Von Willebrand factor (VWF) is composed of a series of multimers, the sizes of which are regulated by the plasma metalloprotease ADAMTS13. Reports suggest that a transient increase in VWF levels, triggered for instance by DDAVP treatment, will result in a decrease in ADAMTS13 activity. This phenomenon is widely believed to be due to ADAMTS13 being exhausted when confronted with an excess of substrate. Treating patients who have type 3 von Willebrand disease with a recombinant human von Willebrand factor (rVWF) that has not yet been exposed to ADAMTS13 might thus cause a drop in ADAMTS13 levels. We therefore tested whether a rise in VWF concentrations to supraphysiological levels caused by administration of rVWF could influence endogenous ADAMTS13 levels in animal models. Various doses of human rVWF (300, 600, and 1200 RCo IU/kg BW) were injected into rabbits and cynomolgus monkeys and plasma samples were collected at a range of time points. As expected, VWF antigen rose sharply in a dose-dependent manner (~25 IU/ml VWF:Ag for the highest dose, 15 min after injection) and then declined gradually (~7 IU/ml VWF:Ag for the highest dose, 18 hours after injection). When these samples were tested for ADAMTS13 activity using the FRETS assay, no relevant changes were observed throughout the entire test period in the rabbit or in the monkey samples, indicating that rVWF, even at high doses, did not compromise ADAMTS13 activity. Both rabbit and cynomolgus ADAMTS13 recognized human rVWF as the specific cleavage products were detectable by electrophoresis at all doses administered. These animal studies thus indicate that an excess of intravenously administered rVWF leading to supraphysiological levels does not exhaust ADAMTS13.

Disclosures: Varadi:Baxter : Employment. Rottensteiner:Baxter: Employment. Muchitsch:Baxter: Employment. Weber:Baxter: Employment. Gritsch:Baxter : Employment. Turecek:Baxter : Employment. Ehrlich:Baxter : Employment. Schwarz:Baxter : Employment.

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