Thein Vitroprocoagulant Effects of Standard and Extended Half-Life Recombinant Factor IX Concentrates in Patients on Prophylaxis with Emicizumab

Introduction:

Emicizumab is a humanized, monoclonal, bispecific antibody that binds factor (F) FX and FIXa allowing thrombin generation in the absence of FVIII, which is used for routine treatment of patients with Hemophilia A (HA) with and without inhibitors.

Plasma level of FIX will be an important limiting factor for the formation of the FX-FIXa-emicizumab ternary complex in the absence of FVIII, suggesting the potential use of FIX concentrates to regulate the procoagulant function of emicizumab and therefore, to use it as an alternative treatment in certain circumstances to stop or prevent bleedings in patients on prophylaxis with emicizumab.

At the present time there are several recombinant FIX concentrates with differences in their content of activated FIX (FIXa) and in their half-life (standard or extended) that may differ in their procoagulant effects when combined with emicizumab.

Aim:

The aim of this study was to evaluate if there were differences in thein vitroprocoagulant effects of two recombinant FIX concentrates, one with standard half-life (rFIX Nonacog Alfa, BeneFIX^®, Pfizer) and the other with extended half-life (rFIX fused to rAlbumin, Albutrepenonacog Alfa, Idelvion^®, CLS Behring) in samples from patients on prophylaxis with Emicizumab.

Methods:

This is a prospective and transversal pilot study that was approved by the Ethics Committee from La Paz University Hospital. Two patients with haemophilia A (HA) with inhibitors in prophylaxis with emicizumab were recruited and one haemophilia B (HB) patient was included as a control for the effects of FIX. Blood samples were collected in tubes with corn trypsin inhibitor (CTI, Haematologic Technologies, USA), to block thecontact phase and to only evaluate coagulation mediated by the extrinsic pathway.

Levels of FIXa in concentrates of FIX were quantified using the Spectrozyme^® FIXa chromogenic substrate (LOXO) and measuring the increase in OD at 405 nm.

Rotational thromboelastometry (ROTEM) was performed using whole blood activated by a low concentration of tissue factor solution (final dilution 1:50,000 of EXTEM reagent) plus recalcification. Parameters evaluated in ROTEM were CT (cloting time), defined as time until detection of a clot firmness of 2 mm; and CFT (clot formation time), defined as time between detection of a clot firmness from 2 to 20 mm.

Calibrated automated thrombogram (CAT)was performed using platelet free plasma (PFP) activated by low concentration of tissue factor plus phospholipids (PPP-Reagent LOW®, Stago). Parameters evaluated in CAT were: Peak, defined as maximum thrombin concentration reached, in nM; and ETP, defined as the total amount of thrombin generated over time, in nMxmin.

Results:

The presence of FIXa activity assayed by a chromogenic substrate was not detectable with 20 IU of Idelvion while BeneFIX^® showed a specific concentration-dependent FIX activity that was blocked with the serine protease inhibitor EGR-chloromethylketone (Figure 1).

ROTEM (Figure 2) and CAT (Figure 3) results showed that the addition of increased concentrations of both concentrates of rFIX produces an enhanced procoagulant effect of Emicizumab similar to the effect produced by the addition of the bypassing agent rFVIIa (NovoSeven®, NovoNordisk).

These results also showed that it is necessary three-four times higher concentration (U/dl) of Idelvion^® to get similar procoagulant effects that those obtained with BeneFIX^®. The higher procoagulant effects of BeneFIX^® were also observed in samples of a patient with severe HB.

Conclusion:

Global coagulation assays suggest that increasing endogenous FIX levels with two rFIX concentrates that have different FIXa content and half-life, produce an enhanced procoagulant effect of Emicizumab opening the idea of the use of these concentrates as an alternative treatment for bleedings in patients with inhibitors on prophylaxis with Emicizumab.

Further studies need to be performed to evaluate the procoagulant activity of the concomitant use of different rFIX concentrates and Emicizumab, and to assess security of this therapeutic approach.

This work was supported by grants from FIS-FONDOS FEDER (PI19/00631 and P19/00772).EMM holds a predoctoral fellowship from Fundación Española de Trombosis y Hemostasia (FETH-SETH).

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