Synergism between recombinant human interferon and nucleoside antiviral agents against herpes simplex virus: examination with an automated microtiter plate assay.

An automated, quantitative, cytopathic effect (CPE) inhibition assay with human fibroblasts in 96-well microtiter plates was used to examine the combination of recombinant human interferon-alpha (rIFN-alpha A) and acyclovir, vidarabine, or dihydroxypropoxymethyl guanine against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) in vitro. Fifty percent CPE (CPE50) end points, calculated from optical density readings of crystal violet-stained monolayers in an automated spectrophotometer, represented 1.7 log reduction in viral yield (50-fold or 98% decrease). Using CPE50 end points of drugs alone and in combination, we defined synergism, additivism, or antagonism with an isobologram plot and a combination index equation. The combinations of rIFN-alpha A plus acyclovir and rIFN-alpha A plus dihydroxypropoxymethyl guanine were highly synergistic against both HSV-1 and HSV-2, whereas the combination of rIFN-alpha A plus vidarabine was additive to mildly synergistic. Combinations of antiviral agents synergistic in cell cultures should be pursued with further studies in animal models of human viral disease and potentially in clinical trials.