LOBUCAVIR IS PHOSPHORYLATED IN HUMAN CYTOMEGALOVIRUS-INFECTED AND CYTOMEGALOVIRUS-UNINFECTED CELLS AND INHIBITS THE VIRAL-DNA POLYMERASE

Lobucavir (LBV) is a deoxyguanine nucleoside analog with broad-spectru m antiviral activity. LBV was previously shown to inhibit herpes simpl ex virus (HSV) DNA polymerase after phosphorylation by the HSV thymidi ne kinase. Here we determined the mechanism of action of LBV against h uman cytomegalovirus (HCMV). LBV inhibited HCMV DNA synthesis to a deg ree comparable to that of ganciclovir (GCV), a drug known to target th e viral DNA polymerase. The expression of late proteins and RNA, depen dent on viral DNA synthesis, was also inhibited by LBV. Immediate-earl y and early HCMV gene expression was unaffected, suggesting that LBV a cts temporally coincident with HCMV DNA synthesis and not through cyto toxicity. In vitro, the triphosphate of LBV was a potent inhibitor of HCMV DNA polymerase with a K-i of 5 nM. LBV was phosphorylated to its triphosphate form intracellularly in both infected and uninfected cell s, with phosphorylated metabolite levels two-to threefold higher in in fected cells. GCV-resistant HCMV isolates, with deficient GCV phosphor ylation due to mutations in the UL97 protein kinase, remained sensitiv e to LBV. Overall, these results suggest that LBV-triphosphate halts H CMV DNA replication by inhibiting the viral DNA polymerase and that LB V phosphorylation can occur in the absence of viral factors including the UL97 protein kinase. Furthermore, LBV may be effective in the trea tment of GCV-resistant HCMV.