Enzyme inhibitors thrombin

Zwitterionic character is notable in several therapeutic area series, e.g. in angiotensin-converhng enzyme inhibitors, quinolone anhbacterials and thrombin inhibitors. The aqueous solubiUty measurement of zwitterions is very pH dependent as might be expected. The relationship of aqueous solubiUty to ionization state is extraordinarily complex if the zwitterion is of the type capable of an equi-Ubrium between true zwitterion and formally neutral forms (e.g. as in a quinolone antibacterial). For these types of complex equilibria, salt effects on solubility may be unexpectedly large, e.g. solubility unexpectedly may track with the chaotropic character of the salt... 

Other non-covalent interactions such as C=0 F-C type, between a fluorine atom and the carbonyl of an amino acid, may take place for the stabilisation of enzyme-inhibitor supramolecular structures [28,30]. It is why the 4-fluorophenyl group is an important motif for binding pocket, as shown by the enhancement of one order of magnitude of the K by introducing one fluorine atom on thrombin inhibitor (Fig. 5) [30],... 

J.A. Olsen, D.W. Banner, P. Seiler, U.O. Sander, A. d Arcy, M. Stihle, K. Muller, F. Diederich, A fluorine scan of thrombin inhibitors to map the fluorophilicity fluoropho-bicity of an enzyme active site Evidence for C-F- C = O interactions, Angew. Chem. Int. Ed. 42 (2003) 2507-2511. 

In the P 1 position, the natural cleavage sites use Thr and lie while Ecotin has Met. In contrast to thrombin, FXa lacks the 60-insertion loop and can accommodate large groups at this position. The BPTI mutants, however, are forced to use a small residue (Ala) because of steric hindrance from the Cys58-Cys42 group residue 61 in the enzyme. The inhibitor TFPI-II has a Gly at PT. 

I I Gustafsson D, Antonsson X Bylund R, et al. Effects of melagatran, a new low-molecular-weight thrombin inhibitor, on thrombin and fibrinolytic enzymes. Thromb Haemost I 998 79 1 10-1 18. 

Pharmacologic inhibitors of thrombin. Thrombin is a key enzyme in the hemostatic system in that it leads to the formation of fibrin strands and is a potent stimulus for platelet activation. Thrombin inhibitors, factor Xa inhibitors, and antiplatelet drugs act at different points in the hemostatic system to regulate the amount of thrombin that is generated. 

Many pharmaceutical companies are actively working on the development of orally active inhibitors of the serine protease thrombin as one important enzyme in the blood coagulation cascade. In many cases, however, the systemic exposure to thrombin inhibitors... 

Thrombin is a proteolytic enzyme and has a remarkable similarity in its overall three-dimensional structure to the digestive serine proteases, trypsin, and chymotrypsin [11-13]. Trypsin and thrombin share a common primary specificity for proteolysis next to arginine or lysine residues. Structural data of thrombin and trypsin have demonstrated strong resemblance in their substrate sites, and many small organic inhibitors are comparably active against both the enzymes [14,15]. For this reason, no or low inhibition of trypsin is viewed as a required condition for a compoimd to be a successful orally bioavailable thrombin inhibitor [16]. 

The hydrophobicity plays only a partial role in the QSAR of thrombin inhibitors. It appears that some of the molecules must be interacting with a hydrophobic portion of the enzyme. The negative hydrophobic effect is... 

The development of thrombin inhibitors that lack the functionalized TSA highlights a major new approach to type I peptidomimetics. In 1995, a Lilly group found that D-Phe-Pro-Agmatine analogs showed increased selectivity for thrombin over other fibrinolytic enzymes despite a 100-fold loss in potency caused by the removal of the aldehyde group (160). These studies paved the way for Merck s development of picomolar thrombin inhibitors (161,162), which use a similar motif Removal of an a-ketoamide transition state mimic from L-370,518 (89) (Fig. 15.39,... 

Thrombin inhibitors (92) and (93) illustrate a novel type III peptidomimetic. Most protease inhibitors bind in an extended jS-strand conformation that is stabilized by multiple enzyme ligand hydrogen bonds. 

For example, a thrombin inhibitor 25 (Kt = 0.25 pM) is more potent than the nonfluo-rinated counterpart (A) = 1.6 pM) and the X-ray crystal structure of the inhibitor-enzyme complexes showed remarkable conformational differences between the two inhibitors. This conformational change is caused by the dipolar C-F—N(H)(Gly216) interaction with a F-N distance of 3.5 A, as illustrated in Figure 1.13a [25]. 

Table 1.7 Enzyme inhibitory activity of tricyclic thrombin inhibitors [105]...

Olsen, J. A., Banner, D. W., Seiler, P., et al. (2003) A fluorine scan of thrombin inhibitors to map the fluorophilicity/fluorophobicity of an enzyme active site evidence for C-F C=0 interactions. Angewandte Chemie International Edition, 42(22), 2507-2511. 

The described work has also significant impact on the design of inhibitors for other enzymes. The first published nonpeptidic inhibitors of the cysteine protease interleukin converting enzyme (ICE) have been designed following the Zeneca approach [12]. Similarly, thrombin inhibitors have recently been disclosed that were designed along the same lines [13]. 

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