The effect of protamine sulphate on plasma tissue factor pathway inhibitor released by intravenous and subcutaneous unfractionated and low molecular weight heparin in man.

Heparin, a negatively charged sulphated glycosaminoglycan, is clinically the most important antithrombotic drug. Heparin augments the inhibitory activity of antithrombin (AT) towards thrombin, factor Xa (FXa) and other activated clotting enzymes. Tissue factor pathway inhibitor (TFPI) is an endogenous heparin releasable three domain Kunitz-type coagulation inhibitor which inhibits the crucial tissue factor-factor VIIa (TF-FVIIa) dependent coagulation pathway in the presence of FXa. The importance of the TF-FVIIa pathway and TFPI has recently been reviewed (1). TFPI is located to different vascular pools, the largest being the vascular endothelium from where TFPI can be released dose-dependently to the blood by heparins (2). TFPI is speculated to contribute to the anticoagulant properties of heparins, but to which degree is not yet fully understood. In recent years low molecular weight heparins (LMWH) have proven to be effective and safe both for prophylactic (3) and therapeutic treatment (4) of deep vein thrombosis (DVT). Protamine is the least toxic and clinically most commonly used antidote to heparin. However, in vitro and in vivo LMW heparinized blood is not fully neutralized by protamine, as substantial anti-Xa activity remains following neutralization (5). This post-protamine effect has been shown to be partly TFPI dependent when measured in a dilute TF-dependent assay (6,7). We undertook this in vivo study on healthy volunteers in order to investigate whether TFPI released by UH or LMWH (intravenous (iv) or subcutaneous (sc)) remains in the circulation following neutralization of the heparin activity with protamine sulphate (PS). We measured TFPI by three different methods-chromogenic activity, anticlotting activity and a new antigen assay specific for full-length and three-domain TFPI.