between technical information and practical applications of pharmacokinetic modeling. It is hoped that drug researchers from all disciplines would be able to get a flavor of the function and capacity of pharmacokinetic modelers and their contribution to drug development. While this review is not intended to be a technical reference on modeling approaches, the roles of statistical applications and population methodologies are discussed where appropriate.

Plasma amprenavir pharmacokinetics and tolerability following administration of 1,400 milligrams of fosamprenavir once daily in combination with either 100 or 200 milligrams of ritonavir in healthy volunteers

Once-daily (QD) fosamprenavir (FPV) at 1,400 mg boosted with low-dose ritonavir (RTV) at 200 mg is effective when it is used in combination regimens for the initial treatment of human immunodeficiency virus infection. Whether a lower RTV boosting dose (i.e., 100 mg QD) could ensure sufficient amprenavir (APV) concentrations with improved safety/tolerability is unknown. This randomized, two 14-day-period, crossover pharmacokinetic study compared the steady-state plasma APV concentrations, safety, and tolerability of FPV at 1,400 mg QD boosted with either 100 mg or 200 mg of RTV QD in 36 healthy volunteers. Geometric least-square (GLS) mean ratios and the associated 90% confidence intervals (CIs) were estimated for plasma APV maximum plasma concentrations (Cmax), the area under the plasma concentration-time curve over the dosing period (AUC0-tau), and trough concentrations (Ctau) during each dosing period. Equivalence between regimens (90% CIs of GLS mean ratios, 0.80 to 1.25) was observed for the plasma APV AUC0-tau (GLS mean ratio, 0.90 [90% CI, 0.84 to 0.96]) and Cmax (0.97 [90% CI, 0.91 to 1.04]). The APV Ctau was 38% lower with RTV at 100 mg QD than with RTV at 200 mg QD (GLS mean ratio, 0.62 [90% CI, 0.55 to 0.69]) but remained sixfold higher than the protein-corrected 50% inhibitory concentration for wild-type virus, with the lowest APV Ctau observed during the 100-mg QD period being nearly threefold higher. The GLS mean APV Ctau was 2.5 times higher than the historical Ctau for unboosted FPV at 1,400 mg twice daily. Fewer clinical adverse drug events and smaller increases in triglyceride levels were observed with the RTV 100-mg QD regimen. Clinical trials evaluating the efficacy and safety of FPV at 1,400 mg QD boosted by RTV at 100 mg QD are now under way with antiretroviral therapy-naïve patients.

Determination of Amprenavir Total and Unbound Concentrations in Plasma by High-Performance Liquid Chromatography and Ultrafiltration

Amprenavir is an HIV-1 protease inhibitor with high protein binding (90%) in human plasma. This study was designed to develop an assay to measure the concentration of unbound amprenavir, to study variation with time in patients, and to investigate whether ritonavir and lopinavir, other protease inhibitors that could be combined, interact with amprenavir protein binding in vitro. A reverse-phase high-performance liquid chromatography assay to UV detection was developed and validated to measure total amprenavir in plasma, and this method was adapted to quantitate low concentrations of unbound amprenavir in ultrafiltrate aqueous fluid. Equilibrium dialysis and ultrafiltration were used and compared with separate unbound fraction. The latter method was easier to use and was, therefore, subsequently adopted. In 10 patients who received amprenavir 600 mg bid combined with ritonavir, mean amprenavir free-fraction in plasma was 8.6% (range, 4.4-20%). When added to pooled human plasmas at concentrations close to those found in treated patients, the unbound amprenavir fraction was increased in the presence of lopinavir, but remained unaffected by ritonavir. It remains to be seen whether measurement of unbound concentrations, rather than total concentrations, could improve therapeutic drug monitoring.

