**2.1. Molecules with activity in the hemostatic system from leeches**

Among different anticlotting molecules from leeches and involved in the coagulation cascade, fibrinolysis, or on the platelet aggregation process, three substances have been the main focus of investigation. They are hirudin (a thrombin inhibitor) [29], antistasin (factor Xa inhibitor) [30], and decorsin (an antagonist of the llb-llla glycoprotein of the platelet membrane) [31]. The amino acid sequences of these substances, together with studies of inhibitory activities from different molecules and designs of the three-dimension structure have been determined, and, then, the structural similarity of these molecules was observed, allowing for the design of a structure motif (L.A.P.: Cys-X6-12-Cys-X-Cys-X3-6-Cys-X3-6-Cys8-14) [32]. However, the mechanisms of action of these inhibitors and important epitopes for the connection to their respective targets are distinct [32], demonstrating the relevance of the many inhibition mechanisms on clotting processes, as well as the evolution of these processes. Many of these substances that come from leeches have been developed by the industry, as targets for different therapies and in different clinical trial stages.

A significant advance was reached with the resolution of the tridimensional structure of hirudin, which allowed for the understanding and development of recombinants equivalent to this protein (rH). The increase of interest on protein inhibitors also was due to studies that demonstrated thrombocytopenia induced by heparin. These new agents produce a direct anticlotting response, having thrombin as target, and they also inhibit the activation of platelets and the increase of thrombin's activity on the coagulation cascade, as thrombin is a multifunctional enzyme responsible for the activation of manly factors, for example, factor V, VIII, and XI [45]. The use of rH has been promising in patients with unstable angina [46].

Anticoagulants from Hematophagous http://dx.doi.org/10.5772/intechopen.78025 43

Lepirudin (Refludan) is an rH, and it was the first direct thrombin inhibitor (DTI) licensed for treatment of thrombosis complicating HIT and associated thromboembolic disease in order to prevent further thromboembolic complications [47]. It is given as an intravenous infusion with or without a bolus, and its dosing is dependent on body weight. It is renally excreted and dose adjustments are required in patients with renal impairment [48]. Significant limitations to its use are its narrow therapeutic window and potential for increased bleeding events [49]. Besides, it is a drug that forms immunogenic complexes and causes a delay in renal excretion causing its accumulation [50, 51]. Therefore, during the treatment, the dose adjustment based on aPTT is recommended. Although not common, anaphylaxis can also occur in patients with hirudin-induced antibodies during the re-exposition to drug [52]. To date, there are no reports of antidotes that reverse these effects of DTIs [53]. There are recent reports that lepirudin has

Desirudin (Iprivask) is also an rH, with very similar characteristics as lepirudin. Both rH are structurally identical except for their N-terminus sequences, which are Leu1-Tyr2 in lepirudin and Val1-Val2 in desirudin. It reversibly binds to the active thrombin site of free and clot-associated thrombin. Desirudin is able to inhibit different actions of thrombin as fibrin formation, activation of coagulation factors V, VII, and XIII, and platelet aggregation, resulting in a dose-dependent prolongation of aPTT. It is the only fixed-dose subcutaneously administered DTI approved by FDA for postoperative prevention of VTE in patients undergoing elective hip replacement surgery [56]. Eriksson and collaborators published two clinical studies comparing the efficacy and safety of desirudin (15 mg s.c. twice daily injections) with unfractionated heparin (5000 units s.c. three times daily) and enoxaparin (40 mg s.c. daily), for the prophylaxis of DVT in patients undergoing major orthopedic surgeries. After 8–12 days of treatment, desirudin proved to be superior to both heparin anticoagulants, while showing a similar safety profile [57, 58]. Recently, desirudin was also under investigation as a potential anticoagulant for patients with heparin induced-thrombocytopenia (HIT) with or without thrombosis. Desirudin was also compared with argatroban in PREVENT-HIT study. This is a small, randomized, open-label trial comparing the clinical efficacy, safety, and economic utility of fixed-dose s.c. of drugs. However, just as lepirudin, desirudin is also renally excreted;

Bivalirudin, formerly named Hirulog, is not properly a molecule from leech, but is a synthetic peptide (20 amino acids) [60] and bivalent analog of hirudin with a thrombin inhibition activity nearly 800 times weaker than that of hirudin [61]. Unlike the rH, the binding of bivalirudin to thrombin is reversible, and after the binding, the inhibitor is slowly cleaved by thrombin. Then, thrombin activity is only transiently inhibited and its enzymatic activity is restored. This reversible relationship between bivalirudin and thrombin can be seen as a benefit, once may contribute

there is still a risk of accumulation if the renal function is impaired [59].

been discontinued from the market [54, 55].
