Synergistic Combination Effect of Cidofovir and Idoxuridine on Vaccinia Virus Replication

Abstract

In view of the potential menace of a terrorism attack with smallpox virus, an intensive search of chemotherapeutic agents active against orthopoxviruses is underway. We comparatively studied the antiviral activity of cidofovir (CDV) and idoxuridine (IUdR) against two vaccinia virus (VV) strains, Bratislava and RIIPD, in cell cultures of chick embryo fibroblasts (CEF). The investigations were carried out according to cytopathic effect (CPE) inhibition assay protocols. To determine the cytotoxicity of the compounds, maximal tolerated concentration (MTC) was calculated in CEF cell monolayers and 50% cell growth inhibitory concentration (CGIC50) was calculated in growing cell cultures. It was found that the antiviral effects were strongly dependent on virus inoculum size. There were no marked differences in the susceptibility to CDV and IUdR between the two VV strains. The individual half maximal inhibitory concentration (IC50) for CDV varied from 7.1–8.5 µM at 10/100 virus 50% infectious dose (ID50) to 13.6–26.5 µM at 10,000 ID50. The CDV selectivity index was also virus dose-dependent with MTC/IC50 and CGIC50/IC50 values ranging between 37.8–141.4 and 33.3–124.6, respectively. For IUdR, IC50 ranged from 0.58 to 0.85 µM, but the selectivity index for monolayer CEF and growing cell cultures produced substantial different results with MTC/IC50 and CGIC50/IC50 values between 117.7–172.4 and 20.4–33.3, respectively. The combination effects of CDV and IUdR against VV Bratislava strain in the CPE inhibition test were also determined. The test design of both combination antiviral effect and combined cytotoxicity followed a three-dimensional model. The combined effect of CDV and IUdR on VV replication in monolayer CEF cultures was characterized as a markedly synergistic one. In contrast, CDV and IUdR together reduced cytotoxicity in both monolayer and growing CEF cells.