1) What is drug resistance?

• There are many types of germs, or pathogens, that can enter the human body. These include viruses, fungi, bacteria, and protozoa. Once inside the body, the primary goal of a germ is to survive and reproduce.
  
  
• Pharmaceutical drugs are designed to target these germs and kill them or prevent them from reproducing inside the body. If a germ continues to reproduce during treatment, it can alter itself— or mutate— to avoid the drugs. This is called drug resistance.
  
  
• When drug resistance occurs, the drug— or combination of drugs— loses its ability to block the germ from reproducing. Over time, the treatment can stop working completely. It is important to prevent germs from reproducing during treatment to prevent drug resistance from occurring.

2) How does HIV drug resistance occur?

• As discussed in Question 1, HIV drug resistance means a reduction in the ability of a drug— or combination of drugs— to block HIV reproduction in the body.
  
  
• Drug resistance occurs as a result of mutations in HIV's genetic structure. HIV's genetic structure is in the form of RNA, a tight strand of proteins needed by the virus to infect cells and produce new virus.
  
  
• Mutations are very common in HIV. HIV reproduces at an extremely rapid rate and does not contain the proteins needed to correct mistakes made during the copying of genetic material.
  
  
• Two of the most important HIV enzymes are reverse transcriptase and protease.
  
  
• Nucleoside analogues [Retrovir (AZT), Videx (ddI), Hivid (ddC), Zerit (d4T), Epivir (3TC), and Ziagen (abacavir)] and non-nucleoside reverse transcriptase inhibitors [Viramune (nevirapine), Rescriptor (delavirdine), and Sustiva (efavirenz)] target the reverse transcriptase enzyme.
  
  
• Protease inhibitors [Fortovase (invirase), Crixivan (indinavir), Norvir (ritonavir), Viracept (nelfinavir), Agenerase (amprenavir), and Kaletra (lopinavir)] target the protease enzyme.
  
  
• In order for these antiretroviral drugs to be effective, they must attach themselves to the necessary enzyme. Certain mutations can prevent a drug from binding with the enzyme and, as a result, make the drug less effective against the virus.
  
  
• HIV drug-resistance mutations can occur both before and during therapy.

3) How do mutations occur before antiretroviral therapy is started?

• Mutations that occur before antiretroviral therapy is started can happen in two ways: natural selection and transmission of drug-resistant virus.
  
  
• Natural selection: Soon after HIV enters the body, the virus begins reproducing at a rapid rate (approximately 10 billion new viruses every day). In the process, HIV produces both perfect copies of itself (wild-type virus) and copies containing errors (mutated virus). In other words, there is no single virus in the body but, instead, a large population of mixed viruses called quasi-species.
  
  
• When HIV reproduces, it wants to be wild-type virus. This is the most natural and usually most powerful form of the virus and, as a result, reproduces the best. Before antiretroviral therapy is started, wild-type virus is the most abundant in the body and dominates all other quasi-species in the body.
  
  
• When HIV makes mistakes during copying, mutated viruses— called variants— are produced. Some variants are too weak to survive and/or cannot reproduce. Other variants are strong enough to reproduce but still are not able to compete with the more fit wild-type virus. As a result, their numbers are less than wild-type virus in the body.
  
  
• Some variants have mutations (sometimes called polymorphisms) that allow the virus to partly, or even fully, resist an antiretroviral drug. This is why people living with HIV should never take just one antiretroviral drug (monotherapy). For example, HIV only requires one mutation— M184V— to become completely resistant to Epivir. The same goes for the non-nucleoside reverse transcriptase inhibitors Viramune, Rescriptor, and Sustiva. The K103N mutation can cause the virus to become highly resistant to these three drugs.
  
  
• These HIV mutations occur randomly and there is no proven way to prevent them from occurring. Variants containing these mutations usually don't go on to develop additional mutations; doing so compromises their ability to stay alive in the body. So while these variants may be completely resistant to one antiretroviral drug, they are almost always sensitive to other drugs used in a regimen. This is why three-drug regimens work better: a variant may be resistant to one of the drugs but doesn't stand much of a chance when facing two other drugs that bind to different parts of the virus.
  
  
• Transmission of drug-resistant virus: This is a growing problem and is being reported by many researchers.
  
