Amidino-Containing Schiff Base Copper(II) and Iron(III) Chelates as a Thrombin Inhibitor

Abstract

Thrombin is a trypsin-like serine proteinase that plays a critical role in the blood coagulation cascade. Thrombin converts fibrinogen into fibrin which forms part of the blood clot, 1,2) and activates other blood coagulation factors such as V, VIII, XIII, and protein C.3) Moreover, thrombin also activates blood platelets, 4,5) and acts as a mitogen for various cell types. [6][7][8] Thus, thrombin has become a special target in the design and development of antithrombotic agents.Various naturally occurring thrombin inhibitors have been found: hirudin (from the leech Hirudo medicinalis), 9,10) cyclotheonamide A, B, and nazumamide A (from the marine sponge Theonella sp.), [11][12][13][14] aeruginosin 298-A, aeruginosins 98-A and 98-B (from the blue-green alga Microcystis aeruginosa). [15][16][17] On the other hand, many compounds having benzamidine or arginine moiety as a partial structure have been synthesized and their inhibitory effects on thrombin examined. [18][19][20][21][22][23] Argatroban (MD-805) proposed by Okamoto et al. 18) is one of most potent synthetic thrombin inhibitors (K i ϭ1.9ϫ10 Ϫ8 M) and is clinically used. Complexes of human or bovine a-thrombin with synthetic arginyl peptides have been analyzed by X-ray diffraction studies. [24][25][26][27][28][29][30] The X-ray data suggested that the thrombin active site consists of the specificity (S1) pocket, the hydrophobic proximal pocket (P-pocket), and the hydrophobic distal pocket (D-pocket). The guanidino nitrogen atoms of inhibitors form a salt bridge with Asp189 (numbering of the thrombin amino acid residues is based on the chymotrypsinogen nomenclature introduced by Bode et al.) 24,28) in S1 pocket, and additional favorable interactions between the inhibitor and D-pocket or P-pocket enhance the inhibitory potency.Cationic organic molecules such as benzamidine and phenylguanidine derivatives are potent inhibitors of trypsin and trypsin-like enzymes, [31][32][33] owing to the electrostatic interaction between the cationic group of inhibitor and an anionic residue at S1 site in the trypsin active site. It is known that salicylaldehyde spontaneously forms a very stable Schiff base metal chelate with an a-amino acid and metal ion. Therefore, we designed and synthesized amidino-or guanidino-containing Schiff base metal chelates (Chart 1) as trypsin-like enzyme inhibitors. [34][35][36] The manifold compounds in the series of A and B can be prepared by changing the combination of salicylaldehyde derivatives, various aamino acids, and metal ions, respectively.Recently we proposed a new binding mode between trypsin active site and amidino-containing Schiff base metal chelate by X-ray diffraction study.37) The cationic amidino group of chelates was shown to form a salt bridge with the carboxylate of Asp189 in the S1 pocket of trypsin as expected. In addition, it was evidenced that the metal ion and the phenolic oxygen of the Schiff base contribute additional interactions with His57 and Ser195 of trypsin.In this report, the inhibitory effect of amidin...

Development of copper based drugs, radiopharmaceuticals and medical materials

“…It is also a rare example of Cu(I) compound proposed for medical use. Toyota et al (2005) described a series of copper and iron complexes acting as thrombin inhibitors. One of these compounds, Cu(II) complex with 4-formyl-3-hydroxybenzamidine and d -tryptophane ( 49 ), had the highest inhibitory activity (K i value 2.7 × 10 −8  M), comparable to registered anticoagulant drug, argatroban (K i 1.9 × 10 −8  M) (Toyota et al 2005).…”

Direct thrombin inhibitors – a survey of recent developments

“…Interestingly, there is experimental evidence indicating that HPW-RX2 has a dual effect inhibiting not only thrombin but also cyclooxygenase [179]. Toyota et al [180] revived the concept of metal chelates as protease inhibitors [181] previously realized by Axys constructing thrombin inhibitors such as 39 (Fig. 12) [182,183].…”

“…The metal ion thus acts not only as a stabilizer of the inhibitor structure but actively participates as a cofactor in inhibition. A different type of chelate was realized by Toyota et al [180] by synthesizing Schiff bases from formyl-hydroxybenzamidines and various amino acids. Depicted are structural models of two of the most active inhibitors, 40 and 41 (Fig.…”

Organometallics as Structural Scaffolds for Enzyme Inhibitor Design

“…Tanizawa et al designed Schiff base chelate complexes of Cu(II) and Fe(III) as inhibitors of the serine proteases trypsin and thrombin [15,16]. The Schiff base ligands were obtained by condensing 4-formyl-3-hydroxybenzamidines or 3-formyl-4-hydroxybenzamidines with amino acids.…”