Report 2

HIV Drug Resistance Testing and Update from Resistance Workshop- San Diego June 23

What is Genotypic Resistance Testing?

There are 2 types of resistance testing-genotypic testing and phenotypic testing.

Genotypic resistance testing looks at a person's HIV genes (enzymes) to detect if there have been changes to it. These changes are called mutations. Generally, changes in these HIV genes are called mutations, and when these changes occur HIV drugs are not as effective. There are certain mutations that are known to be associated with resistance to specific drugs. Mutations to the protease enzyme can result in a protease inhibitor not binding to the protease enzyme as well. The protease inhibitor has to bind to the enzyme to prevent the protease enzyme from doing its job in the production process of HIV. Resistance leads to drug failure (viral load rebound). Less is known about how resistance to AZT develops. The emergence of certain known mutations leads to AZT resistance. AZT and other NRTIs (nucleoside reverse transcriptase inhibitors) inhibit the reverse transcriptase enzyme from performing its role in viral replication. When AZT mutations develop, AZT is less able to inhibit the reverse transcriptase enzyme, and permits viral replication. For example, if your HIV viral gene changes (mutations) at positions 70, 215, and 219 researchers know this means you may have resistance to AZT. But the process appears to be more complicated and is not well understood. Although a person can fail d4T and ddI, mutations are not necessarily detectable. Resistance to 3TC is clear when the 184 mutation is seen. Regarding protease inhibitors, if you have mutations in the protease gene at positions 48 and 90 you have resistance to saquinavir. For indinavir any combination of several mutations from a group of a number of well known mutations indicate resistance to indinavir. So genotypic resistance looks to see if you have mutations known to be associated with resistance to specific drugs. In effect it is an indirect measure of resistance. Resistance to efavirenz (Sustiva) is associated with the K103N mutation. If your genotypic resistance test reports a K103N mutation, you have resistance to efavirenz. Generally, nevirapine resistance is also associated with the 103 mutation. As wel, additional NNRTI mutations can develop. Generally, the development of the 103 mutation results in cross-resistance to other available NNRTIs. Improper adherence, such as missing doses or eating fatty foods with indinavir, is a leading cause of resistance. People can fail a drug for other reasons than resistance. For inexplicable reasons, a person may not reach and maintain adequate blood levels of a drug. Their metabolism could theoretically clear a drug too quickly.

Testing Companies (You want to use a reliable lab)

Virco - based in Europe but available in the USA only through LabCorp. Virco provides their own phenotypic test (Antivirogram)and they use the ABI test for genotyping (VircoGEN). Samples are shipped to LabCorp and then to Virco for processing.

ViroLogic - based in South San Francisco, CA. They provide a phenotypic test (PhotoSense)now. Plan to have genotypic test in future.

Visible Genetics - provides only a genotypic test. Located in Norcross, GA.

Resistance testing can be used to check for pre-existing resistance prior to starting any therapy. This would be to check for resistant virus transmitted by sex or IV drug use.

What is Phenotypic Testing?

With Phenotypic testing-you take virus from a person and put it in a test tube and add increasing amounts of a drug. You want to see how much the virus is able to reproduce itself as you add increasing amounts of drug. And how much drug is required to stop virus reproduction. If all you need to stop HIV from reproducing is an amount of drug equivalent to the amount a person would take that person does not have phenotypic resistance and that drug should work for that person. If you need 4 times that amount of drug to suppress HIV from reproducing then your virus is 4-fold resistant to that drug. If you need 10 times the amount of drug than your virus has 10 fold resistance to that drug. Exception ABT-378 because drug levels are very high.

Phenotypic testing actually measures more directly if you are resistant to a drug. Several studies have been reported, a few at the Resistance Mtg, that if you detect genotypic resistance that may be predictive of phenotypic resistance but possibly not always. Virus with 5-fold phenotypic resistance had no genotypic resistance in 4 cases (n=38).

Strengths & Weaknesses of Genotypic & Phenotypic Testing

Studies Showing Benefits of Resistance Testing

There have been a number of studies. Following are two high-profile studies I selected.The follow-up results of these 2 were just reported at Resistance Workshop.

GART - (genotypic anti-retroviral resistance testing)

Prevalence of Transmitted Resistant Virus

One study of 133 people (56 days after seroconversion within last 12 months in USA) reported major resistance among 3% (using ViroLogic phenotype testing- >10-fold-- and ABI genotype testing):

Moderate reduction in susceptibility or moderate resistance (2.5 to 10-fold resistance) to 1 or more drugs among 29%--

* (numerous polymorphisms but no primary drug mutations)

A second study reported at Resistance Workshop on 230 samples using VircoGEN and Antivirogram.

For genotype, resistant (R) (>10 fold) and intermediate (I) (4-10 fold)-

For phenotype testing-

1% were resistant to 2 classes of drugs. 10% had significant phenotypic change or ant genotypic change.