Monte Carlo estimates of natural variation in HIV infection

We describe a Monte Carlo simulation of the within-host dynamics of human immunodeficiency virus 1 (HIV-1). The simulation proceeds at the level of individual T-cells and virions in a small volume of plasma, thus capturing the inherent stochasticity in viral replication, mutation and T-cell infection. When cell lifetimes are distributed exponentially in the Monte Carlo approach, our simulation results are in perfect agreement with the predictions of the corresponding systems of differential equations from the literature. The Monte Carlo model, however, uniquely allows us to estimate the natural variability in important parameters such as the T-cell count, viral load, and the basic reproductive ratio, in both the presence and absence of drug therapy. The simulation also yields the probability that an infection will not become established after exposure to a viral inoculum of a given size. Finally, we extend the Monte Carlo approach to include distributions of cell lifetimes that are less-dispersed than exponential.

Use of nonlinear mixed effect modeling for the intestinal absorption data: Application to ritonavir in the rat

The aim of this study is to investigate in situ the mechanisms involved in the gastrointestinal absorption of ritonavir in the rat, as an animal model for preclinical studies of anti-HIV agents in vivo. Four ritonavir solutions (40, 27, 13 and 7 microM) in the presence of 1% dimethylsulfoxide (DMSO) were perfused in the small intestine of anaesthetised rats. Effects of DMSO on the intestinal permeability were investigated using solutions containing antipyrine 1.33 mM and ritonavir 7 microM with and without 1% of DMSO. Antipyrine and ritonavir transport was not modified in the presence of 1% of DMSO. The population pharmacokinetic parameters of the ritonavir intestinal transport were obtained by means of nonlinear mixed effect modelling approach according to a nonlinear absorption and nonlinear secretion. The absorption and secretion kinetic parameters for ritonavir were: Vm=47.6 microM/h; Km=8.77 microM; Vms=3.66 microM/h and Kms=0 microM. The interindividual variability found to ritonavir Vm 13.1%, and the residual variability was 8.98%. The Kms value support the saturation of the carrier at the range of concentrations of ritonavir assayed. The interindividual variability value of the Vm could explain, at least in part, the variability in absorption rate constants observed.

Application and impact of population pharmacokinetics in the assessment of antiretroviral pharmacotherapy

Population pharmacokinetics has been an important technique used to explore and define relevant sources of variation in drug exposure and response in patient populations. This has been especially true in the area of antiretroviral therapy where the assurance of adequate and sustained drug exposure of multiple agents is highly correlated with therapeutic success. Population pharmacokinetic analyses across the four drug classes and 20 US FDA-approved products used to treat HIV have been published to date. The published reports were predominantly based on actual clinical trials conducted in HIV-infected patients with one or more agents administered. Modelling and simulation approaches have been used in the evaluation of antiretroviral agent outcomes incorporating problematic design and analysis factors such as sparse plasma sampling, data imbalance and censored data. Additional benefits of population modelling approaches applied to the investigation of antiretroviral agents include the ability to assess dosing compliance, understanding and quantifying drug-drug interactions in order to select dosing regimens and the screening of new drug candidates. Pharmacokinetic/pharmacodynamic models have been used to characterise the relationship between drug exposure and virological and immunological response, and to predict clinical outcome. These models offer the best opportunity for individualising and optimising patient therapy, particularly when adjusted for adherence/compliance. The impact of population pharmacokinetics in the area of antiretroviral therapy can be directly assessed by its role in the validation of surrogate markers such as viral RNA load, therapeutic drug monitoring and the management of individual patient outcomes via exposure-toxicity relationships. Each of these population pharmacokinetic outcomes has contributed to the current regulatory environment, specifically in the area of accelerated approval of new antiretroviral agents.

Development and validation of a population pharmacokinetic model for ritonavir used as a booster or as an antiviral agent in HIV-1-infected patients

AIMS

The aim of this study was to develop and validate a population pharmacokinetic model of ritonavir, used as an antiviral agent or as a booster, in a large patient population and to identify factors influencing its pharmacokinetics.

METHODS

Ambulatory HIV-1-infected patients from the outpatient clinic of the Slotervaart Hospital, Amsterdam, the Netherlands, who were being treated with a ritonavir-containing regimen were included. During regular visits, blood samples were collected for the determination of ritonavir plasma concentrations and several clinical chemistry parameters. Furthermore, complete pharmacokinetic curves were available in some patients. Single and multiple compartment models with zero-order and first-order absorption, with and without absorption lag-time, with linear and nonlinear elimination were tested, using nonlinear mixed effect modelling (NONMEM). Pharmacokinetic parameters and interindividual, interoccasion and residual variability were estimated. In addition, the influence of several factors (e.g. patient characteristics, comedication) on the pharmacokinetics of ritonavir was explored.

