ANTITHROMBOTIC EFFECT OF A RECOMBINANT VON-WILLEBRAND-FACTOR, VCL, ONNITROGEN LASER-INDUCED THROMBUS FORMATION IN GUINEA-PIG MESENTERIC-ARTERIES

To assess the antithrombotic effectiveness of blocking the platelet gl ycoprotein (GP) Ib/IX receptor for von Willebrand factor (VWF), the an tiaggregating and antithrombotic effects were studied fn guinea pigs u sing a recombinant fragment of vWF, Leu 504-Lys 728 with a single intr achain disulfide bond linking residues Cys 509-Cys 695. The in inhibit ory effect of this peptide, named VCL, was tested in vitro on ristocet in- and botrocetin-induced platelet aggregation and compared to the AD P-induced platelet aggregation. In vivo, the antithrombotic effect of VCL was tested in a model of laser-injured mesentery small arteries an d correlated to the ex vivo ristocetin-induced platelet aggregation. I n this model of laser-induced thrombus formation, five mesenteric arte ries were studied in each animal, and the number of recurrent thrombi during 15 min, the rime to visualization and time to formation of firs t thrombus were recorded. In vitro, VCL totally abolished ristocetin- and botrocetin-induced platelet aggregation, but had no effect on ADP- induced platelet aggregation. Ex vivo, VCL (0.5 to 2 mg/kg) administer ed as a bolus i. v. injection inhibits ristocetin-induced platelet agg regation with a duration of action exceeding 1 h. The maximum inhibiti on was observed 5 min after injection of VCL and was dose related. The same doses of VCL had no significant effect on platelet count and ble eding lime. In vivo, VCL (0.5 to 2 mg/kg) had no effect on the appeara nce of the thrombi formed but produced dose-dependent inhibition of th e mean number of recurrent thrombi (the maximal effect was obtained at 5 min following i. v. injection of the highest dose: 0.8 +/- 0.2 thro mbi versus 4 +/- 0.4 thrombi in controls). The three doses of VCL incr eased the time in which the first thrombus in a concentration-dependen t manner was formed. However, the time to visualize the first thrombus was only prolonged in the higher dose-treated group. These in-vivo st udies confirm that VCL induces immediate, potent, and transient antith rombotic effects. Most importantly, this inhibition was achieved witho ut inducing thrombocytopenia nor prolongation of the bleeding time.