Inhibition of serine proteases by low molecular weight peptides and their derivatives

Peptides with 6-Aminohexanoic Acid: Synthesis and Evaluation as Plasmin Inhibitors

Fifteen new peptide derivatives of ɛ-aminocaproic acid (EACA) containing the known fragment –Ala–Phe–Lys– with an affinity for plasmin were synthesised in the present study. The synthesis was carried out a solid phase. The following compounds were synthesised: H–Phe–Lys–EACA–X, H–d-Ala–Phe–Lys–EACA–X, H–Ala–Phe–Lys–EACA–X, H–d-Ala–Phe–EACA–X and H–Ala–Phe–EACA–X, where X = OH, NH₂ and NH–(CH₂)₅–NH₂. All peptides, except for those containing the sequence H–Ala–Phe–EACA–X, displayed higher inhibitory activity against plasmin than EACA. The most active and selective inhibitor of plasmin was the compound H–d-Ala–Phe–Lys–EACA–NH₂ which inhibited the amidolytic activity of plasmin (IC₅₀ = 0.02 mM), with the antifibrinolytic activity weaker than EACA. The resulting peptides did not affect the viability of fibroblast cells, colon cancer cell line DLD-1, breast MCF-7 and MDA-MB-231 cell lines.

Antithrombin Agents as Anticoagulants and Antithrombotics: Implications in Drug Development

Antithrombin agents of recombinant and synthetic origin are now validated in experimental models as useful an ticoagulant and antithrombic drugs. Several clinical trials in cluding surgical anticoagulation, management of coronary syn dromes, adjunct to thrombolytic agents and treatment of throm boembolism have also shown the comparative efficacy of these agents in reference to heparin. Argatroban and hirudin are now available for specific clinical indications such as thrombotic and ischemic stroke and alternate anticoagulants for heparin- induced thrombocytopenia (HIT) patients in Japan and Euro pean countries, respectively. While these agents produce strong anticoagulant effects, their mechanism of action is distinct from that of heparins, thus these agents should be used carefully using specific guidelines provided for each product. Thrombin inhibitors are effective anticoagulants however, their therapeu tic index is narrower than heparin and as such their nonopti mized use is potentially associated with hemorrhagic compli cations.

Fibrinolytic Compromise by Synthetic and Recombinant Thrombin Inhibitors: Implications in the Treatment of Thrombotic Disorders

Site-directed synthetic and recombinant antithrombin agents are widely used to prevent reocclusion during thrombolytic therapy in acute myocardial infarction and other vascular occlusive disorders. However, minimal studies have been conducted to examine the interactions between these anticoagulant and thrombolytic agents. Because of the structural homology of serine proteases, some of these newly developed antithrombin agents are also capable of inhibiting fibrinolytic enzymes, which may lead to fibrinolytic compromise during thrombolytic therapy. In addition, inhibition of thrombomodutin-bound thrombin may also result in fibrinolytic deficit. Several thrombin inhibitors were studied in in vitro systems to assess whether their inactivating properties extend to fibrinolytic and profibinolytic enzymes, such as kallikrein, plasmin, urokinase, streptokinase, and tissue plasminogen activator. The thrombin inhibitors studied included hirudin, hirulog-I, argatroban, D-MePhe-Pro-Arg-H, and Ac-D-Phe-Pro-boroArg-OH. Their activities were compared with those of aprotinin, which is currently used clinically as an antifibrinolytic agent. Although argatroban, hirulog-I, and hirudin exhibited minimal inhibition of the nonthrombin enzymes studied, the tripeptide derivatives showed variable inhibitory activities, with the boronic acid derivative being the most potent and universal inhibitor. The in vivo antifibrinolytic activities of these thrombin inhibitors were also studied in a rabbit model of jugular vein clot lysis. In agreement with the in vitro studies, argatroban, hirulog-I, and hirudin exhibited minimal antifibrinolytic activities, while Ac-D-Phe-Pro-boroArg-OH and D-MePhe-Pro-Arg-H showed marked inhibition of the thrombolytic process. The results of these studies indicate that newly developed thrombin inhibitors with a broader serine protease spectrum may exhibit fibrinolytic compromise resulting in diminishment of the expected thrombolytic outcome. Key Words: Thrombolysis—Fibrinolysis—Thrombin inhibitors—Argatroban—Hirudin—Hirulog-I—Aprotinin.

