Effect of cyclophosphamide in vitro and on vaccinia virus replication in tissue culture

The effect of cyclophosphamide on the growth of Vero, BSC-1, and HeLa cells in monolayer cultures was studied. By using hemocytometer counts and tritiated thymidine uptake as indicators of growth, it was found that cyclophosphamide significantly interfered with the metabolism of Vero and BSC-1 cells when sustained in Leibovitz medium. Vero cells and HeLa cells grown in Eagle medium were not affected by exposure to cyclophosphamide. Vaccinia virus replication in Vero cell monolayer cultures incubated with cyclophosphamide was markedly augmented, and this enhanced growth was reflected by virus quantitation techniques and metabolic studies using tritiated thymidine uptake. No difference in the distribution of infectious particles was found when cyclophosphamide-treated and control infected cultures were compared. Pathways other than through hepatic enzymes appear available to activate cyclophosphamide in vitro. These effects are dependent on both the cell type and the medium in which the cells are grown. Cyclophosphamide can facilitate vaccinia virus replication in vitro through metabolic interactions at the cellular level. The precise mechanisms underlying this effect require further study.