The normalized inhibitory quotient of boosted protease inhibitors is predictive of viral load response in treatment-experienced HIV-1-infected individuals

Therapeutic drug monitoring of antiretroviral drugs in HIV-infected patients

INTRODUCTION

Therapeutic drug monitoring (TDM) may be beneficial when applied to antiretroviral (ARV). Even though TDM can be a valuable strategy in HIV management, its role remains controversial. Areas covered: This review provides a comprehensive update on important issues relating to TDM of ARV drugs in HIV-infected patients. Articles from PubMed with keywords relevant to each topic section were reviewed. Search strategies limited to articles published in English. Expert commentary: There is evidence supporting the use of TDM in HIV treatment. However, some limitations need to be considered. The evidence supporting the use of routine TDM for all patients is limited, as it is not clear that this strategy offers any advantages over TDM for selected indications. Selected groups of patients including patients with physiological changes, patients with drug-drug interactions or toxicity, and the elderly could potentially benefit from TDM, as optimized dosing is challenging in these populations.

Differences in the Prediction of Area Under the Curve for a Protease Inhibitor Using Trough Versus Peak Concentration: Assessment Using Published Pharmacokinetic Data for Indinavir

In the present day antiretroviral therapy, Ctrough is a key tool for efficacy assessment. The present work explored the feasibility of using Ctrough or Cmax in the area under the concentration-time curve (AUC) prediction of indinavir. A simple unweighted linear regression model was developed to describe the relationship between Cmax versus AUC (r = 0.8101, P < 0.001) and Ctrough versus AUC (r = 0.8127, P < 0.001) for indinavir. The regression lines were used to predict the AUC values from literature Cmax or Ctrough data of indinavir in HIV and healthy subjects. The fold difference, defined as the quotient of the observed and predicted AUC values, was evaluated along with statistical comparison, including root mean square error (RMSE) prediction for the 2 models. The correlation between Cmax versus AUC and Ctrough versus AUC was established. Majority of the predicted values for Cmax versus AUC were within 0.75- to 1.5-fold differences. However, the Ctrough versus AUC model showed larger variability with approximately one-third of the predictions within 0.75- to 1.5-fold differences. The r value and %RMSE for observed versus predicted AUC for Ctrough (r = 0.5925, n = 65, P < 0.001, and RMSE: 67%) were inferior to the Cmax (r = 0.8773, n = 86, P < 0.001, and RMSE: 46%). In conclusion, Cmax versus AUC and Ctrough versus AUC relationships were established for indinavir showing the utility of a single concentration time point for therapeutic drug monitoring purpose. The Cmax model for indinavir may be more relevant for AUC prediction as determined by the statistical criteria.

High-Dose Lopinavir/Ritonavir in Highly Treatment-Experienced HIV-1 Patients: Efficacy, Safety, and Predictors of Response

OBJECTIVE

To investigate the efficacy and safety of high-dose lopinavir/ritonavir (LPV/r) therapy in multiple protease inhibitor, non-nucleoside reverse transcriptase inhibitor (NNRTI)-experienced subjects.

METHOD

Thirty-six HIV-1-infected subjects were randomized to LPV/r 400/300 mg or 667/167 mg bid in a 48-week, open-label study. Subjects also received investigator-selected nucleoside reverse transcriptase inhibitors (NRTIs). Primary outcomes were the proportion of subjects with HIV-1 RNA levels <50 copies/mL at week 24 and time until loss of virologic response through week 48.

RESULTS

Six of 17 (35%) and 10 of 19 (53%) subjects in the 400/300 and 667/167 groups, respectively, completed 48 weeks of treatment. Median durations of follow-up in discontinued subjects and all subjects were 15 weeks and 32 weeks, respectively. Forty-four percent of subjects achieved HIV-1 RNA <50 copies/mL at least once; 18% (400/300 mg) and 21% (667/167 mg) of subjects achieved HIV-1 RNA <50 copies/mL at week 24 (intent-to-treat analysis). Corresponding results at week 48 were 18% (400/300 mg) and 26% (667/167 mg). No statistically significant differences in adverse event incidence occurred between treatment groups, except for a higher vomiting rate in the 400/300 mg dose group. Predictors of response included baseline LPV inhibitory quotient and number of active NRTIs.

