Edward G. Bridges

Antiviral l-Nucleosides Specific for Hepatitis B Virus Infection

ABSTRACT A unique series of simple “unnatural” nucleosides has been discovered to inhibit hepatitis B virus (HBV) replication. Through structure-activity analysis it was found that the 3′-OH group of the β-l-2′-deoxyribose of the β-l-2′-deoxynucleoside confers specific antihepadnavirus activity. The unsubstituted nucleosides β-l-2′-deoxycytidine, β-l-thymidine, and β-l-2′-deoxyadenosine had the most potent, selective, and specific antiviral activity against HBV replication. Human DNA polymerases (α, β, and γ) and mitochondrial function were not affected. In the woodchuck model of chronic HBV infection, viral load was reduced by as much as 108 genome equivalents/ml of serum and there was no drug-related toxicity. In addition, the decline in woodchuck hepatitis virus surface antigen paralleled the decrease in viral load. These investigational drugs, used alone or in combination, are expected to offer new therapeutic options for patients with chronic HBV infection.

Pharmacology of β-l-Thymidine and β-l-2′-Deoxycytidine in HepG2 Cells and Primary Human Hepatocytes: Relevance to Chemotherapeutic Efficacy against Hepatitis B Virus

ABSTRACT β-l-Thymidine (l-dT) and β-l-2′-deoxycytidine (l-dC) are potent and highly specific inhibitors of hepatitis B virus (HBV) replication both in vivo and in vitro (50% effective concentrations, 0.19 to 0.24 μM in 2.2.15 cells). The intracellular metabolisms of l-dT and l-dC were investigated in HepG2 cells and primary cultured human hepatocytes. l-dT and l-dC were extensively phosphorylated in both cell types, with the 5′-triphosphate derivative being the predominant metabolite. In HepG2 cells, the 5′-triphosphate levels were 27.7 ± 12.1 and 72.4 ± 1.8 pmol/106 cells for l-dT and l-dC, respectively. In primary human hepatocytes, the 5′-triphosphate levels were 16.5 ± 9.8 and 90.1 ± 36.4 pmol/106 cells for l-dT and l-dC, respectively. Furthermore, a choline derivative of l-dCDP was detected at concentrations of 15.8 ± 1.8 and 25.6 ± 0.1 pmol/106 cells in human hepatocytes and HepG2 cells, respectively. In HepG2 cells exposed to l-dC, the 5′-monophosphate and 5′-triphosphate derivatives of β-l-2′-deoxyuridine (l-dUMP and l-dUTP, respectively) were also observed, reaching intracellular concentrations of 6.7 ± 0.4 and 18.2 ± 1.0 pmol/106 cells, respectively. In human hepatocytes, l-dUMP and l-dUTP were detected at concentrations of 5.7 ± 2.4 and 43.5 ± 26.8 pmol/106 cells, respectively. It is likely that deamination of l-dCMP by deoxycytidylate deaminase leads to the formation of l-dUMP, as the parent compound, l-dC, was not a substrate for deoxycytidine deaminase. The intracellular half-lives of l-dTTP, l-dCTP, and l-dUTP were at least 15 h, with intracellular concentrations of each metabolite remaining above their respective 50% inhibitory concentrations for the woodchuck hepatitis virus DNA polymerase for as long as 24 h after removal of the drug from cell cultures. Exposure of HepG2 cells to l-dT in combination with l-dC led to concentrations of the activated metabolites similar to those achieved with either agent alone. These results suggest that the potent anti-HBV activities of l-dT and l-dC are associated with their extensive phosphorylation.

