Preclinical Safety of Baxter's Recombinant ADAMTS13

Abstract 3381

Baxter is currently developing a recombinant ADAMTS13 (rADAMTS13, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) product for treatment, and routine prophylaxis of acute episodes of thrombotic thrombocytopenic purpura (TTP) in patients with ADAMTS13 deficiency. TTP is a rare, life-threatening microvascular disease characterized by single or recurrent episodes of thrombocytopenia, microangiopathic haemolytic anaemia and widespread microvascular thrombosis, which leads to the ischemic damage of multiple organs.

To characterize the safety and the no observed adverse effect level (NOAEL) of Baxter's rADAMTS13, a 1-month repeat dose toxicity study in rats, an escalating dose and pilot 4-week repeat dose toxicity study in cynomolgus monkeys, and a 1-month repeat dose toxicity study including cardiovascular/pulmonary safety pharmacology in cynomolgus monkeys was conducted. Species suitability was shown by demonstrating that human rADAMTS13 can cleave monkey and rat VWF in vitro and in vivo under native conditions to present a worst case scenario for repeat dose toxicity studies. A local tolerance study was conducted in rabbits.

Baxter's rADAMTS13 was well tolerated in rats at doses of 80, 400 and 800 FRETS-U/kg administered every third day for one month. Thus, the NOAEL in rats was the highest dose of 800 FRETS-U/kg. In cynomolgus monkeys, no adverse effects were detected at 800 FRETS-U/kg after intravenous treatment with rADAMTS13 once a week for 5 weeks (pilot repeat dose study). The NOAEL for the escalating dose phase of the study was 1790 FRETS-U/kg. Treatment of cynomolgus monkeys with bolus injection of Baxter's rADAMTS13 at doses of 80, 200, and 400 FRETS-U/kg once a week for 29 days did not reveal adverse findings. Thus, the NOAEL for this study was the highest dose of 400 FRETS-U/kg. As expected for a heterologous human protein drug, repeat doses of rADAMTS13 resulted in the formation of anti-drug antibodies specific for human ADAMTS13 and in neutralising human ADAMTS13 activity in animal models. No injection site reactions were observed in animals of either species. rADAMTS13 was well tolerated after intravenous (intended clinical administration route), intraarterial and paravenous administration in rabbits.

In conclusion, Baxter's rADAMTS13 was well tolerated in rats, monkeys and rabbits, with no toxicity observed even at the highest dose levels.