Relative Inhibition of Thrombin Activatable Fibrinolytic Inhibitor by Newly Developed Thrombin Inhibitors: Impact on Bleeding and Antithrombotic Actions.

Thrombin activatable fibrinolytic inhibitor (TAFI) or carboxypeptidase U plays a key role in the fibrinolytic cascade. It acts as an inhibitor of fibrinolysis by cleaving the arginine and lysine amino acid residues from the carboxy terminus of fibrin. The cleavage then renders the fibrin resistant to digestion by plasmin, forming a clot that cannot be easily digested. This stabilized thrombus can then cause myocardial infarction, stroke or pulmonary embolism. It has been shown that individuals with acute coronary syndrome tend to have elevated TAFI, which may lead to further complications. Heparin and low molecular weight heparins (LMWHs) are the most common agents given to those at risk of thrombotic complications either during surgery, therapeutically or prophylacticly. Thrombin inhibitors are being developed to provide an alternative means of anticoagulation if a patient cannot be given heparin or LMWHs due to heparin induced thrombocytopenia (HIT) or some other condition. This study was done to determine the effect of different thrombin inhibitors on TAFI functionality and determine if there is a correlation to the thrombin inhibitors anti-IIa effects. Argatroban, one of the thrombin inhibitors used in this study, has been approved for use in patients who have a history of HIT. In addition to argatroban, the following thrombin inhibitors were tested: melagatran, angiomax, LU 208791, hirudin and dabigatran. All thrombin inhibitors were supplemented to normal human pooled plasma (NHP) in various concentrations (10 – 0.16 μg/ml). The functional TAFI levels were determined using a chromogenic substrate based method developed by Pentapharm Inc. (Basel, Switzerland), the Pefakit^® TAFI. The anti-IIa (thrombin inhibition) was determined using a chromogenic substrate based method by American Diagnostica (Stamford, CN). All of the thrombin inhibitors were capable of inhibiting TAFI functionality; LU 208791 being the strongest inhibitor with an IC₅₀ of 0.09 μg/ml while angiomax was the weakest with an IC₅₀ of 6.6 μg/ml respectively. The other thrombin inhibitors showed IC₅₀s ranging from 0.38 – 1.4 μg/ml. These results for the most part corresponded to the anti-IIa results. LU 208791 had the strongest anti-IIa with an IC₅₀ of 0.39 μg/ml while angiomax had an IC₅₀ >10 μg/ml. However, the IC₅₀s for TAFI inhibition by argatroban and hirudin (1.3 and 1.2 μg/ml) were not significantly different, their IC₅₀s for thrombin inhibition were (>10 and 3.0 μg/ml). This indicates that argatroban may inhibit TAFIa directly. The anti-IIa rank order was as follows: LU 208791 > dabigatran > hirudin > melagatran > angiomax = argatroban. These results indicate that the bleeding complications observed when using thrombin inhibitors may be due to the indiscriminate inhibition of TAFI yielding fibrin that is easily digested. It is vital that the balance between coagulation and fibrinolysis be maintained in patients on anticoagulants. When this balance is disrupted either bleeding or thrombosis can occur. This study indicates that TAFI may be the key to maintaining this balance. Too much TAFI inhibition may lead to bleeding while not enough inhibition may lead to thrombosis. Persistent inhibition of TAFI by thrombin inhibitors may compromise both their safety and efficacy.