2'-C-Methyl branched pyrimidine ribonucleoside analogues : potent inhibitors of RNA virus replication

RNA viruses are the agents of numerous widespread and often severe diseases. Their unique RNA-dependent RNA polymerase (RDRP) is essential for replication and, thus, constitutes a valid target for the development of selective chemotherapeutic agents. In this regard, we have investigated sugar-modified ribonucleoside analogues as potential inhibitors of the RDRP. Title compounds retain ‘natural’ pyrimidine bases, but possess a β-methyl substituent at the 2′-position of the D- or L-ribose moiety. Evaluation against a broad range of RNA viruses, either single-stranded positive (ssRNA), single-stranded negative (ssRNA−) or double-stranded (dsRNA), revealed potent activities for D-2′-C-methyl-cytidine and -uridine against ssRNA+, and dsRNA viruses. None of the L-enantiomers were active. Moreover, the 5′-triphosphates of the active D-enantiomers were found to inhibit the bovine virus diarrhoea virus polymerase. Thus, the 2′-methyl branching of natural pyrimidine ribonucleosides transforms physiological molecules into potent, broad-spectrum antiviral agents that merit further development.

ANTI-HBV SPECIFIC β-L-2′-DEOXYNUCLEOSIDES

A unique series of simple unnatural L-nucleosides that specifically inhibit hepatitis B virus (HBV) replication has been discovered. These molecules have in common a hydroxyl group in the 3′-position (3′-OH) of the β-L-2′-deoxyribose sugar that confers antiviral activity specifically against hepadnaviruses. Replacement of the 3′-OH broadens activity to other viruses. Substitution in the base decreases antiviral potency and selectivity. Human DNA polymerases and mitochondrial function are not effected. Plasma viremia is reduced up to 8 logs in a woodchuck model of chronic HBV infection. These investigational drugs, used alone or in combination, are expected to offer new therapeutic options for patients with chronic HBV infection.

Nonclinical Safety Profile of Telbivudine, a Novel Potent Antiviral Agent for Treatment of Hepatitis B

ABSTRACT Telbivudine is a novel nucleoside drug recently approved for the treatment of patients with chronic hepatitis B. Its nonclinical safety was evaluated in a comprehensive program of studies, including safety pharmacology, acute and chronic toxicity, reproductive and developmental toxicity, genotoxicity, and carcinogenicity. There were no test article-related effects observed in an in vitro hERG assay or in a core battery of safety pharmacology studies (central nervous system, respiratory, and cardiovascular safety pharmacology studies). Telbivudine was well tolerated in rats and in monkeys following single oral doses up to 2,000 mg/kg/day. Except for equivocal axonopathic findings in monkeys and occasional incidences of emesis, soft feces, and minor changes in body weight and food consumption, there was no target organ toxicity observed in mice, rats, or monkeys following oral administration for up to 3, 6, or 9 months, respectively, at doses up to 3,000 mg/kg/day. Axonopathy in the sciatic nerves and in the spinal cords of monkeys dosed at 1,000 mg/kg/day observed in a 9-month study was considered equivocal, as the role of telbivudine in the injury could not be determined. Slightly higher incidences of abortion and premature delivery observed in rabbits dosed at 1,000 mg/kg/day were considered secondary to maternal toxicity. There was no evidence of genotoxicity or carcinogenicity. These results suggest that telbivudine has a favorable safety profile and support its use in patients with chronic compensated hepatitis B viral infection.

Monoval-LdC: Efficient Prodrug of 2′-Deoxy-β-L-cytidine (L-dC), A Potent and Selective Anti-HBV Agent

Abstract In order to improve the oral bioavailability of LdC, valinyl esters were prepared as prodrugs. We report here the syntheses of the 3′-mono-, 5′-mono, and 3′,5′-di-O-valinyl esters of LdC. The comparison of their ease of synthesis, their physicochemical properties, as well as their pharmacokinetic parameters in cynomologus monkeys has revealed 3′-mono-O-valinyl derivative as the most promising of the studied prodrugs. This compound is being developed as a new anti-HBV agent.

