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Serine proteinases are involved in several physiological processes and elicit profound cellular effects in a variety of tissues. Besides the thrombin receptor a second receptor, activated by trypsin, the proteinase-activated receptor 2 (PAR-2), was cloned and characterized. Both enzymes generate a new extracellular N-terminus by limited proteolytic cleavage which functions as tethered ligand to activate the receptor. Synthetic peptides corresponding to the sequences of the newly generated N-terminus are able to mimic the effects of the enzymes. In porcine pulmonary arteries trypsin and the receptor-derived peptide SLIGRL elicited an endothelium-dependent transient relaxation of PGF2alpha-precontracted vessels. The EC50 values for trypsin and SLIGRL amounted to 1.1 +/- 0.2 nM and 5.4 +/- 0.6 microM, respectively. Trypsin and SLIGRL caused a homologous desensitization but thrombin and the thrombin receptor-activating peptide SFLLRN were still able to elicit pronounced relaxant effects. The trypsin- and SLIGRL-induced relaxant responses were markedly diminished after blockade of the nitric oxide synthesis by N(G)-nitro-L-arginine methyl ester (200 microM) and were absent in endothelium-denuded vessels. Indomethacin and hirudin did not influence the relaxant effects. The effect of trypsin but not that of SLIGRL was blocked by the proteinase inhibitor aprotinin suggesting that only proteolytically active trypsin activates the receptor. Benzamidine derivatives of the 3-amidinophenylalanine type with different affinity for trypsin and thrombin inhibited the vascular effects of trypsin (IC50 0.007-0.7 microM) correlating with its antitrypsin activity. The data suggest that the vascular effects of trypsin and SLIGRL are mediated through activation of PAR-2 which differs from the thrombin receptor.

Besides guanidino compounds and amines structurally related to arginine and lysine, compounds with an amidino moiety are inhibitors of trypsin-like enzymes. However, in most cases ordinary benzamidine derivatives are not selective inhibitors. Relatively selective inhibitors of some enzymes were found among amino acids containing a benzamidine moiety at the side chain. For example, derivatives of phenyl-alpha-aminobutyric acid with one or two amidino moieties such as the 4'-amidinoanilide of N alpha-[4-amidinophenylsulfonyl]-phenyl-alpha- aminobutyric acid are potent and selective inhibitors of plasma kallikrein (Ki = 0.15 microM). Using N alpha-arylsulfonylated 3-amidinophenylalanine as the key building block, novel inhibitors of plasma kallikrein were developed. Several esters and amides of the nipecotic acid derivative were synthesized which inhibit plasma kallikrein competitively with Ki values between 0.1 and 1.0 microM. The compounds prolonged aPTT in micromolar concentration, indicating an inhibition of the intrinsic coagulation pathway.

In studies on the inhibition of activated protein C (APC) by benzamidine derivatives potent inhibitors of APC were found among anilides of 4-amidinophenyl-alpha-aminobutyric acid (Ki = 0.58 mumol/l). Several bis-benzamidine derivatives containing a cycloalkanone linking bridge inhibit APC with Ki values near the micromolar range. Potent and selective inhibitors of thrombin derived from 3- and 4-amidinophenylalanine do not inhibit APC. This is of great importance for further development of these inhibitors as potential anticoagulant drugs.

Considerable circumstantial evidence has been provided by in vitro studies that tryptase (EC 3.4.21.59), a neutral serine proteinase stored in large amounts in mast cell granules, may play an important pathogenetic role in mast cell-dependent diseases. However, a definitive role has not yet been ascribed to this trypsin-like enzyme with restricted substrate specificity as natural or synthetic inhibitors of tryptase applicable for in vivo studies are not available so far. Therefore, we have studied structure-activity relationships for inhibition of tryptase by benzamidine derivatives, compounds known to be potent inhibitors of various trypsin-like enzymes. Among the benzamidine derivatives 4-amidinophenylpyruvic acid exerts a striking inhibitory activity with a Ki of 0.71 mumol/l. Several additional inhibitors of tryptase with Ki values in the micromolar range were found among bis-benzamidines. Derivatives of N alpha-arylsulfonyl-omega-amidinophenyl-alpha-aminoalkylcarboxylic acids are only weak inhibitors of tryptase, although members of this group are potent and selective inhibitors of several other trypsin-like enzymes. Comparison of the inhibition of tryptase and trypsin revealed that the affinities of the benzamidine derivatives to both proteinases are closely correlated (correlation coefficient r = 0.702; n = 37; p < 0.001). These results demonstrate that 4-amidinophenylpyruvic acid may be useful as a pharmacologic tool for the investigation of the (patho)physiological role of tryptase. In addition, benzamidine derivatives may be applicable to probe the active site topography of tryptase isoenzymes.

Besides guanidino compounds and amines structurally related to arginine and lysine, compounds with other cationic groups are inhibitors of trypsin-like serine proteinases. Particularly aromatic ring structures with an amidino moiety have high affinity for these enzymes. In most cases ordinary benzamidine derivatives are no selective inhibitors, however, among derivatives of N alpha-arylsulfonyl-omega-amidinophenyl-alpha-amino-alkylcarboxylic acids selective competitive inhibitors of several enzymes were found. Amides of phenyl-alpha-aminobutyric acid containing an amidino moiety are inhibitors of plasma kallikrein. The p-amidinoanilide of 2-tosylamino-4-phenylbutyric acid inhibits selectively plasma kallikrein with a Ki of 0.70 mumol/l. In contrast, potent and selective inhibitors of glandular kallikrein were hardly found among benzamidines.