Background: Emicizumab binds FX and FIXa allowing thrombin generation in the absence of FVIII. FIX plasma levels will be a limiting factor suggesting the potential use of recombinant FIX (rFIX) to regulate the procoagulant effect of emicizumab. Emicizumab recognizes both FIX and FIXa with similar affinity, indicating that a small amount of FIXa is enough to initiate emicizumab-mediated hemostasis.

Aims: Our goal was to study the ex-vivo procoagulant effects of two rFIX products, with different FIXa content, in samples from severe hemophilia (SHA) patients receiving emicizumab prophylaxis.

Methods: Blood from 4 SHA patients on prophylaxis with emicizumab was collected with CTI (Corn trypsin Inhibitor). The effect of rFIX (Nonacog-alfa and Nonacog-gamma), rFVIIa, and aPCC were assessed by rotational thromboelastometry (ROTEM) and thrombin generation test (TGT). ROTEM was performed with EXTEM-diluted 1:50,000 and TGT was triggered by PPP-Low (1pM TF, 4mM phospholipids). rFIX concentrates were incubated with EGR-chloromethylketone (ck) to block FIXa, and following dialysis, FIXa activity in EGRck-treated or untreated products were quantified using the Spectrozyme-FIXa substrate.

Results: Spiking of increasing concentrations of both rFIX products produced an enhanced procoagulant effect of emicizumab similar to that obtained with 90 μg/kg rFVIIa and 0.5 IU/kg aPCC (Fig.1). Compared to Nonacog-gamma, lower concentrations of Nonacog-alfa were required for recovering a normal coagulation profile.

Fig.1: Ex vivo effect of rFIX concentrates or bypass agents in clotting time and thrombin peak of emicizumab-treated patient samples assayed with ROTEM and TGT.
Nonacog-alfa showed at least 9-times higher FIXa activity than Nonacog-gamma (Fig.2A). After blocking FIXa with EGRck, both EGRck-treated rFIX products showed similar procoagulant profiles and lower effects than the untreated ones, demonstrating FIXa contribution in the procoagulant effects of these concentrates (Fig.2B).

Fig.2: Activity assay of rFIX concentrates (A) and haemostatic effect of EGRck-treated or untreated rFIX products on clotting time and thrombin peak in samples from emicizumab-treated patients assayed with ROTEM and TGT (B).

Conclusions: Global assays suggest that rFIX concentrates produce an enhanced procoagulant effect of emicizumab opening the idea of its usage as alternative treatment for bleedings in emicizumab-treated patients and prove that FIXa levels of rFIX concentrates have an essential contribution in their procoagulant function.
Research funded by FIS_Fondos_FEDER_PI19/00631

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/ex-vivo-assessment-of-the-importance-of-activated-factor-ix-fixa-levels-in-the-procoagulant-effect-of-recombinant-fix-concentrates-in-emicizumab-treated-patients/