Difference in the Heparin-Neutralizing Effect of Protamine and Polybrene as Tested by Thrombotest

Regulation of thrombin generation by TFPI in plasma without and with heparin

The purpose of this study was to investigate the impact of recombinant glycosylated TFPI (rg-TFPI) from BHK cells, nonglycosylated TFPI (r-TFPI) from Escherichia coli, and truncated TFPI (1-161) on thrombin generation (TG) in plasma treated with and without heparin in vitro and ex vivo. Fasting plasma samples were collected from 6 healthy persons. TG was assessed by the calibrated automated thrombography (CAT) method. The addition of increasing concentrations (0-200 ng/mL) of different TFPI caused a 5% to 30% prolongation of lag time for TF (3.0 pM) induced TG, with the most pronounced effect for rg-TFPI and the least pronounced effect for truncated TFPI, but without affecting endogenous thrombin potential (ETP) in TF-induced coagulation. Removal of native TFPI from plasma by anti-TFPI IgG treatment shortened lag time by 35 +/- 4% without affecting ETP. Increasing concentrations (0-200 ng/mL) of various TFPI in the presence of low heparin concentrations (0.1 IU/mL) prolonged lag time and decreased ETP by 25% to 75% with the most prominent effect promoted by glycosylated full-length TFPI. The effect of neutralizing antibodies against TFPI and antithrombin (AT) was studied in plasma in the presence of heparin administered in vitro or ex vivo. The results revealed that TFPI and AT acted in synergy as inhibitors of coagulation in terms of the effect on both initiation (lag time) and propagation (ETP). Our data demonstrated that the CAT assay appropriately assessed the impact of TFPI on initiation and propagation of TG in a physiological plasma milieu with and without heparin. TFPI contributed significantly to regulation of coagulation initiation (lag time). The C-terminal region and, to a lesser extent, glycosylation of the TFPI molecule were essential for its anticoagulant function in the absence and presence of heparin.

Effect of cholesterol lowering on intravascular pools of TFPI and its anticoagulant potential in type II hyperlipoproteinemia

Tissue factor pathway inhibitor (TFPI) inhibits the extrinsic coagulation system. A major pool of TFPI is associated with the vascular endothelium and can be mobilized into the circulation by heparin. In circulating blood, TFPI is mainly associated with LDL (80%), whereas 10% to 20% is carrier free. In this study, heparin administration caused a 2.2-fold and a 7.5-fold increase in TFPI activity and TFPI antigen, respectively, in 25 patients with phenotypes IIa and IIb hyperbetalipoproteinemia. Because the antigen determination of TFPI almost exclusively measures carrier-free TFPI, more than 90% of the heparin-induced increase in TFPI activity was caused by mobilization of carrier-free TFPI from the vascular endothelium. Therapeutic lowering of total cholesterol (a decrease of 31.1 +/- 11.6%, P < .001) by 40 mg/d lovastatin in 17 patients with hyperbetalipoproteinemia was accompanied by a parallel decrease in TFPI activity (of 27.7 +/- 24.2%, P < .001) because of a reduction in LDL-TFPI complexes. However, drug intervention did not affect carrier-free TFPI or the magnitude of the vascular pool of TFPI that could be mobilized into the circulation by heparin. Moreover, this reduction of LDL-TFPI complexes did not reduce the anticoagulant potency of TFPI in plasma or of the vascular endothelial pool. The results of this study may imply that the anticoagulant potency of TFPI is associated with its carrier-free form in plasma or on the endothelium and that downregulation of LDL affects neither the size nor the anticoagulant potency of the endothelial pool of TFPI.

