A comparative study of the inhibition of herpes virus types 1 and 2 by nucleoside analogues.

Assessment of the relative potencies of compounds which inhibit the multiplication of herpes virus is rendered difficult by the fact that they have been investigated by different workers who were not using comparable test systems. The aim of this work was therefore to evaluate such test systems as could be used for studying the chemotherapy of herpes virus infections with particular reference to their suitability for determining relative potencies. Both in vitro and in vivo systems were investigated. Tissue culture methods included parallel observations of the time of appearance of cytopathic effect in the presence and absence of the test compound, carried out in tubes and microtitre plates. The compounds were also examined in plaque inhibition and plaque reduction tests, by the gradient plate technique, and by determining the reduction in virus yield produced by the test compound in single-cycle experiments. The gradient plate technique gave a visual indication of the minimal inhibitory concentration. With the other methods dose-response lines were obtained, the most satisfactory being given by the plaque reduction method. Cytarabine was the most active compound, the ID50 with a standard strain of type 1 herpes being 0. 23 muM. The values for the other compounds were 0. 76 muM for trifluorothymidine, 0. 89 muM for idoxuridine, 14. 96 muM for vidarabine and 20. 89 muM for diaminopurine arabinoside. The compounds fell in the same order in tests with type 2 strains, except that 2 strains were found to be insensitive to trifluorothymidine and diaminopurine arabinoside. The in vivo models used were herpetic encephalitis in mice and herpetic keratitis in rabbits. Dose-response lines could be obtained in mice by taking the mean reciprocal survival time as the response, but they were not as satisfactory as those obtained by plaque reduction. The keratitis model could not be used for determinations of relative potency, but it was satisfactory for assessing the therapeutic effect of different concentrations of the test compound, and thus for ranking comparable solutions of the different compounds in order of activity. Satisfactory clinical cure was obtained with all compounds regardless of their relative potencies as determined in tissue culture. From the results of both in vitro and in vivo test systems it was possible to demonstrate variations in sensitivity between types and between strains of the same type. There are several reports in the literature that type 2 strains of herpes virus are less sensitive than type 1. In the present work this was found to be not the case, since there was a wide overlap in the sensitivities of the strains of the two types, with some type 2 strains being considerably more sensitive than those of type 1. The present studies have thus established satisfactory test methods for the quantitation of the relative potency of the standard anti-herpes compounds, which will be valuable in the assessment of the antiviral potential of compounds to be discovered in the future