Inhibitor development in patients with hemophilia A after continuous infusion of FVIII concentrates.

Continuous infusion (CI) of coagulation factor concentrates has been used since the early 1990s. Recent reports of the occurrence of an inhibitor after CI have raised concerns about this method of treatment. We conducted a retrospective study to investigate the development of inhibitors after CI of Factor VIII concentrates in Germany. So far, 13 hemophilia centers have been contacted, and data have been collected by a questionnaire. Of the 13 centers, CI had never been performed in three, no inhibitors had been detected in five, and inhibitor development after CI was recorded in 10 patients in the remaining five centers. Of these 10 patients (ages 7 months to 57 years), five were suffering from severe, one from moderate, and four from mild hemophilia. Indications for treatment were major bleeds and surgical procedures. Plasma-derived (6 cases) and recombinant (4 cases) factor concentrates were given in various infusion sets. Data concerning amount infused (4300 to > 100,000 IU), number of days of exposure to factor concentrates (1 to > 100), and inhibitor characteristics (alloantibodies, 3 LR, 7 HR) were collected. Regarding hemophilia genotype, we found missense mutations in four patients, intron-22 inversions in two, and one small deletion in one; the genotype in three was unknown. In conclusion, only 3 out of 10 patients who developed an inhibitor after CI showed the typical risk profile for inhibitor formation, which is severe hemophilia A with a severe gene defect and less than 50 days of exposure to coagulation factor concentrates. Especially striking was the finding that 50% of the patients who developed inhibitors had mild to moderate hemophilia A. Our data point to the existence of a so-far unknown factor, related to CI, that might lead to inhibitor formation.

Platelet-dependent coagulation assays for factor VIII efficacy measurement after substitution therapy in patients with haemophilia A.

FVIII therapy for haemophilia A is safe and effective, with the problem of individually sufficient efficacy unsettled. Routine one-stage clotting assays and tests employing chromogenic substrates poorly detect individual haemostatic effects of FVIII due to artificial test conditions. In particular, the use of cell-free and diluted plasma samples neglect the crucial role of platelets for thrombin and fibrin formation. To optimize FVIII substitution therapy, we measured in 40 patients with severe to mild haemophilia A before and after FVIII substitution the FVIII activity in cell-free plasma samples using a one-stage clotting assay as well a chromogenic substrate assay and compared the data with those obtained with cell-based coagulation tests, i.e. thrombin generation in platelet-rich plasma (PRP) and thromboelastography (TEG) in samples of citrated whole blood (WB). To determine the maximum ex vivo haemostatic effect we added 1 unit/ml of FVIII to samples of PRP and WB and measured the maximum thrombin generation in the thrombin generation test (TGT) and the maximum clot firmness (MCF) in TEG. After FVIII substitution we observed a nearly linear relation between the individual FVIII activities administered to the patients and the activities measured in the plasma samples. However, data obtained with TGT and TEG revealed a high inter-individual variation and a very poor correlation to the administered FVIII activity. Actually, it could be shown that FVIII substitution yielding in a FVIII plasma activity of about 30% is sufficient to get an ex vivo haemostatic effect of more that 90% as measured by maximum thrombin generation and MCF. FVIII substitution up to a plasma activity of more than 90% did not further enhance the haemostatic effect. Our data clearly demonstrate that the haemostatic effect of FVIII is not only dependent on the activity that is measured in plasma but also depends on the interplay between coagulation and blood cells, in particular with platelets. The use of cell-based coagulation tests such us TGT or TEG may help to optimize FVIII therapy by determining the individual FVIII dosage that produces a maximum haemostatic effect.