  
• Many HIV-positive people now take or have taken antiretroviral therapy. If someone has developed resistance to one or more of the antiretrovirals and has unprotected sex or shares needles with someone who is not infected with the virus, it is possible that they can infect their partner with a drug-resistant variant— a strain of HIV containing mutations that cause resistance to one or more antiretroviral.
  
  
• An example: Person A is HIV-positive and has been taking a triple-drug antiretroviral regimen consisting of Crixivan, Retrovir, and Epivir. He does not know it, but his virus contains mutations associated with resistance to these three drugs. He has unprotected sex with Person B, an HIV-negative woman. Person A's virus enters Person B's body and begins reproducing. The end result is that Person B has been infected with a multiple-drug-resistant (MDR) variant of HIV.
  
  
• At first, the MDR variant in Person B's body would dominate all other viruses that are produced during copying. Over time, wild-type virus will emerge and usually dominates the MDR variant. But this does not mean that the MDR variant is gone; it has merely become a minority member of the entire population of HIV.
  
  
• If Person B were to start therapy a few years later with Crixivan, Retrovir, and Epivir, the wild-type HIV would diminish quickly, but would probably be replaced with the MDR variant already in her body. As a result, Person B might have a difficult time reducing her viral load or keeping her viral load undetectable.
  
  
• According to some studies, between 10% and 30% of all new HIV infections (defined as people infected with HIV over the past two years) involve strains resistant to at least one antiretroviral drug. Some researchers expect that this percentage will increase in the years to come.
  
  
• It might also be possible for someone who is already infected with HIV to be infected, again, with a (multiple) drug-resistant strain of HIV. This is sometimes referred to as reinfection or superinfection. There has been at least one report demonstrating that this may be possible.

4) How do mutations occur during antiretroviral therapy?

• Soon after antiretroviral therapy is started, the amount of virus in the body is reduced dramatically. Unfortunately, no antiretroviral drug— or combination of drugs— is able to completely stop HIV from reproducing. In other words, there is always a small population of virus in the body that continues reproducing, despite the presence of antiretroviral drugs.
  
  
• As discussed in Question 3, there is a large mixture of virus in an HIV-infected person's body. Antiretroviral-drug therapy reduces the amount of all HIV quasi-species in the body. The amount of wild-type virus is dramatically reduced, and the number of variants is also decreased.
  
  
• Wild-type virus is the most sensitive to antiretroviral drugs. Because of this, HIV variants in the body have a survival advantage over that of wild-type virus. In the presence of antiretroviral drug therapy, variants can become the dominant strain of HIV, even though there is a much smaller amount of HIV in the body.
  
  
• Over time, variants accumulate additional mutations. Some of these mutations will harm the virus, while others will further limit a drug's ability to stop it from reproducing. Once the virus has accumulated enough mutations, the antiretroviral drugs lose their ability to bind to it and prevent it from reproducing. As the drugs become weaker, the amount of drug-resistant virus in the body increases, causing an undetectable viral load to become detectable again and increase over time. If the drug-resistant virus continues to reproduce, it can acquire even more mutations to resist the antiretroviral drugs completely.
  
  
• Mutations that emerge during therapy can be divided into two groups: primary mutations and secondary mutations. Each antiretroviral drug is associated with at least one primary mutation. Primary mutations are of greatest concern, as they are the ones that cause the greatest amount of drug resistance. Secondary mutations do not cause drug resistance unless a primary mutation is present. If both primary and secondary mutations are present, drug resistance can become more complicated.
  
  
• While primary and secondary mutations can cause the virus to become resistant to anti-HIV drugs, they usually have a negative effect on the power of the virus. This is why some people who are experiencing an increase in their viral load might not see a decrease in their CD4+ cell counts, at least not at first. In other words, the virus loses its ability to cause damage to the immune system if it contains drug-resistance mutations. However, some studies show that certain primary and secondary mutations can cause the virus to regain its power and, quite possibly, become even more powerful than wild-type virus. Because of this, most experts recommend switching therapies before the virus accumulates any additional mutations.
  
  
• Cross-resistance can also occur during therapy. When HIV becomes resistant to one drug, it can automatically become resistant to other drugs in the same class. For example, the primary and secondary HIV mutations that occur in someone who is taking the protease inhibitor Crixivan are the same mutations that cause resistance to the protease inhibitor Norvir. Even though the person hasn't yet taken Norvir, he or she will likely be cross-resistant to the drug and will not likely benefit from it.
• The key to avoiding the accumulation of mutations that cause resistance and cross-resistance is to keep the amount of virus in the body as low as possible, for as long as possible.

