**3. Anticoagulants from ticks**

XIIIa with about 7 kDa, this inhibitor works with effective concentrations in the nanomolar range [113]. It was also related the presence of transcripts similar to tridegin in some transcriptome analysis of other leeches specie [12], but the obtaining of new molecules from leeches with this function was not yet published. Some tridegin analog peptides have been synthesized and analyzed for their action improvement, but so far, nothing very relevant has been exposed [114]. Although not used in clinical trials on its recombinant form (T087), a derivative of tridegin is being marketed by more than one company for use in laboratory research.

Hementin is responsible for proteolysis of blood fibrinogen with formation of products which block conversion of fibrinogen into fibrin catalyzed by thrombin; this molecule was discovered in salivary gland from *Haementeria ghilianii* [115]. Since fibrinogen is involved in the formation of platelet clot, hementin is able to prevent the platelet aggregation induced by ADP and collagen; on the other hand, it can also induce disaggregation of platelet aggregation induced by ADP, but not collagen [116]. Hementin can lyse fibrin clots; but its fibrinolytic activity is less potent than the fibrinogenolytic one. It does not influence the activity of other

It was also demonstrated that plasma clots formed in the presence of tridegin are more sensitive to lyses by hementin (time required for 50% lysis in the presence and absence of hementin was 16 and about 22 h, respectively) [118]. Study of lysis of clots formed from PRP revealed that in the presence of tridegin the effect of fibrinolytic enzymes was the same as in PPP, whereas lysis of platelet-containing clots occurred slower. Thus, the importance of the platelets in the resistance of plasma clots to fibrinolytic enzymes and also the importance of

Considering that both molecules are obtained in the same leech species, it was suggested that hementin and tridegin have a synergic action in feeding process of *Haementeria ghilianii*. They

Hementerin (HT) is a single-chain 80 kDa, Ca++-dependent metalloproteinase, which specifically degrades fibrin(ogen) through a plasminogen-independent pathway. The amino terminal sequence of 8 residues shows 80% similarity with hementin. However, their activities differ somewhat in terms of kinetics and with regard to the structure of the fibrin(ogen) fragments they may produce. Cleavage by HT of fibrinogen A-alpha, gamma, and B-beta chains, in that order, produces fragments differ from those produced by plasmin. HT was also able to degrade cross-linked fibrin although at a lower rate as compared to fibrinogen. HT is a plasminogenindependent fibrino(geno)lytic metalloproteinase that degrades fibrinogen faster than fibrin, prevents the coagulation and destroys fibrin clots *in vitro* [120]. The action of HT was also studied in different platelet assays and the studies have indicated that HT is an effective inhibitor of human platelet aggregation, presumably through activation of the platelet's nitridergic pathway [121].

Destabilase was discovered in salivary glands from *Hirudo medicinalis* and it was able to hydrolyze the epsilon-(gamma-glutamyl)-lysine bonds as a result of fibrin stabilization by FXIIIa in the presence of calcium ions [122]. It was characterized as a polyfunctional molecule

*2.1.6.2. Fibrino(geno)lytics molecules*

cross-linking in this process [119].

may be considered as promising thrombolytic agents.

plasma proteins [117].

48 Anticoagulant Drugs

Most anticoagulants from ticks are produced for the salivary glands and play essential functions during feeding. Ticks inject the saliva into the skin of a wide range of terrestrial vertebrates and absorb it along with the blood of the animal. Faced with an injury inflicted by tick bite, the animal respond by activating blood coagulation, vasoconstriction, inflammation, and tissue remodeling related to wound healing. However, these ectoparasites have a complex and potent pharmacological mechanism to overcome the host defenses, blocking pain and itch and facilitating blood flow to allow the feeding [25, 127, 128].

Differences in the composition of tick saliva are reflected in the co-evolution between ticks and their host, the feeding strategies, the tick developmental stage, the process of penetration of the host skin, and the duration of the feeding. This can be observed between the two major families, Argasidae and Ixodidae. The first family (family Argasidae) is called soft ticks. They feed fast, less than 1 h, for multiple times causing profound damage to the host skin due the deep mouthparts penetration, while hard ticks (family Ixodidae) feed for a prolonged period (days to weeks) in each developmental stage. Hard ticks have strategies to firmly attach to its host, producing large amount of cement or glue to penetrate the host skin and cause a superficial damage (Metastriata ticks, e.g., *Dermacentor* or *Rhipicephalus* genera), or by attaching more deeply to the host skin by physical mechanisms using longer, barbed mouthparts. Females hard tick feed only once and may ingest more blood than 100-times their initial body weight to die later after oviposition (Prostriata, e.g., *Ixodes*, *Metastriata*, and *Amblyomma* genera) [127, 129–131].
