Variables influencing the in vitro susceptibilities of herpes simplex viruses to antiviral drugs.

Some of the variables in a microtiter tissue culture method for estimating antiviral susceptibility of herpes simplex viruses (HSV) were studied. Six antiviral drugs (5-iodo-2'-deoxyuridine, cytosine arabinoside, 9-beta-arabinofuranosyladenine, isoprinosine, virazole, and 2-deoxy-d-glucose) and three tissue culture cell lines (primary rabbit kidney, WI-38, and rat brain) were studied. Four different inocula (10, 100, 1,000, and 2,000 mean tissue culture doses) and nine isolates of HSV were tested. Minimal inhibitory concentration was defined as a complete inhibition of cytopathogenic effect in tissue culture cells after 72 h of incubation. There was a direct relationship between the viral inocula and the minimal inhibitory concentration. There were also differences between the three tissue culture cell lines and among the different strains of herpesvirus. This was not type specific, and the strain differences were not consistent in different tissue culture lines. Cytosine arabinoside appeared to be the most active drug in vitro, with 5-iodo-2'-deoxyuridine intermediate in activity between cytosine arabinoside and 9-beta-arabinofuranosyladenine; no significant cytotoxicity was encountered with these drugs. Isoprinosine, virazole, and 2-deoxy-d-glucose were not effective against any of the nine strains of HSV tested in this in vitro system; a comparison with previous studies indicates that their activity may be cell dependent. These results emphasize the importance of careful evaluation of in vitro test methods for the susceptibility testing of antiviral agents against HSV. Marked variability between strains and in vitro test systems caution against the application of these methods to in vivo situations until further studies are carried out.