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, hepar
in elicited a greater prolongation of APTT than PT. These results sugg
est that FL-TFPI and heparin exert their anticoagulant activity primar
ily through inhibition of the extrinsic and intrinsic pathways, respec
tively. Using a dilute thromboplastin-induced clotting assay, it was f
ound that FL-TFPI was similar to 37-fold more potent than the carboxyl
terminus truncated form (CT-TFPI) in prolonging the clotting time, wh
ich 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 a
nd heparin were added sequentially to plasma. However, when FL-TFPI wa
s 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 al
one suggesting that neutralization of the positively charged carboxyl
terminus of FL-TFPI by heparin could also decrease its anticoagulant a
ctivity. 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 rat
ios to plasma caused only inhibition of the anticoagulant activity of
FL-TFPI. Pharmacokinetic studies of FL-TFPI and heparin:FL-TFPI comple
x were carried out in rabbits. Both pharmacokinetic data could be fitt
ed into a biexponential clearance model. Full-length TEPI had a very s
hort 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 pr
olonged 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-f
old faster for FL-TFPI than heparin:FL-TFPI complex. Constant infusion
studies confirmed that it was possible to achieve the same steady sta
te level of TFPI in the circulating plasma by infusing 2.6-fold less c
omplex than infusion of the FL-TFPI alone.