Today, a significant proportion (20–30%) of HIV-infected patients experience treatment failure despite the effectiveness of multiple drug combination therapy [1–3]. Reasons for treatment failure are multiple and include suboptimal adherence to the treatment, suboptimal drug exposure, and development of mutants with amino acid substitutions that confer both resistance to the drugs in the current regimen and variable levels of cross-resistance to other members of the same drug class . Perhaps, one of the main reasons for virological failure within the HIV-infected population is the presence of a high proportion of patients who were administered suboptimal treatments [i.e., mono- or dual-drug antiretroviral therapy (ART)] available before the introduction of HAART into clinical practice.
Developing effective combination therapies for treatment-experienced patients, especially those harbouring viruses resistant to three or four ART classes, is one of the greatest challenges in the clinical management of HIV-infected patients. The major barrier is the unavailability of a sufficient number of new drugs to construct the new regimen. Addition of only one active drug to a failing regimen is generally not suggested because of the high possibility of developing resistance to that drug .
Not surprisingly, the more active the drugs in a regimen are the greater the likelihood of virological response .
Therefore, significant improvement in the management of treatment-experienced patients with no current therapeutic options may result from the development of new ART agents and ideally from the administration of ART regimens containing combinations of these agents. This is rarely possible. Current drug development programmes involve testing new agents against placebo or existing licenced therapies, as often required by regulatory authorities, rather than studying the potential effects of more than one investigational agent. Furthermore, drug–drug interactions  and difficulty in distinguishing the side effect profiles of more than one investigational agent adds to the complexity of combining experimental agents for the treatment of resistant viruses.
Etravirine (TMC125) is a novel nonnucleoside reverse transcriptase inhibitor (NNRTI) with potent in-vitro activity against both wild-type HIV and HIV resistant to currently approved NNRTI .
Etravirine is currently in phase III clinical trials and has proven to have potent short-term activity and be generally well tolerated in both ART-naive subjects and in those with NNRTI resistance [7–10]. These studies were conducted with a formulation of etravirine that generated a high-pill burden. This issue has now been resolved, and a compact dose (F060 formulation) of 200 mg twice daily, which provides a comparable exposure to the 800 mg twice daily dose studied, is available .
The oral bioavailability of etravirine (F060 formulation) has been recently shown to be less affected by food intake than the previously studied formulations. However, lower exposures have been observed in a fasted state; therefore, etravirine should be taken with a meal . Etravirine is mainly metabolized by cytochrome P450 3A4 and 2C isoenzymes; it induces 3A4 and is eliminated as glucuronide conjugates. It is characterized by an elimination half life of approximately 30–40 h .
The protease inhibitor (PI) darunavir (TMC114) in combination with low-dose ritonavir (600/100 mg twice daily) has shown potent antiviral activity and good tolerability in three-class-experienced HIV-infected individuals with limited or no treatment options .
As with the other PI drugs, darunavir is metabolized via cytochrome P450 3A4 . Therefore, its coadministration with low-dose ritonavir twice daily ensures the achievement and maintenance of a therapeutic darunavir plasma concentration over the dosing interval . Darunavir, when combined with low-dose ritonavir, is characterized by a half life of approximately 15 h and its exposure increases by 30% in the presence of food [14,15].
The aims of the present study were in the context of a salvage regimen for HIV-1-positive patients who had failed several lines of ART therapy and had no viable treatment options; the study examined the pharmacokinetics of the two investigational drugs, etravirine and darunavir/ritonavir, when dosed together and assessed the safety and efficacy of such a combination plus nucleoside analogues with or without enfuvirtide over 24 weeks.
After providing written informed consent, HIV-1-infected patients were screened for 14 days before the study started. These were adults (aged 18–65 years) of either sex who had been failing several successive lines of ART in the past and who demonstrated extensive resistance to the majority of currently available drugs.
None of the patients was permitted to take another NNRTI or PI during the study period, any hepatic enzyme inducer or inhibitor within 14 days of study entry, or any other investigational drug or antineoplastic radiotherapy/chemotherapy other than local skin radiotherapy within 12 weeks of starting study medication. The patients were allowed to receive other nonstudy medications provided that, in the opinion of the study investigators, these did not have any significant interaction with the study drugs.