Natural and synthetic inhibitors of kallikrein-related peptidases (KLKs)

Including the true tissue kallikrein KLK1, kallikrein-related peptidases (KLKs) represent a family of fifteen mammalian serine proteases. While the physiological roles of several KLKs have been at least partially elucidated, their activation and regulation remain largely unclear. This obscurity may be related to the fact that a given KLK fulfills many different tasks in diverse fetal and adult tissues, and consequently, the timescale of some of their physiological actions varies significantly. To date, a variety of endogenous inhibitors that target distinct KLKs have been identified. Among them are the attenuating Zn(2+) ions, active site-directed proteinaceous inhibitors, such as serpins and the Kazal-type inhibitors, or the huge, unspecific compartment forming α(2)-macroglobulin. Failure of these inhibitory systems can lead to certain pathophysiological conditions. One of the most prominent examples is the Netherton syndrome, which is caused by dysfunctional domains of the Kazal-type inhibitor LEKTI-1 which fail to appropriately regulate KLKs in the skin. Small synthetic inhibitory compounds and natural polypeptidic exogenous inhibitors have been widely employed to characterize the activity and substrate specificity of KLKs and to further investigate their structures and biophysical properties. Overall, this knowledge leads not only to a better understanding of the physiological tasks of KLKs, but is also a strong fundament for the synthesis of small compound drugs and engineered biomolecules for pharmaceutical approaches. In several types of cancer, KLKs have been found to be overexpressed, which makes them clinically relevant biomarkers for prognosis and monitoring. Thus, down regulation of excessive KLK activity in cancer and in skin diseases by small inhibitor compounds may represent attractive therapeutical approaches.

Antithrombin agents: the new class of anticoagulant and antithrombotic drugs

Antithrombin drugs represent a wide group of natural agents, recombinant agents equivalent to some of the naturally occurring proteins, and synthetic agents. This group of drugs is characterized by marked structural and functional heterogeneity. Several of these drugs are currently in various phases of development. Argatroban represents the first clinically approved antithrombin agent, which was made available in Japan several years ago. Two recombinant hirudin preparations, Revasc (Novartis) and Refludan (Aventis), are available for postsurgical DVT prophylaxis and alternate anticoagulant use in patients with heparin-induced thrombocytopenia. A synthetic antithrombin agent based on the combined structures of hirudin and antithrombin peptides, hirulog (Bivalirudin), is undergoing clinical trials in cardiovascular indications. Additional studies on the hirudins are being carried out to test their efficacy as surgical and interventional anticoagulants as replacements for heparin. However, the need for a proper antagonist is one of the limiting factors for the optimal development of hirudin in this indication. Several of the synthetic thrombin inhibitors are also being developed for oral use for the prophylaxis of DVT in surgical patients. Since the therapeutic index of thrombin inhibitors is narrower than that of heparin, this route may not be an optimal approach for the development of these agents. Despite several unresolved developmental issues, the thrombin inhibitors provide a useful alternative to heparin anticoagulation and may prove to be useful in validated clinical use.

Antithrombin agents as anticoagulants and antithrombotics. Implications in drug development

Synthetic and recombinant thrombin inhibitors have undergone several clinical evaluations for thrombotic and cardiovascular indications. While the initial trials were focused in coronary indications, more recently, these agents are also developed for the prophylaxis and therapeutic management of thromboembolic disorders. Hirudin, PEG-hirudin and argatroban are in advanced clinical development. Recombinant hirudin has been approved in Europe as a substitute anticoagulant for the management of HIT patients. Several additional clinical trials are currently carried out to demonstrate the usefulness of these agents in thrombotic and cardiovascular indications. Despite these developments such issues as dosage optimization, laboratory monitoring, neutralization and drug interactions require additional studies for the optimal development of these drugs.