RESULTS

From 186 patients 505 ritonavir plasma concentrations at a single time-point and 55 full pharmacokinetic profiles were available, resulting in a database of 1228 plasma ritonavir concentrations. In total 62% of the patients used ritonavir as a booster of their protease inhibitor containing antiretroviral regimen. First order absorption in combination with one-compartment disposition best described the pharmacokinetics of ritonavir. Clearance, volume of distribution and absorption rate constant were 10.5 l h(-1) (95% prediction interval (95% PI) 9.38-11.7), 96.6 l (95% PI 67.2-121) and 0.871 h(-1) (95% PI 0.429-1.47), respectively, with 38.3%, 80.0% and 169% interindividual variability, respectively. The interoccasion variability in the apparent bioavailability was 59.1%. The concomitant use of lopinavir resulted in a 2.7-fold increase in the clearance of ritonavir (P value < 0.001). No patients characteristics influenced the pharmacokinetics of ritonavir.

CONCLUSIONS

The pharmacokinetic parameters of ritonavir were adequately described by our population pharmacokinetic model. Concomitant use of the protease inhibitor lopinavir strongly influenced the pharmacokinetics of ritonavir. The model has been validated and can be used for further investigation of the interaction between ritonavir and other protease inhibitors.

Pharmacologic Perspectives for Once-Daily Antiretroviral Therapy

OBJECTIVE

To contrast available once-daily antiretroviral agents and combinations of these agents from a clinical pharmacologic viewpoint.

DATA SOURCES

Data were extracted from publications and major HIV conference proceedings cited in MEDLINE (1966–March 2004) using the search terms antiretroviral therapy, combination therapy, once-daily therapy, and pharmacokinetics. Additional references were obtained from the bibliographies of these sources.

STUDY SELECTION AND DATA EXTRACTION

Information pertaining to pharmacologic perspectives for once-daily antiretroviral agents was selected.

DATA SYNTHESIS

Maximal and durable suppression of plasma HIV RNA, the principal goal of therapy, depends on the intrinsic antiviral activity of the antiretroviral regimen. A favorable tolerability/toxicity profile is also fundamentally important. All once-daily agents exhibit some pharmacologic limitations or lack adequate long-term follow-up. Of available agents, efavirenz has a long and distinguished efficacy record, with reasonable safety and moderate tolerability. Lamivudine, and newer agents such as atazanavir (or atazanavir/ritonavir), emtricitabine, fosamprenavir/ritonavir, and tenofovir, may offer pharmacologic advantages in the current state of once-daily therapy. Important considerations exist for coadministering once-daily agents including drug–drug interactions, drug–food incompatibilities, and synergistic toxicities. Few controlled studies have compared once-daily regimens with conventional regimens.

CONCLUSIONS

Progress has been made toward once-daily therapy, but more clinical experience with available agents is needed, including comparative studies of entirely once-daily regimens versus conventional regimens. Limitations of currently available agents signify a need for improved antiretroviral utilization or new alternative agents.

Steady-State Pharmacokinetics of Saquinavir Hard-Gel/Ritonavir/Fosamprenavir in HIV-1???Infected Patients

BACKGROUND

In vitro synergy and complementary resistance profiles provide a strong rationale for combining fosamprenavir with saquinavir as part of a potent double-boosted protease inhibitor regimen. This study evaluated the steady-state pharmacokinetics of saquinavir 1000 mg twice daily (bid) and fosamprenavir 700 mg bid administered with 2 different doses of ritonavir (100 and 200 mg bid) in HIV-1-infected subjects.

