IntroductionMuscle hematomas are the second most common complication of hemophilia and insufficient treatment may result in serious and even life-threatening complications. Hemophilic dogs and rats do experience spontaneous muscle bleeding, but currently, no experimental animal model is available specifically investigating spontaneous muscle bleeds in a hemophilic setting.AimThe objective of this study was to develop a model of spontaneous muscle bleeds in hemophilia B mice. We hypothesized that treadmill exercise would induce muscle bleeds in hemophilia B mice but not in normal non-hemophilic mice and that treatment with recombinant factor IX (rFIX) before treadmill exercise could prevent the occurrence of pathology.MethodsA total of 203 mice (123 F9-KO and 80 C57BL/6NTac) were included in three separate studies: (i) the model implementation study investigating the bleeding pattern in hemophilia B mice after treadmill exercise; (ii) a study evaluating the pharmacokinetics of recombinant FIX (rFIX) in hemophilia B mice and based on these data; (iii) the treatment study, which tested therapeutic intervention with rFIX. At termination of the treadmill studies the presence of bleeds was evaluated.ResultsTreadmill exercise resulted in a high incidence of muscle bleeds in F9-KO mice but not in C57BL/6NTac mice. Treating hemophilia B mice with rFIX before treadmill exercise prevented muscle bleeds.ConclusionA novel model of muscle bleeds in hemophilia B mice, responsive to rFIX, has been developed.

Background Following induced joint hemorrhage, hemophilia B results in the abnormal persistence of iron deposition, inflammation, and neovascularity of the synovial tissue, as well as deterioration of the bone articular surface and strength. Previously, we demonstrated that a factor IX (FIX) replacement protein with extended circulating FIX activity, glycoPEGylated FIX nonacog beta pegol (N9‐GP), could improve synovial and osteochondral parameters in F9 knockout mice when administered after joint injury. Objective We explored the use of N9‐GP prior to unilateral joint hemorrhage and compared to unmodified recombinant FIX (rFIX). Methods Pharmacodynamics, histology, and microcomputed tomography were used to assess the effects of prophylactic administration of glycoPEGylated FIX. Results In comparison to rFIX, N9‐GP significantly improved soft tissue histological parameters, as well as bone outcome at 2 weeks post injury, while performing equally in reduction of blood present in the joint space assessed 1 day after injury. Conclusions These results indicate that, in comparison to rFIX, the prophylactic use of extended half‐life FIX provides superior protection from bleeding‐induced joint damage, manifested by improved correction of histologic parameters.