Different factor VIII neutralizing effects on anti-factor VIII inhibitor antibodies associated with epitope specificity and von Willebrand factor.

Inhibitor neutralization therapy based on factor (F)VIII replacement is used for haemostatic treatment in haemophilia A patients with inhibitors on low responder, but effects appear to depend on various properties of inhibitors. We investigated this nature by evaluating the global coagulation function in timed-reactions after mixing FVIII (1 U/ml) with anti-FVIII alloantibodies containing distinct epitopes (2·5 Bethesda units/ml). Thrombin generation assays showed that peak thrombin and mean velocity to peak thrombin were depressed by anti-C2 type 1 inhibitors to significantly greater extents than by anti-A2 type 1 and anti-C2 type 2 (2- to 6-fold and 10- to 20-fold, respectively). In the presence of FVIII-von Willebrand Factor (VWF) complex, the anti-C2 type 1-mediated decreased thrombin generation was reduced by 20-40%, reflecting the protective function of VWF. However, the activities of anti-A2 type 1 were little affected, and that of anti-C2 type 2 was rather enhanced by c. 2·5-fold, relative to FVIII. Clot waveform analysis also showed similar patterns. Anti-FVIII monoclonal antibodies with well-defined characteristics demonstrated similar reactions to those with polyclonal inhibitors. In conclusion, the neutralizing effects of FVIII(-VWF) depending on epitopes could have significant therapeutic implications, and it could be important to determine inhibitor properties in order to predict the effects of infused FVIII in neutralization therapy.