A Subset of High Titer Acquired Hemophilia Patients May Benefit from Treatment with High Dose Factor VIII.

Abstract 3476

Poster Board III-413

Approximately 30% of patients with severe hemophilia A will develop inhibitory antibodies (Abs) to factor VIII (fVIII inhibitors). The immune response to fVIII currently is the most significant complication in the management of patients with hemophilia A. In addition, autoimmune Abs to fVIII can develop in non-hemophiliacs, producing acquired hemophilia A, which frequently produces life- or limb-threatening bleeding. Patients with autoimmune hemophilia often have Abs with type II kinetics in which there is incomplete inactivation of fVIII at saturating concentrations of inhibitor. We have characterized the antibody response to the C2 domain of human fVIII in a murine hemophilia model and described 5 structural groups of Abs. Groups A, AB, and B are classical anti-C2 Abs that block fVIII and fVIIIa binding to phospholipid. Groups BC and C consist of non-classical anti-C2 Abs that inhibit the proteolytic activation of fVIII but do not block the binding of fVIII to phospholipid. Subsequently, we identified classical and non-classical anti-C2 Abs in human fVIII inhibitor plasmas. Most murine non-classical Abs have inhibitor titers greater than 10,000 Bethesda units/mg IgG. In a murine in vivo bleeding model, both type I classical C2 Abs, type II non-classical C2 Abs, and a type I anti-A2 Ab produced similar amounts of blood loss that were significantly greater than control mice injected with 180 U/kg of fVIII alone. Increasing the dose of fVIII to 360 U/kg overcame the bleeding diathesis produced by the type II MAbs, but not the type I Abs. These results were consistent with the in vitro Bethesda assay in which a type I anti-A2 Ab, 4A4, completely inhibited both 1 U/mL and 3 U/mL fVIII, while there was 40% residual activity at saturating concentrations of a type II anti-C2 Ab, 2-77, at either concentration of fVIII. To determine if similar in vitro characteristics exist in patients with acquired hemophilia, plasmas from 3 patients with high titer type II inhibitors were studied. All 3 plasmas primarily had C2 domain epitope specificity that included non-classical Abs. Plasma A7 additionally had detectable anti-A2 activity. Recovery of fVIII activity after a 2 h incubation at 37 °C at nominal added concentrations of 1 mL and 3 U/mL fVIII was compared (Table 1). At 3 U/mL added fVIII, recovery of activity in plasmas A4 and A5 was 1.1 U/mL and 0.51 U/mL, respectively, despite the presence of inhibitor titers of 18 and 11 Bethesda units (BU) per mL. The presence of anti-A2 Abs, which typically have type I kinetics, may have contributed to the overall lower recovery of activity in plasma A7. These results suggest that treatment with high-dose fVIII rather than bypassing agents may be warranted in patients with an inhibitor response dominated by non-classical anti-C2 Abs.