Effects of purine nucleoside analogues with a cyclobutane ring and erythromycin A oxime derivatives on duck hepatitis B virus replication in vivo and in cell culture and HIV-1 in cell culture

New therapeutic strategies in the treatment of hepatitis B virus infection

Principally, because of the association of the chronic carrier state with the development of cirrhotic liver disease and hepatocellular carcinoma, chronic hepatitis B infection is a public health problem of global significance. In the main, therapy for chronic hepatitis B is limited to the use of alpha interferon for a limited number of chronic hepatitis B virus (HBV) carriers who have chronic hepatitis with active viral replication. The development of antiviral nucleoside analogues for the herpes viruses and human immunodeficiency virus (HIV) has resulted in the identification of several compounds which also have activity against HBV. Unfortunately, these agents have not been associated with the clearance of hepatitis B infection, but rather only the suppression of active infection while the patient is receiving medication. In addition, the development of drug-resistance to these agents by the virus will most likely limit their long-term efficacy. Gene therapy has recently been applied to HBV both in vitro and in vivo. This has included the use of antisense oligodeoxynucleotides and RNA, ribozymes, dominant negative mutants and therapeutic HBV vaccines. These newer therapeutic modalities may hold promise as effective treatments for chronic hepatitis B, but to date, have been limited by the problem of delivery to the target cell population or infected organ in vivo. Combination nucleoside analogue therapy may also provide an important treatment modality for chronic hepatitis B, although this will require further investigation.

Antiviral chemotherapy for the treatment of hepatitis B virus infections

Approximately 5% of the world's human population have an increased risk for developing liver cancer and cirrhosis as a direct consequence of chronic infection with the hepatitis B virus (HBV). Antiviral chemotherapy remains the only option for controlling infection in these individuals, for whom the current licensed hepatitis B vaccines provide no benefit. Interferon (IFN)-alpha has proven benefit in a well-defined group of those with hepatitis B but has made little impact on the global burden of chronic liver disease. The development of more effective chemotherapy for treatment of chronic hepatitis B infection has proven to be extremely challenging, the result of both virus- and host-dependent factors, which will be reviewed in this article. Past attempts to treat chronic hepatitis B infection using nucleoside analogues were disappointing, but more recently, several nucleoside (or nucleotide) analogues have been identified that are potent and selective inhibitors of HBV replication. These agents fall into two broad categories: (1) nucleoside/nucleotides that have modified sugar residues in either cyclic or acyclic configurations and (2) stereoisomers of nucleosides in the "unnatural" L-configuration. Of the analogues that have been used clinically, representatives of the first category are purine derivatives, e.g., adefovir dipivoxil and famciclovir, whereas representatives of the second category are pyrimidine derivatives, such as lamivudine.

Sensitivity of L-(-)2,3-dideoxythiacytidine resistant hepatitis B virus to other antiviral nucleoside analogues

L-(-)2',3'-Dideoxythiacytidine (L(-)SddC, Lamivudine) resistant hepatitis B virus (HBV) develops in patients after prolonged treatment. Point mutations detected in the viral genome from these patients have been shown to be responsible for L(-)SddC resistance. Therefore, new drugs active against L(-)SddC resistant HBV are needed. Using a transient transfection system, we studied the sensitivity of L(-)SddC resistant HBV to other anti-HBV nucleoside analogues. It was found that the L526M mutation alone caused greater resistance to penciclovir (PCV) than did the V553I mutation alone. Both mutations also caused the virus to be less sensitive to L(-)SddC and 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU), although the degree of resistance was much less than that to PCV. The A546V mutation had no impact on the sensitivity to L(-)SddC, L-FMAU, and PCV. When these single mutations were coupled with the M550V/I mutation, all the double mutants were resistant to those drugs. Although 2',3'-dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine (L(-)Fd4C) was also less active, the IC50 of L(-)Fd4C against the L(-)SddC resistant mutant was at least fifty times lower than that against cell growth in culture. DNA polymerase associated with L(-)SddC resistant virions was also found to be less sensitive than that with wild-type HBV to those L-nucleoside triphosphates. All the L(-)SddC resistant mutants were still sensitive to 9-(2-phosphonylmethoxyethyl)-adenine (PMEA). These results suggest that different mutations in the HBV genome have a different impact on its sensitivity to those compounds, and L(-)SddC resistant HBV may also be resistant to PCV, L-FMAU, and L(-)Fd4C. A nucleoside analogue less toxic than PMEA could be developed against L(-)SddC resistant HBV.

