Dr. Petropoulos and colleagues presented data from the Virologic, Inc. resistance database, which contains samples of both genotypic and phenotypic resistance test results. They have recently discovered that several resistance reports derived from patient samples contain discordant results showing phenotypic resistance (or loss of susceptibility) but not genotypic resistance to the nonnucleoside reverse transcriptase inhibitor (NNRTI) class of medications. Phenotypic resistance, or loss of susceptibility, is usually defined as the increase in drug concentration required to inhibit a patient's virus compared to a wild type strain of virus. The increase in drug concentration required to inhibit a resistant virus is referred to as the fold increase or fold change (FC) of drug. Every HIV drug has a unique FC in drug concentration that defines its loss of susceptibility or resistance. This has primarily been determined using laboratory strains or viral strains from patients who have never been exposed to HIV drugs. The FC above which resistance is defined is referred to as the biological cutoff. The clinical cut offs, or fold change that defines treatment success or failure in patients for phenotypic susceptibility, have not been defined for the NNRTI class.

Dr. Petropoulos reviewed the relevant codon changes that confer resistance to the NNRTI class. It is widely known that even a single mutational change in the HIV-1 reverse transcriptase (RT) gene can confer high-level resistance to the NNRTI class. The mutations, which confer resistance, occur clustered in three separate locations, located between codons 98-106, 181-190 and 225-236 of the RT gene. He further presented prevalence data from samples collected from January 2002 to the present, showing that of those with phenotypic NNRTI resistance 34 percent had a K103N mutation, 14 percent had an Y181C mutation, 11 percent had a 190A mutation, followed by several others with much less common prevalence.

Next he presented examples of their PhenoSense phenotypic resistance assay, in which results demonstrated a phenotypic loss of susceptibility, and yet the genotype did not demonstrate any of the classic genotypic NNRTI-associated mutations. They have collected and analyzed 180,034 samples as of April 2003, of which 8,673 had no NNRTI mutations, 146 had at least one NNRTI with a FC >5 and of those, 48 had at least one NNRTI with a FC >10. Next they performed an analysis of these 48 samples and determined that some unique mutations or combinations of mutations resulted in a loss of susceptibility to the NNRTI class. Ten were associated with a K101P mutation and 11 were associated with mutations K103R + V179D. Twenty-seven could not be explained by any other common or unique mutation known to confer NNRTI resistance.

The conclusion from this presentation was that NNRTI resistance occurs in the absence of known mutations and can occur with unique single or combination mutations, which explains some but not all NNRTI resistance in the absence of known mutations. The data presented is important because the mutations described in this report are not reported on current clinical genotype resistance reports. Unless clinicians are aware of these new findings, a patient's NNRTI resistance or loss of susceptibility might go unnoticed.