Treatment of patients with hemophilia A and B has undergone significant advances during the past 2 decades. However, despite these advances, the development of antibodies that inhibit the function of infused clotting factor remains a major challenge and is considered the most significant complication of hemophilia treatment. This chapter reviews current tools available for the care of patients with inhibitors and highlights areas where progress is imminent or strongly needed. For management of bleeding, bypassing agents remain the mainstay of therapy. Recombinant factor VIIa and activated prothrombin complex concentrates are similarly effective in populations of patients with hemophilia and inhibitors; however, individuals may show a better response to one agent over another. Recent studies have shown that prophylaxis with bypassing agents can reduce bleeding episodes by ∼50%-80%. The prophylactic use of bypassing agents is an important tool to reduce morbidity in patients before they undergo immune tolerance induction (ITI) and in those with persistent high titer inhibitors, but cost and lack of convenience remain barriers. Because of the significant burden that inhibitors add to the individual patient and the health care system, inhibitor eradication should be pursued in as many patients as possible. ITI is an effective tool, particularly in patients with severe hemophilia A and good risk profiles, and leads to a return to a normal factor VIII response in ∼60% of patients. However, for the group of patients who fail to respond to ITI or have hemophilia B, new and improved tools are needed.

The pharmaceutical properties of coagulation factor VIII are improved by reconstructing the protein's ancestral sequence. Optimization of a protein's pharmaceutical properties is usually carried out by rational design and/or directed evolution. Here we test an alternative approach based on ancestral sequence reconstruction. Using available genomic sequence data on coagulation factor VIII and predictive models of molecular evolution, we engineer protein variants with improved activity, stability, and biosynthesis potential and reduced inhibition by anti-drug antibodies. In principle, this approach can be applied to any protein drug based on a conserved gene sequence.

The immune response to infused factor concentrates remains a major source of morbidity and mortality in the treatment of patients with hemophilia A and B. This review focuses on current treatment options and novel therapies currently in clinical trials. After a brief review of immune tolerance regimens, the focus of the discussion is on preventing bleeding in patients with hemophilia and inhibitors. Recombinant factor VIIa and activated prothrombin complex concentrates are the mainstays in treating bleeds in patients with inhibitors. Both agents have been shown to reduce bleeding episodes to a similar degree when infused prophylactically; however, individual patients may respond better to one agent over the other at any given time. The international immune tolerance trial revealed that a high-dose factor VIII regimen provided significantly better bleeding protection than the low-dose regimen. Given the high cost of treatment and the potential for a high-dose immune tolerance regimen to prevent bleeding in some patients, we discuss how we treat patients to maximize the prevention of bleeds while minimizing cost. Novel approaches to treatment of these patients are in development. These include agents that mimic factor VIII or augment thrombin generation by bypassing the inhibitor, as well as agents that inhibit the natural anticoagulants.

The diversity of factor VIII (fVIII) C2 domain antibody epitopes was investigated by competition enzyme-linked immunosorbent assay (ELISA) using a panel of 56 antibodies. The overlap patterns produced 5 groups of monoclonal antibodies (MAbs), designated A, AB, B, BC, and C, and yielded a set of 18 distinct epitopes. Group-specific loss of antigenicity was associated with mutations at the Met2199/Phe2200 phospholipid binding β-hairpin (group AB MAbs) and at Lys2227 (group BC MAbs), which allowed orientation of the epitope structure as a continuum that covers one face of the C2 β-sandwich. MAbs from groups A, AB, and B inhibit the binding of fVIIIa to phospholipid membranes. Group BC was the most common group and displayed the highest specific fVIII inhibitor activities. MAbs in this group are type II inhibitors that inhibit the activation of fVIII by either thrombin or factor Xa and poorly inhibit the binding of fVIII to phospholipid membranes or von Willebrand factor (VWF). Group BC MAbs are epitopically and mechanistically distinct from the extensively studied group C MAb, ESH8. These results reveal the structural and functional complexity of the anti-C2 domain antibody response and indicate that interference with fVIII activation is a major attribute of the inhibitor landscape.

Key Points: The multivariate mechanism of FeCl3-induced thrombosis is rooted in colloidal chemistry, mass transfer, and biological clotting. FeCl3-induced thrombosis is mediated by charge-based binding of proteins (cell surface bound and soluble) to the Fe3+ ion.