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Viral Resistance
Jan 20, 2002

What exactly is Viral Resistance?

Response from Dr. Little

Viral resistance means that the viral has developed the ability to reproduce (or replicate) in the presence of a particular drug. For instance, if you have 3TC (lamivudine) resistance, it means that the virus will be able to continue replicating even if exposed to therapeutic levels of 3TC. In some circumstances, you may be on enough other drugs to overcome the effect of drug resistance to one drug. However, in many circumstances, a person may develop resistance to multiple drugs and will then "fail" their treatment regimen (ie. have a detectable viral load even though they are taking all of their drugs as prescribed).

The next question is then how resistance develops. There are two ways. First, the most common - if the level of drugs in a treated person's blood stream becomes lower than expected for any reason (skipping doses is the most common reason), then you have lower than desired levels of drug in the blood stream. This lower level of drug may not be enough to prevent HIV replication. In this case, the virus may start to replicate again and over time, may develop substitutions in its genetic structure (this is a natural and predictable feature of all HIV replication) which by chance make it better able to replicate in the presence of a certain drug. If the level of the particular drug is low (as above - resulting from noncompliance, poor drug aborption, or a drug-drug interaction), this "resistant" virus will replicate better than the other viruses present and over more time, the resistant virus becomes the predominant form present (ie and therefore the only type detected).

The second way that drug resistance develops - is that someone with drug resistant virus (which has arisen as described above) passes that virus on to someone else (ie a newly infected person). This newly infected person now has drug resistant virus, even though they have never taken any antiretroviral drugs.

Fortunately, drug resistance tests are now available so that both of these types of resistance can be detected and treatment decisions modified based upon these test results.

Drug holiday?
So many studies.....what to believe?

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