Capacity of peptides selected by phage display to neutralize Bo2C11 to FVIII. (A) Inhibition of the binding of Bo2C11 to immobilized FVIII by increasing concentrations of specific peptides (peptides 103-109) or irrelevant ones (controls 1 and 2). (B) Bo2C11 preincubated overnight at 4°C with increasing concentrations of peptide 107 or an irrelevant peptide was incubated in the presence of FVIII for 2 hours at 37°C. Residual FVIII activity was then measured in a 1-stage clotting assay. Results are expressed as percentage residual FVIII activity, which represents the percentage of neutralization of Bo2C11 by peptide 107. The concentration of Bo2C11 (3.5 nM) was chosen as providing close to complete (98%) inhibition of the FVIII residual activity (data not shown). (C) Bo2C11 alone (16.7 nM) or after preincubation with peptide 107 or a control peptide was injected into the tail vein of FVIII KO mice. Recombinant human FVIII (0.5 IU) was injected 30, 60, and 120 minutes later. Residual FVIII activity in plasma was measured in a chromogenic assay 15 minutes later. One hundred percent activity corresponded to 0.3 IU/mL.