CONCLUSION

Higher doses of LPV/r may provide substantial antiviral activity in multiple class-experienced subjects.

Novel method to assess antiretroviral target trough concentrations using in vitro susceptibility data

Durable suppression of HIV-1 replication requires the establishment of antiretroviral drug concentrations that exceed the susceptibility of the virus strain(s) infecting the patient. Minimum plasma drug concentrations (C(trough)) are correlated with response, but determination of target C(trough) values is hindered by a paucity of in vivo concentration-response data. In the absence of these data, in vitro susceptibility measurements, adjusted for serum protein binding, can provide estimations of suppressive in vivo drug concentrations. We derived serum protein binding correction factors (PBCF) for protease inhibitors, nonnucleoside reverse transcriptase inhibitors, and an integrase inhibitor by measuring the effect of a range of human serum concentrations on in vitro drug susceptibility measured with the PhenoSense HIV assay. PBCFs corresponding to 100% HS were extrapolated using linear regression and ranged from 1.4 for nevirapine to 77 for nelfinavir. Using the mean 95% inhibitory concentration (IC(95)) for ≥1,200 drug-susceptible viruses, we calculated protein-bound IC(95) (PBIC(95)) values. PBIC(95) values were concordant with the minimum effective C(trough) values that were established in well-designed pharmacodynamic studies (e.g., indinavir, saquinavir, and amprenavir). In other cases, the PBIC(95) values were notably lower (e.g., darunavir, efavirenz, and nevirapine) or higher (nelfinavir and etravirine) than existing target recommendations. The establishment of PBIC(95) values as described here provides a convenient and standardized approach for estimation of the minimum drug exposure that is required to maintain viral suppression and prevent the emergence of drug-resistant variants, particularly when in vivo concentration-response relationships are lacking.

A randomized clinical trial evaluating therapeutic drug monitoring (TDM) for protease inhibitor-based regimens in antiretroviral-experienced HIV-infected individuals: week 48 results of the A5146 study

BACKGROUND

We devised an open-label, randomized trial to evaluate whether therapeutic drug monitoring (TDM) of protease inhibitors (PIs) and dose escalation based upon a normalized inhibitory quotient (NIQ), which integrates PI trough concentration and drug resistance, could improve virologic outcome in PI-experienced patients with treatment failure. Secondary analyses through 48 weeks are presented.

METHODS

Eligible HIV-infected subjects with a screening viral load of ≥ 1000 copies/mL initiated a new PI-based regimen at entry and had NIQ performed at week 2. Subjects with an NIQ ≤1 were randomized at week 4 to a standard-of-care (SOC) arm or TDM arm featuring PI dose escalation.

RESULTS

One hundred and eighty-three subjects were randomized. There was no significant treatment difference in change from randomization to week 48 in HIV-1 RNA [ P = .13, median (25th, 75th percentile log10 copies/mL change): -0.03 (-0.74, 0.62) with TDM and 0.11 (-2.3, 0.82) with SOC]. In subgroup analysis, patients with ≥ 0.69 active PIs benefited from TDM compared to those with <0.69 active PIs ( P = .05).

CONCLUSIONS

While the TDM strategy of PI dose escalation did not improve virologic response at week 48 overall, in subgroup analysis, TDM favorably impacted virologic outcome in subjects taking PI-based regimens with moderate antiviral activity.