5) What are some of the factors that contribute to the accumulation of drug-resistance mutations during therapy?

• Don't forget the golden rule: the less virus there is in the body, the less likely it is that the virus will continue reproducing and mutating. A powerful antiretroviral regimen is the most effective way to keep the level of virus low— preferably undetectable (< 50 copies/mL)— and to delay additional mutations from occurring.
  
  
• There are a number of factors that can prevent an antiretroviral drug regimen from being as powerful as it can be. These include:
  
  
• Poor adherence or compliance. In order for antiretroviral drugs to work correctly, they must be taken exactly as prescribed. This means taking the correct number of pills each day, being careful to take them a certain number of hours apart, while at the same time following dietary requirements (see "poor absorption" below).
  
  
• Skipping doses or not taking medication correctly can cause the trough level of an antiretroviral drug to decrease in the body. The trough level refers to the amount of drug left in a person's body just before another dose of the drug is taken by mouth. If the trough level becomes too low, HIV can reproduce more freely and accumulate additional mutations.
  
  
• According to a few research reports, an HIV-positive person must be more than 95% adherent with his or her antiretroviral drug regimen in order for it to continue working properly. This means missing less than one dose a month.
  
  
• Poor absorption. Not only must antiretroviral drugs be taken on schedule, they also need to be absorbed effectively into the bloodstream. A drug-or combination of drugs-that is not absorbed properly can result in trough levels that are too low and, ultimately, allow HIV reproduction and the accumulation of drug-resistance mutations.
  
  
• Certain drugs have specific dietary requirements. For example, people taking standard doses of the protease inhibitor Crixivan must take the drug three times a day (every eight hours) on an empty stomach. This means not eating within two hours before or one hour after taking the drug. (Note: If Crixivan is taken in combination with Norvir, another protease inhibitor, food restrictions do not apply.) The same goes for people who take the buffered formulation of Videx (the tablets that need to be chewed or mixed in water). Conversely, the protease inhibitor Fortovase should be taken with food, preferably food containing a moderate amount of fat. If dietary requirements are not followed while taking any of these drugs, drug levels in the body will decrease.
  
  
• People with HIV can also experience diarrhea and vomiting. These can cause antiretroviral drugs to be expelled from the gut too quickly, reducing the amount of drug absorbed into the bloodstream.
  
  
• Varying pharmacokinetics. Pharmacokinetics is a term used by researchers to mean how a drug is absorbed, distributed, metabolized, and removed from the body.
  
  
• Even though two people might receive the exact same dose of a drug, the amount of drug may be higher in one person's bloodstream than in the other person's bloodstream. Factors that can contribute to this difference include their body weight, height, and age. Some people also process, or metabolize, drugs faster or slower than others do. This can speed up— or slow down— the rate at which a drug is cleared from the body.
  
  
• It is important to remember that a drug's correct dose— the dose dispensed by pharmacists— is determined in clinical trials based on the average dose found to be safe and effective. In other words, some people may be able to keep their viral load undetectable using lower doses of the drug, while some people might require higher doses of the drug to keep their viral load undetectable.
  
  
• In the future, healthcare providers may begin performing blood tests to measure the amount of drug in their patients' bodies. This is called therapeutic drug monitoring and it may help determine whether or not a person has a correct trough level of each medication to ensure that viral load remains low or undetectable.

6) Does a rebound in viral load mean that drug resistance has occurred?

• Figuring out if an antiretroviral drug regimen is not working properly can be determined in three ways:
  
  
  1. A viral load that fails to go undetectable within the first few months of therapy.
  2. A viral load that goes from being undetectable to detectable (note: a one-time "blip" in viral load is not usually a sign that a drug regimen is no longer working).
  3. A detectable viral load continues increasing, even though antiretroviral drug therapy is still being taken.
     
     
• While viral load can help determine whether or not an antiretroviral drug regimen is still working correctly, it cannot explain why a regimen is no longer working the way it should.
• A detectable or increasing viral load does not necessarily mean that drug-resistance mutations have occurred. A detectable viral load may be due to poor adherence or poor absorption. While these can eventually lead to the emergence of drug-resistance mutations, viral load can become detectable before they develop. Thus, it is important to act quickly and determine the reason why viral load is increasing soon after it becomes detectable.
  
