Antiviral nucleoside analogues have been developed to inhibit the enzymatic activities of the hepatitis B virus (
HBV) polymerase, thereby preventing the replication and production of HBV. However, the usage of these analogues can be limited by
drug toxicity because the 5'-triphosphates of these
nucleoside analogues (
nucleotide analogues) are potential substrates for human
DNA polymerases to incorporate into host
DNA. Although they are poor substrates for human replicative
DNA polymerases, it remains to be established whether these
nucleotide analogues are substrates for the recently discovered human X- and Y-family
DNA polymerases. Using presteady state kinetic techniques, we have measured the substrate specificity values for human
DNA polymerases β, λ, η, ι, κ, and Rev1 incorporating the active forms of the following anti-HBV
nucleoside analogues approved for clinical use:
adefovir,
tenofovir,
lamivudine,
telbivudine, and
entecavir. Compared to the incorporation of a natural
nucleotide, most of the
nucleotide analogues were incorporated less efficiently (2 to >122,000) by the six human
DNA polymerases. In addition, the potential for
entecavir and
telbivudine, two drugs which possess a 3'-hydroxyl, to become embedded into human
DNA was examined by primer extension and
DNA ligation assays. These results suggested that
telbivudine functions as a chain terminator, while
entecavir was efficiently extended by the six
enzymes and was a substrate for human
DNA ligase I. Our findings suggested that incorporation of anti-HBV
nucleotide analogues catalyzed by human X- and Y-family polymerases may contribute to clinical toxicity.