Mode of action of acyclovir triphosphate on herpesviral and cellular DNA polymerases

The effect of 5'-triphosphate of acyclovir (ACV) on DNA polymerases of two human herpes-viruses, herpes simplex virus type-1 (HSV-1) and Epstein-Barr virus (EBV) as well as human cellular DNA polymerases alpha and beta has been examined. Of the enzymes tested, HSV-1 DNA polymerase was the most sensitive to inhibition by acyclovir triphosphate (ACVTP). The EBV DNA polymerase and DNA polymerase beta were less sensitive. ACVTP inhibition was competitive with dGTP with Ki values of 0.03, 0.15, 9.8 and 11.9 microM for HSV-1 DNA polymerase, DNA polymerase alpha, EBV DNA polymerase and DNA polymerase beta, respectively. Substituting a synthetic primer template (dG) approximately 15 x (dC)n for activated DNA template did not alter the pattern of inhibition. In a time course experiment, addition of ACVTP instead of dGTP did not increase DNA synthesis and it appeared to act as a chain terminator in DNA replication catalyzed by either HSV-1 DNA polymerase or DNA polymerase alpha. Although EBV DNA polymerase was less sensitive to ACVTP inhibition, the nucleoside analog itself was inhibitory to EB virus production by P3HR1 cell line as determined by a reduction in the percentage of cells expressing virus capsid antigen (VCA). On day 4, ACV at 10 and 25 micrograms/ml reduced the cell growth by 10% and 32%, respectively, while it reduced the VCA-positive cells by 80% and 84%, respectively. These results indicate that inhibition of EBV DNA polymerase activity by ACVTP may not be the primary mechanism responsible for ACV inhibition of EBV replication.