ANTITHROMBOTIC EFFICACY OF INACTIVATED ACTIVE-SITE RECOMBINANT FACTORVIIA IS SHEAR DEPENDENT IN HUMAN BLOOD

Several studies have indicated a profound role for factor VII(a) [FVII (a)] in venous and arterial thrombogenesis. In the present study, we q uantified the inhibitory efficacy of dansyl-glutamyl-glycyl-arginyl-re combinant FVIIa (DEGR-rFVIIa) on acute thrombus formation. Thrombus fo rmation was elicited by immobilized tissue factor (TF) in a parallel-p late perfusion chamber device at blood flow conditions characterized b y wall shear rates of 100 s(-1) (veins) and 650 s(-1) (medium-sized he althy arteries). Native human blood was drawn directly from an antecub ital vein by a pump into a heparin-coated mixing device in which DEGR- rFVIIa (0.09 to 880 nmol/L final plasma concentration) or buffer was m ixed homogeneously with flowing blood. Subsequently, the blood was pas sed over a plastic coverslip coated with TF and phospholipids in the p arallel-plate perfusion chamber. Fibrin deposition, platelet-fibrin ad hesion, and platelet thrombus volume triggered by this surface were me asured by morphometry. DEGR-rFVIIa inhibited thrombus formation in a d ose-dependent manner, but the efficacy was shear rate dependent. At a wall shear rate of 100 s(-1), the IC50 (50% inhibition) was 30 nmol/L, whereas at 650 s(-1), the IC50 was 0.6 nmol/L. Binding studies to imm obilized TF under flow conditions using surface plasmon resonance reve aled a significantly higher on-rate for DEGR-rFVIIa and FVIIa than for FVII, 2.8x10,(5) 2.6x10(5), and 1.8x10(5) M-1 s(-1), respectively. Th is indicates that a contributing factor to the shear-dependent efficac y may be a differential importance of on-rates at arterial, and venous blood flow conditions.