  
• If resistance mutations have developed, viral load tests cannot determine whether or not the virus is resistant to one specific drug or the entire regimen. Moreover, in a person with drug-resistant HIV, viral load testing cannot determine which drug or combination of drugs is likely to be the most effective in the future.
  
  
• To look for drug resistance, there are two tests, or assays, available to people living with HIV and their healthcare providers. The first is called genotypic testing. Genotypic tests can help determine whether specific mutations are causing drug resistance and drug failure. The second method, called phenotypic testing, is a more direct measure of resistance and, more specifically, of the sensitivity of a person's HIV to particular antiretroviral drugs.

7) What is genotypic testing?

• Genotypic resistance testing examines the actual structure— or genotype— of HIV taken from a patient (a standard blood sample is all that is required). The HIV is examined for the presence of specific mutations that are known to cause resistance to certain drugs.
  
  
• An example: As discussed in Question 3, researchers know that Epivir is not effective against forms of HIV that contain the mutation M184V in its reverse transcriptase enzyme. If a genotypic resistance test discovers a mutation at position M184V, chances are that the person's HIV is resistant to Epivir and is not likely to respond to the drug.
  
  
• For many drugs, including the protease inhibitors, complex patterns of mutations are required for resistance to occur.
  
  
• To conduct a genotypic test, laboratories use PCR technology to make many copies of, or "amplify," the HIV genetic material. Once amplification has been completed, the genetic sequences of particular viral enzymes— such as reverse transcriptase and protease— can be examined carefully for mutations. Depending on the type and number of mutations found, the laboratory can determine whether someone has developed resistance to a specific drug, since almost all drugs follow a set pattern of mutations.
  
  
• There are actually two types of genotypic tests: sequencing assays and point-mutation assays. Sequencing assays look for any mutation in either the reverse transcriptase or protease enzymes. Point-mutation assays look for key mutations in these enzymes that are known to cause drug resistance. Most laboratories use point-mutation assays, as they are easier (and cheaper) to perform and their results are easier to interpret.
  
  
• For genotypic tests to be accurate, they generally require the use of a blood sample from a person who is actively taking antiretroviral medication and has a viral load higher than 1,000 copies/mL.
  
  
• If therapy is stopped before blood is drawn for a genotypic test, the wild-type virus in the body may outgrow the mutant virus. In turn, the results may not show any drug-resistant mutations, but the drug-resistant strain may still remain at very low numbers in the person's body and may quickly increase when therapy with the same drugs is restarted.
  
  
• Genotypic resistance testing can take as little as a few days to complete. A single genotypic test can cost between $300 and $500.

8) How are genotypic test results reported?

• When a genotypic testing report comes back from the lab, it contains a listing of the mutations that were found in the virus' reverse transcriptase and protease enzymes. It's important to understand how these mutations are reported.
  
  
• An example: The M184V mutation is responsible for causing resistance to Epivir. The 184 refers to the amino acid position, or codon, in the reverse transcriptase enzyme. The M— which stands for methionine— is the amino acid at position 184 of a wild-type virus' reverse transcriptase enzyme. The V— which stands for valine— refers to the mutation that results in drug resistance. In other words, the amino acid methionine at position 184 has been replaced by a valine. This change prevents Epivir from binding with the enzyme to prevent the virus from reproducing.
  
  
• While researchers have identified a number of mutations that can cause drug resistance, they don't know everything there is to know about these mutations. We know that some combinations of mutations cause the virus to become more resistant to antiretroviral drugs than other combinations of mutations. Researchers are still trying to determine which sequences of mutations are the most important.
  
  
• Some genetic mutations have yet to be fully identified by researchers. Such is the case with drugs like Videx and Zerit. In people who take these two drugs, resistance certainly does occur. However, researchers are only beginning to determine the exact genetic mutations that cause HIV to become less sensitive to these drugs.
  
  
• Mutations known to cause resistance to Retrovir and Epivir can also be misleading. For example, a genotypic resistance test may show that a person's HIV has several genetic mutations that cause resistance to Retrovir. However, if the person is also taking Epivir-which appears to increase HIV's sensitivity to Retrovir-such genetic mutations may not accurately reflect the amount of Retrovir resistance.
  
