N-terminal requirements of small peptide anticoagulants based on hirudin54-65

Argatroban, a direct thrombin inhibitor for heparin-induced thrombocytopaenia: present and future perspectives

Heparin remains the most commonly used anticoagulant in the treatment of patients with acute vascular syndromes, including myocardial infarction, unstable angina and ischaemic stroke. However, heparin therapy is not always associated with a significant improvement of clinical outcomes, is linked with enhanced bleeding risk and can occasionally provoke the development of heparin-induced thrombocytopaenia, the most devastating complication of conventional therapy with unfractioned heparin. Understanding the key role of thrombin in clot formation and platelet activation has stimulated the development of a new class of drugs - direct thrombin inhibitors. The direct thrombin inhibitor argatroban has been known for decades. Similar to the unfractioned heparin, argatroban requires intravenous administration and activated partial prothrombin time-dependent dose adjustment; however, this pharmacological agent has a relatively short half-life that broadens its safety margins, as well as its low antigenic potential due to the small molecular weight of the compound. The efficacy of argatroban has been demonstrated among patients with acute coronary syndromes and stroke. However, this drug is currently approved by the FDA only for the treatment of patients with heparin-induced thrombocytopaenia. Indeed, in such patients, argatroban significantly improves clinical outcomes, and is associated with reduced mortality. Further clinical studies are needed to present more clinical evidence necessary to broad the indication spectrum of this agent.

Conformationally restricted thrombin inhibitors resistant to proteolytic digestion

A new type of thrombin exo-site inhibitor has been designed with enhanced inhibitory potency and increased metabolic stability. With the aid of the model of the structure of the thrombin-hirudin fragment complex [Yue, S.-Y., DiMaio, J., Szewczuk, Z., Purisima, E. O., Ni, F., & Konishi, Y. (1992) Protein Eng. 5, 77-85], cyclic analogs of the hirudin fragment (hirudin55-65) were designed and synthesized. In these analogs, the side chains of appropriately substituted residues, 58 and 61, were joined in order to restrict the conformation of the inhibitor. An analog with an 18-membered lactam ring showed higher antithrombin activity (IC50 = 0.57 microM) than the corresponding analogs with 17- or 16-membered rings and was 2-fold more potent than its linear counterpart. Even 4-fold greater enhancement was obtained when a shorter fragment, hirudin 55-62, was cyclized. This cyclization not only improved the potency but, more importantly, dramatically increased the resistance to proteolytic digestion. Remarkable enhancement of stability to proteolysis was observed for peptide bonds located in the exocyclic linear peptide segments. These results are discussed using molecular modeling.

Chemical modifications and amino acid substitutions in recombinant hirudin that increase hirudin-thrombin affinity

Recombinant hirudin (r-hirudin), unlike the naturally occurring leech protein, lacks a sulfate ester on Tyr-63 which reduces its binding affinity to thrombin by 3-10-fold. We demonstrate that nitration or iodination of Tyr-63 restores hirudin-thrombin affinity to levels similar to or exceeding that of the natural inhibitor. In contrast, nitration of Tyr-3 reduces the affinity of hirudin for thrombin. These chemical modifications results in multiple reaction products that are readily separated by reverse-phase HPLC. The mechanism of the observed changes in thrombin affinity may involve a reduction in the pK of the hydroxyl group of tyrosine due to substitution of the electrophilic iodo or nitro group on the phenyl ring, resulting in an increased negative charge at neutral pH. For Tyr-63, this effect mimics the sulfatotyrosine of natural hirudin, leading to an increased thrombin affinity at the anion-binding exosite. For Tyr-3, the increased polarity may destabilize its interaction within the apolar-binding site of thrombin. Substitution of the highly conserved Tyr-3 residue with Phe or Trp not only enables specific and quantitative chemical modification at Tyr-63 but also independently increases hirudin-thrombin affinity. Kinetic analysis of thrombin inhibition showed that enhanced binding by r-hirudin(nitro-Tyr-63) is due to an increase in the association rate between hirudin and thrombin whereas the reduced binding of r-hirudin(nitro-Tyr-3) results from a large increase in the dissociation rate. These observations indicate that specific segments within both the amino- and carboxy-terminal regions of hirudin interact with thrombin.

C-terminal peptide alcohol, acid and amide analogs of desulfato hirudin54-65 as antithrombin agents

Analogs of the antithrombin peptide hirudin54-65 with C-terminal modifications have been synthesized in order to examine the requirements for alpha-thrombin inhibition. The C-terminal residue, Gln65, could be replaced with L-amino acids or amino alcohols with neutral or charged hydrophilic side chains without greatly affecting the peptide's antithrombin potency as determined by inhibition of thrombin-induced clot formation in human plasma in vitro. Derivatives with D- or L-amino carboxamides at position 65 had significantly reduced potency, but still retained activity. Deletion of residue 65 with conversion of residue 64 to the amide or alcohol derivative resulted in a three-fold loss of potency. In addition to these results the solid-phase synthesis of peptide alcohols via direct displacement of p-nitrobenzhydrylideneisonitroso resin attached peptides with the desired C-terminal amino alcohol is reported.

Design, synthesis and antithrombin activity for conformationally restricted analogs of peptide anticoagulants based on the C-terminal region of the leech peptide, hirudin

Synthetic peptides cyclized via disulfide linkages have been synthesized as conformationally restricted analogs of a novel class of antithrombotic peptides that inhibit fibrinogen cleavage by binding to a non-enzymatic site on thrombin. Several conformational models for these inhibitors have been considered and cyclic analogs were synthesized to test their validity. Compounds designed on an alpha-helical model yielded several cyclic analogs that retained antithrombin activity. [D-Cys58, Cys61]-hirudin54-65, 5, and [D-Cys60, Cys63]-hirudin54-65, 6, had IC50 values of 26 and 30 microM, respectively, in an in vitro clot assay compared with a value of 3.7 microM for the linear hirudin54-65.