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.

Effect of oxetanocin G, a novel nucleoside analog, on DNA synthesis by hepatitis B virus virions

The novel nucleoside oxetanocin G, 9-(2-deoxy-2-hydroxymethyl-beta-D-erythro-oxetanosyl)guanine (OXT-G), that is a derivative of oxetanocin A, was studied in relation to its action on the synthesis of hepatitis B virus (HBV) DNA and cellular DNA in an HBV-producing cell line, HB611 (T. Tsurimoto, A. Fujiyama, and K. Matsubara, Proc. Natl. Acad. Sci. USA 84:444-448, 1987). The median effective concentration of OXT-G against HBV replication was 1.5 microM, and the median cytotoxic concentration was more than 1,000 microM. At the same concentration, OXT-G did not inhibit cellular DNA synthesis or viral RNA synthesis. Chemically synthesized OXT-GTP inhibited the HBV endogenous DNA polymerase reaction and was incorporated into HBV DNA strands at a low efficiency compared with the incorporation of dGTP. A synthetic primer-template study revealed that OXT-GTP was incorporated into DNA strands at a low efficiency and that further extension of the DNA strand by using the 2' position of the incorporated OXT-G could take place.

Characterization of the antiviral effects of 2' carbodeoxyguanosine in ducks chronically infected with duck hepatitis B virus

This study was carried out to evaluate benefits and limitations of long-term therapy of hepatitis B virus infections with a nucleoside analog inhibitor of virus replication. The model we used was the domestic duck chronically infected with duck hepatitis B virus by in ovo infection. 2' Carbodeoxyguanosine was used as an inhibitor of viral DNA synthesis. In all animals examined there was a reduction in virus production during therapy. A dose of 2' carbodeoxyguanosine of 10 micrograms/kg every other day reduced the number of infected hepatocytes from greater than 95% to 25% to 50% in less than 3 mo, whereas a 10-fold higher dose produced a decline to less than 10%. Histological evaluation revealed mild to moderate liver injury in ducks receiving the higher dose of 2' carbodeoxyguanosine, suggesting that disappearance of infected hepatocytes may have been accelerated by a toxic effect of the drug. Drug treatment did not completely eliminate duck hepatitis B virus from any duck, and replication was restored in all hepatocytes within a few weeks to several months after antiviral therapy was terminated. Our results suggest that elimination of a chronic infection with a single inhibitor of replication may be difficult in a host that lacks an antiviral immune response capable of eliminating at least a portion of the infected hepatocytes and of ultimately producing antibodies capable of neutralizing residual virus.

Analysis of the earliest steps of hepadnavirus replication: genome repair after infectious entry into hepatocytes does not depend on viral polymerase activity

Hepadnaviruses contain a relaxed circular DNA genome (RC-DNA) with discontinuities in both strands. Upon infectious entry into a host cell, this genome is converted into a covalently closed superhelical form (CCC-DNA), which later serves as the template for transcription. Here we examined whether the viral polymerase activity is required for this repair reaction. Primary hepatocytes prepared from embryonated duck eggs were infected with the duck hepatitis B virus. Conversion of the RC-DNA into the CCC-DNA was then analyzed by a newly developed polymerase chain reaction technique. This method allows the efficient discrimination between the two DNA forms and is sensitive enough to monitor repair of the infecting viral DNA in the absence of replication and amplification. Thus, we were able to monitor this process in the presence of a potent inhibitor of the viral polymerase, the nucleoside analog 2',3'-dideoxyguanosine. The data show that inhibition of the viral polymerase activity has no influence on genome repair, suggesting that this enzymatic function is not required for conversion of the RC-DNA into the CCC-DNA. Consequently, antiviral drugs blocking the polymerase activity cannot prevent the infectious entry of the virus into a host cell.