  
• Another limitation: genotypic tests do not evaluate the genetic structure of small HIV populations found in a blood sample. For example, there might be a population of HIV that contains a mutation at position M184V (the mutation that causes resistance to Epivir). Unless this particular strain accounts for more than 20% of the HIV population found in a blood sample, chances are that it will not be recognized by the test.

9) What is phenotypic testing?

• Unlike genotypic testing, which looks for particular genetic mutations that causes drug resistance, phenotypic testing directly measures the sensitivity-or phenotype-of a patient's HIV in response to particular antiviral drugs.
  
  
• Phenotypic resistance tests measure the concentration of a drug required to inhibit viral replication in the test tube by a defined amount such as 50% or 95%. This is called IC50 or IC95. IC stands for inhibitory concentration. In other words, a laboratory conducting a phenotypic test is trying to determine the amount of drug needed to stop HIV from reproducing. If it only takes a standard amount of the drug— a concentration equal to that used by HIV-positive people— HIV is not resistant to the drug. If higher amounts of the drug are needed to stop HIV from reproducing, HIV is considered to be resistant to the drug being tested.
  
  
• The concentration of drug necessary to inhibit virus replication is expressed in units called nanomoles (nM). For example, if the IC50 of the wild-type virus is 100nM and that of the test virus is 400nM, the test virus is considered to be fourfold resistant to the drug being tested. In other words, HIV in the patient is fourfold less sensitive to the drug.
  
  
• Unlike genotypic tests, the phenotypic resistance test generally does not require a high viral load. Like genotypic testing, however, it is recommended that patients be taking antiretroviral therapies at the time blood is drawn for the test.
  
  
• Because phenotypic testing directly measures the sensitivity of the virus to particular drugs, many researchers believe that these tests are more comprehensive and trustworthy than genotypic tests.
  
  
• Phenotypic resistance testing procedures are relatively complex and can take longer than genotypic tests to produce accurate results— from ten days to several weeks. They are also more expensive than genotypic tests. A single phenotypic test can cost between $700 and $900.
  
  
• Phenotypic tests cannot evaluate the sensitivity of small HIV populations found in a blood sample. For example, there might be a population of HIV that is not sensitive to Epivir. Unless this particular strain accounts for more than 10% to 20% of the HIV population found in a blood sample, chances are that it will not be recognized by the test.
  
  
• Another challenge is that researchers still do not fully understand what level of resistance translates into a failure of treatment. For example, a five-, six-, or sevenfold reduction in the sensitivity of HIV to a protease inhibitor is considered "moderate." But is there a significant difference between a fivefold reduction and a sevenfold reduction? Researchers are still trying to figure out what level of resistance determines that a drug is no longer useful.

10) What about using genotypic and phenotypic tests together?

• Using both tests together could certainly help deal with some of the weaknesses of each test administered individually. However, this can be very expensive and time consuming.
  
  
• A test is now being developed in Europe that combines genotypic testing results with phenotypic testing results. This test, called VirtualPhenotype, first analyzes HIV's genotype. Once the genotype has been determined, the laboratory searches a database containing the genotypes of more than 65,000 HIV samples collected from other patients. Then it retrieves the phenotypes— the IC50s and IC90s— that correspond to these samples, averages the information together, and predicts the drugs that the current sample might be sensitive to or resistant to.

11) Can drug-resistance tests be used before someone starts antiretroviral therapy for the first time?

• Maybe. Based on what is known about HIV's error-prone reproduction process (see Question 3), it is safe to assume that all HIV-infected people have at least a few forms of HIV that are resistant to individual drugs before therapy is started. However, these strains are often too limited in number and strength to compete with wild-type virus, and they stand a good chance of being killed off by starting combination antiretroviral therapy. In other words, genotypic or phenotypic testing might not provide an accurate picture of drug resistance before therapy is started.
  
  
• Drug-resistance tests might prove to be useful for people infected with multiple-drug-resistant (MDR) strains of HIV. Soon after an MDR strain enters the body, it begins reproducing. Over time, a wild-type strain of HIV emerges and dominates the viral population. Thus, in order for drug-resistance tests to be used, blood will probably need to be drawn soon after infection takes place (i.e., within a few weeks after infection occurs). Unfortunately, only a small percentage of people know when they are infected or immediately go to see a healthcare provider.

12) Can drug-resistance tests be used to choose a new drug regimen after an initial one fails?

