Sustained inhibition of circulating FXI procoagulant activity after administration of aXIMab. An intravenous injection of aXIMab (2 mg/kg) was given over 5 minutes to a single baboon. Plasma samples were collected into citrate anticoagulant and tested over 4 weeks for (A) FXI procoagulant activity, FXI antigen (FXI:Ag), and (B) inhibitor levels, with each time point being the mean of duplicate measurements. The FXI:Ag ELISA could be used to detect both free and complexed FXI. Because the Bethesda assay detected only free FXI inhibitor (aXIMab), FXI:Ag and FXI activity at low inhibitor levels did not correlate until all complexes were cleared from circulation. The 1-hour time points for all levels were omitted for clarity. (C) A Western blot of 1-μL NHP and NBP samples size-fractionated by nonreducing 7.5% SDS-PAGE. Detection was with a polyclonal antibody against human FXI. The 5 lanes on the right represent samples before (0) or 1, 6, 8, and 27 days after infusion of aXIMab. XI indicates 100 ng purified human FXI; NHP, normal human plasma; NBP, normal baboon plasma; and XI-DP, FXI-deficient human plasma.