Monitoring the bioavailability of FEIBA with a thrombin generation assay

Background: Hemophilia A patients with inhibitors are generally treated with preparations containing activated coagulation factors to achieve hemostasis by bypassing factor (F)VIII. Objectives: We developed an assay for monitoring the kinetic of thrombin generation in human FVIII inhibitor plasma reconstituted in vitro with activated prothrombin complex concentrate, FEIBA, and in plasma samples from hemophilia A patients taken after FEIBA treatment. Patients and methods: For pharmacokinetic studies three patients with severe hemophilia A and with a high-titer inhibitor received a single dose of FEIBA. Repeated FEIBA treatment was monitored in one patient with acquired hemophilia A. Coagulation was triggered in citrated plasma by adding a low concentration of tissue factor/phospholipid complex and CaCl2 in the presence of a fluorogenic thrombin substrate. The intensity of the fluorescence signal (FU) was continuously monitored, and the rate of increase in the fluorescence signal for every time point, which reflects the actual thrombin concentrations, was calculated. Results: The maximum rate of substrate conversion, which indicates the highest thrombin concentration, was approximately 1900 FU min(-1) in a normal plasma pool. Practically no thrombin generation was observed in the FVIII inhibitor plasma, but when it was spiked with FEIBA, the rate and the peak of thrombin generation increased dose-dependently to close to normal. Plasma samples from FVIII inhibitor patients treated with a single dose of FEIBA had an improved thrombin maximum within an hour after treatment, which gradually returned to baseline values with a half-life of 4-7 h. Changes in the characteristic parameters of thrombin generation coincided with the repeated administration of FEIBA in a patient with acquired hemophilia A. Conclusions: This assay enables the pharmacodynamic and pharmacokinetic properties of bypassing therapies to be monitored, thus helping to optimize treatment.