Antithrombotic effect of platelet glycoprotein Ib-blocking monoclonal antibody Fab fragments in nonhuman primates

Platelet adhesion in arterial blood flow is mainly supported by the platele t receptor glycoprotein (GP) Ib, which interacts with von Willebrand factor (vWF) that is bound to collagen at the site of vessel wall injury. Antibod y 6B4 is a monoclonal antibody (MoAb) raised against purified human GPIb, M oAb 6B4 inhibits both ristocetin- and botrocetin-induced, vWF-dependent hum an platelet agglutination, MoAb 6B4 furthermore blocks shear-induced adhesi on of human platelets to collagen I. We studied the antithrombotic effect o f this inhibitory murine MoAb 6B4 in a baboon model of arterial thrombosis. When injected into baboons, intact IgG and its F(ab')(2) fragments caused almost immediate thrombocytopenia, whereas injection of the Fab fragments a lone did not. Fab fragments were subsequently used to investigate their in vivo effect on platelet deposition onto a thrombogenic device, consisting o f collagen-rich, glutaraldehyde-fixed bovine pericardium (0.6 cm(2)), at a wall shear rate ranging from 700 to 1000 s(-1). Baboons were either pretrea ted with Fabs to study the effect of inhibition on platelet adhesion or tre ated 6 minutes after placement of the thrombogenic device to investigate th e effect on interplatelet cohesion. Pretreatment of the animals with bolus doses ranging from 80 to 640 mu g/kg Fab fragments significantly reduced In -111-labeled platelet deposition onto the collagen surface by approximate t o 43% to 65%. Only the highest dose caused a significant prolongation (doub ling) of the bleeding time. Ex vivo ristocetin-induced platelet agglutinati on was equally reduced, Treatment with a bolus of 110 mu g/kg Fab fragments after a thrombus was allowed to form for 6 minutes had no effect on furthe r platelet deposition. We therefore conclude that Fab fragments or derivati ves of inhibitory anti-GPIb antibodies may be useful compounds to prevent t hrombosis.