RESISTANCE TESTING: New Information from the Durban Conference and the 2000 HIV Treatment Guidelines

BY: Bill Valenti, MD

In May, 2000 The International AIDS Society (IAS)USA revised their guidelines to include a discussion of the use of resistance testing in HIV care. The guidelines are available on the World Wide Web and are also reprinted in the Journal of the American Medical Association (JAMA). Also included in the JAMA report is additional information on resistance testing that was discussed at the recent XIIIth International AIDS Conference, held July 9-14 in Durban, South Africa.

By way of review, the guidelines discuss the current standard of care, i.e., the use of CD4 counts and viral load testing. They note that "decisions regarding initiation or changes in antiretroviral therapy should be guided by monitoring the laboratory parameters of plasma HIV RNA (viral load) and CD4+T-cell count, as well as the clinical condition of the patient. Results of these two laboratory tests give the physician important information about the virologic and immunologic status of the patient and the risk of disease progression to AIDS." The guidelines go on to say that "HIV viral load testing has been approved by the FDA for determining prognosis and for monitoring the response to therapy only for the RT-PCR assay (Roche)." A discussion of the extensive literature on the use of viral load and CD4 monitoring is included.

One section of the guidelines, "Testing for Drug Resistance," is entirely new, and reviews the background and rationale for resistance testing as well as the use of these assays in clinical practice.

Rationale
Resistance testing to antiretroviral drugs, according to the IAS, is a rational adjunct to guide antiretroviral therapy. When combined with a detailed drug history and efforts aimed at maximizing drug adherence, these assays may help to maximize the benefits of antiretroviral therapy. One comment made at the recent Durban Conference was that resistance testing allows for truly individualized therapy for patients.

There are two types of resistance testing in common clinical use today: genotype assays and phenotype assays.

Genotyping assays detect drug-resistance mutations that are present in the relevant viral genes (i.e., RT and protease). Some genotyping assays involve sequencing of the entire RT and protease genes, while others use probes to detect selected mutations known to confer drug resistance. Genotyping assays can be performed relatively rapidly, such that results can be reported within a week or less after sample collection, depending on whether testing is done locally or by a reference laboratory. Interpretation of test results requires an appreciation of the range of mutations that are selected for by various antiretroviral drugs, as well as the potential for cross-resistance to other drugs conferred by some of these mutations (for more information, visit the website). Consultation with an expert in HIV drug resistance is encouraged to facilitate interpretation of genotypic test results. This is one of the limitations of genotype testing: i.e., difficulty with interpretation and the need to "consult with an expert" who is familiar with the wide range of mutations related to resistance in the currently used antiretrovirals.

Phenotyping assays measure the ability of viruses to grow in various concentrations of antiretroviral drugs. Automated, recombinant phenotyping assays have recently become commercially available, but the turnaround time is usually longer than for genotyping (e.g., 2 to 3 weeks) In addition, phenotyping assays are generally more costly to perform compared with genotypic assays. The concentrations of drugs that inhibit 50% and 90% of viral replication (i.e., the so-called IC50 and IC90) are calculated, and the ratio of the IC50s of the test and reference viruses is reported as the fold increase in IC50, or fold resistance. Interpretation of phenotyping assay results is complicated by the paucity of data on the specific level of resistance (fold increase in IC50) that is associated with failure of different drugs; again, consultation with an expert may be helpful for interpretation of test results.

Other limitations of both genotyping and phenotyping assays include the lack of uniform quality assurance for all assays that are currently available, relatively high cost, and insensitivity for minor viral species; if drug-resistant viruses are present but constitute less than 10% to 20% of the circulating virus population, they will probably not be detected by current assays. This limitation is of particular importance when interpreting data about susceptibility to drugs that the patient has taken in the past but that are not part of the current antiretroviral regimen.

If drug resistance has developed to a drug that was subsequently discontinued, the drug-resistant virus can become a minor species because its growth advantage is lost. Consequently, resistance assays should be performed while the patient is taking their antiretroviral regimen, and data suggesting the absence of resistance should be interpreted carefully in relation to the prior treatment history. One final limitation is the difficulty of performing these assays at all in patients with low viral loads, e.g., generally <1000 copies per mL.

Clinical use of resistance assays
Resistance testing is recommended by the IAS in two clinical situations: in the setting of virologic failure on antiretroviral therapy, and in acute HIV infection. A presentation at the Durban Conference also listed selection of therapy for pregnant women according to Dr. Doug Mayers.

The guidelines do not recommend the use of resistance testing for antiretroviral selection in treatment-naive patients with established infection, category DIII in the guidelines (see table) for the rating scheme used for clinical practice in the guidelines). The use of resistance testing prior to initiation of antiretroviral therapy in chronic HIV infection is not generally recommended because of uncertainty about the prevalence of resistance in treatment-naive individuals and because currently available resistance assays may fail to detect drug-resistant species that were transmitted at the time of primary infection but which became a minor species in the absence of selective drug pressure.

The currently favored approach would be to reserve resistance testing for cases in which viral load suppression was suboptimal after initiation of therapy, although this may change as more information becomes available on the prevalence of resistant virus in antiretroviral-naive individuals.

The guidelines also refer to the current dogma regarding changing more than one component of a regimen in the setting of treatment failure. They state that virologic failure in the setting of HAART (Highly Active Antiretroviral Therapy) is in some instances associated with resistance only to one component of the regimen. In this situation, it may be possible to substitute individual drugs in a failing regimen. Although this concept requires clinical validation, it does leave the door open for use of this technology for more individualized drug selection in the future.

