The antiviral activity of podophyllotoxin against herpes simplex type 1 virus (HSV-1) grown in Vero cells was studied by a simple microtitration assay. Antiviral effects were induced at similar concentrations as direct cellular toxicity, as characterised by a time-dependent loss of cell monolayer. Podophyllotoxin-mediated toxicity arises from cytoplasmic microtubular, and hence cytoskeletal, decay. Some degree of selectivity was seen for inhibition of virus replication over direct cellular toxicity. Podophyllotoxin acted against an early viral process, as an antiviral effect was still seen if drug was removed 2 h after infection. Similar effects were seen with colchicine, a classical tubulin-binding compound, but not with bromovinyldeoxyuridine. Podophyllotoxin was capable of inducing a cytoprotective effect in Vero cells, as pre-treatment of cells abrogated virus growth for up to 90 min after removal of drug. This is coincident with the repolymerisation of cellular microtubules and re-formation of the cytoskeleton. We conclude that HSV-1 relies upon a functional cellular cytoskeleton for efficient completion of an early replicative event. Such a process may be the transport of viral material to the nucleus or inhibition of the formation of intranuclear viral ‘replication factories’, bodies containing cytoskeletal fragments constructed after viral infection.


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