The antiviral activity of scopadulciol (SDC), a tetracyclic diterpenoid with a chemical structure related to that of aphidicolin, isolated from Scoparia dulcis, was studied in vitro against herpes simplex virus type 1 (HSV-1). SDC was found to inhibit the virus replication as shown by reduction of virus production. The action was not due to the inhibition of viral DNA polymerase activity and virus penetration, but might involve, at least in part, a virucidal effect. SDC did not suppress the viral protein synthesis of infected cells when added at an early stage of HSV-1 replication, but did when added later. When aciclovir (ACV) and SDC were evaluated in combination for antiviral activity against HSV-1 replication and cytotoxicity, these drugs inhibited viral replication in HeLa cells synergistically, but the same combination did not produce synergistic cytotoxicity in HeLa cells. Studies of the deoxynucleotide pool sizes revealed that SDC increased the intracellular dNTP pools and ACV triphosphate level significantly in infected cells when the cells were treated with the combination. These results could account for the synergistic action between SDC and ACV.
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