HIV Drug Resistance and Resistance Testing
Can Drug-Resistance Tests Be Used to Choose a New Drug Regimen After an Initial One Fails?
Yes. As discussed in "Does a Rebound in Viral Load Mean That Drug Resistance Has Occurred?", viral load tests can help determine whether or not drug failure is occurring. Drug resistance tests, on the other hand, may play an invaluable role in helping doctors and their patients understand why failure has occurred and what treatment options are still available.
If viral load fails to become undetectable or becomes detectable again after a period of being undetectable, drug-resistance testing may help determine the cause. If no mutations are present (using genotypic assays) or the HIV is still sensitive to the drugs being used (using phenotypic assays), the problem might be poor adherence/compliance or poor absorption. It is best to remedy these problems before resistance mutations develop.
If mutations are found or HIV is determined to be losing sensitivity to the drugs being used, drug-resistance tests can help determine which of the remaining antiretroviral drugs might be effective against the virus.
If drug-resistance tests are not used, it is recommended that anyone who appears to be failing a particular combination should switch to an entirely new batch of drugs. This can be frustrating, as many HIV-positive people do not have three or more untried drugs from which to choose. It may also be a wasteful decision for those who do have several remaining options.
There have been a number of studies demonstrating that both genotypic tests and phenotypic tests can help patients and their healthcare providers choose a new regimen after an initial regimen has failed. Patients who use drug-resistance tests may be able to keep their viral load undetectable for a longer period of time than those who do not use the tests.
With drug resistance testing, it might also be possible to weed out the ineffective drug or drugs in a given combination. For example, in a study published in the Journal of the American Medical Association in January 2000 involving people taking an antiretroviral combination of Crixivan, Retrovir, and Epivir, 17 patients experienced viral load increases while receiving therapy. Although it would make sense to blame such viral load increases on multiple-drug resistance, resistance tests demonstrated that 14 patients had developed resistance to Epivir only; HIV in these patients could generally still be blocked by Crixivan.
Drug-resistance testing can also help determine what can be done about partial resistance. For example, a phenotypic test might determine that HIV is partially -- as opposed to completely -- resistant to a certain protease inhibitor (e.g., Crixivan). In this case, it might be possible to simply add another drug (e.g., a low dose of Ritonavir) to increase the amount of Crixivan in the body. By increasing the amount of Crixivan, there is more drug available to combat the partially resistant virus.
This article was provided by AIDS Community Research Initiative of America. Visit ACRIA's website to find out more about their activities, publications and services.