METHODS

On day 1, 12-hour pharmacokinetic profiles for saquinavir/ritonavir (1000/100 mg bid) were obtained for 18 subjects. All subjects were receiving ongoing treatment with a saquinavir/ritonavir-containing regimen. Fosamprenavir 700 mg bid was then added to the regimen, and pharmacokinetic sampling was repeated for all 3 agents at day 11. The ritonavir daily dose was then increased to 200 mg bid, and a 3rd pharmacokinetic profile was obtained at day 22.

RESULTS

The coadministration of fosamprenavir 700 mg bid with saquinavir/ritonavir 1000/100 mg bid resulted in a statistically nonsignificant decrease in saquinavir concentrations (by 14, 9, and 24%, for saquinavir area under the concentration-time curve [AUC]0-12, C(max), and C(trough), respectively). This was compensated for by an increased ritonavir dose of 200 mg bid, which resulted in a statistically nonsignificant increase in saquinavir exposure compared with baseline. Amprenavir levels did not appear to be significantly influenced by coadministration of saquinavir with fosamprenavir. Fosamprenavir significantly reduced ritonavir exposure, but the increased ritonavir dose compensated for this interaction.

CONCLUSIONS

Our findings showed that saquinavir/ritonavir/fosamprenavir was well tolerated over the study period. Saquinavir plasma concentrations were slightly lowered by the addition of fosamprenavir to the regimen. However, the addition of a further 100 mg ritonavir bid restored the small and insignificant decrease.

A 42-Week Open-Label Study to Assess the Pharmacokinetics, Antiretroviral Activity, and Safety of Amprenavir or Amprenavir plus Ritonavir in Combination with Abacavir and Lamivudine for Treatment of HIV-Infected Patients

The pharmacokinetics, antiviral activity, and safety of an amprenavir-ritonavir (APV-RTV) 600/100 mg b.i.d. regimen and an APV-RTV 1200/200 mg q.d. regimen were studied in a human immunodeficiency virus (HIV)-infected population. The geometric least-square mean ratio (90% confidence interval) of steady-state trough concentrations, compared with that of the amprenavir 1200 mg b.i.d. regimen, was 6.08 (4.94-7.49) for the twice-daily APV-RTV regimen, and it was 4.19 (2.90-6.08) for the daily APV-RTV regimen. The regimens were well tolerated, which supports APV-RTV as an option for twice-daily or daily therapy for HIV.

Pharmacokinetic enhancement of protease inhibitor therapy

Combination antiretroviral therapy with two or more protease inhibitors has become the standard of care in the treatment of HIV infection. Dual protein inhibitor (PI) regimens, such as lopinavir/ritonavir, are commonly used as initial PI therapy. As viral resistance increases and the development of mechanistically novel protease inhibitors decreases, clinicians turn to ritonavir-enhanced dual PI therapy to treat salvage patients. Potency of these combination regimens is increased while pill burden, food restrictions and often, side effects are decreased. These clinical advantages result from the enhancement of their pharmacological properties, including alterations in the absorption and metabolism process. Alterations in the absorption and metabolism of protease inhibitors when co-administered with a cytochrome P450 (CYP) enzyme inhibitor, such as low dose ritonavir, are reflected by impressive changes in pharmacokinetic parameters. For example, the addition of ritonavir 100 or 200 mg to saquinavir 1200-1800 mg has been shown to increase saquinavir area under the concentration-time curve (AUC) by approximately 300-800% compared with saquinavir alone. The ability of ritonavir to increase plasma trough concentrations (C(min)) of concomitantly administered PIs is perhaps the greatest clinical benefit of dual or ritonavir-enhanced dual PI therapy since inadequate concentrations of antiretrovirals may support long term antiretroviral resistance. For example, lopinavir 400mg alone in healthy volunteers produced plasma concentrations that briefly exceeded the concentration required to inhibit 50% of viral replication (IC(50)). Yet, when low doses of ritonavir were added, C(min) values were 50- to 100-fold greater than the concentration required to produce 50% of the maximum effect for wild-type HIV (EC(50)). The following manuscript will discuss the rationale for combining protease inhibitors and will review pertinent pharmacokinetic and clinical data on these combination regimens.

