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Coverage provided by Calvin J. Cohen, M.D., M.S.

July 10, 2001

• Comparison of Drug Susceptibility in HIV-1 from Antiretroviral (ART) Experienced Subjects as Assessed by the Antivirogram™ and PhenoSense™ Assays (Poster 588)
  Authored by Hernandez, J.; Ross, L.; Henry, K.; Scarcella, A.; Raffanti, S.; Shalit, P.; Fisher, R.; GlaxoSmithKline Reserch Trial, North Carolina, USA

There are two primary companies offering phenotypic resistance testing. They are known by their corporate names as Virco, whose test is called the AntiVirogram, and ViroLogic, whose test is called a PhenoSense assay. Both are currently in use clinically. As these have different methods, it is important for clinicians to know how these tests might differ in conclusions, since it is not yet clear which result would be considered more accurate if they did differ. Thus, this study was done to evaluate how much these two tests do differ.

HIV from seventy antiviral-experienced patients was assessed using both methods, and the researchers compared the results for all available drugs tested by both methods. One of the key issues in these comparisons is that these tests have differed in the degree of resistance above which the result is considered consistent with reduced susceptibility that is clinically meaningful.

In other words, the first challenge to these tests is to define the degree of resistance that is sufficient to blunt the response expected when that drug is used. Only in recent years have two drugs in particular -- abacavir and lopinavir/ritonavir (known as Kaletra) -- defined from actual clinical studies what the true "cut-off" is for the drug, based on a comparison of how patients did when the drug was actually used. In these cases, the cutoff is defined as the point above which only a small minority of people had much, if any, response from the drug.

Of note, however, is that these cutoffs are not usually able to define an "all or none" threshold for response. For most of the antivirals, there is a continuum of how likely someone is to respond to a drug based on the results, as there are some people with higher levels of resistance who still may have some viral suppression when the drug is used. (One reason for this is the varying drug levels in people, as for most of these agents; the response varies depending on how much of the drug is actually present in the bloodstream.) For most antivirals, however, the cutoff has been primarily defined by looking at untreated HIV, and determining where the upper limit is for the vast majority of sensitive strains.

The results demonstrated that the vast majority of the samples were in agreement between the two testing approaches -- 90% of results were considered "concordant," in that both called the sample either sensitive or resistant. In the other samples, the differences were in both directions, i.e. in about one third, the Virco test called a sample resistant but the Virologic test called it sensitive, while for the other two thirds, the Virco test called a result sensitive, but the ViroLogic test called it resistant. The majority of samples that had this discordant evaluation were those with only minimal degrees of resistance, and the different result is therefore more a result of how the result is judged, rather than a difference in measuring reduced susceptibility.

These results suggest that at this point, either test can be useful for clinicians and patients when making treatment decisions. The degree of difference between the tests, as seen here, is small, and largely explained by differences for viral isolates that are near to the cutoffs. When a result in either of these tests is just above or below a cutoff for resistance, this study confirms that we should not rely on the interpretation reported with the test, but instead realize that some isolates are close enough to the threshold of resistance to still lead to uncertainty about the usefulness of a specific drug.

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