Summary: Resistance to anti-HIV drugs

• Antiretroviral drugs are intended to prevent HIV from reproducing.
• Mutated variants of HIV can emerge which are able to replicate despite the presence of a drug.
• These drug-resistant viruses then replicate into a large drug-resistant viral population.
• Resistance is one of the main reasons for drug treatment failure.
• Virus populations can develop resistance to more than one drug. This is why combination therapy may fail.
• Once resistance to one drug has emerged, this virus population may also be resistant to other drugs in the same class, even if they have never been taken before. This is called cross-resistance.
• The highest chance of resistance occurs when drug levels in the bloodstream are not high enough to stop HIV from reproducing. This can happen if doses are missed, taken improperly, or not absorbed properly, or due to drug interactions.
• The chance of resistance is therefore reduced by taking every dose of anti-HIV drugs as prescribed. The more doses are missed, the more likely it is that resistant viruses will emerge.
• Continuing to take the same drugs after treatment begins to fail, i.e. when viral load begins to go up, also encourages the development of resistance to those drugs.
• People who inject drugs such as heroin may develop resistance more quickly than people who do not inject, probably due to poor adherence.
• Tests have been developed to detect which drugs a person's HIV is resistant to, and the level of resistance to them. British and European guidelines recommend the use of resistance testing to guide choice of therapy in people starting or changing treatment and in recently infected individuals.
• The results of resistance tests are best interpreted by clinicians who are experts in the area of drug resistance.
• The case for resistance testing to guide the choice of first-line therapy in people who have been infected for several years is less strong than in people who are recently infected because tests taken in later disease are unlikely to detect minority resistant viruses. Storing samples from early in infection is desirable where possible.
• Resistance testing may also be useful in pregnant women and newborn infants.
• Transmission of drug-resistant virus appears to be increasing in the United Kingdom and the United States.
• The standardisation of these tests and interpretation of results is an ongoing process.