Explain susceptibility testing for antiviral agents.
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Abstract
Despite increasing concern about drug-resistant herpes simplex virus (HSV), antiviral susceptibility testing is not routinely performed by most clinical virology laboratories. This omission is in large part because the most widely accepted method, the plaque reduction assay (PRA), is cumbersome to perform and results are rarely available in time to influence treatment. We report here the development of a sensitivity test for HSV which utilizes a cell line (VeroICP6LacZ#7) that expresses beta-galactosidase activity after infection with HSV such that infected cells can be detected by histochemical staining. We designed an assay in which 10-fold dilutions of virus stocks with undetermined titers were inoculated onto VeroICP6LacZ#7 cells in a 24-well tissue culture dish. Forty-eight hours after infection, the cell monolayers were histochemically stained. Plaques appear blue against a clear background and are thus easily visualized at 48 h. As with the standard PRA, the 50% inhibitory concentration (IC50) was reported as the concentration of an antiviral drug that reduces the number of plaques by 50%. Evaluation of 10 well-characterized laboratory strains and 12 clinical HSV isolates showed that the IC50 determined by this method correlated in all instances with the IC50 determined by the PRA. This method is easy to use and eliminates the need to determine the titer of the virus, and results are available within 48 h of the detection of the virus. VeroICP6Lac#7 cells are a useful tool for performing HSV antiviral susceptibility testing and could be used in a number of different formats to facilitate the identification of drug-resistant isolates of HSV.
Antiviral agents
Tests for antiviral susceptibility are still far from routine, being readily available only through reference or specialist laboratories. Experience with these assays is steadily increasing, and the clinical benefits of measuring antiviral susceptibility are gradually being established.
The detection of mutations associated with drug resistance is now a routine procedure in the management of HIV infection. Some reference laboratories have built up a great deal of experience with in-house sequencing of HIV genes and have generated large databases, which are shared on an international basis to allow for prediction of reduced susceptibility. Expert opinion on the significance of the mutations may usefully guide therapeutic decisions and is invaluable for selection of therapeutic combinations.