Viral resistance, clinical experience.

Over 1500 herpes simplex virus isolates from over 600 patients have been examined in the Wellcome Research Laboratories during the past 5 years using the dye uptake method in Vero cells to determine acyclovir sensitivity. No significant change in sensitivity of those isolates to acyclovir has been noted during that period, and the few isolates whose sensitivity significantly diminished during therapy were generally not associated with a clinical lack of response to therapy. In patients who were severely immunocompromised and who had received prolonged or repeated courses of therapy, however, less sensitive viruses were occasionally associated with poorly healing ulcers. The significance of these findings are discussed, as are problems in the interpretation of results of in vitro antiviral sensitivity testing and their extrapolation to clinical practice.