Resistance Testing: What is it? What does it mean?

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Resistance Testing: What is it? What does it mean?. How does drug resistance emerge? Overview of methods Advantages and disadvantages Current recommendations for use Jill Taylor, Ph.D. Director Viral Genotyping Laboratory Wadsworth Center New York State Department of Health. - PowerPoint PPT Presentation


<ul><li><p>Resistance Testing: What is it? What does it mean? How does drug resistance emerge?Overview of methodsAdvantages and disadvantagesCurrent recommendations for use</p><p>Jill Taylor, Ph.D.DirectorViral Genotyping LaboratoryWadsworth CenterNew York State Department of Health</p><p>TPED</p></li><li><p>Why do mutations occur in the HIV genome?HIV makes copies of itself very rapidly ~ 1-10 billion new virus particles/dayDuring its replication, HIV is prone to make errors when copying itself This results in mutations or errors in the genetic material of the virus which make the structure of the offspring virus slightly different to that of the parent virusSome of these mutations will result in an increased ability of the virus to grow in the presence of antiretroviral drugs</p><p>TPED</p></li><li><p>How drug resistance arises. Richman, DD. Scientific American , July 1998How drug resistance arises</p><p>TPED</p></li><li><p>How does this lead to drug resistance?When HIV replication is not completely blocked Sub-optimal therapy regimens e.g. monotherapyAdherence problemsPharmacokinetic problems: poor drug absorption, inadequate dosing or drug-drug interactions</p><p>These conditions can allow drug-resistant virus, already present in the population to dominate</p><p>TPED</p></li><li><p>Situations in which to consider drug resistance testing Patients on HAART showing inadequate viral load suppression or viral load increase after previous suppressionAcute infection - possibility of transmission of resistant virusHIV-infected pregnant women BUTOther factors to consider before resorting to resistance testing: adherence profile, toxicity, pharmacokinetic issues</p><p>TPED</p></li><li><p>How do you measure drug resistance?Phenotyping:Direct assay: Measures the ability of the virus to grow in various concentrations of antiretroviral drugs.Genotyping:Indirect assay: Detects drug resistance mutations that are present in the relevant virus genes.</p><p>Both assays focus on the reverse transcriptase (RT) and protease genes of the virus where many of the mutations occur</p><p>TPED</p></li><li><p>Basis of phenotyping assaysHIV circulating in the clients plasma is isolated and the RT and protease genes are copied. The copies are inserted into another strain of HIV which scientists are able to grow in the laboratory. This is now called a recombinant virus.</p><p>The recombinant virus is then grown in presence of known concentrations of antiretroviral drugs and its ability to grow in the presence of these drugs is compared to that of a reference strain of HIV that is known to be sensitive to the drugs.</p><p>TPED</p></li><li><p>Basis of phenotyping assays (cont.)The concentrations of drug required to inhibit the replication of the test and the reference virus are calculated. For example: </p><p>Concentration of Indinavir required to inhibit growth of the reference strain = 10MConcentration of Indinavir required to inhibit growth of the test strain = 100MTest virus is ten-fold less sensitive to Indinavir than reference strain</p><p>A report is generated documenting the decreased sensitivity to particular drugs</p><p>13-15 drugs are assayed, cost $880-$955 Turnaround time is 2 to 3 weeks</p><p>TPED</p></li><li><p>Phenotyping: AdvantagesProvides resistance information on each drug regardless of the presence of multiple mutationsInterpretation may be more intuitive than for genotype assayVery useful in patients with complex drug history and complicated mutation profileVery useful for deciphering cross-resistanceMay be more useful than genotyping for new drugs until appropriate mutations are established by clinical data</p><p>TPED</p></li><li><p>Phenotyping: DisadvantagesIf drug resistant population is minor the phenotypic effect may not be detectedCurrent limitation of use is that viral load needs to 1000 copies/ml - need greater sensitivityVery expensive and time-consumingThe relevance of small changes in drug sensitivity not yet fully determined - drugs to which patient is actually still sensitive may be unnecessarily eliminated</p><p>TPED</p></li><li><p>Sequence-based genotyping assaysHIV circulating in the clients plasma is isolated. The RT and protease genes are copied, amplified and sequencedThe test sequence is then compared to the sequence of a reference HIV strain and all changes (mutations) documented.Computer software then compares these changes to a list of the mutations known to be associated with drug resistance.Based on the presence or absence of particular mutations, a report is produced documenting the mutations and (optimally) the antiretroviral drugs to which the patient is sensitive or resistant</p><p>TPED</p></li><li><p>Genotyping: AdvantagesIdentification of all nucleotides, amino acid differences, deletions &amp; insertionsGenotyping has the ability to detect resistant virus that constitutes only a small proportion (about 20%) of the viral population. This can provide predictive early warning of developing resistance before full resistance developsFaster and less expensive than phenotype assay - cost is $400-$500 and turnaround time &lt; 2weeks</p><p>TPED</p></li><li><p>Genotyping: DisadvantagesReports may be difficult to interpret unless clinician is very experiencedLabs use different software programs to predict resistance - need a consensus on which mutations are importantThere is a lot of variation in the quality of the product from different laboratories especially in the ability to detect minority species in the populationCurrent limitation of use is that viral load needs to 1000 copies/ml - need greater sensitivity</p><p>TPED</p></li><li><p>Current recommendations for use of resistance testsGuidelines for use of antiretroviral agents in HIV-infected adults and adolescents. DHHS Panel on Clinical Practices for Treatment of HIV Infection and Kaiser Family Foundation.</p><p>Antiretroviral Therapy in Adults. Updated recommendations of the International AIDS Society-USA Panel. JAMA May 10, 2000 283(18): 2417-2426</p><p>TPED</p></li><li><p>Current recommendations (DHHS)Useful for selection of active drugs when changing antiretroviral regimens if viral load does not decrease Both phenotyping and genotyping may be useful in patients with complex prior treatment historyResistance testing may be useful in cases of acute infectionResistance testing not yet recommended at initiation of therapy in treatment-nave individuals until more information available on prevalenceDo recommend use of resistance testing in cases where viral load suppression is sub-optimal in initial regimenRecommendations are the same for pregnant and non-pregnant patients</p><p>TPED</p></li><li><p>What is the benefit for the client?If a client is failing on his current drug regimen these tests can help the physician determine whether the cause of treatment failure is the presence of drug resistant virus or some other reason e.g. adherence problems, toxicity problems</p><p>If there is resistant virus present, these tests help the physician understand which of the drugs the client is resistant to. The physician can then substitute drugs to which the client is sensitive. </p><p>Resistance testing may allow the physician to detect emerging drug resistance early and change the drug regimen before it becomes a big problem. This also saves drugs for potential later use.</p><p>TPED</p></li><li><p>Useful websites for further</p><p>TPED</p></li><li><p>For more HIV-related resources, please visit</p><p>TPED</p></li></ul>