G-Quadruplex Aptamers to Human Thrombin Versus Other Direct Thrombin Inhibitors: The Focus on Mechanism of Action and Drug Efficiency as Anticoagulants

Thrombin is a key enzyme of blood coagulation system which has multiple functions including pro- and anticoagulant, platelet aggregating and inflammatory activities. Unsurprisingly, this enzyme has been a target for anticoagulant drug development for decades. Among the most interesting direct thrombin inhibitors with intravenous administration route are the following ones: 1) hirudins, proteins with bivalent binding mode to the thrombin, 2) bivalirudin, the peptide with bivalent binding mode to the thrombin, 3) argatroban, the chemical that binds to the thrombin active site, and 4) G-quadruplex DNA aptamers, structured oligonucleotides with an affinity to protein-binding site of the thrombin. Efficiency of all these inhibitors has been studied in vivo in preclinical and clinical trials, as well as in vitro with various tests, allowing to compare them thoroughly. In the review three levels of comparison were used to highlight the features of each inhibitor: 1) thrombin inhibition constants as a characteristic of inhibitor potency in simple enzymatic system; 2) inhibition of fibrin fiber formation and thrombin generation in coagulation cascade as a characteristic of anticoagulant potency in human blood plasma; and 3) therapeutic doses used and therapeutic profiles obtained after intravenous administration into animals and humans. The data clearly demonstrate weak and strong aspects of thrombin binding aptamers providing a solid background for further novel anticoagulant development.