Investigating the Impact of FIX Fc Fusion on Its Extravascular Distribution and Hemostatic Properties

Background: Previous studies have demonstrated that factor IX (FIX) has unique characteristics in vivo where it distributes into the extravascular space and binds to collagen IV (Col IV). These properties can influence the pharmacokinetic (PK) parameters of FIX and its ability to protect against bleeding. To improve clinical outcomes and reduce treatment burden, extended half-life FIX products have been developed to treat patients with hemophilia B. However, it is still unclear whether these modifications to extend the half-life of FIX can impact the extravascular distribution and Col IV binding. Methods: FIX-KO male mice 8-12 weeks of age were employed in all in vivo experiments. A single dose of 50 IU/kg of recombinant FIX Fc fusion (rFIX-Fc; Alprolix) was administered retro-orbitally to each mouse. Blood samples were collected at timepoints (3 mice for each timepoint) ranging from 5 minutes up to 120 hours post-injection and assayed for FIX activity. To investigate the extravascular distribution of rFIX-Fc in relation to Col IV, tissue samples were harvested 48h post-injection after perfusion and analyzed using immunofluorescence staining. The hemostatic potential of extravascular rFIX-Fc was evaluated in a modified exercise-induced musculoskeletal bleeding model. In brief, 48h post rFIX-Fc (or saline) injection, when plasma FIX activity is <1%, FIX-KO mice were exposed to 5 consecutive days of moderate treadmill running then evaluated for musculoskeletal bleeding. To investigate the binding between Col IV and rFIX-Fc, varying concentrations of rFIX-Fc (0-3000 nM) were used in a plate-based binding assay developed in-house to determine the optimal parameters for the binding between Col IV and rFIX-Fc. Results: The PK curve obtained by testing FIX-KO mice blood plasma samples demonstrated the gradual clearance of rFIX-Fc with FIX activity persistently <1% after 48 hours. Results of the treadmill study showed a lower bleeding score for the rFIX-Fc treated group compared to the control group (mean = 1.08, SD = 1.44 vs mean = 4, SD = 3.86; P < 0.05) even though the plasma FIX activity was < 1% at the beginning of the hemostatic challenge in both groups. Interestingly, the confocal microscopy analysis of the muscle tissue collected 48h post-injection of rFIX-Fc demonstrated the presence of rFIX-Fc in similar spatial locations as Col IV within the endomysium surrounding each muscle fibre, while the binding assay demonstrated an interaction between rFIX-Fc and Col IV. Conclusion: These findings suggest that the extravascular distribution of rFIX-Fc and its localization in the tissues could explain the discordance between the lower bleeding scores in the rFIX-Fc treated group compared to the control group despite undetectable FIX activity in both at the start of the bleeding challenge. In sum, our data demonstrates the localization of rFIX-Fc in seemingly identical spatial regions as Col IV, and this appears to be the result of a specific interaction between these two proteins as confirmed in vitro in our binding assay. Overall, this suggests that the extension of FIX half-life via Fc-fusion technology does not impair its extravascular distribution and Col IV binding.