Quality assessment for therapeutic drug monitoring in AIDS Clinical Trials Group (ACTG 5146): a multicenter clinical trial

In a randomized trial, AIDS Clinical Trials Group (ACTG) protocol 5146 (A5146) investigated the use of therapeutic drug monitoring (TDM) to adjust doses of HIV-1 protease inhibitors (PIs) in patients with prior virologic failure on PI-based therapy who were starting a new PI-based regimen. The overall percentage of "PI trough repeats" such as rescheduled visits or redrawn PI trough specimens increased from 2% to 5% to 10% as the process progressed from the clinical sites, the pharmacology specialty laboratory, and the study team, respectively. Cumulatively, this represents a 17% rate of failure to obtain adequate PI trough sample. While targeting a turnaround of 7 days or less from sample receipt to a drug concentration report, 12% of the received specimens required a longer period to report concentrations. The implementation of dosing changes in the TDM arm were achieved within 7 days or less for 56% of the dose change events and within 14 days or less for 77% of dose change events. This quality assurance analysis provides a valuable summary of the specific points in the TDM process that could be improved during a multicenter clinical trial including: 1) shortening the timeline of sample shipment from clinical site to the laboratory; 2) performing the collection of PI trough specimen within the targeted sampling window by careful monitoring of the last dose times and collection times by the clinicians; 3) increasing patient adherence counseling to reduce the number of samples that are redrawn due to suspecting inconsistent adherence; and 4) decreasing the time to successful TDM-based dose adjustment. The application of some of these findings may also be relevant to single-center studies or clinical TDM programs within a hospital.

[AIDS Study Group/Spanish AIDS Plan consensus document on antiretroviral therapy in adults with human immunodeficiency virus infection (updated January 2010)]

OBJECTIVE

This consensus document is an update of antiretroviral therapy recommendations for adult patients with human immunodeficiency virus infection.

METHODS

To formulate these recommendations a panel made up of members of the Grupo de Estudio de Sida (Gesida, AIDS Study Group) and the Plan Nacional sobre el Sida (PNS, Spanish AIDS Plan) reviewed the advances in the current understanding of the pathophysiology of human immunodeficiency virus (HIV) infection, the efficacy and safety of clinical trials, and cohort and pharmacokinetic studies published in biomedical journals or presented at scientific meetings. Three levels of evidence were defined according to the data source: randomized studies (level A), cohort or case-control studies (level B), and expert opinion (level C). The decision to recommend, consider or not to recommend ART was established in each situation.

RESULTS

Currently, the treatment of choice for chronic HIV infection is the combination of three drugs of two different classes, including 2 nucleosides or nucleotide analogs (NRTI) plus 1 non-nucleoside (NNRTI) or 1 boosted protease inhibitor (PI/r), but other combinations are possible. Initiation of ART is recommended in patients with symptomatic HIV infection. In asymptomatic patients, initiation of ART is recommended on the basis of CD4 lymphocyte counts, plasma viral load and patient co-morbidities, as follows: 1) therapy should be started in patients with CD4 counts below 350 cells/microl; 2) When CD4 counts are between 350 and 500 cells/microl, therapy should be started in case of cirrhosis, chronic hepatitis C, high cardiovascular risk, HIV nephropathy, HIV viral load above 100,000 copies/ml, proportion of CD4 cells under 14%, and in people aged over 55; 3) Therapy should be deferred when CD4 are above 500 cells/microl, but could be considered if any of previous considerations concurs. Treatment should be initiated in case of hepatitis B requiring treatment and should be considered for reduce sexual transmission. The objective of ART is to achieve an undetectable viral load. Adherence to therapy plays an essential role in maintaining antiviral response. Therapeutic options are limited after ART failures but undetectable viral loads maybe possible with the new drugs even in highly drug experienced patients. Genotype studies are useful in these situations. Drug toxicity of ART therapy is losing importance as benefits exceed adverse effects. Criteria for antiretroviral treatment in acute infection, pregnancy and post-exposure prophylaxis are mentioned as well as the management of HIV co-infection with hepatitis B or C.

CONCLUSIONS

CD4 cells counts, viral load and patient co-morbidities are the most important reference factors to consider when initiating ART in asymptomatic patients. The large number of available drugs, the increased sensitivity of tests to monitor viral load, and the ability to determine viral resistance is leading to a more individualized therapy approach in order to achieve undetectable viral load under any circumstances.