This study was an open-label, single-arm investigation carried out at the Pharmacokinetic Unit of the St Stephen's Centre, Chelsea and Westminster Hospital, London, UK. The study protocol was reviewed and approved by the Riverside Research Ethics Committee.
At screening, subjects had a clinical assessment and routine laboratory tests performed. Plasma viral loads were measured by quantitative RNA polymerase chain reaction (Roche Taqman HIV-1 test, version 1.5; Roche Molecular Systems, Branchburg, New Jersey, USA) and T lymphocyte subset counts and percentages were determined by flow-cytometric testing.
Phenotypic and genotypic determinations were performed using the Antivirogram and Virtual Phenotype tests, respectively (Virco BVBA, Mechelen, Belgium). Primary PI mutations were identified according to the International AIDS Society Guidelines 2005 . Phenotypic results were expressed as fold change in IC50 (concentration of drug needed to inhibit 50% of viral replication) relative to a reference wild-type virus. In-vitro IC50 values for HIV wild type for efavirenz, nevirapine and etravirine were 3.4, 5.2 and 1.0, respectively. Those for saquinavir, amprenavir, lopinavir, atazanavir, tipranavir, and darunavir were 1.7, 1.8, 1.6, 2.0, 1.6 and 2.8, respectively.
The safety and tolerability of study medications were evaluated throughout the study on the basis of clinical adverse events (using the US Division of AIDS table for grading the severity of adult and paediatric adverse events to characterize abnormal findings; published December, 2004), clinical laboratory tests, vital signs, electrocardiograph and physical examinations. The severity of adverse events and the investigator's assessment of their causality to etravirine and darunavir/ritonavir were recorded. Viral load and lymphocyte subset determinations were performed 2 and 4 weeks following study initiation and every 8 weeks after that.
On day 1, all subjects received etravirine (F060 formulation) 200 mg, darunavir 600 mg and ritonavir 100 mg, all twice daily, in combination with two or more nucleotide reverse transcriptase inhibitors (NRTI), and with or without enfuvirtide.
Serial blood samples, for the determination of etravirine and darunavir plasma concentrations were collected on days 7 and 28 predose and 0.5, 1, 1.5, 2, 3, 4, 6, 9, and 12 h after the administration of the ART regimen with a standard meal containing 5 g fat and 250–500 Kcal, along with 240 ml of water.
Concentrations of etravirine and darunavir in plasma were measured using validated high-pressure liquid chromatography–tandem mass spectrometry methods. The lower limits of quantification for etravirine and darunavir were 2 and 10 ng/ml, respectively.
Plasma pharmacokinetic parameters [area under the curve from 0 to 12 h (AUC12h), maximum concentration (C max), predose concentration (C 0h) and minimum concentration (C min)] for etravirine and darunavir were calculated using noncompartmental methods utilizing a pharmacokinetic data analysis program (WinNonLin version 4.01a, Mountain View, California, USA). The AUC was calculated using the linear trapezoidal rule.
Etravirine and darunavir pharmacokinetics were compared with historical data obtained from subjects taking etravirine in the absence of darunavir and darunavir in the absence of etravirine.
Pharmacokinetic, safety and efficacy variables were summarized using median, mean, geometric mean, SD and range. Interindividual variability in etravirine and darunavir pharmacokinetic parameters was expressed as a coefficient of variation [(SD/mean) × 100].
Predictors of change from baseline to day 7 and week 24 HIV RNA were determined using linear regression modelling. The potential predictors assessed were darunavir and etravirine plasma exposure (day 7 plasma exposure associations with day 7 HIV RNA change and day 7 and 28 plasma exposure associations with week 24 HIV RNA change), number of NNRTI and PI mutations, darunavir and etravirine fold change and the use of enfuvirtide.
All P values were two-tailed with values < 0.05 regarded as statistically significant. Data were analysed using SPSS statistical software (Chicago, Illinois, USA).
Both pharmacokinetic phases were complete by 10 of the 12 patients; baseline demographic and clinical characteristics are summarized in Table 1. Two patients withdrew consent; one on screening and one on day 7 for personal reasons.
