Thrombin Activatable Fibrinolytic Inhibitor Modulation in Heparin Supplemented Plasma: Clinical Considerations.

Thrombin activatable fibrinolytic inhibitor (TAFI) is a procarboxypeptidase that cleaves the arginine and lysine amino acid residues off the carboxyl terminus of fibrin strands rendering fibrin resistant to the digestive actions of plasmin. Anticoagulant drugs such as the heparins and antithrombin drugs (hirudin, angiomax, argatroban and ximelagatran) by virtue of their inhibition of thrombin and its complex formation with thrombomodulin can down regulate the functionality of TAFI. It is now known that there are several TAFI polymorphisms, but the exact function of each form of TAFI is unclear at this time. Heparin-antithrombin complex produces strong inhibition of thrombin and thrombin-thrombomodulin complex and thereby reduces the conversion of TAFI to TAFIa. However, despite anticoagulation some of the individuals do not exhibit decreased TAFI functionality. In order to determine if a population of normal healthy volunteers exhibits differences in TAFI functionality upon anticoagulation with heparin, plasma samples were divided into two aliquots; saline was added to one set and 2.5 U heparin was added to the second set. This resulted in a final concentration of 0.25 U of heparin, which mimics therapeutic levels. Functional TAFI levels were determined using a synthetic substrate based method from Pentapharm Inc. (Basel, Switzerland), functional tissue factor pathway inhibitor (TFPI) levels and anti-Xa levels were obtained using a chromogenic-based method from American Diagnostica (Stamford, CT). TAFI levels in the saline supplemented samples were 89.9 ± 16.0% (range; 58.9 – 126.7%) normal human pooled plasma (NHP). After heparin supplementation, TAFI levels were reduced to 45.6 ± 16.1% NHP (range: 24.2 – 139.8% NHP). Upon further analysis it was determined that 45 (33%) individuals showed TAFI levels > 50% NHP (62.7 ± 14.8; range: 51.5 – 139.8% NHP) and 16 (11.8%) showed TAFI levels > 65% NHP (74.0 ± 17.8; range: 65.1 – 139.8% NHP). The anti-Xa level in the heparin supplemented samples was 0.181 ± 0.179 U (range: 0 – 0.61 U/ml) of heparin showing that the samples were anticoagulated. Comparing the TAFI levels obtained from the individuals that showed higher than expected levels to the anti-Xa showed that although TAFI levels were higher, the anti-Xa levels showed that the individuals were adequately anticoagulated. Interestingly functional TFPI levels did not differ in the heparin supplemented (1.35 ± 0.4 U/ml) and controls (1.73 ± 0.6 U/ml). These studies suggest that normal individuals, and different patient populations, may respond differently to anticoagulation using heparin, as functional TAFI levels may remain normal even after heparinization. This may be due to TAFI polymorphism in some individuals that allow the activation of TAFI to TAFIa even though only a small amount of thrombin is present. It is also plausible that this form of TAFI is more readily activated by plasmin or that the TAFIa itself may be more efficient in cleaving the arginine and lysine amino acid residues from fibrin. This suggests that patients undergoing surgical procedures where anticoagulation with heparin is necessary may be at risk of a fibrinolytic deficit if their functional TAFI levels remain high. This may predispose these patients to impaired fibrinolysis leading to further complications.