Pharmacokinetics of β-L-thymidine and β-L-2′-deoxycytidine in woodchucks and monkeys

Publisher Summary Pharmacokinetic studies in woodchucks and monkeys indicated that the disposition of β-L-thymidine (L-dT) is comparable to the pharmacokinetic characteristics of other nucleoside analogs after administration to these animals. Woodchuck is a useful animal model for studying anti-hepatitis B virus (HBV) agents because of many similarities between woodchuck hepatitis virus (WHV) and HBV, hence providing important information on the in vivo efficacy of anti-HBV candidates. The compound has good oral bioavailability and is eliminated and unchanged in urine. 3-L-2’-deoxycytidine (L-dC) exhibits lower oral bioavailability in both species. Several prodrugs that improve the oral bioavailability of L-dC are currently under evaluation. Chronic HBV infection is a major global health problem, affecting approximately 5% of the worlds population. Currently, the only approved treatment options include alpha interferon (INF-α) and lamivudine. Potent in vitro and in vivo efficacy coupled with favorable pharmacokinetic disposition make both L-dT and L-dC promising antiviral candidates for the treatment of chronic HBV infection.

Synthesis, Physicochemical and Pharmacokinetic Studies of Potential Prodrugs of β-L-2′-Deoxycytidine, a Selective and Specific Anti-HBV Agent

β-L-2′-Deoxycytidine (β-L-dC) is a potent, selective and specific anti-hepatitis B virus (HBV) agent. To improve its oral bioavailability, several derivatives involving sugar or base acylation, as well as N4-derivatization with an N,N-(dimethyl-amino)methylene function, were synthesized. The physicochemical characteristics (including chemical stabilities, solubilities and distribution coefficient values) and pharmacokinetics of these compounds were determined and compared with those of the parent drug, β-L-dC. Presented in part at the 14th International Conference on Antiviral Research, Seattle, Washington, USA, 8–13 April 2001. Antiviral Reseach 2001; 50:A79.

Toxicological study of the anti-HBV agent β-L-thymidine

Publisher Summary β-L-thymidine (L-dT) is a potent antiviral nucleoside against hepatitis B virus (HBV) replication in cell culture, and it has a favorable in vitro toxicity profile. L-dT is an unsubstituted, unmodified P-L-deoxynucleoside with potent antiviral activity against hepadnaviruses in vitro and in vivo and a favorable pharmacological profile. As part of its development as a potential oral anti-HBV agent, a series of preclinical studies in vitro and in laboratory animals were conducted to delineate the cytotoxic, genotoxic, toxicologic, and toxicokinetic profiles of L-dT. In vivo efficacy studies conducted on the chronically infected woodchucks indicated that L-dT has a potent antihepadnaviral activity, decreasing plasma viral DNA loads by up to 8 logs. L-dT is nontoxic and nonclastogenic at doses much greater than those most likely to be used clinically. The potent antiviral efficacy and low toxicity in vitro and in vivo make L-dT an attractive candidate for further development as an anti-HBV agent.

Chapter 21 – Antiviral β-L-nucleosides specific for hepatitis B virus infection

Publisher Summary For the treatment of chronic hepatitis B virus (HBV) infection with the goal of eliminating persistent viral replication and preventing progression to chronic active hepatitis, cirrhosis, and hepatocellular carcinoma (HCC), approved treatments that are used are alpha interferon (IFN- α) and lamivudine. The response rate to IFN is low and drug-associated side effects are significant. Individuals who are immunosuppressed rarely respond to IFN therapy. A complete antiviral response, as assessed by HBe seroconversion, is seen in only a minority of patients after one year of treatment. With long-term survival increasing for human immunodeficiency virus (HIV)-infected patients, chronic HBV infection is becoming an important determinant of disease outcome in the coinfected patient population. Nucleoside analogs currently under evaluation for the treatment of HBV include adefovir dipivoxil that may be beneficial against lamivudine-resistant mutants and emtricitabine that is similar to lamivudine. Development of famciclovir and lobucavir have been discontinued because of limited antiviral activity and adverse side effects, respectively. Two other potent nucleoside analogs—L-FMAU and BMS200-458—are currently in human trials, although clinical data have not yet been reported.