Neutralization of antifactor VIII inhibitors by recombinant porcine factor VIII

Background: Inhibitory antibodies to factor (F) VIII (FVIII inhibitors) present a major clinical challenge as a complication of hemophilia A and as acquired autoantibodies in non-hemophiliacs. Porcine FVIII is a potentially useful therapeutic agent because of its low crossreactivity with many inhibitors. Recombinant porcine FVIII (rpFVIII) is undergoing clinical trials in inhibitor patients. Objectives: The goals of this study were to neutralize human FVIII inhibitors in vitro with rpFVIII and to characterize the relationship between recovery of FVIII activity and the antiporcine FVIII inhibitor titer. Methods: rpFVIII was added to 28 antihuman FVIII inhibitor plasmas and assayed either immediately or after a 2 h preincubation at 37 degrees C. The concentration of rpFVIII required for recovery of FVIII activity to 50% of normal (EC(50)) was determined by polynomial regression analysis and compared with the antiporcine FVIII inhibitor titer measured by Bethesda assay. Results: Six of nine plasmas classified as non-crossreactive by Bethesda assay had detectable inhibitory activity in the FVIII recovery assay. Recovery was decreased following a 2 h preincubation compared with immediate assay for the 19 plasmas with detectable antiporcine FVIII inhibitors. There was a linear relationship between EC(50) and inhibitor titer for plasmas with antiporcine FVIII titers ranging from 0.6 to 10 BU per milliliter in both the immediate and the 2 h preincubation assays. Conclusion: A quantitative method for analysis of inhibitor neutralization in vitro has been developed, which may be useful for comparison with pharmacodynamic studies of rpFVIII in FVIII inhibitor patients and subsequent rational dosing in this patient population.