Clinical application of the inhibitory quotient: is there a role in HIV protease inhibitor therapy?

PURPOSE OF REVIEW

The inhibitory quotient of boosted protease inhibitors is a measurement incorporating both plasma drug exposure and resistance testing. Inhibitory quotients have been assessed in over 50 clinical studies to date, many of these studies describe highly significant associations between the inhibitory quotient and virological outcomes. This review will focus on studies that assess the inhibitory quotient in a prospective fashion and with the use of newly available boosted protease inhibitors.

RECENT FINDINGS

Atazanavir, darunavir and tipranavir are the three most recently licensed protease inhibitors. Studies have described associations between the inhibitory quotient and outcomes for all of these agents with cutoff inhibitory quotient values now proposed for all of these protease inhibitors.A prospective study has assessed the use of inhibitory quotient across many different protease inhibitor regimens and although no overall benefit was described, improved clinical outcomes were observed in specific patient groups.

SUMMARY

Although ongoing challenges need to be overcome, recent studies have described associations between this useful parameter and modern day antiretroviral regimens.

Clinical cut-offs for HIV-1 phenotypic resistance estimates: update based on recent pivotal clinical trial data and a revised approach to viral mixtures

The clinical utility of HIV-1 resistance testing is dependent upon accurate interpretation and application of results. The development of clinical cut-offs (CCOs) for most HIV antiretroviral drugs assessed by the vircoTYPE HIV-1 resistance test has been described previously. Updated CCOs based on new methodology and new data from clinical cohorts and pivotal clinical studies are presented in this communication. Data for analysis included the original records for CCO derivation from eight clinical trials and two cohort studies plus new records from the clinical cohorts and from the TITAN, POWER, and DUET clinical studies. Drug-specific linear regression models were developed to describe the relationship between baseline characteristics (phenotypic resistance as estimated by virtualPhenotype-LM using methods revised recently for handling mixed viral sequences; viral load; and treatment history), new treatment regimen, and 8-week virologic outcome. The clinical cut-offs were defined as the estimated phenotypic resistance levels (fold change, FC) associated with a 20% and 80% loss of drug activity. The development dataset included 6550 records with an additional 2299 reserved for validation. The updated, v.4.2 CCOs were generally close to the v4.1 values, with a trend observed toward marginally higher cut-offs for the NRTIs. These results suggest that the updated CCOs provide a relevant tool for estimating the contribution to virological response of individual antiviral drugs in antiretroviral drug combinations as used currently in clinical practice.

Use of different inhibitory quotients to predict early virological response to tipranavir in antiretroviral-experienced human immunodeficiency virus-infected patients

Information about the relationship between pharmacological parameters and an early virological response to tipranavir (TPV) is scarce. Human immunodeficiency virus (HIV)-infected patients who had received TPV as part of a salvage regimen were analyzed retrospectively. A virological response was defined as a decline in the HIV RNA level of > or = 1 log unit or to <50 copies/ml between weeks 4 and 12 of therapy. The virtual inhibitory quotient (vIQ) was calculated as the ratio of the TPV plasma trough concentration (C(trough))/virtual change in the 50% inhibitory concentration. Three genotypic inhibitory quotients (gIQs) were calculated by using different TPV resistance mutation scores (from the International AIDS Society-USA [IAS-USA], Randomized Evaluation of Strategic Intervention in Multidrug-Resistant Patients with Tipranavir [RESIST], and Agence Nationale de Recherches sur le Sida et les Hépatites Virales [ANRS] trials). The sensitivities, specificities, positive predictive values (PPVs), negative predictive values (NPVs), and likelihood ratios for a positive result (LHR+) and a negative result (LHR-) [LHR+ = sensitivity/(1-specificity); LHR- = (1-sensitivity)/specificity] were calculated. A total of 57 HIV-infected patients were analyzed. A virological response was achieved by 77% of the patients. TPV resistance mutations, TPV C(trough), vIQs, and gIQs were all significantly associated with a virological response. The vIQ had the best PPV and NPV (97% and 78%, respectively). The values of the LHR+ were 7.8 for vIQ, 3.4 for the RESIST gIQ, 3.3 for the IAS-USA gIQ, 3.1 for the ANRS gIQ, 2.2 for TPV C(trough), and 1.3 for the IAS-USA and RESIST scores. The values of LHR- were 0 for the RESIST score, 0.07 for vIQ, 0.09 for the IAS-USA score, 0.27 for the RESIST gIQ, 0.32 for the IAS-USA gIQ, 0.37 for the ANRS gIQ, and 0.48 for TPV C(trough). HIV-infected patients who initiate a salvage regimen based on TPV may benefit from baseline drug resistance testing and TPV plasma concentration determination, as vIQ is the best predictor of a virological response.

