Boronic acids thrombin inhibitors

Schematic representation of the active-site orientation of a neutral PI boronic acid thrombin inhibitor.

A group of peptide derivatives such as peptide arginals and boronic acid peptide derivatives belong to another class of reversible thrombin inhibitors. One such inhibitor is PPACK (D-Phe-Pro-Arg chloromethyl ketone), which functions as a powerful irreversible thrombin inhibitor by alkylating the histidine residue at the catalytic site of thrombin (58). It, however, is unstable in neutral solution, as it undergoes cyclization and inactivation. However, the D-methyl derivative of D-Phe-Pro-Arg-H (D-Mephe-Pro-Arg-H) called efegatran, with a molecular mass of 515 Da, is a stable selective reversible inhibitor of thrombin with a K. of approximately 100 nM. The basic amino terminus in this compound is responsible for promoting the specificity toward thrombin (63). 

Some boronic acid-based enzyme inhibitors undergo strong yet reversible covalent attachment to a nucleophile at the enzyme s active site, while others simply act as competitive inhibitors in their borate conjugate base form. Boronic acid-based inhibition of thrombin has been achieved <93MI109>, and that of P-lactamases has been particularly effective <95TL8399, 96M1688>. When compared to other covalent transition-state analog inhibitors of P-lactamases like phos-... 

Many research groups have started with PPACK and produced inhibitors of a less peptidic nature (to improve in vivo stabihty), and/or with conformational restriction (to tackle the entropy loss problem), and/or with a variety of serine trap functionalities, for example, aldehydes, boronic acids, a-keto amides and acids, a-keto heterocycles, polyfluorinated ketones, and phosphonates. Structures of many of these are known in complex with thrombin but will not be reviewed here (see e.g., pubhca-tions of C.A. Kettner and coworkers). As well as lack of specificity, these potential drugs suffer from slow on-rates and have not progressed to the market. 

The very first examples were some simple boronic acid compoimds [5, 6]. For example, phenylboronic acid and substituted phenylboronic acids were found to be strong competitive inhibitors of subtilisin and chymotrypsin [6]. More recendy, boronic acids have been used for the synthesis of inhibitors against thrombin [50,51], lactamases [52], dipeptidyl peptidases [53], and others [54, 55]. In this section, thrombin inhibitors will be used as examples. The rest will be summarized in a table at the end of the section. 

Figure 13.1 Boronic acid-based thrombin inhibitors (Part A).

There have also been efforts in designing selective thrombin inhibitors by varying the PI position. For example, incorporation of m-cyano-substituted phenylalanine boronic acid analogues into R-(D)Phe-Pro-OH dipeptides produced several highly effective thrombin inhibitors such as H-(D)Phe-Pro-boroPhe(m-CN)-OH [62]. The cyano group enhances binding by several orders of magnitude. Because of its struc-... 

Figure 13.7 Boronic acids and esters as inhibitors of nitric oxide synthase, glycosidases, thrombin, and HIV protease.

---