OTHER MODES OF INHIBITION OF WNV Ribavirin in .NET

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OTHER MODES OF INHIBITION OF WNV Ribavirin
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Ribavirin was discovered in 1972 and exhibits antiviral activity against a broad range of RNA viruses. Ribavirin exerts its antiviral effects by different mechanisms of action: In chronic viral infections like HCV infection, the therapeutic effect of ribavirin seems to rely mainly on immunomodulating mechanisms. It could be shown that ribavirin modulated interleukin-10 expression in mice. The combination of interferon and ribavirin is the standard therapy of chronic HCV infection. Acute viral infections like Lassa and RSV infections could be treated ef ciently by ribavirin monotherapy. Ribavirin represents a compound that contains no phosphate groups and may be regarded as an analogue of adenosine in which the C(2) N(3) fragment of adenosine is removed (Figure 12.5).
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O N HO O N N NH2
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FIGURE 12.5
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1-b-D-Ribofuranosyl-1,2,4-triazole-3-caboxamide (ribavirin).
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OTHER MODES OF INHIBITION OF WNV
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Ribavirin can be incorporated either as ITP or ATP analogue. Interestingly, the Lenantiomer of ribavirin does not have antiviral activity in cell culture but does retain the immunomodulatory properties. Ribavirin is a potent mutagen of poliovirus in cell culture and mutagenic activity correlated directly with its antiviral activity. This observation led to the hypothesis that ribavirin s primary antiviral mechanism of action could be lethal mutagenesis of the viral RNA genomes.34 There are reports of signi cantly higher survival and eradication of WNV from brain in mice after intraperitoneal injection of ribavirin and in vitro studies showing that ribavirin inhibited WNV replication in human oligodendral cells. Recent studies with Vero cells showed that interferon a-2b inhibited viral cytotoxicity when applied after or before cells were infected with WNV. Ribavirin had a protective, but not therapeutic effect in vitro35. In vivo, the need to use very high doses of ribavirin in WNV infections proved too toxic to be clinically useful. 12.10.2 Inhibition of NTP Synthesis
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Certain known inhibitors of orotidine monophosphate decarboxylase showed activity in viral infection assays against a New York isolate of WNV.36 Among them, 6-azauridine, 6-azauridine triacetate, pyrazofurin, and 2-thio-6-azauridine had the most signi cant anti-WNV activity. Mycophenolic acid and cytopentenylcytosine were found to have antiviral effects as inhibitors of inosine monophosphate dehydrogenase and CTP synthetase, respectively. Interestingly, some differences in drug sensitivities were observed between the New York and the Uganda isolate of WNV. 12.10.3 Neplanocin
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( )-Neplanocin A (NPA) is a naturally occurring carbocyclic nucleoside in which a methylene group replaces the oxygen atom in the furanose ring. The absence of a true glycosidic bond makes carbocyclic nucleosides like NPA chemically more stable, as they are not susceptible to enzymatic cleavage of the glycosidic linkage. Several related substances, among them abacavir and carbovir, have been shown to have antiviral activity and are clinically used for the treatment of HIV infection (Figure 12.6).
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H2N N HO N N N
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FIGURE 12.6
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( )-Neplanocin A (NPA).
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The antiviral effect of NPA seems to be due at least partially to inhibition of S-adenosylhomocysteine hydrolase (SAH hydrolase).37 This enzyme has been studied as an attractive target for the design of antiviral agents for a long time.38 S-adenosylmethionine acts as a methyl donor in transmethylation reactions. The RNA-dependent RNA polymerase NS5 of aviviruses presents a characteristic motif of S-adenosyl-L-methionine dependent methyltransferases at its N terminus. The NS5 domain of dengue virus type 2 includes a typical methyltransferase core and exhibits methyltransferase activity on capped RNA.8 Cellular and viral methyltransferases are susceptible to inhibition by SAH, and speci c SAH inhibitors were able to block the replication of RNA viruses.39 However, the therapeutic use of NPA is limited because of its signi cant cytotoxicity, which could be attributed to the phosphorylation of the primary hydroxyl group at the 60 position by adenosine kinase. The subsequent phosphorylation to the triphosphate may inhibit cellular polymerases. The clinical use of NPA is further reduced by its rapid deamination by adenosine deaminase to its therapeutically inactive inosine congener. Therefore, NPA itself does not seem to be useful as an antiviral agent but may be useful as a lead substance for the development of structurally related antivirals. Recently, the synthesis and antiviral activity of D- and L-cyclopentenyl nucleosides have been described. D-Cytosine and D-5- uorocytosine analogues exhibited the most potential antiviral activity against WNV in vitro but also had signi cant cytotoxicity.37
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