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netLibrary - eBook Summary Structure-based Drug Design by ...

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Page 477<br />

fluenza, as it shows weak inhibition to bacterial, para influenza, and mammalian neuraminidases<br />

[91,93]. This is possibly due to the specific interactions of the 4-guanidino group within the subpocket<br />

of the active site of influenza neuraminidase that is not conserved in other neuraminidases. In bacterial<br />

neuraminidases [51] this pocket is much smaller, and would prevent the binding of the 4-guanidino<br />

group in this region of the active site. This is consistent with the proposition that the interactions of<br />

sialic acid with the active site of neuraminidases are function specific at the C4, C5, and C6 position of<br />

sialic acid and that modification at these positions confer specificity to the target enzyme [72]. Other<br />

approaches [94] to structure-<strong>based</strong> design of inhibitors have to date produced only millimolar inhibition<br />

of neuraminidase activity.<br />

V. Antiviral Activity<br />

In vitro inhibition of viral replication in tissue culture was demonstrated earlier [34] for the trifluro<br />

derivative of Neu5Ac2en, but its antiviral activity in vivo was not demonstrated [77]. As a consequence<br />

of this, efforts were directed towards hemagglutinin, which was then considered a better target for<br />

antiinfluenza drugs. The interest in neuraminidase inhibitors as anti-influenza drugs has only been<br />

revived with the success of 4-guanidino-Neu5Ac2en and its analogs in attenuating viral titer in mice<br />

when administered directly into the lungs [91,95].<br />

A. Inhibition In Vitro<br />

Von Itzstein and co-workers [91] have shown that the 4-amino-and 4-guanidino-Neu5Ac2en inhibit<br />

influenza strains A/Singapore/1/57 and B/Victoria/102/95 in MDCK cells with IC 50 values (the<br />

concentration required to inhibit plaque formation in MDCK cells <strong>by</strong> 50%) of 1.5 mM and 0.065 mM (4amino)<br />

and 0.014 mM and 0.005 mM (4-guanidino) respectively. These IC 50 values, in particular for the<br />

4-guanidino compound, are well below those found for amantadine, ribovarin, and Neu5Ac2en.<br />

Furthermore in comparison to Neu5Ac2en, the 4-guanidino-Neu5Ac2en inhibitor was 100-fold less<br />

active against human lysosomal sialidase and over 1000-fold more active against a wide range of<br />

clinical isolates of influenza A and B, including amantidine and rimantadine resistant variants [93].<br />

The inhibition of virus replication in MDCK cells has been confirmed [96] and this knowledge<br />

prompted the extension of the inhibition studies to human respiratory epithelium cells in vitro [97],<br />

which indicated high antiviral activity for strains of A(HINI) and A(N3N2) isolates. They also found<br />

that delayed administration of the drug—after viral replication was well estab-<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_477.html [4/9/2004 12:26:55 AM]

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