• Yes. As discussed in Question 6, 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.

13) Do specialists recommend drug-resistance tests?

• Yes. Two important groups of medical experts now recommend that drug resistance tests be used in helping HIV-positive people plan their treatment regimens, especially if a switch in therapies is needed. One group that recommends drug resistance testing is the United States Department of Health and Human Services (DHHS), a branch of the federal government that oversees public health in the United States. A second group that recommends these tests is the International AIDS Society-USA (IAS-USA), a private medical organization made up of many leading HIV/AIDS experts in the United States and elsewhere.

14) How can drug resistance be avoided?

• There are a number of steps that HIV-positive people can take to prevent-or at least slow down-the development of resistance:
  
  
• Learn as much as possible about anti-HIV drugs. The more people with HIV know, the easier it will be to make treatment choices that can help avoid drug resistance.
  
  
• Start treatment with a powerful anti-HIV regimen. The first antiretroviral drug regimen an HIV-positive person takes may be their best chance to fully suppress the virus and prevent the development of drug resistance.
  
  
• Be sure to follow instructions. As discussed in Question 5, it is very important that HIV-positive people take their antiretroviral medication exactly as prescribed. Missing doses, not taking the right number of pills, or eating when pills need to be taken on an empty stomach, can all cause viral load to increase and cause drug-resistance mutations to develop.
  
  
• Good communication with a healthcare provider. HIV-positive people should understand their doctor's instructions on how antiretroviral medication should be taken. Asking questions and reporting any problems to a healthcare provider are important for avoiding drug resistance.
  
  
• Regular viral load testing matters. An increasing viral load is often the first sign that drug resistance is developing. Monitoring viral load regularly is a good way to guard against drug resistance.

No resistance tests have been approved by the Food and Drug Administration (FDA) so far. There are a number of tests currently being used, however. Many health insurance plans, including private insurance and some state ADAP (AIDS Drug Assistance Program) and Medicaid programs cover them.

Resistance Tests in Use:

Genotype Tests
TrueGene HIV-1 (Visible Genetics)
VircoGEN (Virco) (processed in US by LabCorp)
INNO-LiPA (Line Probe Assay) (Innogenetics)
HIV-1 GentypR (Specialty Labs)

Phenotype Tests
PhenoSense HIV (Virologic)
Antivirogram (Virco)

GLOSSARY

Adherence
The degree to which a patient exactly follows a prescribed treatment regimen. Poor adherence may negatively impact a drug's effectiveness. Compliance is an alternate term.

Amino Acid
A nitrogen-containing molecule that serves as a building block for proteins, including enzymes, muscles, and structural molecules. The human body uses twenty of the eighty amino acids found in nature.

Antiretroviral
A substance that stops or suppresses the activity of a retrovirus such as HIV. AZT (Retrovir), ddC (Hivid), ddI (Videx), and d4T (Zerit) are examples of antiretroviral drugs. Antiviral is sometimes used as an alternate term.

Assay
A test.

bDNA (branched DNA)
A test for measuring the amount of HIV and other viruses in the blood. Test results are reported in numbers of virus particle equivalents per milliliter of plasma. See also PCR.

Codon
A three-nucleotide genetic subunit that determines which amino acid is placed at one point in a protein chain. Mutations at specific HIV codons are associated with changes in the amino acid sequence of HIV's proteins and enzymes. Such mutations can cause HIV to become resistant to antiretroviral drugs.

Cross-Resistance
The phenomenon by which HIV and other disease-causing organisms become resistant to more than one drug after a single therapy. For example, people who develop resistance from taking one non-nucleoside reverse transcriptase inhibitor (NNRTI) are likely to be cross-resistant to other drugs in the same class.

DNA (Deoxyribonucleic Acid)
A double-stranded molecule that makes up the chromosomes in the center of a cell and that carries genetic information in the form of genes.

Enzyme
A cellular protein whose shape allows it to hold together several other molecules in close proximity to each other. Enzymes also induce chemical reactions in other substances.

First-Line Treatment
The best starting therapy for someone who has never received therapy before. Because of the potential for the development of cross-resistance by HIV and other microbes, the choice of first-line medication(s) affects the efficacy of subsequent therapies.

Gene
A unit of DNA in the chromosomes that determines the structure of a specific protein or enzyme. Genes regulate the metabolism of individual cells and the development and specialization of body cells and tissues.