Plasma TFPI activity after intravenous injection of pentasaccharide (PS) and unfractionated heparin in rabbits

Tissue Factor Pathway Inhibitor (TFPI), is known, to be released in plasma after injection of intravenous or subcutaneous injection of heparin or low molecular weight heparin (1,2). Addition of protamine or polybrene in plasma can neutralize anticoagulant effects of heparin in vitro but not completely ex vivo in patients treated with heparin (3). It was believed for many years that this post-heparin anticoagulant effect was due to another activity released by heparin. More recently it has been shown that post-heparin anticoagulant effect could be inhibited by anti-TFPI antibodies (4). Since we have shown in a previous work (5), that in healthy volunteers, pharmacokinetics of TFPI were more closely related to anti IIa activity than anti Xa activity we hypothesized that fragments with anti-IIa activity may be required for this release, possibly from vascular wall. In order to determine if a very low molecular weight glycosaminoglycan can release TFPI in plasma, in the present study, we compared plasma TFPI amidolytic activity after intravenous injection in rabbits of pentasaccharide (PS), a synthetic fragment of very low molecular weight and with a strong and exclusive anti Xa activity, and unfractionated heparin (UFH).

The anticoagulant effect in heparinized blood and plasma resulting from interactions with extrinsic pathway inhibitor

The influence of Extrinsic pathway inhibitor (EPI) on global clotting times of plasma was studied using activity-blocking IgG antibodies. Dilute tissue thromboplastin (TP) clotting times in plasma collected after intravenous injection of heparin were dramatically shortened by the addition of anti-EPI IgG. Anti-EPI IgG shortened the TP times to a lesser degree in plasma heparinized in vitro. Compared to plasma heparinized in vitro, the TP clotting times were markedly prolonged in post-heparin plasma of equal heparin concentration. Addition of anti-antithrombin IgG reduced the clotting times somewhat more than did anti-EPI IgG, particularly in normal plasma. In plasma from patients with cancer, about equal effect was obtained by blocking either EPI or antithrombin. These clotting time studies suggested that much of the anticoagulant effect caused by injection of heparin depended on EPI. This was confirmed by recording the release of fibrinopeptide A (FPA), as marker of thrombin generation, following addition of TP and CaCl2 to citrated blood. Thrombin generation was delayed and markedly reduced in post-heparin blood compared to that in normal blood. After incubating post-heparin citrated blood with anti-EPI IgG, the generation of FPA was more rapid; the amounts released 30 seconds after addition of TP were 6 times greater (36 vs 6 ng/ml) than in post-heparin blood without anti-EPI IgG. The subsequent FPA values were midway between pre-injection and post-heparin values. In conclusion, between one third and one half of the inhibition of TP-initiated coagulation in post-heparin plasma depends on EPI. This inhibition is mainly due to inactivation of the factor VIIa-TP complex. A small, but distinct contributing effect observed in the APTT assay (and hence no TP) indicates that even increased inactivation of activated factor X contributes. In cancer patients, these EPI-heparin interactions contribute even more to the anticoagulant effects of heparin.

Extrinsic pathway inhibitor (EPI) and the post-heparin anticoagulant effect in tissue thromboplastin induced coagulation

It is known that the anticoagulant effect of blood or plasma is greater when heparin is given in vivo than when added in similar heparin concentrations in vitro. In this study, we neutralized heparin in citrated blood with polybrene, and then triggered coagulation with dilute tissue thromboplastin (TTP) and CaCl2. The clotting time was longer and the release of fibrinopeptide A (FPA) was retarded in the post injection samples compared to samples spiked with heparin in vitro. We have earlier reported that the extrinsic pathway inhibitor (EPI) is released to the blood after heparin injection. This was demonstrated here also for LMW heparin Enoxaparine both after intravenous and subcutaneous administration. Polyclonal blocking antibodies to EPI were added to blood or plasma heparinized in vivo or in vitro, and the direct heparin effect was neutralized with polybrene. When TTP and CaCl2 now were added and clotting time and the release of FPA recorded, the postheparin effect was greatly reduced by the antibodies. Addition of EPI antibodies to post-heparin plasma samples from cancer patients caused a marked reduction in the thromboplastin clotting times. We conclude that the release of EPI to the blood contributes significantly to the anticoagulant effect of heparin ex vivo.