A randomized trial of therapeutic drug monitoring of protease inhibitors in antiretroviral-experienced, HIV-1-infected patients

OBJECTIVE

Whether therapeutic drug monitoring of protease inhibitors improves outcomes in HIV-infected patients is controversial. We evaluated this strategy in a randomized, open-label clinical trial, using a normalized inhibitory quotient (NIQ), which incorporates drug exposure and viral drug resistance. NIQs < or = 1 may predict poor outcome and identify patients who could benefit from dose escalation.

DESIGN/METHODS

Eligible patients had a viral load > or =1000 copies/ml on a failing regimen, and began a new protease inhibitor containing regimen at entry. All FDA-approved protease inhibitors available during the study recruitment (June 2002-May 2006) were allowed. One hundred and eighty-three participants with NIQ < or = 1, on the basis of their week 2 protease inhibitor trough concentration and pre-entry drug resistance test, were randomized at week 4 to standard of care (SOC) or protease inhibitor dose escalation (TDM). The primary endpoint was change in log10 plasma HIV-1 RNA concentration from randomization to 20 weeks later.

RESULTS

Ninety-one patients were randomized to SOC and 92 to TDM. NIQs increased more in the TDM arm compared to SOC (+69 versus +25%, P = 0.01). Despite this, TDM and SOC arms showed no difference in outcome (+0.09 versus +0.02 log10, P = 0.17). In retrospective subgroup analyses, patients with less HIV resistance to their protease inhibitors benefited from TDM (P = 0.002), as did black and Hispanic patients (P = 0.035 and 0.05, respectively). Differences between black and white patients persisted when accounting for protease inhibitor susceptibility.

CONCLUSIONS

There was no overall benefit of TDM. In post hoc subgroup analyses, TDM appeared beneficial in black and Hispanic patients, and in patients whose virus retained some susceptibility to the protease inhibitors in their regimen.

Determination of clinically relevant cutoffs for HIV-1 phenotypic resistance estimates through a combined analysis of clinical trial and cohort data

BACKGROUND

Clinically relevant cutoffs are needed for the interpretation of HIV-1 phenotypic resistance estimates as predicted by "virtual" phenotype HIV resistance analysis.

METHODS

Using a clinical data set containing 2596 treatment change episodes in 2217 patients in 8 clinical trials and 2 population-based cohorts, drug-specific linear regression models were developed to describe the relation between baseline characteristics (resistance, viral load, and treatment history), new treatment regimen selected, and 8-week virologic outcome.

RESULTS

These models were used to derive clinical cutoffs (CCOs) for 6 nucleoside/nucleotide reverse transcriptase inhibitors (zidovudine, lamivudine, stavudine, didanosine, abacavir, and tenofovir), 3 unboosted protease inhibitors (PIs; indinavir, amprenavir, and nelfinavir), and 4 ritonavir-boosted PIs (indinavir/ritonavir, amprenavir/ritonavir, saquinavir/ritonavir, lopinavir/ritonavir). The CCOs were defined as the phenotypic resistance levels (fold change [FC]) associated with a 20% and 80% loss of predicted wild-type drug effect and depended on the drug-specific dynamic range of the assay.

