We have examined the kinetics of incorporation of
acyclovir triphosphate by the
herpes simplex virus-1
DNA polymerase holoenzyme (Pol-UL42) and the human
mitochondrial DNA polymerase using transient kinetic methods. For each
enzyme, we compared the kinetic parameters for
acyclovir to those governing incorporation of
dGTP. The favorable ground state dissociation constant (6 microM) and rate of polymerization (10 s(-1)) afford efficient incorporation of
acyclovir triphosphate by the Pol-UL42
enzyme. A discrimination factor of approximately 50 favors
dGTP over
acyclovir triphosphate, mostly due to a faster maximum rate of
dGTP incorporation. Once incorporated,
acyclovir is removed with a half-life of approximately 1 h in the presence of a normal concentration of deoxynucleoside triphosphates, leading to a high toxicity index (16,000) toward viral replication. To assess the potential for toxicity toward the host we examined the incorporation and removal of
acyclovir triphosphate by the human
mitochondrial DNA polymerase. These results suggest moderate inhibition of
mitochondrial DNA replication defining a toxicity index of 380. This value is much higher than the value of 1.5 determined for
tenofovir, another acyclic
nucleoside analog. The enzymatic therapeutic index is only 42 in favoring inhibition of the viral polymerase over polymerase gamma, whereas that for
tenofovir is greater than 1,200. Mitochondrial toxicity is relatively low because
acyclovir is activated only in infected cells by the promiscuous viral
thymidine kinase and otherwise, mitochondrial toxicity would accumulate during long term treatment.