Design of novel affinity adsorbents for the purification of trypsin-like proteases

A number of ligands for the selective purification by affinity chromatography of the trypsin-like protease, porcine pancreatic kallikrein, were designed de novo by computer-aided molecular design. The ligands were designed to mimic the side-chains of a number of arginyl dipeptides and included a benzamidine moiety substituted on a triazine ring. The ligands displayed inhibitory activities against pancreatic kallikrein which mirrored the specificity constants of the dipeptides they were designed to mimic. The ligand with the highest affinity for the enzyme, an analogue of a Phe-Arg dipeptide, when immobilized to Sepharose CL-4B via a hexamethylene spacer arm, purified pancreatic kallikrein 110-fold in one step from a crude pancreatic acetone extract.

Development of a photometric assay for activated partial thromboplastin time and its application to the Cobas Bio centrifugal analyzer

We describe a two-step procedure for APTT that can be performed on photometric devices. It includes preincubation of diluted plasma with ellagic acid and phospholipids and a starting reagent that contains calcium and a chromogenic peptide substrate for thrombin, Tos-Gly-Pro-Arg-pNA. Reaction time is recorded from addition of the starting reagent until thrombin formation occurs, and a prefixed amount of substrate is cleaved. The pattern of sensitivity to clotting factors and heparin was similar to clotting assays and the substrate used did not interfere with the activity of factor Xa. An application of the method was made for the Cobas(R) Bio centrifugal analyzer. Absorbance readings were sent to an external computer and were transformed into reaction times by a computer program. Although the results are independent on fibrinogen concentrations, from kinetic data of the reaction curve fibrinogen concentrations can be estimated. Correlation studies showed good correspondence to clotting methods (r = 0.92, n = 53) as well as an excellent precision (CV 3% for inter-assays, n = 15) and high throughput of samples (greater than 100/h) in the automated assay.

Inhibition of platelet aggregation and endogenous lectin activity by oligoamines

Amino sugars and basic amino acids inhibit platelet aggregation and the activity of the endogenous platelet lectin, yet, relatively high concentrations (approximately 30 mM) are required for the inhibition. If cooperative interactions are involved in these platelet surface activities, oligomers of primary amines should be more potent inhibitors than their individual component amines. Accordingly, series of oligomers of basic amino acids, of the polyamines (putrescine, spermidine and spermine) and of aliphatic diamines differing in chain length were tested for potency of inhibition of platelet aggregation and endogenous platelet lectin activity. Indeed, oligoamines were much more potent inhibitors of platelet aggregation than their corresponding monomers or shorter oligomers, more than accountable by an additive effect. For example, 60, 3 and 0.3 mM were needed for 50% inhibition of platelet aggregation by lysine, (lys)3 and (lys)5, respectively. A similar pattern was observed for the effect of the oligoamines on the activity of the endogenous platelet lectin. The inhibition of platelet aggregation by spermine is competitive, since the effect of a given dose of spermine decreased with increasing platelet concentration. Neither inhibition of platelet-inducer interaction nor Ca2+ insufficiency explain the inhibitory effects of the oligoamines. The results are consistent with the hypothesis that cooperative surface interactions underlie platelet aggregation and platelet lectin activity. The cooperative effects may reflect the formation of patches or clusters of positively charged groups on the surface of activated platelets.

Transition-state affinity chromatography of trypsin-like proteinases with dipeptidyl argininal ligands

Dipeptidyl argininal (arginine aldehyde) affinity resins of general formula R-(X-Y-argininal) (where R = resin matrix and X, Y = amino acids of varied structure) are synthesized in a solid-phase procedure in which the dipeptide (-X-Y-) is first attached to the resin, followed by the joining of the Y amino acid to argininal semicarbazone, and decomposition of the semicarbazone in a methanol/acetic acid/formaldehyde reagent. An R-(Gly-Gly-argininal) resin binds urokinase tightly, but does not bind thrombin. However, thrombin binds strongly to R-(Phe-Pro-argininal), whereas urokinase does not bind. Accordingly, the X-Y-argininal ligands selectively bind proteinases of identical primary binding site specificity to arginine, but different secondary site specificity in -X-Y-. The selectivity is due to an amplification of peptide binding specificity caused by the transition-state analog properties of the ligands. While the affinity constants between peptide aldehyde and proteinase approach those of antibody-antigen interactions, the elution with semicarbazide (aldehyde-trapping reagent) buffers easily remove tightly bound proteinases without proteinase inhibitors or denaturation. Conditions for the binding and elution of proteinases, methods of regeneration and other characteristics of the resins are described.