Venous and arterial thromboembolic diseases are still the most frequent causes of death and disability in high-income countries. an important role in different biological phenomena, such as hemostasis, thrombosis, inflammation, and proliferative response [2, 3]. Thrombin is the key enzyme of the blood coagulation system, presenting many important biological UR-144 functions, such as the activation of platelets, conversion of fibrinogen to fibrin, and feedback amplification of coagulation. The precise generation of thrombin at vascular injury sites is the result of an ordered series of reactions collectively referred to as blood coagulation cascade [4, 5]. 2. Blood Coagulation The hemostatic process is a host defense mechanism to preserve the integrity of the closed high-pressure circulatory system. This process must remain inactive but poised to immediately minimize extravasations of blood from the vasculature following tissue injury [6]. The blood coagulation cascade is initiated when subendothelial tissue factor is exposed to the blood flow following either the damage or activation of the endothelium [7, 8]. After cellular activation by vascular trauma or an inflammatory stimulus, tissue factor becomes exposed and binds to a serine protease, factor VIIa, already present in blood [9, 10], and forms the factor VIIa-tissue factor complex, in the presence of phospholipid and calcium (extrinsic factor tenase), which activates the zymogens factor IX and factor X [11]. The limited amounts of the serine protease factor Xa produced generate picomolar concentrations of thrombin, which initiates several positive feedback reactions that sustain thrombin’s own formation and facilitates the rapid growth of the UR-144 blood clot or thrombus around the area of vascular damage [12]. Thrombin partially activates platelets and cleaves the procofactors factor V and factor VIII generating the active cofactors factor Va and factor VIIIa, respectively [13]. Factor VIIIa forms the intrinsic factor tenase complex with the serine protease, factor IXa, phospholipid, and calcium, on a membrane UR-144 surface provided by platelets and endothelial and other cells [14], and activates factor X at a 50C100-fold higher rate than the factor VIIa-tissue factor complex [15]. Factor Xa forms the prothrombinase complex with the cofactor, factor Va, phospholipid, and calcium on the membrane surface, which is the primary activator of prothrombin [16]. The thrombin produced further amplifies its own generation by activating factor XI [17] and completing the activation of platelets and factors V and VIII [13]. Thrombin also cleaves fibrinogen [18] and factor XIII [19] to form the insoluble cross-linked fibrin clot [20] that forms the backbone of a thrombus or blood clot [21, 22]. 3. Thrombin Thrombin plays a vital role in blood coagulation by promoting platelet aggregation and by converting fibrinogen to form the fibrin clot in the final step of the coagulation cascade. In addition, thrombin influences a number of other cellular effects. Besides promoting platelet aggregation, thrombin also stimulates platelets to release mediators including thromboxane A2, platelet factor 4, PDGF (platelet-derived growth factor), UR-144 and TGF-(transforming growth factor-[57]. It is a polypeptide composed by 65 amino acids, which tightly and specifically binds to -thrombin, in a 1?:?1 stoichiometry with Ki about 20?fM. It interacts with thrombin catalytic site and exosite-1, preventing fibrinogen cleavage and consequently clot formation. Hirudin also inhibits thrombin agonist action upon the platelet aggregation and the activation of factors V, VIII, and XIII [58C60]. Other hirudin variants have been isolated from different species of leeches. These variants differ from hirudin in both TMOD4 length and amino-acid composition, even though they show the same high inhibitory potency [61C63]. Hirudin itself is not commercially available; however, its discovery.