All patients had viruses with high levels of resistance for all ART classes (Tables 1 and 2). Phenotypic resistance analysis also evidenced high levels of resistance to all ART classes: the median fold change for all available NNRTI and PI ranged from 0.6 to 104.9 and 0.4 to 137.6, respectively. Median patient fold changes for etravirine and darunavir were 1.25 (range, 0.3–3.8) and 3.6 (range, 0.4–46.8), respectively.
Two subjects showed wild type at screening but had extensive archived resistance documented by different tests performed in the past (Table 2).
Six of the patients had prior exposure to tipranavir and to enfuvirtide; only two used enfuvirtide for the first time (Table 2).
Table 3 shows the pharmacokinetics of etravirine and darunavir measured on days 7 and 28. Compared with historical controls, these reflect modestly reduced (30%) exposure to etravirine. However, values for etravirine pharmacokinetic parameters were similar to those previously measured when etravirine was coadministered with a boosted PI (median etravirine AUC and C 0h from historical data were 3556 ng.h/ml and 196 ng/ml, respectively) .
All subjects had an etravirine C min higher than the median etravirine IC50 for HIV wild type (2 ng/ml) .
Exposure to darunavir was similar to values previously measured in the absence of etravirine . All subjects had a darunavir C min higher than the median darunavir IC50 for HIV wild type and PI-resistant HIV-1 strains when corrected for protein binding (21.9 ng/ml) .
Coefficient of variation in etravirine and darunavir pharmacokinetic parameters ranged from 27 to 70% (Table 3).
At week 24, 9/10 patients had achieved an undetectable viral load; only one patient exhibited a viral load of 722 copies/ml, which decreased to 59 copies/ml at week 32 (Table 2 and Fig. 1). Median HIV RNA decline was 2.7 log10 copies/ml (range, 2.3–3.9) and CD4 cell count increase was 113 cells/μ1 (range, 41–268).
No new AIDS events, serious adverse events, or changes in laboratory safety were reported. Possibly drug-related adverse events were mild diarrhoea (one), headache (one) and grade 1 skin rash (one), which all resolved with continuous dosing.
No statistically significant associations were observed between the log10 change in viral load at day 7 or week 24 and etravirine or darunavir pharmacokinetic parameters, the use of enfuvirtide, baseline etravirine, and darunavir fold change or NNRTI- and PI-associated mutations (all P > 0.20).
As the paradigm continues to shift, so must the design of clinical trials assessing the use of investigational agents as novel therapies. The so-called sequential monotherapy studies where HIV-1-infected patients are exposed to only one new agent, encouraging the development of further drug resistance, has become unacceptable. Future drug development and study design must take into account not only the requirements of the appropriate regulatory boards for licensing drugs but also the needs of our patients and how best to optimize their future treatment options.
Our study has explored, albeit in a small population, the potential to use two investigational agents as a viable treatment option in HIV-infected subjects with very limited treatment options. We have observed an exceptional virological response given that this patient population was infected by viruses that, according to the results of previous and current resistance tests, were not sensitive to any of the triple antiretroviral drug combinations available in clinical practice. We acknowledge this approach of utilizing two experimental agents is not without risks, such as unfavourable pharmacokinetic interactions and unforeseen side effects. However, these risks can be minimized by studying a small cohort with the greatest need for these novel agents, as we have undertaken, before embarking on larger studies.
This study evaluated for the first time the pharmacokinetics, the safety, and the efficacy of the investigational agents etravirine and darunavir/ritonavir when coadministered twice daily to ART-experienced HIV-infected patients.
No clinically significant pharmacokinetic interaction between etravirine and darunavir was observed. The combination was well tolerated and showed impressive short-term efficacy against three-class-resistant HIV over 24 weeks of therapy.
Strategies for the management of patients with extensive prior treatment and drug resistance are very limited. With the exhaustion of interventions to optimize the remaining virological activity of available drugs, through reassessment of treatment history, resistance data , dual PI boosting , or strategies known to preserve fitness-reducing mutations (e.g., M184V by lamivudine and emtricitabine) , the use of experimental agents (where available as part of a clinical trial or in expanded access programmes) are imperative.
The authors would like to thank Tibotec BVBA, Mechelen, Belgium for providing an educational grant for the study.
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