There are currently no prospective data to support the use of one type of resistance assay over the other (i.e., genotyping vs. phenotyping) in different clinical situations. Therefore, one type of assay is generally recommended per sample; however, in the setting of a complex prior treatment history, both assays may provide important and complementary information, according to the guidelines.

In some clinical situations, however, one assay may be preferred over another. The guidelines also describe clinical situations in which treatment should not be delayed while waiting for the results of resistance testing. For example, transmission of drug-resistant strains of HIV has been documented, and may be associated with a suboptimal virologic response to initial antiretroviral therapy. Treatment of acute HIV infection is associated with improved immunological outcome, and optimization of the initial antiretroviral regimen through the use of resistance testing is a reasonable albeit untested strategy (see guidelines category CIII). Because of its more rapid turnaround time, the use of a genotypic assay may be preferred in this setting; however, therapy should not be withheld while awaiting the results of resistance testing.

More from the Durban Conference
Recent prospective studies supporting the use of resistance testing in clinical practice have come from trials in which the utility of resistance tests was assessed in the setting of virologic failure. The VIRADAPT and GART studies compared virologic responses to antiretroviral treatment regimens when genotyping resistance tests were used to help guide therapy compared to with those observed when changes in therapy were guided solely by clinical judgment or the current "standard of care."

The results of both studies indicated that the short-term virologic response to therapy was significantly greater when results of resistance testing were available.

More recently, two important trials discussed in Durban have added to our knowledge of the value of resistance testing. In VIRA 3001, patients failing their first PR-inhibitor regimen were randomized to receive a viral phenotype or to standard of care. Patients in the phenotyping arm received more new antiretroviral drugs than did patients in the control arm. Patients in the phenotyping arm had a significantly greater decrease in virus load compared to the control arm; this translated into a decrease in viral load of 0.7 logs using a cut-off of 50 copies/mL. In addition, a greater proportion of patients in the phenotyping arm achieved a plasma HIV-1 RNA level of less than 400 copies/mL by week 16, but this result did not achieve statistical significance.

The NARVAL study randomized patients to phenotype, genotype, or standard of care. Patients in this study had more extensive prior antiretroviral therapy than in the studies discussed above. Not surprisingly, this study failed to find a significant benefit of resistance testing in the study population as a whole, but it did demonstrate a benefit in less experienced patients.

To address the issue of interpretation of results, the technology has risen to the occasion with more widespread use of the so-called "Virtual Phenotype." The Virtual Phenotype (Virco, Mechelen, Belgium) predicts phenotype based on a large relational database that links together genotypes and phenotypes from thousands of patient specimens for which both assays have been run. The Virtual Phenotype's accuracy in predicting the true measured phenotype ranges from 80% to more than 95%, depending on the drug in question.

Summary
The use of resistance testing to antiretroviral drugs is an important new technology that has been correlated with improved outcomes for patients in certain situations. There is very little argument today that these assays are essential when selecting new drugs for patients on a failing regimen.

As we gain more experience with these assays, they will doubtless be used in other clinical situations as well. Interpretation and so called technical issues, such as quality control, reproducibility, and interpretation still need to be resolved. However, the Virtual Phenotype seems to be a good start toward resolving some of these problems by making interpretation of results almost as seamless as antibacterial resistance reporting.

Bill Valenti, MD, is Clinical Associate Professor of Medicine at the University of Rochester School of Medicine and has been involved in AIDS treatment, research, and policy since the early 1980s. Dr. Valenti is also co-founder of Community Health Network, Rochester, New York, where he continues to see patients.

References

The following sources and resources were used in the preparation of this paper:

The AIDS Treatment Information Service provides a living document with changes made as guidelines are updated. It can be accessed at: <http://www.hivatis.org>

Hirsch M, et al. Antiretroviral drug resistance testing in adult HIV-1 infection: Recommendations of an International AIDS Society-USA Panel. JAMA. 2000;283:. Available at: <http://jama.amaassn.org/issues/v283n18/full/jst90018.html>

Mayers D. Keynote: HIV drug resistance testing is worth the cost. Program and Abstracts of the XIII International AIDS Conference, July 9-14, 2000; Durban, South Africa. Session MoOr31.

The AIDS 2000 Web site: <http://64.28.82.59/>

Baxter JD, Mayers DL, Wentworth DN, et al. A randomized study of antiretroviral management based on plasma genotypic antiretroviral resistance testing in patients failing therapy. AIDS. 2000;14:F83-F92.

Durant J, Clevenbergh P, Halfon P, et al. Drug-resistance genotyping in HIV-1 therapy: The VIRADAPT randomised controlled trial. Lancet. 1999;353:.

Cohen C, Kessler H, Hunt S, et al. Phenotypic resistance testing significantly improves response to therapy: final analysis of a randomized trial (VIRA3001). Antiviral Ther. 2000;5(suppl 3):67.

Meynard JL, Vray M, Morand-Joubert L, et al. Impact of treatment guided by phenotypic or genotypic resistance tests on the response to antiretroviral therapy: A randomized trial (NARVAL, ANRS 088). Antiviral Ther. 2000;5(suppl 3):67-68.

Viral Resistance Web Site: <http://www.viral-resistance.com> HIV Sequence Database: <http://hiv-web.lanl.gov> HIV Resistance web site HIVresistance.com <http://www.hivresistance.com>