Kyoichi A. Watanabe

Ribonucleoside Analogue That Blocks Replication of Bovine Viral Diarrhea and Hepatitis C Viruses in Culture

ABSTRACT A base-modified nucleoside analogue, β-d-N4-hydroxycytidine (NHC), was found to have antipestivirus and antihepacivirus activities. This compound inhibited the production of cytopathic bovine viral diarrhea virus (BVDV) RNA in a dose-dependant manner with a 90% effective concentration (EC90) of 5.4 μM, an observation that was confirmed by virus yield assays (EC90 = 2 μM). When tested for hepatitis C virus (HCV) replicon RNA reduction in Huh7 cells, NHC had an EC90 of 5 μM on day 4. The HCV RNA reduction was incubation time and nucleoside concentration dependent. The in vitro antiviral effect of NHC was additive with recombinant alpha interferon-2a and could be prevented by the addition of exogenous cytidine and uridine but not of other natural ribo- or 2′-deoxynucleosides. When HCV RNA replicon cells were cultured in the presence of increasing concentrations of NHC (up to 40 μM) for up to 45 cell passages, no resistant replicon was selected. Similarly, resistant BVDV could not be selected after 20 passages. NHC was phosphorylated to the triphosphate form in Huh7 cells, but in cell-free HCV NS5B assays, synthetic NHC-triphosphate (NHC-TP) did not inhibit the polymerization reaction. Instead, NHC-TP appeared to serve as a weak alternative substrate for the viral polymerase, thereby changing the mobility of the product in polyacrylamide electrophoresis gels. We speculate that incorporated nucleoside analogues with the capacity of changing the thermodynamics of regulatory secondary structures (with or without introducing mutations) may represent an important class of new antiviral agents for the treatment of RNA virus infections, especially HCV.

A practical synthesis of 2-deoxy-2-fluoro-D-arabinofuranose derivatives.

A seven-step synthesis of 1,3-di-O-acetyl-5-O-benzoyl-2-deoxy-2-fluoro-D-arabinofuranose, a versatile intermediate in the synthesis of chemotherapeutically important nucleosides, was achieved from 1,2:5,6-di-O-isopropylidene-3-O-tosyl-alpha-D-allofuranose. The crucial steps were the fluorination by use of potassium fluoride in acetamide and the conversion of 6-O-benzoyl-3-deoxy-3-fluoro-D-glucofuranose into 5-O-benzoyl-2-deoxy-2-fluoro-3-O-formyl-D-arabinofuranose by periodate oxidation. Also described is the synthesis of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)cytosine. This procedure affords good overall yields of products without formation of undesirable, isomeric intermediates and is suitable for large-scale preparations.

Inhibition of the Subgenomic Hepatitis C Virus Replicon in Huh-7 Cells by 2′-Deoxy-2′-Fluorocytidine

ABSTRACT 2′-Deoxy-2′-fluorocytidine (FdC) is a potent inhibitor of the hepatitis C virus RNA replicon in culture, and FdC-5′-triphosphate is an effective inhibitor of the NS5B polymerase. Dynamic profiling of cell growth in an antiviral assay showed that FdC caused cytostasis due to an S-phase arrest. These observations demonstrate that FdC treatment is affecting both a viral target and a cellular target.

5′-Hydrogenphosphonates and 5′-Methylphosphonates of Sugar Modified Pyrimidine Nucleosides as Potential Anti-HIV-1 Agents.1

Abstract A number of nucleoside 5′-hydrogerphosphonates and nucleoside 5′-methylphosphonates were prepared, to study their ability to inhibit replication of HIV-1. Two compounds, the 5′-hydrogenphosphonate of 3′-azido-3′-deoxythymidine (AZT-HP, IVc) and of 3′-deoxy-3′-fluorothymidine (FLT-HP, IVa), exhibit potent anti-HIV-1 activity with selectivity indices similar to or better than those of their parent nucleosides.

Synthesis of Oligodeoxynucleotides Containing the C-Nucleoside and 2′- Deoxy-2′-Fluoro-ara-Nucleoside Moieties by the H-Phosphonate Method.1,2

Abstract A module type, computer-controlled, multipurpose synthesizer displaying a novel device for the transport of liquids, was constructed and used in the synthesis of oligomers containing some C-nucleosides and 2′-deoxy-2′-fluoro-ara-nucleoside moieties. H-Phosphonate method was applied in terms of a further adjustment of construction features of the synthesizer versus chemistry of the process. Results of preliminary studies on the effects of the modified nucleosides on the stability of duplexes showed a clear tendency of destabilization of duplexes in the case of C-nucleosides while fluorinated nucleosides in most cases stabilize the formed duplexes.

Stereospecific fluorination of 1,3,5-tri-O-benzoyl-α-d-ribofuranose-2-sulfonate esters: preparation of a versatile intermediate for synthesis of 2′-[18F]-fluoro-arabinonucleosides

Abstract A detailed investigation on fluorination of 1,3,5-tri-O-benzoyl-α- d -ribofuranose-2-sulphonate esters is reported. Various combinations of sulfonate esters, fluorinating agents and solvents were evaluated in this study. Organic ammonium fluoride, in particular n-Bu4NF, was found to be better fluorinating agent than inorganic fluoride, and 1,3,5-tri-O-benzoyl-α- d -ribofuranose-2-trifluoromethylsulphonate ester appeared to be the best substrate. The developed method is suitable for stereospecific (arabino) incorporation of radiofluorine ( 18 F ) into the sugar moiety.