In vitro-in vivo model for evaluating the antiviral activity of amprenavir in combination with ritonavir administered at 600 and 100 milligrams, respectively, every 12 hours

The study objective was to evaluate the pharmacodynamics of amprenavir in an in vitro system, develop an exposure target for maximal viral suppression, and determine the likelihood of target attainment based on the pharmacokinetics of amprenavir and ritonavir in human immunodeficiency virus (HIV)-infected patients. Population pharmacokinetic data were obtained from 13 HIV-infected patients receiving amprenavir and ritonavir in doses of 600 and 100 mg, respectively, every 12 h. A 2,500-subject Monte Carlo simulation was performed. Target attainment was also estimated for a target derived from clinical data. Maximal viral suppression (in vitro) was achieved when amprenavir free-drug concentrations remained greater than four times the 50% effective concentration (EC(50)) for 80% of the dosing interval. At an amprenavir EC(50) of 0.03 microM, the likelihood of target attainment is 97.4%. For reduced-susceptibility isolates for which the EC(50)s are 0.05 and 0.08 microM, target attainment is 91.0 and 75.8%, respectively. For the clinical target of a trough concentration/EC(50) ratio of 5, the target attainment rates were similar. Treatment with amprenavir and ritonavir at doses of 600 and 100 mg, respectively, twice a day provides excellent suppression of wild-type isolates and reduced-susceptibility isolates up to an EC(50) of 0.05 micro M. Even at 0.12 microM, target attainment likelihood exceeds 50%, making this an option for patients with extensive exposure to protease inhibitors when this treatment is used with additional active antiretroviral agents.

Pharmacokinetics and safety of GW433908 and ritonavir, with and without efavirenz, in healthy volunteers

OBJECTIVE

To evaluate the safety and pharmacokinetic interaction between GW433908, ritonavir (RTV), and efavirenz (EFV).

METHODS

In period 1, subjects received either a once daily (QD) regimen of GW433908 1395 mg + RTV 200 mg (Study 1) or a twice daily (bid) regimen of GW433908 700 mg + RTV 100 mg (Study 2) for 14 days. In period 2, subjects received EFV 600 mg QD with either the same GW433908 + RTV regimen as in period 1 (arm 1) or with a GW433908 + RTV regimen that included an additional 100 mg of RTV (arm 2) for 14 days. Amprenavir (APV) pharmacokinetic sampling and safety assessments were performed on the last day of each period.

RESULTS

Plasma APV exposure was not significantly altered when EFV was coadministered with GW433908 700 mg twice daily (BID) + RTV 100 mg BID. Plasma APV exposure was decreased when EFV was coadministered with GW433908 1395 mg QD + RTV 200 mg QD. However, administration of EFV with GW433908 1395 mg QD + RTV 300 mg QD (i.e., adding an extra 100 mg of RTV) was able to negate this interaction. Adverse events were consistent with prior data for each of the separate agents.

CONCLUSION

When EFV is coadministered with the GW433908 700 mg + RTV 100 mg BID regimen, no dosage adjustment is recommended. However, when EFV is coadministered with the GW433908 1400 mg + RTV 200 mg QD regimen, an increase to RTV 300 mg QD is needed to maintain plasma APV exposure.

New anti-HIV protease inhibitors provide more treatment options

For several years, protease inhibitor (PI)-containing antiretroviral treatment (ART) regimens have demonstrated long-term virologic and immunologic benefits and good durability of response. However, first-generation PIs have been associated with high pill burdens, gastrointestinal side effects, perturbation of lipid levels and glucose metabolism, and, in some cases, food and hydration requirements. Coadministration of low-dose ritonavir with PIs has enhanced their pharmacokinetic profile (lower doses, fewer pills, less frequent dosing schedules) and pharmacodynamics (increased potency, especially against resistant viruses) but has also been associated with increases in lipid levels. Two new PIs, atazanavir and 908 (fosamprenavir), may offer salvageable PI treatment options and may also address issues of potency, tolerability, and convenience by requiring fewer pills and causing fewer lipid and glucose perturbations than current PI options. The availability of these novel PIs may improve longterm treatment options for many patients.

