Concizumab is an anti-Tissue Factor Pathway Inhibitor (TFPI) monoclonal antibody in development as a once-daily subcutaneous prophylaxis for bleeding episodes in patients with hemophilia A or B with or without inhibitors. Understanding how the activity of novel, non-factor therapies compare to traditional Factor (F) VIII/FIX replacement therapies is of interest to physicians but remains complex. Thrombin Generation Assays (TGAs) can measure the in vitro activity of FVIII/FIX as well as non-factor products.

To compare the activity of concizumab to FVIII using TGAs initiated with combinations of Tissue Factor (TF) and activated Factor XI (FXIa).

TGAs (Thrombinoscope) were performed using hemophilia A plasma (George King Bio-Medical) and triggered with 1 pM TF (Thrombinoscope), with or without FXIa (Enzyme Research Laboratories) at 0.1, 0.5, 1.0 and 8.0 mU/mL. Under these conditions, FVIII (NovoEight, Novo Nordisk; 0.03–2.0 IU/mL) was compared with 4,000 ng/mL concizumab. Results: The activity of FVIII increased with increasing FXIa concentrations, whereas the activity of concizumab was consistent under all conditions tested, independent of FXIa concentration. Consequently, the level of FVIII similar in activity to 4,000 ng/mL concizumab decreased as the FXIa concentration increased. In the absence of supplemented FXIa, the thrombin peak with 4,000 ng/mL concizumab corresponded to 0.9 IU/mL FVIII, and the Endogenous Thrombin Potential (ETP) to 1.3 IU/mL FVIII. In the presence of lower FXIa concentrations (0.1 and 0.5 mU/mL), the thrombin peak with concizumab corresponded to 0.25 and 0.08 IU/mL FVIII, respectively, and the ETP to 0.47 and 0.18 IU/mL FVIII, respectively. When FXIa was further increased, the activity of concizumab was < 0.05 IU/mL FVIII. Conclusion: The relative activity of FVIII and concizumab is highly sensitive to the exact TGA conditions used. Comparing activity of concizumab with that of FVIII in TGA assays can therefore be misleading if translated into potential clinical efficacy.