Evaluation of the in Vitro Procoagulant Effect of Factor IX Concentrates in Patients on Prophylaxis with Emicizumab

Introduction: The incidence of bleeding in patients with severe (S) haemophilia A (HA) on prophylaxis with emicizumab is similar to that seen in patients with moderate-mild HA, therefore these patients will require administration of factor (F) VIII concentrates (patients without inhibitors) or bypassing agents (patients with inhibitors) in case of acute bleeding or surgery. In the absence of FVIII, thrombin generation is guided by the FIX plasma levels that becomes the limiting factor for the formation of the FX-FIXa-emicizumab ternary complex. We hypothesize that the increment of level of FIX in patients on prophylaxis with emicizumab would increase their procoagulant function.

Objectives: To measure the in vitro procoagulant effect of increasing factor IX activity in patients on prophylaxis with emicizumab.

Material and Methods: We performed a study in 6 patients on prophylaxis with emicizumab (2 patients with inhibitors) and 20 healthy controls. We evaluated the in vitro procoagulant effect of therapeutic concentrations of recombinant activated FVII (rFVIIa), activated prothrombin complex concentrate (aPCC) and several concentrations of FIX using global assays as thrombin generation test (CAT) and thromboelastometry (ROTEM).

Results: In correspondence with our hypothesis, our ROTEM results indicated that increasing concentration of FIX up to 110% was able to normalize the procoagulant capacity of all patients. Further increment of in vitro FIX concentrations up to 125% had a procoagulant effect similar to what would be obtained after one standard dose of 90 mcg/kg rFVIIa. In regard to aPCC, we needed to increase FIX levels up to 200 IU/dl to achieve a procoagulant effect similar to what would be obtained with a dose of 2.5 IU/kg of aPCC. CAT showed total normalization of thrombin generation in all patients with levels of 125 IU/dl of FIX which support the idea of normalization of procoagulant function of patients on prophylaxis with emicizumab in response to low increments of FIX. Moreover, similar to ROTEM, 200 IU/dl of FIX had comparable procoagulant effect to concentrations of aPCC that would be obtained after one dose of 2.5 IU/kg aPCC. With these results we might speculate on a possible good safety profile of this high level of FIX in patients on prophylaxis with emicizumab if we take into account that 2.5 IU/kg aPCC is 20 times lower than aPCC dose [50 IU/kg] used in HAVEN-1 with no associated incidence of thrombotic events, however, more studies are needed to explore this hypothesis.

Conclusions: In vitro increment of FIX plasma levels enhances in vitro procoagulant function of patients on prophylaxis with emicizumab opening the idea of an alternative hemostatic treatment in this type of patient. The use of FIX concentrates with much longer half-life compared to FVIII concentrates or bypassing agents in patient on prophylaxis with emicizumab might produce longer period of time with a normalized haemostatic function which might speed up the recovery, might reduce the incidence of bleeding complication and would require much less number of administrations, this later of paramount importance in patients with limited venous access. However, more studies should be addressed to confirm these results. Moreover, and even more important, prothrombotic risk of combinatory therapy with FIX and emicizumab should be carefully studied in pre-clinical studies.

NB holds a tenure track grant from FIS-FONDOS FEDER (CP14/00024).