Antithrombotic agents under flow conditions

Thomas, Stephen (2003). Antithrombotic agents under flow conditions. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d389

Abstract

Haemostasis is the result of a fine balance of interactions between the endothelium, plasma proteins and blood cells under a wide range of flow rates. To mimic these conditions in vitro, a parallel plate flow chamber with human umbilical vein endothelial cells (HUVEC) or extracellular matrix (ECM) has been developed. A method to investigate thrombin generation (TG) in platelet rich defibrinated plasma was validated and used to investigate inhibition by two distinct classes of antithrombotic agents: anti-platelet antibodies and anticoagulants, including unfractionated heparin (UFH), low molecular weight heparin (LMWH) and hirudin.

Increasing flow rates increased TG, which was higher in the presence of ECM than in the presence of HUVEC. All anti thrombotic agents investigated were less effective in the presence of ECM.

The monoclonal anti-platelet glycoprotein (GP) IIb/IIIa antibody, RFGP56, partially inhibited TG under static or arterial flow conditions and was less effective under venous flow conditions. The monoclonal anti-platelet GP Ibα antibody, RFGP37, did not inhibit TG under flow or static conditions. A combination of the two antibodies showed no further activity than RFGP56 alone.

UFH, which has equal anti-factor Xa and anti-thrombin activity, was able to inhibit TG under static and flow conditions. On an anti-Xa unit basis, comparatively more LMWH (with a 10: 1 ratio of anti-factor Xa to anti-thrombin activity) was required to inhibit TG under static and venous conditions, but under arterial conditions LMWH was as effective as UFH. Hirudin, a thrombin specific inhibitor, was totally effective under static conditions, but was only able to inhibit up to 40 % ofTG under flow.

This study shows that some anti-platelet agents can inhibit coagulation and this may contribute to their antithrombotic efficacy under certain flow conditions. Although both the anti-factor Xa and anti-thrombin activities of anticoagulants are effective, anti-factor Xa activity may play a more important inhibitory role under flow conditions.