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

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involves the catalytic asparates 25/25'. The structural effect of a mutation of Leu90/90' to the larger<br />

methionine is rather difficult to predict since it can either rigidify or destabilize the HIV PR dimer or it<br />

may have an effect on the catalytic efficiency of the “resistant” enzyme.<br />

Page 32<br />

The complicated pattern of HIV resistance to protease inhibitors, in particular the appearance of<br />

compensatory mutations that alone do not confer any resistance, suggests that the key to understanding<br />

the basis of decreased susceptibility of the virus to a given drug is the kinetics of specific processing of<br />

the GAG and GAG-POL polyproteins. The reduction in sensitivity of a mutant HIV PR towards any<br />

inhibitor can be conveniently reflected <strong>by</strong> the ratio of K i mutant/K i wild type. However, this reduced<br />

inhibitor sensitivity is only one component that distinguishes mutant-form from wild-type proteases. For<br />

virus encoding of a mutant HIV PR to be viable, the mutant protease must be capable of a minimal<br />

(although not yet quantified) level of enzymatic activity towards all substrates required for maturation of<br />

the virions. This proteolytic efficiency is reflected in the specificity constants (K cat/K m) as determined for<br />

mutant and wild type HIV PRs. In order to rationalise these potentially conflicting relationships between<br />

enzymes, substrates, and inhibitors, Gulnik and his colleagues [66] introduced the term “Vitality<br />

Factor,” in which<br />

In order for the “Vitality Factor” to be predictive for the level of resistance expected for a particular drug<br />

or combination of drugs for a given resistant strain of HIV, the determination of the specificity constants<br />

(K cat/K m for mutants) must be repeated for all nine known substrates processed <strong>by</strong> HIV PR. The<br />

inhibition constants of a given compound should not depend on the substrate, but the K cat/K m ratios do<br />

and therefore vitality values will differ for different substrates. It will be expected that the mean for all<br />

nine “vitality” values will be predictive for the change in antiviral activity for a particular compound.<br />

Although those data will be derived from in vitro experiments and are clearly not without some<br />

limitations, they may help in understanding the molecular basis of resistance and may contribute some<br />

value to possible multidrug strategies for the clinical management of AIDS.<br />

III. Perspective<br />

HIV PR inhibitors with acceptable oral availability and pharmacokinetic properties offer great promise<br />

for the treatment of HIV infection and AIDS. Efficacy studies of indinavir, ritonavir, or nelfinavir using<br />

plasma viral RNA as a marker have demonstrated up to three log reductions in RNA copy numbers that<br />

are<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_32.html [4/5/2004 4:46:54 PM]

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