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Document<br />

Page 462<br />

strains of influenza. The discovery of the active site of influenza neuraminidase and the exploitation of<br />

its structural conservation shall be discussed in terms of the design of potent neuraminidase inhibitors.<br />

The potential therapeutic use of these inhibitors as antiviral drugs against influenza virus infections shall<br />

be examined.<br />

A. Antigenic Variation<br />

The plethora of different strains of virus that are responsible for the continued reinfection of virus in<br />

humans is primarily related to mutations in the viral genes of two surface glycoproteins, hemagglutinin<br />

and neuraminidase [9]. The current paradigm for this genetic variation [10,11] is that these mutations<br />

arise primarily from incremental changes in the amino acid sequences of these glycoproteins <strong>by</strong><br />

selection pressure of the immune system of the infected host. This mechanism termed “Antigenic Drift”<br />

accounts for most of the strain variation within a particular subtype of influenza.<br />

However, infrequently a mutation arises <strong>by</strong> genetic reassortment of viruses from different animal hosts<br />

(“Antigenic Shift”) where<strong>by</strong> an entirely new gene for one of the surface glycoproteins is generated that<br />

is significantly different (~50%) in amino acid sequence from the parent virus. This is the mechanism <strong>by</strong><br />

which new subtypes of influenza arise and are primarily responsible for the major pandemics that occur.<br />

Strains of influenza virus are classified <strong>by</strong> type (A, B, or C), geographic location, date of original<br />

isolation, and the subtype of the hemagglutinin and neuraminidase antigens. There exist 9 known<br />

subtypes (N1 to N9) of neuraminidase and 13 known subtypes (H1 to H13) of hemagglutinin for<br />

influenza A in all animal populations. Two neuraminidase (N1 and N2) and three hemagglutinin (H1,<br />

H2, and H3) subtypes of influenza A have occurred in strains that have infected humans since 1933<br />

when isolates were first characterized [12]. Prior to 1933 there is indirect evidence of antigenic shift<br />

occurring in human populations [13]. The N1 subtype was associated with virus isolated between 1933<br />

and 1957, after which time the N2 subtype appeared in the Asian influenza. No major change in the<br />

structure of neuraminidase has occurred since, although the hemagglutinin subtype has changed from H2<br />

to H3 in 1968 in the Hong Kong pandemic, and H1N1 reappeared in 1978 as the Russian pandemic.<br />

Influenza B, which infects only human hosts, has only one subtype, but like type A undergoes continual<br />

antigenic drift.<br />

B. Current Therapeutics<br />

Amantidine and Rimantidine are the only class of drugs that have been approved for therapy. At high<br />

concentration (>50 mg/mL) Amantidine is thought to buffer the pH of the endosome and prevent the<br />

conformational<br />

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