CONCLUSIONS

The proposed CCOs were better correlated with virologic response than were biological cutoffs and provide a relevant tool for estimating the resistance to antiretroviral drug combinations used in clinical practice. They can be applied to diverse patient populations and are based on a consistent methodologic approach to interpreting phenotypic drug resistance.

The design and implementation of A5146, a prospective trial assessing the utility of therapeutic drug monitoring using an inhibitory quotient in antiretroviral-experienced HIV-infected patients

The AIDS Clinical Trials Group designed and implemented a prospective, randomized, strategy trial in antiretroviral-experienced, HIV-infected patients to evaluate the virologic impact of protease inhibitor dose escalation in response to therapeutic drug monitoring (TDM) with an inhibitory quotient, which integrates both drug exposure and viral drug resistance. In the process of developing this clinical trial, several unique challenges were identified that required innovative solutions. The major challenge was the need to integrate resistance testing, pharmacokinetic data, medication adherence, toxicity data, clinical assessments, randomization assignment, and protocol-specified clinical management in a way that could be utilized in real time by the protocol team, communicated promptly to the clinical sites, and transmitted accurately to the study database. In addition, the protocol team had to address the relative lack of commercially available TDM laboratories in the United States that were experienced in antiretroviral drug assays and a lack of familiarity with the principles of pharmacokinetic monitoring at participating clinical sites. This article outlines the rationale for the design of this strategy trial, specific barriers to implementation that were identified, and solutions that were developed with the hope that these experiences will facilitate the design and conduct of future trials of TDM.

[Recommendations from the GESIDA/Spanish AIDS Plan regarding antiretroviral treatment in adults with human immunodeficiency virus infection (update January 2007)]

OBJECTIVE

This consensus document is an update of antiretroviral therapy (ART) recommendations for adult patients infected with the human immunodeficiency virus (HIV-1).

METHODS

To formulate these recommendations, a panel composed of members of the Grupo de Estudio de Sida (GESIDA; AIDS Study Group) and the Plan Nacional sobre el Sida (PNS; Spanish AIDS Plan) reviewed the advances in the current understanding of the pathophysiology of HIV, the safety and efficacy findings from clinical trials, and the results from cohort and pharmacokinetic studies published in biomedical journals or presented at scientific meetings over the last years. Three levels of evidence were defined according to the source of the data: randomized studies (level A), cohort or case-control studies (level B), and expert opinion (level C). The decision to recommend, consider or not recommend ART was established in each situation.

RESULTS

Currently, the treatment of choice for chronic HIV infection is the combination of three drugs of two different classes, including 2 nucleosides or nucleotide analogs (NRTI) plus 1 non-nucleoside (NNRTI) or 1 boosted protease inhibitor (PI/r). Initiation of ART is recommended in patients with symptomatic HIV infection. In asymptomatic patients, initiation of ART is recommended on the basis of CD4+ lymphocyte counts and plasma viral load, as follows: 1) therapy should be started in patients with CD4+ counts of < 200 cells/microl; 2) therapy should be started in most patients with CD4+ counts of 200-350 cells/microl, although it can be delayed when CD41 count persists at around 350 cells/microL and viral load is low, and 3) initiation of therapy can be delayed in patients with CD4+ counts of > 350 cells/microL. The initial objective of ART is to achieve an undetectable viral load. Adherence to therapy plays an essential role in maintaining the antiviral response. Therapeutic options are limited with the development of cross resistance and ART failure. Genotype studies are useful in these cases. More information regarding the studies analyzed and the panel recommendations for adherence, toxicity, treatment during pregnancy, patients with hepatitis B or C virus co-infection, and post-exposure prophylaxis can be accessed at www.gesida.seimc.org.

