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Document<br />

Page 514<br />

sis. Large numbers of compounds, which might have been synthesized in the absence of any structural<br />

knowledge, have been eliminated from the list of inhibitors to be created, because these compounds<br />

would not have fit well into the VP1 pocket due to its finite size and hydrophobic nature.<br />

The goal of being able to predict with greater accuracy the potency and spectrum of compounds before<br />

they are synthesized awaits three developments: the structures of more HRVs and compounds, the<br />

ability to more accurately model hydrophobic interactions, and probably the most difficult, the ability to<br />

predict changes in the HRVs that occur due to drug binding.<br />

V. Viral Resistance<br />

As would be expected for any virus, particularly an RNA virus, resistance to capsid-binding compounds<br />

has been observed in the laboratory. The effect such mutations have on the life cycle of the virus is<br />

important. Mutations that occur in regions where changes are poorly tolerated, because that region<br />

serves a vital function in the viral life cycle, are likely to lead to less virulent viruses. The almost<br />

universal inhibition of rhino- and enteroviruses <strong>by</strong> capsid-binding compounds, coupled with the<br />

observation that all of the viruses with determined structures have a VP1 hydrophobic pocket, suggests<br />

that these pockets serve an important and similar function. It seems unlikely that so many viruses would<br />

maintain such a pocket if it were not a selective advantage. <strong>Drug</strong>-resistance mutants have been classified<br />

into two groups, exclusion mutants and compensation mutants.<br />

A. Exclusion Mutants<br />

The exclusion mutants' behavior has been readily and adequately determined <strong>by</strong> biochemical and<br />

crystallographic means [91,92]. The mutations occur within the hydrophobic pocket of VP1 and<br />

thermostabilization studies have shown that these mutations preclude the binding of drug in the VP1<br />

pocket. One mutation site in HRV14 has been located at position 1188, which is on the side of the<br />

pocket closest to the viral interior, away from the canyon. Mutations at this site that convey resistance<br />

are Val rarrow.gif Leu or Val rarrow.gif Met. In both cases the mutation is to a larger side chain,<br />

which would be expected to fill the pocket. The crystal structure of the Val rarrow.gif Leu mutation<br />

confirms this hypothesis and demonstrates that the Leu side chain occupies space that would normally<br />

be occupied <strong>by</strong> an antiviral drug.<br />

The second site found in HRV14 is at Cys1199. Mutations to Phe, Tyr, Trp, and Arg all confer<br />

resistance at this site. Again, all of these mutations are to larger side chains. The hypothesis that these<br />

mutations function <strong>by</strong> excluding<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_514.html [4/9/2004 12:45:35 AM]

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