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Figure 9<br />

Peptide scaffold- and nonpeptide template-<strong>based</strong> design strategies:<br />

gpIIb/IIIa antagonists<br />

Page 573<br />

peptidomimetic 37 was found to be a moderately potent SRIF-like agonist (partial) in cellular assays<br />

[41]. This discovery extends previous studies on TRH (see peptidomimetic 30, Figure 8) which utilized<br />

a cyclohexane ring system as a nonpeptide template to functionalize with the pyroglutamic acod and His<br />

side chains as well as the C-terminal carboxamide group of the parent peptide ligand [13]. However, in<br />

the comparative analysis of analogs of the SRIF-mimetic 37 it was also found that N-acetylation of the<br />

Lys side chain moiety yielded a potent antagonist of substance-P (NK 1 receptor). This indicated that<br />

slightly different functionalization of the nonpeptidic glucopyranoside template was quite compatible<br />

with NK 1 receptor molecular recognition. Intuitively, a “reversed design” strategy to convert the latter<br />

glucopyranoside-<strong>based</strong> NK 1 ligand to a cyclic peptide was next investigated (Figure 10), and<br />

successfully led to the discovery of a novel cyclic peptide ligand also exhibiting potent NK 1 receptor<br />

binding and antagonism [46].<br />

The above examples of peptide scaffold- or nonpeptide template-<strong>based</strong> peptidomimetic agonists or<br />

antagonists illustrate various strategies to elaborate bioactive conformation and/or pharmacophore<br />

models of peptide ligands at their receptors. In many cases, receptor subtype selectivity has also been<br />

achieved <strong>by</strong> systematic structural modifications of prototypic leads of peptidomimetics. Thus, although<br />

the 3D structures of G-protein-coupled receptors (GPCRs) remain as elusive (except for models<br />

constructed from homology-<strong>based</strong> low-<br />

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