Uptake of [125I]Iododeoxycytidine by Cells Infected with Herpes Simplex Virus: A Rapid Screening Test for Resistance to Acyclovir

A rapid screening test for resistance to acyclovir, mediated by a lack of thymidine kinase (TK) activity in herpes simplex virus (HSV), was developed by utilizing the uptake of [125I]iododeoxycytidine (IdC) by infected Vero cells. Cells infected with TK+ virus demonstrate uptake of IdC within 3 hr of infection. The assay can detect as few as 690 pfu of virus. Cells infected with TK virus have an uptake of IdC similar to that of uninfected cells, whereas cells infected with TK+ generally have an uptake greater than 10 times that of uninfected cells if tested when obvious viral cytopathic effect is present. All 19 clinical HSV isolates tested were correctly identified as TK+. Of 17 blinded HSV isolates tested, all six TK- isolates were correctly identified. A single strain with an ID50 of 3.4 micrograms/ml and an altered TK substrate specificity was incorrectly classified as TK+. The assay is a useful, rapid screening test for viral TK activity.