Inhibitors of the IMPDH enzyme as potential anti-bovine viral diarrhoea virus agents

Ribavirin and mycophenolic acid (MPA) are known inhibitors of the IMPDH enzyme (E.C. 1.1.1.205). This enzyme catalyzes the conversion of inosine monophosphate to xanthine monophosphate, leading eventually to a decrease in the intracellular level of GTP and dGTP. The antiviral effect against bovine viral diarrhoea virus (BVDV) of 15 analogues related to MPA was determined. MDBK cells were infected with the cytopathic strain of BVDV in presence or absence of test compounds. Viral RNA was extracted from the cell supernatant fluids and quantified by RT-PCR. Ribavirin showed a potent antiviral effect against BVDV with 90% effective concentration (EC90) of 4 μM. MPA along with several analogues, including both its corresponding aldehyde and alcohol, and modifications in the length of the side chain (C2- and C4-derivatives) were tested. We have identified previously unreported IMPDH inhibitors that have potent anti-BVDV activity, namely: C6-MPAlc (5), C6-MPA-Me (7), C4-MPAlc (8), C4-MPA (10) and C2-MAD (20). Most of these compounds inhibited the IMPDH enzyme in the nanomolar range (4–800 nM) in cell-free assays. Some compounds, such as mizoribine, which is a potent inhibitor of IMPDH in vitro (enzyme 50% inhibitory concentration IC50=4 nM), had no detectable anti-BVDV activity up to 100 μM. The compounds were essentially non-toxic to a confluent monolayer of MDBK cells. However, in exponentially growing cells, they showed minimal toxicity at 100 μM over a 24 h period, but the toxicity was more pronounced after 3 days [50% cytotoxic concentration (CC50) value ranged from 5 to 30 μM].

2′-Fluoro-arabinosyl Pyrimidine Nucleosides: Chemistry, Antiviral, and Potential Anticancer Activities

ABSTRACT A series of 2′-fluoro-5-substituted-arabinofuranosyl-cytosines and -uracils were synthesized. Two of these, FIAC and FMAU, were found to be very potent and highly selective against herpes simplex virus (HSV) types 1 and 2 at very low drug levels. Cytotoxicity to uninfected Vero or human fibroblast cell proliferation was minimal. The selectivity of FIAC against HSV versus its low cytotoxicity against Vero cells is shown to be due, at least in part, to a virus-specified thymidine kinase. Structure-activity studies demonstrate that the 2′-fluoro substituent in the up (arabino) configuration is essential for this potent antiviral activity. Substitution of the 2′-fluoro group by chloro or bromo reduces the antiviral potency. FIAC is also active in a plaque reduction assay against herpes zoster virus at concentrations of 0.01 μM and against cytomegalovirus plaque formation at 0.1 μM. In vivo studies in mice inoculated with 20 LD50 of HSV-1 show that FIAC and FMAU are effective, the latter giving 60% cures at dose levels as low as 1 mg/kg/day x5. The selective cytotoxicity of FIAC against human tumor cell lines but not against normal human cells is discussed.

The chemistry of nicotinamide adenine dinucleotide (NAD) analogues containing C-nucleosides related to nicotinamide riboside.

Oncolytic C-nucleosides, tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide) and benzamide riboside (3-beta-D-ribofuranosylbenzamide) are converted in cell into active metabolites thiazole-4-carboxamide- and benzamide adenine dinucleotide, TAD and BAD, respectively. TAD and BAD as NAD analogues were found to bind at the nicotinamide adenine dinucleotide (cofactor NAD) site of inosine monophosphate dehydrogenase (IMPDH), an important target in cancer treatment. The synthesis and evaluation of anticancer activity of a number of C-nucleosides related to tiazofurin and nicotinamide riboside then followed and are reviewed herein. Interestingly, pyridine C-nucleosides (such as C-nicotinamide riboside) are not metabolized into the corresponding NAD analogues in cell. Their conversion by chemical methods is described. As dinucleotides these compounds show inhibition of IMPDH in low micromolar level. Also, the synthesis of BAD in metabolically stable bis(phosphonate) form is discussed indicating the usefulness of such preformed inhibitors in drug development. Among tiazofurin analogues, Franchetti and Grifantini found, that the replacement of the sulfur by oxygen (as in oxazafurin) but not the removal of nitrogen (tiophenfurin) of the thiazole ring resulted in inactive compounds. The anti cancer activity of their synthetic dinucleotide analogues indicate that inactive compounds are not only poorly metabolized in cell but also are weak inhibitors of IMPDH as dinucleotides.

Inhibition of Simian varicella virus infection of monkeys by 1-(2-deoxy-2-fluoro-1-β-d-arabinofuranosyl)-5-ethyluracil (FEAU) and synergistic effects of combination with human recombinant interferon-β

1-(2-Deoxy-2-fluoro-1-beta-D-arabinofuranosyl)-5-ethyluracil (FEAU) has been shown to be a highly effective inhibitor of Simian varicella virus infection in African green monkeys. Administration of FEAU by either intravenous injection or gavage at doses as low as 1 mg/kg/day prevented the development of rash and reduced viremia. The effective dose could be further reduced to 0.2 mg/kg/day when administered in combination with a sub-effective dose of human recombinant interferon-beta. No evidence of toxicity was seen in monkeys treated for 10 days with FEAU doses of 10 mg/kg/day when they were monitored by hematology and clinical chemistry tests and by clinical observations.