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Figure 5<br />

Schematic representation of the principal intermolecular<br />

interactions of a fibrinogen peptide A mimetic within the active site<br />

of thrombin.<br />

Page 252<br />

Obst et al. [13], departing from the peptide template, designed and synthesized novel nonpeptide<br />

inhibitors of thrombin. They began with a cyclic template having attachment sites for three side chains<br />

that would be complementary to the S1, S2, and S3 sites in thrombin. Important to the design was a rigid<br />

template that would avoid hydrophobic collapse of the side chains and loss of conformational degrees of<br />

freedom upon complex formation with thrombin. Using computational approaches (Insight<br />

II/Discover/CVFF force field), possible templates were modeled within the active site of thrombin.<br />

These studies resulted in the synthesis of thirteen analogs that shared a common template.<br />

The most active molecule (K i = 90 nM, 8-fold selective versus trypsin) was studied further <strong>by</strong> x-ray<br />

crystallography (Figure 6). The positively charged benzamidine binds into the S1 pocket of thrombin<br />

forming a bidentate hydrogen bond with Asp189. The proximal carbonyl of the rigid template acts as a<br />

hydrogen-bond acceptor for the amide NH of Gly216. The methylene dioxybenzyl group at P3 interacts<br />

with thrombin in two ways. An edge-to-face interaction was observed with Trp215, and an oxygen of<br />

the methylenedioxy group acts as an acceptor for a hydrogen bond with the OH hydrogen of Tyr60A.<br />

Recent communications from Bristol-Myers Squibb [14,15] describe peptidomimetic inhibitors (Figure<br />

7) that were designed to bind thrombin with an N- to C-polypeptide chain sense opposite that of the<br />

substrate and form interactions similar to those made <strong>by</strong> the first three residues of hirudin (lle1, Thr2,<br />

Tyr3). In the x-ray crystal structure of BMS-183507 (K i = 17.2 nM) with thrombin [15], the N terminus<br />

is facing the catalytic site while the methyl ester is<br />

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