Platelet-Targeted rFVIIa-Xten Improves Thrombin Generation and Fibrin Formation Compared to Recombinant FVIIa

The development of inhibitors to replacement plasma factors in hemophilia is an ongoing clinical complication.  Bypass therapies, such as recombinant factor VIIa (rFVIIa), have emerged as important alternative on-demand strategies for hemophilic patients with inhibitors to treat spontaneous bleeds and prevent bleeding during surgery.  However, general prophylaxis strategies for hemophilia inhibitor patients are lacking.  An attractive approach for effective prophylaxis and on-demand treatment includes engineering potent rFVIIa variants with prolonged pharmacokinetics.  Because the primary mechanism of action of rFVIIa is thought to occur on the platelet surface, we combined rFVIIa with platelet-targeting and XTEN half-life extension technologies to improve its pharmacokinetic profile and procoagulant activity.  Platelet-targeting was achieved by recombinant fusion of an antibody fragment which binds the human alpha IIb integrin.  Half-life extension was achieved by fusion of an XTEN polypeptide which increases the hydrodynamic radius, and therefore half-life, of rFVIIa.  We have shown that these rFVIIa modifications improve the pharmacokinetics and efficacy of rFVIIa in vivo in humanized alpha IIb transgenic hemophilia A mice.  The goal of the current study was to evaluate and compare thrombin generation and fibrin formation kinetics in hemophilic platelet-rich plasma in the presence of platelet-targeted rFVIIa-XTEN or rFVIIa.  To achieve this, platelet-rich plasma from normal human donors was treated with an inhibitory factor VIII antibody to model hemophilia A and spiked with doses of platelet-targeted rFVIIa-XTEN or rFVIIa.  Reactions were triggered with low tissue factor and recalcification.  Thrombin generation (n=9) was monitored by calibrated automated thrombography and fibrin formation (n=7) was monitored optically on a plate reader.  Non-linear regression analysis of dose responses was used to determine EC₅₀ values for each parameter for each donor.  Platelet-targeted rFVIIa-XTEN increases the rate and peak of thrombin generation with 2- to 6-fold lower EC₅₀ values (peak and rate, respectively) than rFVIIa.  These data were confirmed by thrombin generation in platelet-rich plasma from 1 hemophilia A donor and 1 hemophilia B donor which demonstrated similar responses to platelet-targeted rFVIIa-XTEN with 2.5- to 5-fold lower EC₅₀ values (peak and rate, respectively) than rFVIIa.  Notably, platelet count-dependent changes in thrombin generation activity were similar between platelet-targeted rFVIIa-XTEN and rFVIIa. Fibrin formation analysis indicated platelet-targeted rFVIIa-XTEN increases the rate and time to plateau of fibrin formation with 5- to 13-fold lower EC₅₀ values (rate and time to plateau, respectively) than rFVIIa.  Analysis of fibrin network structure by confocal microscopy indicated platelet-targeted rFVIIa-XTEN increases fibrin network density in platelet-rich plasma clots. Together, these data show platelet-targeted rFVIIa-XTEN has more procoagulant activity than rFVIIa by supporting more thrombin generation and faster fibrin formation and suggest our approach has the potential to be an effective alternative for the treatment and prevention of bleeds in hemophilia patients with inhibitors.