A Once-Daily Haart Regimen Containing Indinavir + Ritonavir plus One Or Two Nucleoside Reverse Transcriptase Inhibitors (Pipo Study)

Introduction

There is an increased interest in developing once-daily regimens for the treatment of HIV-infected patients. A Phase II study was conducted to investigate the pharmacokinetics, and short-term safety and efficacy of an indinavir/ritonavir combination as part of a once-daily regimen.

Methods

HIV-infected patients with either proven poor compliance to HAART regimens in the past or an anticipated poor compliance to such a regimen in the future were eligible for this study. They received a once-daily regimen consisting of indinavir 1200 mg, ritonavir 400 mg, and one or two nucleoside reverse transcriptase inhibitors (NRTIs), also administered once daily with food. A 24 h pharmacokinetic profile was constructed in a subset of patients. Short-term safety and efficacy were evaluated at 4, 12 and 24 weeks after initiation of treatment.

Results

A total of 64 patients were included in this study, of whom 27 (42.2%) were treatment-naive. The geometric mean (+95% CI) of indinavir AUC0–24h, Cmax and Cmin as determined in an unselected group of 16 patients were 84.9 (69.7–103.5) mg/l.h, 12.0 (10.2–14.1) mg/l and 0.15 (0.09–0.26) mg/l, respectively. A large interpatient variability was observed, with five out of the 16 subjects having a Cmin value below the minimum effective concentration of 0.10 mg/l. During the 24 weeks of follow-up nine patients (14.1%) discontinued study medication, two due to medication-related toxicity. Gastrointestinal adverse events were reported most frequently (50.0%), followed by skin effects (45.3%), joint pain (9.4%) and urological complaints (7.8%). No patient developed nephrolithiasis. The median (+interquartile range) serum creatinine level in the 64 patients increased slightly from 74 (63–88) μmol/l to 79 (66–92) μmol/l during the 24 weeks of follow-up. One new patient reached a grade 1 elevation in serum creatinine, which normalized during the follow-up; five other patients with elevated serum creatinine at baseline remained stable. During the 24 weeks of follow-up, the proportion of patients with a viral load <500 copies/ml increased from 35.1% at baseline to 71.4% (ITT NC=F analysis) or 83.3% (OT analysis), and from 0% at baseline to 76.2% (ITT NC=F analysis) or 100.0% (OT analysis) in treatment-experienced and -naive patients, respectively. This was accompanied by a mean increase in CD4 cell count of 52 and 220 cells/mm3 in these two sub-groups, respectively.

Conclusion

The 24-week follow-up data of this study indicate favourable pharmacokinetics of an indinavir/ritonavir 1200/400 mg combination as part of a once-daily regimen consisting also of one or two NRTIs. Short-term safety and efficacy were also satisfactory. Long-term follow up is planned to evaluate the durability of these results.

The role of pharmacological enhancement in protease inhibitor-based highly active antiretroviral therapy

Having changed the landscape in the treatment of HIV infection, the functional efficacy of current protease inhibitors (PIs) remains limited by their pharmacokinetic and pharmacodynamic profiles. Complex metabolism by the cytochrome P450 system (particularly the 3A4 isoenzyme), action of membrane drug transporter elements (such as P-glycoprotein and multi-drug resistance-associated proteins) and activation of the nuclear receptor steroid xenobiotic receptor may alter exposures and compromise the antiretroviral activity of these drugs. These factors, as well as inadequate adherence, can facilitate the emergence of PI resistance and lead to regimen failure. Coadministration of ritonavir can enhance exposures of a primary PI by inhibiting CYP3A4 metabolism, P-glycoprotein activity and multi-drug resistance protein-1-mediated efflux. Adding ritonavir, however, is not without cost. Dyslipidaemia (possibly increasing the risk of cardiovascular events), gastrointestinal intolerance, multiple drug-to-drug interactions and activation of steroid xenobiotic receptor can all result and must be balanced against the pharmacokinetic improvement rendered by the addition of ritonavir. Understanding the pharmacological origins for the variations in exposures of PIs, both between and within patients, is important for the successful use of these agents.

