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C. <strong>Structure</strong>-Activity Relationships<br />

Page 502<br />

Structural features, as stated above, have been found that are common to the known HRV structures.<br />

These features place constraints on compounds that have been demonstrated in a large number of<br />

structure—activity relationships. Shape considerations, hydrophobicity requirements, hydrogen-bonding<br />

requirements and compound flexibility will be discussed.<br />

Pocket-Shape Considerations<br />

Compound Length. The results of x-ray studies on several HRV serotypes have shown that the<br />

hydrophobic pockets vary in size, but all pockets impose structural constraints on compounds. The<br />

pockets, which almost completely enclose the compounds, require that both drug length and width must<br />

be limited. The results of structure—activity relationships for one series of WIN compounds have shown<br />

that the optimum length of the aliphatic linking region between the phenoxy (B ring) and isoxazole (C<br />

ring) ring is between 3 and 6 carbons (Table 2). This optimum length will vary in other series <strong>based</strong> on<br />

the extent of substitution on the A and C rings (see below) [54,80–82]. This effect on linker length is<br />

obvious both for individual serotypes like HRV14 and for the MIC 80, which measures many serotypes.<br />

This effect of pocket length can also be seen when substituents are added to either the A or C ring which<br />

would lengthen the compounds. Additions of alkyl chains of four or more carbons tend to decrease<br />

potency, whereas shorter chains may increase potency (Table 3) [54,83].<br />

In studies on compound length, HRV14 appears to be more sensitive to longer compounds than are<br />

many other rhinoviruses. This is consistent with the observation that the hydrophobic pocket of HRV14<br />

is longer than that of HRVs<br />

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