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Genotype
and Phenotype
Resistance
testing generally involves determining the genotype or phenotype
of a virus. The genotype refers to the sequence of nucleotides in
the viral genome that determine the genes of the virus. Genotypic
assays are assays that determine the nucleotide sequence of specific
genes, or parts of genes. Viruses that have the same gene sequence
as other viruses found in nature (for example, from patients who
never received antiretroviral therapy) are considered to be Awild
type@. Genetic differences (mutations) that arise during the course
of antiretroviral therapy are considered to be drug resistance mutations
if the presence of these mutations reduces the susceptibility of
the virus to a particular drug. In some cases the nucleotide sequence
at a particular position in a gene may vary from one isolate to
the next, even in the absence of any drug treatment. Such differences
are often referred to as polymorphisms.
The phenotype
refers to the characteristics or properties of the virus. Phenotypic
assays for drug susceptibility determine the amount of drug needed
to inhibit viral growth in tissue culture. The amount of drug needed
to inhibit virus growth by 50% is called the 50% inhibitory concentration,
or IC50; similarly, the concentration of drug that inhibits virus
growth by 95% is known as the IC95. Testing a particular drug against
a large number of isolates from patients who never received antiretroviral
therapy can determine average IC50 for wild-type isolates of HIV-1.
Viruses that are inhibited by similar concentrations of that drug
are considered susceptible or sensitive; those that are inhibited
only at higher drug concentrations are considered resistant.
Results of
phenotypic assays are sometimes expressed as Afold-resistance@ by
comparing the IC50 of the patient=s virus to that of a control isolate.
For example, if the IC50 for zidovudine of the control isolate is
2 nM and the patient isolate has an IC50 of 20 nM, then the patient=s
virus would be 10-fold resistant as compared to the control. However,
the definition of Aresistant@ also needs to consider the concentration
of drug that can be achieved in the plasma and the relationship
between IC50 or fold-resistance and clinical response to the drug
in question.
4/15/01
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by HIV and Hepatitis.com. All Rights Reserved
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