Pharmacokinetics, food intake requirements and tolerability of once-daily combinations of nelfinavir and low-dose ritonavir in healthy volunteers

AIMS

This study was performed to evaluate the steady-state pharmacokinetics, food intake requirements and short-term tolerability of once-daily combinations of nelfinavir and low-dose ritonavir.

METHODS

Twenty-seven healthy volunteers were randomized over three groups to receive a once-daily regimen of nelfinavir/ritonavir 2,000/200 mg (group 1), 2,000/400 mg (group 2) or 2,500/200 mg (group 3) with food for 14 days. Pharmacokinetic parameters for nelfinavir and its active metabolite M8 were assessed on study days 15 and 16, after administration of the regimens with a full (610 kcal) or light (271 kcal) breakfast, respectively.

RESULTS

Pharmacokinetic data were evaluable for eight volunteers in group 1, eight in group 2 and four in group 3. Administration of nelfinavir/ritonavir with a full breakfast resulted in geometric mean (GM) nelfinavir AUC(24h) values of 76.8, 51.3, and 61.9 h*mg/l in group 1, 2 and 3, respectively. GM 24-h Cmin concentrations of nelfinavir were 0.76 mg l(-1), 0.43 mg l(-1) and 0.47 mg l(-1), respectively. Co-administration of ritonavir increased M8 concentrations more than nelfinavir concentrations, resulting in GM AUC(24h) and Cmin values for nelfinavir plus M8 that were higher than or comparable to reference values for the approved regimen of nelfinavir (1,250 mg BID without ritonavir). In the 2,000/200 mg group, seven out of eight subjects had a Cmin value of nelfinavir plus M8 above a threshold of 1.0 mg l-1. Administration of the combinations with a light breakfast resulted in significant decreases in the AUC(24h) and Cmin of nelfinavir and nelfinavir plus M8, compared with intake with a full breakfast. For the Cmin of nelfinavir plus M8, the GM ratio (light/full breakfast) was 0.76 (90% confidence interval 0.67-0.86, participants from all groups combined). Short-term tolerability was satisfactory, apart from a higher than expected incidence of mild rash (12%).

CONCLUSIONS

Administration of nelfinavir in a once-daily regimen appears feasible. A nelfinavir/ritonavir 2,000/200 mg combination appears appropriate for further evaluation. Once-daily nelfinavir/ritonavir should be taken with a meal containing at least 600 kcal.

Genotypic and phenotypic cross-resistance patterns to lopinavir and amprenavir in protease inhibitor-experienced patients with HIV viremia

Genotypic correlates of reduced phenotypic susceptibility to amprenavir (APV) and lopinavir (LPV) were examined in 271 HIV isolates from 207 protease inhibitor (PI)-experienced subjects. All samples were from LPV-naive subjects; two were from APV-experienced subjects. Using a fold resistance (FR) of <2.5, 179 (66%) were APV susceptible. Using FRs of <2.5 and <10, 107 (39%) and 194 (72%), respectively, were LPV susceptible. The I84V mutation was the strongest APV resistance marker in PI-experienced subjects in both univariate and multivariate analyses, with an increased relative incidence (RI) of 6.9 with >2.5 FR. Mutations L10I (RI, 1.7), M46I (RI, 2.3), and L90M (RI, 1.9, but 65% linked with the I84V) were associated with decreased APV susceptibility in the univariate analysis (p < 0.001). Mutations L10I, G48V, I54T, I54V, and V82A were significantly associated with decreased LPV susceptibility (p < 0.001 for each) and had increased RIs of 2.2, 4.4, 13, 4.6, and 3.2, respectively. Decreased susceptibility to LPV (FR, >or=10) was significantly associated with prior exposure to the following PIs: ritonavir (RTV) (p < 0.001), saquinavir (SQV) (p < 0.001), nelfinavir (NFV) (p = 0.008), and indinavir (IDV) (p = 0.028). Decreased APV susceptibility (FR, >or=2.5) was significantly associated with prior exposure to RTV (p = 0.009), NFV (p = 0.003), and IDV (p = 0.021) but not with prior SQV (p = 0.103). These results suggest that APV and LPV have different cross-resistance mutation patterns that may help determine choice of PI therapy after therapy failure.