Antithrombotics from the Sea: Polysaccharides and Beyond

Marine organisms exhibit some advantages as a renewable source of potential drugs, far beyond chemotherapics. Particularly, the number of marine natural products with antithrombotic activity has increased in the last few years, and reports show a wide diversity in scaffolds, beyond the polysaccharide framework. While there are several reviews highlighting the anticoagulant and antithrombotic activities of marine-derived sulfated polysaccharides, reports including other molecules are sparse. Therefore, the present paper provides an update of the recent progress in marine-derived sulfated polysaccharides and quotes other scaffolds that are being considered for investigation due to their antithrombotic effect.

Isolation and Characterization of Poecistasin, an Anti-Thrombotic Antistasin-Type Serine Protease Inhibitor from Leech Poecilobdella manillensis

Antistasin, first identified as a potent inhibitor of the blood coagulation factor Xa, is a novel family of serine protease inhibitors. In this study, we purified a novel antistasin-type inhibitor from leech Poecilobdella manillensis called poecistasin. Amino acid sequencing of this 48-amino-acid protein revealed that poecistasin was an antistasin-type inhibitor known to consist of only one domain. Poecistasin inhibited factor XIIa, kallikrein, trypsin, and elastase, but had no inhibitory effect on factor Xa and thrombin. Poecistasin showed anticoagulant activities. It prolonged the activated partial thromboplastin time and inhibited FeCl₃-induced carotid artery thrombus formation, implying its potent function in helping Poecilobdella manillensis to take a blood meal from the host by inhibiting coagulation. Poecistasin also suppressed ischemic stroke symptoms in transient middle cerebral artery occlusion mice model. Our results suggest that poecistasin from the leech Poecilobdella manillensis plays a crucial role in blood-sucking and may be an excellent candidate for the development of clinical anti-thrombosis and anti-ischemic stroke medicines.

Structure-activity relationship studies for multitarget antithrombotic drugs

Thrombosis is a complex process involving multiple pathways. Currently, therapy relies on the combination of two or more antithrombotic drugs, showing that inhibiting more than one target provides benefits in the prevention and treatment of thrombosis. This review focuses on structure-activity relationship studies of molecules possessing multiple actions against thrombosis, namely, dual inhibitors of coagulation, dual inhibitors of coagulation and platelet aggregation, and also dual inhibitors of platelet aggregation. EP217609 has just entered clinical trials, which raise the expectations on the multitarget strategy to prevent or treat thrombosis.

Direct thrombin inhibitors: patents 2002-2012 (Review)

Acute vascular diseases and other thromboses of the blood system constitute major health risks in developing countries. Thrombin plays a central role in blood coagulation, which is a crucial process involved in thrombosis. Direct thrombin inhibitors (DTIs) such as argatroban, dabigatran, dabigatran etexilate, lepirudin, desirudin and bivalirudin, which bind to thrombin and block its enzymatic activity, are widely and effectively used in the treatment of thromboembolic diseases. DTIs appear to overcome the disadvantages of indirect thrombin inhibitors such as unfractionated heparins (UFH). Although these DTIs show specific advantages over indirect inhibitors, they still present limitations, such as a narrow therapeutic window, and bleeding and anaphylaxis as side-effects. Novel anticoagulant drugs need thus to be developed to overcome these limitations. In the search for additional candidate agents with improved efficacy, safety and high bioavailability in oral administration, a high number of compounds has been identified, such as those derived from the tripeptide template D-Phe-Pro-Arg, aptamers and peptides isolated from blood-sucking animals. These candidates may prove the new agents of choice for the treatment of cardiovascular diseases.

The Direct Thrombin Inhibitors: Their Role and Use for Rational Anticoagulation

Major clinical advantages are achieved when direct thrombin inhibitors are used in venous thromboembolism. These agents provide more reliable anticoagulant response patterns because they are not significantly bound to plasma proteins and few, if any, drug-drug interactions are seen. The studies to date confirm that not all direct thrombin inhibitors are the same. The new reversible, short-acting catalytic site-specific drugs provide an excellent safety profile and high degree of efficacy for the prophylaxis and treatment of venous thromboembolism and pulmonary embolic states. The availability of the oral prodrug ximelagatran allows reproducible, effective, and safe direct thrombin inhibition without the requirement for coagulation laboratory monitoring; it appears destined to be the oral anticoagulant of the future.

Dipetalogastin, a potent thrombin inhibitor from the blood-sucking insect. Dipetalogaster maximus cDNA cloning, expression and characterization

A cDNA coding for the thrombin inhibitor dipetalogastin has been isolated from a stomach library of Dipetalogaster maximus, a blood-sucking insect. The open reading frame of the cloned inhibitor cDNA codes for a protein of 344 amino-acid residues. Sequence analysis reveals the existence of three repeated homologous main regions, indicating that the inhibitor consists of three domains. Each domain shows a double-headed structure with an internal sequence homology like rhodniin, the thrombin inhibitor from the blood-sucking insect Rhodnius prolixus. Peptide sequence comparisons of the deduced amino-acid sequence exhibit a high homology of the domains I and II to the natural inhibitor dipetalogastin from the stomach content of D. maximus and to rhodniin, respectively. Significant sequence similarities to Kazal-type inhibitors, like the conserved sequence CGXDXXTYXNXC and several cysteine residues, indicate that the thrombin inhibitor from D. maximus is a further blood-sucking insect which belongs to the Kazal-type family (besides rhodniin). A biologically active recombinant protein corresponding to domain II of the dipetalogastin cDNA was expressed in Escherichia coli. The isolated recombinant dipetalogastin with a molecular mass of 12.91 kDa has proved to be a specific thrombin inhibitor similar to its natural counterpart as well as rhodniin and hirudin. The Ki value of the recombinant dipetalogastin was determined to be 49.3 +/- 22.28 fM.

Isolation and characterization of americanin, a specific inhibitor of thrombin, from the salivary glands of the lone star tick Amblyomma americanum (L.)

A thrombin (EC 3.4.21.5) inhibitor (americanin) was isolated from the salivary glands of the lone star tick Amblyomma americanum (L.) using reversed-phase chromatography and anion-exchange chromatography. Americanin did not inhibit any other protease tested, including factor Xa, plasmin, trypsin, chymotrypsin, elastase, papain, pepsin, and carboxypeptidase. The inhibition of thrombin by americanin decreased dramatically with dilution of the reaction mixture including thrombin, its substrate, and americanin. When thrombin assays were performed in the presence of americanin, the reaction curve showed a time-dependent inhibition. Significant inhibition was observed when americanin concentration was approximately equal to that of thrombin, with a Ki of 0.073 nM. The results suggest that americanin is a specific, reversible, competitive, slow, tight-binding inhibitor of thrombin.