Activated factor VII: presence in factor IX concentrates and persistence in the circulation after infusion

Abstract

Factor IX concentrates were evaluated for their factor VII content and for the presence of activated factor VII through use of a coupled amidolytic assay insensitive to activated factor VII and a clotting assay sensitive to activated factor VII. The factor VII content of the concentrates studied (except for one concentrate purposely produced to exclude factor VII) varied between 33 and 621 U/vial. All concentrates contained activated factor VII. as indicated by ratios of factor VII clotting activity to factor VII amidolytic activity, of from 1.6 to 21.5. Higher ratios were found in two brands of activated concentrates than in nonactivated concentrates. In patients infused with factor IX concentrates, plasma factor VII activity rose strikingly in the clotting assay but not in the amidolytic assay. Thus, the elevated factor VII levels by the clotting assay after infusion of factor IX concentrates stem from circulating activated factor VII. A mean intravascular half-disappearance time of144 mm was found for activated factor VII. Its persistence in the circulation makes it important to evaluate the possible role of activated factor VII in the thrombogenicity of factor IX concentrates and in their reported effectiveness in treating bleeding in hemophilia A patients with inhibitors. THE PRESENCE of variable amounts of activated clotting factors in factor IX concentrates has hampered accurate measurement of the content of clotting factors in such concentrates, because even small amounts of an activated clotting factor can introduce serious error into test results obtained with conventional clotting factor assays. Factor VII may be present in in vitro materials in at least two forms: as the native one-chain factor VII molecule and, after minimal proteolysis, as a two-chain molecule possessing increased reactivity with tissue factor”2 and therefore referred to as activated factor VII. Some factor IX concentrates, particularly concentrates known to contain large amounts of activated clotting factors (activated factor IX concentrates), have been reported to contain very high levels of factor VII activity, e.g., 6,000 U/vial.3 Moreover, striking increases in plasma factor VII levels have been reported after infusion of factor IX concentrates into patients.35 Until now it has not been possible to evaluate the effect on such test results of the presence of activated factor VII in factor IX concentrates. We have recently described a coupled amidolytic assay for factor VII that is insensitive to the activity state of factor VII.6 When factor VII is measured both by