Genotype
The genetic makeup of an individual organism, determined by the sequence of nucleotides in its genes. See also Phenotype.

Genotypic Assay
A blood test that determines the genetic sequences of an organism. Frequently performed in HIV to establish whether certain mutations conferring drug resistance are present. See also Phenotypic Assay; Resistance.

HAART (Highly Active Antiretroviral Therapy)
Potent antiretroviral treatment usually including a combination of three or more drugs whose purpose is to reduce viral load to undetectable levels.

IC (Inhibitory Concentration)
The amount of drug in the blood needed to suppress the reproduction of a disease-causing microorganism such as HIV. For example, IC95 is the drug level needed to block 95% of HIV's normal replication; IC50 is the drug level needed to block 50% of HIV's normal replication.

NNRTI (Non-Nucleoside Reverse Transcriptase Inhibitor)
A member of a class of compounds - including efavirenz (Sustiva), delavirdine (Rescriptor), and nevirapine (Viramune) - that acts directly to combine with and block the action of HIV's reverse transcriptase to prevent viral RNA from being converted into DNA and integrated into the uninfected cell's nucleus. NNRTIs have suffered from HIV's ability to mutate rapidly and become resistant to their effects.

Nucleoside
The molecular units that serve as the building blocks of DNA and RNA, the genetic material found in living organisms.

Nucleoside Analog
A type of antiviral drug, such as AZT (Retrovir), ddI (Videx), ddC (Hivid), d4T (Zerit), 3TC (Epivir), or abacavir (Ziagen) whose structure constitutes a defective version of a natural nucleoside. Like NNRTIs, these drugs block the viral enzyme responsible for converting HIV RNA into DNA, ultimately preventing the cell from becoming infected.

PCR (Polymerase Chain Reaction)
A sensitive test that amplifies DNA. PCR is a critical part of tests for viral load, genotyping, and phenotyping.

Phenotype
The actual appearance, behavior, or activity of an organism. HIV's phenotype is affected by its genotype; mutations in its genetic structure can alter its behavior. See also Genotype.

Phenotypic Assay
A test that measures the sensitivity of HIV to specific antiretroviral drugs. It is considered more of a direct measure of HIV drug resistance than genotypic tests. See also Genotypic Assay.

Protease
An enzyme that triggers the breakdown of proteins. HIV's protease enzyme breaks apart long strands of viral protein into the separate proteins constituting the viral core and the enzymes it contains. HIV protease acts as new virus particles are budding off a cell membrane.

Protease Inhibitor
A drug that binds to and blocks HIV protease from working, thus preventing the production of new functional viral particles. Examples include saquinavir (Fortovase), ritonavir (Norvir), indinavir (Crixivan), nelfinavir (Viracept), amprenavir (Agenerase), and lopinavir (Kaletra).

Protein
Large molecules made up of long sequences of amino acids. Some hormones and all enzymes and cellular structural components are proteins.

Resistance
Reduction in an organism's sensitivity to a particular drug. Resistance is thought to result mainly from a genetic mutation. In HIV, such mutations can change the structure of viral enzymes and proteins so that an antiviral drug can no longer bind with them. Resistance detected by searching a pathogen's genetic makeup for mutations believed to confer lower susceptibility is called genotypic resistance. Resistance found by successfully growing laboratory cultures of the pathogen in the presence of a drug is called phenotypic resistance.

RNA (Ribonucleic Acid)
A single-stranded molecule composed of nucleotide sequences. It is similar in basic structure to half of the double-stranded DNA. In cells, RNA transmits the code from the DNA-based genes that instruct the cells' chemical machinery to produce structural proteins and enzymes. In retroviruses, RNA is the sole repository of the viral genes.

Sensitivity
The degree to which an organism is affected by a drug. See also Resistance.

Wild-Type Virus
Naturally occurring HIV with an optimal genetic makeup and no laboratory-induced mutational defects. The term also refers to HIV that has not been exposed to antiviral drugs and therefore has not accumulated mutations conferring drug resistance.

Written by: Tim Horn
Edited by: James Learned and Jerome Ernst, MD

This brochure was originally produced with support from the Westchester County Department of Health.

Copyright 2001 Community Research Initiative on AIDS (CRIA). All rights reserved. Reproduction of this booklet is encouraged as long as it is reproduced in its entirety and full credit is given to CRIA.