Lobucavir is phosphorylated in human cytomegalovirus-infected and -uninfected cells and inhibits the viral DNA polymerase.

Lobucavir (LBV) is a deoxyguanine nucleoside analog with broad-spectrum antiviral activity. LBV was previously shown to inhibit herpes simplex virus (HSV) DNA polymerase after phosphorylation by the HSV thymidine kinase. Here we determined the mechanism of action of LBV against human cytomegalovirus (HCMV). LBV inhibited HCMV DNA synthesis to a degree comparable to that of ganciclovir (GCV), a drug known to target the viral DNA polymerase. The expression of late proteins and RNA, dependent on viral DNA synthesis, was also inhibited by LBV. Immediate-early and early HCMV gene expression was unaffected, suggesting that LBV acts temporally coincident with HCMV DNA synthesis and not through cytotoxicity. In vitro, the triphosphate of LBV was a potent inhibitor of HCMV DNA polymerase with a Ki of 5 nM. LBV was phosphorylated to its triphosphate form intracellularly in both infected and uninfected cells, with phosphorylated metabolite levels two- to threefold higher in infected cells. GCV-resistant HCMV isolates, with deficient GCV phosphorylation due to mutations in the UL97 protein kinase, remained sensitive to LBV. Overall, these results suggest that LBV-triphosphate halts HCMV DNA replication by inhibiting the viral DNA polymerase and that LBV phosphorylation can occur in the absence of viral factors including the UL97 protein kinase. Furthermore, LBV may be effective in the treatment of GCV-resistant HCMV.

Inhibition of herpes simplex virus type 1 DNA polymerase by [1R(1 alpha,2 beta,3 alpha)]-9-[2,3-bis(hydroxymethyl)cyclobutyl] guanine.

(+/-)-(1 alpha,2 beta,3 alpha)-9-[2,3-Bis(hydroxymethyl)cyclobutyl] guanine [(+/-)-BHCG] is a nucleoside analog with potent in vitro activity against herpesviruses [Tetrahedron Lett. 30:6453-6456 (1989)]. The two enantiomers have been synthesized, and their biochemical characterization is reported here. [1S(1 alpha,2 beta,3 alpha)]-9-[2,3-Bis(hydroxymethyl)cyclobutyl]guanine [(S)-BHCG] was phosphorylated by herpes simplex virus type 1 (HSV-1) thymidine kinase (Vmax = 8 nmol/hr/micrograms of enzyme), whereas [1R(1 alpha,2 beta,3 alpha)]-9-[2,3-bis(hydroxymethyl)cyclobutyl]guanine [(R)-BHCG] was a poor substrate for the viral thymidine kinase under these conditions. The triphosphate of each enantiomer was enzymatically synthesized, and both enantiomers competitively inhibited HSV-1 DNA polymerase with respect to dGTP. However, the potency of (R)-BHCG-TP was 4 orders of magnitude greater than that of (S)-BHCG-TP. (R)-BHCG-TP inhibited HeLa DNA polymerase alpha, but the inhibition constant was 30-fold higher than that for the viral DNA polymerase. In comparison, (S)-BHCG-TP was a very poor inhibitor of DNA polymerase alpha. (R)-[3H]BHCG-TP could be incorporated into a synthetic DNA template by HSV-1 DNA polymerase at 80% the extent of dGTP under the assay conditions used and, therefore, could act as an alternative substrate. Incorporation of (R)-BHCG-TP was similar to that observed for acyclovir triphosphate and ganciclovir triphosphate, based on maximal velocities. In contrast, HSV-1 DNA polymerase did not incorporate (S)-BHCG-TP into DNA. Compared with dGTP, only limited extension (10%) of the DNA primer by HSV-1 DNA polymerase occurred after incorporation of (R)-BHCG-TP and, therefore, (R)-BHCG-TP acts as a nonobligate chain terminator.

Synthesis and antiviral activity of enantiomeric forms of cyclobutyl nucleoside analogues.

The syntheses of the enantiomeric cyclobutyl guanine nucleoside analogues [1R-1 alpha, 2 beta, 3 alpha]- and [1S-1 alpha, 2 beta, 3 alpha]-2- amino-9-[2,3-bis(hydroxymethyl)cyclobutyl]-6H-purin-6-one (7 and 8, respectively) and the enantiomeric cyclobutyl adenine analogues [1R-1 alpha, 2 beta, 3 alpha]- and [1S-1 alpha, 2 beta, 3 alpha]-6-amino-9-[2,3-bis(hydroxymethyl) cyclobutyl]purine (9 and 10, respectively) are described. trans-3,3-Diethoxy-1,2-cyclobutanedicarboxylic acid (14) was coupled with R-(-)-2-phenylglycinol to provide a mixture of diastereomeric bis-amides, 15a and 15b, which was readily separated by crystallization. Conversion of each bis-amide to the corresponding diol enantiomer, 16a and 16b, respectively, was effected by a facile three-step sequence in high overall yield. Homochiral diol 16a was converted in a straightforward manner to 7 and 9, and homochiral diol 16b was similarly converted to the corresponding optical isomers 8 and 10. Compounds 7 and 9, which mimic the absolute configuration of natural nucleosides, are highly active against a range of herpesviruses in vitro while the isomers of opposite configuration, 8 and 10, are devoid of antiherpes activity. The corresponding triphosphates of 7 and 8 (7-TP and 8-TP) were prepared enzymatically. Compound 7-TP selectively inhibits HSV-1 DNA polymerase, compared to human (HeLa) DNA polymerase, while 8-TP is much less inhibitory than 7-TP against both types of enzymes. Compounds 7 and 9 are efficacious in a mouse cytomegalovirus model infection.

Neutralization of purified herpes simplex virus DNA polymerase by two antipeptide sera.

Synthetic peptides corresponding to amino acid sequences present in the herpes simplex virus type-1 (HSV-1) DNA polymerase (pol) were used to raise polyclonal rabbit antisera. The three peptides described in detail in this report were among seven sequences chosen for initial studies designed to generate reagents capable of recognizing discrete regions of the HSV-1 pol protein from the amino to carboxy termini. Two of the peptides, designated P6 and P7, representing amino acid residues 1100-1108 and 1216-1224 of the deduced HSV-1 (strain KOS) DNA pol sequence (1235 residues) produced antisera that could not only recognize the native HSV-1 pol enzyme but also could specifically neutralize purified HSV-1 pol activity in a dose-dependent manner. An additional peptide, designated P3, representing residues 548-557, produced an antiserum that was unable to recognize the native protein but could react with HSV-1 pol in a denatured form by immunoblot assay.