Sensitivity of HSV strains isolated before and after treatment with acyclovir.

Sequential isolates of herpes simplex virus (HSV) from 5 patients, treated for six episodes with orally administered acyclovir, were tested for in vitro sensitivity to the drug. In addition, isolates from an acyclovir treated immunocompromised patient with oral and genital herpes infections were examined for sensitivity to acyclovir. All isolates were tested in a production (24 h-yield) inhibition assay and defined as sensitive or resistant by the parallel examination of thymidine kinase (TK) positive and negative reference strains of HSV. No evidence for decreased or altered sensitivity to acyclovir was recorded of the genital HSV strains, isolated during seven episodes of recurrences. However, in agreement with the clinical response to therapy, genital isolates obtained from the immunocompromised patient pre- and post-treatment with acyclovir displayed in vitro sensitivity values indicating a rapid and complete development of resistance.