Heparin-tissue factor pathway inhibitor complexes: Anticoagulant and pharmacokinetic properties

Exogenous addition of full-length tissue factor pathway inhibitor (FL-TFPI) to plasma caused a greater prolongation of prothrombin time (PT) than activated partial thromboplastin time (APTT). In contrast, heparin elicited a greater prolongation of APTT than PT. These results suggest that FL-TFPI and heparin exert their anticoagulant activity primarily through inhibition of the extrinsic and intrinsic pathways, respectively. Using a dilute thromboplastin-induced clotting assay, it was found that FL-TFPI was similar to 37-fold more potent than the carboxyl terminus truncated form (CT-TFPI) in prolonging the clotting time, which indicated that the positively charged carboxyl terminus of FL-TFPI was crucial for its anticoagulant activity. Both FL-TFPI and CT-TFPI could exert synergistic anticoagulant action with heparin when TFPIs and heparin were added sequentially to plasma. However, when FL-TFPI was complexed with heparin before addition to the plasma, the effect on anticoagulant activity was dependent on the weight ratio of heparin:FL-TFPI. Addition of the heparin:FL-TFPI complex at weight ratios <1.25:1 gave a dPT clotting time shorter than that of addition of FL-TFPI alone suggesting that neutralization of the positively charged carboxyl terminus of FL-TFPI by heparin could also decrease its anticoagulant activity. Addition of heparin:FL-TFPI complex at weight ratios greater than or equal to 1.25:1 gave an additive or synergistic anticoagulant effect compared to the individual anticoagulant effects of heparin and FL-TFPI. In contrast, addition of preformed N-acetyl heparin:FL-TFPI and low molecular weight heparin:FL-TFPI complexes in above weight ratios to plasma caused only inhibition of the anticoagulant activity of FL-TFPI. Pharmacokinetic studies of FL-TFPI and heparin:FL-TFPI complex were carried out in rabbits. Both pharmacokinetic data could be fitted into a biexponential clearance model. Full-length TEPI had a very short t(1/2 alpha) (1.4 min) and a relatively long t(1/2 beta) (92 min) with AUC(beta) (92%) dominant. Heparin:FL-TFPI, in contrast, had a prolonged t(1/2 alpha) (15 min) and a similar t(1/2 beta) (116 min) with AUC(alpha) (88%) dominant. The overall clearance was similar to 2.6-fold faster for FL-TFPI than heparin:FL-TFPI complex. Constant infusion studies confirmed that it was possible to achieve the same steady state level of TFPI in the circulating plasma by infusing 2.6-fold less complex than infusion of the FL-TFPI alone.