CONCLUSIONS

CD4+ lymphocyte count is the most important reference factor for initiating ART in asymptomatic patients. The large number of available drugs, the increased sensitivity of tests to monitor viral load, and the ability to determine viral resistance is leading to a more individualized approach to therapy.

Role of integrase inhibitors in the treatment of HIV disease

The introduction of highly active antiretroviral therapy (HAART) has produced a dramatic reduction in HIV-related mortality and morbidity among populations with widespread access to drugs. However, the increase in mutated HIV strains with reduced susceptibility to antiretroviral agents and the emergence of HAART-related side effects make it necessary to develop novel compounds characterized by activity against resistant viruses, a high genetic barrier to resistance and favorable pharmacokinetic and toxicity profiles. Integrase is a key enzyme in the life cycle of HIV and has represented an appealing target of antiretroviral therapy for several years. In 2006, after more than a decade of advances, pitfalls and disappointments, large clinical trials with integrase inhibitors in HIV-positive subjects have eventually begun.

A randomised trial comparing genotypic and virtual phenotypic interpretation of HIV drug resistance: the CREST study

Objectives:

The aim of this study was to compare the efficacy of different HIV drug resistance test reports (genotype and virtual phenotype) in patients who were changing their antiretroviral therapy (ART).

Design:

Randomised, open-label trial with 48-week followup.

Setting:

The study was conducted in a network of primary healthcare sites in Australia and New Zealand.

Participants:

Patients failing current ART with plasma HIV RNA > 2000 copies/mL who wished to change their current ART were eligible. Subjects were required to be > 18 years of age, previously treated with ART, have no intercurrent illnesses requiring active therapy, and to have provided written informed consent.

Interventions:

Eligible subjects were randomly assigned to receive a genotype (group A) or genotype plus virtual phenotype (group B) prior to selection of their new antiretroviral regimen.

Outcome Measures:

Patient groups were compared for patterns of ART selection and surrogate outcomes (plasma viral load and CD4 counts) on an intention-to-treat basis over a 48-week period.

Results:

Three hundred and twenty seven patients completing > one month of followup were included in these analyses. Resistance tests were the primary means by which ART regimens were selected (group A: 64%, group B: 62%; p = 0.32). At 48 weeks, there were no significant differences between the groups for mean change from baseline plasma HIV RNA (group A: 0.68 log copies/mL, group B: 0.58 log copies/mL; p = 0.23) and mean change from baseline CD4+ cell count (group A: 37 cells/mm³, group B: 50 cells/mm³; p = 0.28).

Conclusions:

In the absence of clear demonstrated benefits arising from the use of the virtual phenotype interpretation, this study suggests resistance testing using genotyping linked to a reliable interpretive algorithm is adequate for the management of HIV infection.

Background: Antiretroviral drugs are used to treat patients with HIV infection, with good evidence that they improve prognosis. However, mutations develop in the HIV genome that allow it to evade successful treatment—known as drug resistance—and such mutations are known against every class of antiretroviral drug. Resistance can cause treatment failure and limit the treatment options available. Different types of tests are often used to detect resistance and to work out whether patients should switch to a different drug regimen. Currently, the different types of tests include genotype testing (direct sequencing of genes from virus samples infecting a patient); phenotype testing (a test that assesses the sensitivity of a patient's HIV sample to different drugs), and virtual phenotype testing (a way of interpreting genotype data that estimates the likely viral response to different drugs). The researchers of this study did a trial to find out whether providing an additional virtual phenotype report would be beneficial to patients, as compared with a genotype report alone. The main outcome was HIV viral load after 12 months of treatment, but the researchers also looked at differences in drug regimens prescribed, number of treatment changes in the study, and changes in CD4+ (the type of white blood cell infected by HIV) counts.

