Effect of neutral red and light on Herpesvirus hominis type 1 in cell culture

Various concentrations of neutral red were added to monolayers of muscle-skin fibroblasts after adsorption of Herpesvirus hominis type 1. The concentration necessary to reduce plaque counts was found to be 10(-5.5) M. At the same time, the minimal toxic concentration of neutral red for muscle-skin fibroblasts was determined by the concentration that reduced the plaques of a challenge virus, vesicular stomatitis virus, that was applied after treatment with neutral red and light. The minimal toxic concentration was found to be 10(-5) M. Thus, the effective concentration of neutral red for H. hominis in tissue culture appears to be only slightly less than the minimal toxic concentration. The concentrations used for clinical trials in humans have been 10(3)-10(4) times this amount. Any observed efficacy of such treatment may be a reflection of cell toxicity.