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

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Document<br />

Page 501<br />

Because of the large number of serotypes, a successful inhibition strategy needs to consider whether it is<br />

better to have a drug that is exceedingly potent against a small subset of HRV serotypes (e.g. 30%) or a<br />

drug that is somewhat less potent but effective against most serotypes (e.g. 90%). Because of these<br />

considerations, different parameters are required to describe viral inhibition.<br />

Two important values that have been used extensively to describe potency for antipicornaviral<br />

compounds include the mean inhibitory concentration (MIC) and the MIC 80. The MIC is the<br />

concentration that inhibits the viral progeny production <strong>by</strong> 50% in a cell-<strong>based</strong> plaque assay. The mean<br />

MIC is the average MIC over the number of serotypes against which the compounds have been tested.<br />

The MIC 80 is the MIC at which at least 80% of the serotypes tested will be inhibited <strong>by</strong> at least 50%.<br />

Another way to think about the MIC 80 is that it is the MIC value for the serotype that is at the 80th<br />

percentile rank for viruses inhibited. For example, if ten viruses were tested, the MIC 80 would be the<br />

MIC concentration of the drug that inhibits the 8th most sensitive virus [15].<br />

B. Potency and Binding Energetics<br />

It would be reasonable to assume that the activity of a drug (MIC) against a specific serotype would be<br />

related to its binding energy. This is an important consideration because algorithms that are used to<br />

predict potency rely on estimations of binding energy. The only experiment completed to directly study<br />

this correlation suggests a rough correlation in a small number of samples. This study however was<br />

limited to a small number of compounds in a single chemically similar series [78]. It is unclear whether<br />

this relationship will hold over diverse chemical entities.<br />

One could imagine a series of compounds that binds, but is ineffective at stabilizing the virion to any<br />

extent, thus ineffective in inhibiting viral replication. This appears to be what was observed when<br />

fragments of WIN compounds that only contained the A and B rings were examined. These fragments<br />

were less able to stabilize the virus to heat-induced denaturation than intact compounds. Although the<br />

binding constant for these compounds has not been determined, the diminished thermostabilization<br />

occurred at concentrations of compound sufficient to allow the compounds to be seen bound in the VP1<br />

pocket via x-ray crystallography [79]. This observation suggests that the drug binding and inhibition<br />

have been decoupled to some degree.<br />

Another question concerning correlating binding affinity to viral inhibition is the number of sites per<br />

virion required to be occupied before the virion can no longer uncoat: all 60, or a few? Further, is there<br />

any positive or negative cooperativity in the interaction? These questions have not yet been answered.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_501.html [4/9/2004 12:37:57 AM]

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