Neutralization of inhibitory antibodies and restoration of therapeutic ADAMTS-13 activity levels in inhibitor-treated rats by the use of defined doses of recombinant ADAMTS-13.

Background: Acquired thrombotic thrombocytopenic purpura (TTP) is caused by an autoantibody-mediated deficiency of the von Willebrand factor-cleaving protease ADAMTS-13. Acute episodes of the disease are treated with a combination of immunosuppression and repeated cycles of plasma exchange to remove anti-ADAMTS-13 autoantibodies and, at the same time, replenish functional ADAMTS-13. Although this is often effective, the mortality rate has remained between 10% and 20%, highlighting the need for safer treatment options.

Objectives: We previously showed that, in vitro, human recombinant ADAMTS-13 (rADAMTS-13) is able to override neutralizing antibodies and restore ADAMTS-13 activity in plasma from patients with acquired TTP. In the present study, we assessed the in vivo feasibility of this strategy by using a rat model.

Methods: Wild-type rats were adjusted to an ADAMTS-13 inhibitor (inhibitor) titer of ~ 10 BU mL(-1) with goat anti-ADAMTS-13 IgG, and treated with increasing doses of rADAMTS-13. Blood samples were drawn and analyzed for ADAMTS-13-specific parameters, including FRETS-VWF73 activity, inhibitor, and ADAMTS-13-specific immune complexes (ICs). The pharmacokinetics of ADAMTS-13 activity and inhibitors were evaluated.

Results: Administration of inhibitor titer-adjusted doses of rADAMTS-13 to inhibitor-treated rats predictably restored activity. Inhibitors were readily neutralized through formation of ADAMTS-13-specific ICs, which were cleared at a higher rate than the free inhibitor. Surplus protease was enzymatically active in plasma, and showed similar pharmacokinetics to ADAMTS-13 in not inhibitor-treated rats.

Conclusions: Defined doses of rADAMTS-13 neutralized circulating anti-ADAMTS-13 antibodies and enabled reconstitution of ADAMTS-13 activity in plasma in our model, indicating that the protease may be a promising candidate for further exploration in treating acute episodes of acquired TTP.