What this trial shows: The researchers found that the main endpoint of the trial (HIV viral load after 12 months) was no different in patients whose clinicians had received a virtual phenotype report as well as a genotype report, compared with those who had received a genotype report alone. In addition, the average number of drugs prescribed was no different between patients in the two different arms of the trial, and there was no difference in number of drug regimen changes, and no change in immune response (measured using CD4+ cell levels). However, more drugs predicted to be sensitive were prescribed by clinicians who got both a genotype and virtual phenotype report, as compared with clinicians who received only the genotype report.

Strengths and limitations: The size of the trial (338 patients recruited) was large enough to properly test the hypothesis that providing a virtual phenotype report as well as a genotype report would result in lower HIV viral loads. Randomization of patients to either intervention ensured that the comparison groups were well-balanced, and the researchers also tested whether selection bias had affected the results (i.e., testing for the possibility that clinicians could predict which intervention participants would receive, and change recruitment into the trial as a result). They found no evidence for selection bias occurring within the trial. However, interpreting the results is difficult because the trial did not directly compare the two different testing platforms, but rather looked at whether providing a virtual phenotype report as well as a genotype report was better than providing a genotype report alone. The investigators also acknowledge that since the trial was conducted, the cutoffs for interpreting genotype information as resistant have been lowered. The findings may therefore not translate precisely to the current situation.

Contribution to the evidence: Other cohort studies and clinical trials have shown that patients offered resistance testing respond better to antiretroviral therapy compared with those who were not, but the clinical effectiveness of different resistance testing methods is not known. This study provides additional data on the respective benefits of genotype testing versus genotype plus provision of virtual phenotype. Another trial comparing genotype versus virtual phenotype has also found that the different interpretation methods perform similarly.

An Update on Therapeutic Drug Monitoring for Antiretroviral Drugs

There is an increasing uptake of TDM of antiretroviral drugs, particularly in Europe. There is consensus that current antiretroviral drugs meet most of the criteria of drugs that can be considered as candidates for a TDM strategy. This review examines the pharmacokinetic-pharmacodynamic relationship for protease inibitors and non nucleoside reverse transcriptase inhibitor, give an overview of the published randomised clinical trials and then summarises the scenarios for use of TDM. Finally the development of the inhibitory quotient (IQ) concept is discussed.

Clinical pharmacodynamics of HIV-1 protease inhibitors: use of inhibitory quotients to optimise pharmacotherapy

The introduction of HIV-1 protease inhibitors and non-nucleoside reverse transcriptase inhibitors in 1996 began an era described as that of highly active antiretroviral therapy. In addition, the more recent development and availability of HIV-1 genotypic and phenotypic resistance tests and advances in pharmacological assays that support therapeutic drug monitoring (TDM) have created tools that may help clinicians to provide more individualised treatment with HIV-1 protease inhibitors. All current treatment guidelines provide fixed doses of protease inhibitors with vague recommendations for the use of TDM in selected clinical situations. In patients with resistance to protease inhibitors, the combined use of resistance tests with TDM provide a mechanism for individualising the clinical pharmacodynamics of protease inhibitors. Current therapeutic approaches seek to include the monitoring of protease-inhibitor concentrations as part of a TDM programme with phenotypic assays to calculate an inhibitory quotient, virtual inhibitory quotient, or normalised inhibitory quotient, whereas genotypic tests are used with TDM to calculate a genotypic inhibitory quotient. Current investigation is focused on examining the predictive value of this approach for clinical monitoring.

Role of the Inhibitory Quotient in HIV Therapy

A systemic review is presented of all studies that have evaluated the inhibitory quotient (IQ). The IQ is defined as the ratio between (trough) drug concentration and level of drug resistance of the HIV isolate. From the studies presented, it can be concluded that for protease inhibitors (PIs) and efavirenz, the phenotypic IQ is associated with virological response. The genotypic IQ (GIQ) for PIs was also demonstrated to be associated with virological response. An intrinsic limitation of the GIQ is that it is only applicable for PIs, of which resistance is based on the cumulative effect of mutations. As the IQ can be modified by adjustment of the drug dosage, it may be of clinical value. Its application in patient care should therefore be further investigated.