An emicizumab-responsive mouse model. (A) Mice received emicizumab (0-10 mg/kg) via retro-orbital infusion. Plasma samples obtained 24 hours after infusion were used to determine circulating emicizumab levels, expressed as milligrams per deciliter. Each data point represents an individual mouse. (B) FVIII-deficient mice previously infused with emicizumab (1.5-10 mg/kg), received a cocktail containing human FIX (hFIX) and FX (both 100 U/kg) via retro-orbital infusion. Plasma samples obtained 5 minutes after infusion were used to determine circulating levels of human FIX (left y-axis) and FX (right y-axis) antigen (Ag), expressed as units per deciliter. Each data point represents an individual mouse, and pooled data from the various emicizumab doses are shown. (C) Based on the reported affinity of emicizumab (Emi) for FIX (1.85 μM) and FX (1.58 μM) and the respective concentrations of emicizumab (0.7-18.6 mg/dL), FIX (85 U/dL), and FX (100 U/dL) in our model (see panels A-B), the concentration of the ternary emicizumab/FIX/FX complex for each emicizumab dose used in this study (blue symbols) was calculated as described.4 Higher doses of emicizumab will not further increase ternary complex formation, and may even lead to reduced ternary complex formation (red symbols). In contrast, increasing FIX and/or FX concentrations would allow for higher concentrations of ternary complexes (eg, green symbol: 9.6 mg/dL emicizumab, 200 U/kg FIX, 100 U/kg FX)