Testing of herpes simplex virus for resistance to antiviral drugs

Herpes simplex virus (HSV) infections can be treated efficiently by the application of antiviral compounds. Several drugs on the basis of acyclovir (ACV), penciclovir (PCV) and foscarnet (FOS) have been approved. Resistant viral isolates can be observed especially in immunocompromised patients, who are treated with antivirals for long time intervals. That is why methods for analysis of HSV resistance to antiviral drugs have to be available in virological laboratories. We analyzed HSV type 1 (HSV-1) and type 2 (HSV-2) isolates resistant to ACV for correlation between phenotypic and genotypic drug resistance using tetrazolium reduction assay as well as sequencing of thymidine kinase (TK) and DNA polymerase (pol) genes. All strains were characterized as cross-resistant to PCV, brivudin and susceptible to cidofovir. In addition, three sequential isolates were resistant to FOS. Genotypic analysis revealed high polymorphism of TK among HSV-1 isolates and high polymorphism of DNA pol among both HSV-1- and 2 isolates. In nearly half of ACV-resistant strains, nucleotide insertions and deletions, responsible for a frameshift and the synthesis of a non-functional TK could be related to resistance. In the remaining strains, there were non-synonymous nucleotide substitutions which were not known as part of gene polymorphism. In conclusion, for reliable interpretation of genotypic resistance, a database of non-synonymous mutations has to be established.