Fibrinopeptide-A (FPA) Generation by Factor IX Concentrates (F-IX-C) and Feiba (factor VIII inhibitor bypassing activity)
The increase of FPA from 0.4-2.4 to 5.6-80 mg/ml after infusion of F-I-C in 5 hemophiliacs B prompted a study whether thrombin is present or genera ted in F-IX-C and in FEIBA rasp. Purified dialysed fibrinogen served as substrate, FPA-release was measured by RIA and expressed as % of the total FPA in fibrinogen. F-IX-C + fibrinogen produced only traces of FPA. The addition of Ca++ led to a time-dependent increase of FPA-release. 8 batches F-IX-C from 2 sources (Immuno, SwissRedCross) were preincubated with Ca++ for 2 hrs, added to fibrinogen with and without heparin, and FPA measured after 60 s. In absence of heparin 7 F-IX-C produced 5 to 80% FPA, in its presence no FPA was liberated. FEIBA + fibrinogen produced clotting and an 80% FPA-release in 5 min, prevented by prior heparin addition. Preincubation of FEIBA with Ca++ increased FPA-release from 80 to 100%, however, prevention required an ll-fold heparin conc. Hirudin more effectively blocked FPA-release probably due to low AT III in concentrates. Thus, in F-IX-C thrombin is absent or blocked by heparin added by the manufacturer; however, thrombin is rapidly generated in presence of Ca++. The coagulant activity of FEIBA is due to thrombin rather than to a “factor VIII inhibitor bypassing activity”.