Skip to main content
ISTH 2023 Main banner
Back

    Oral Communication

    Hemophilia and Rare Bleeding Disorders

    Hemophilia Basic Science - FVIII and FIX

    OC 04.1 - Exploring the hemostatic properties of extravascular factor IX in a hemophilia B mouse model

    Saturday, June 24, 2023
    13:00 – 13:15 ET
    Room: 520
    Session Available As: On-Demand
    Background: Evidence from animal models and pharmacokinetic studies in hemophilia B patients point to a reservoir of extravascular factor IX (FIX) existing in dynamic equilibrium with circulating FIX. However, the hemostatic properties of extravascular FIX alone remains to be clarified.

    Aims: Exploring the hemostatic properties of extravascular FIX in an exercise-induced musculoskeletal bleeding model comparing FIX plasma activity and tissue localization via confocal microscopy.

    Methods: FIX-KO male mice at age 8-12 weeks were retro-orbitally injected once with 50 IU/kg of recombinant wild-type FIX (rFIX). Blood samples were collected up to 72h post-injection to determine PK parameters of rFIX. Tissues were harvested 72h post-injection and analyzed using immunohistochemistry. In separate experiments, mice were treated with normal saline (control) or rFIX. Mice were exposed to 5 days of moderate treadmill running 24h post-injection. After 5 days, mice were evaluated for the presence of musculoskeletal bleedings and scored in accordance with a previously published protocol with minor modifications.

    Results: Data from PK experiments demonstrate clearance of circulating rFIX and no detectable activity beyond 24h post-injection. In contrast, confocal microscopy analysis of muscle tissue collected 72h post-injection shows rFIX seemingly colocalizing with collagen IV within the endomysium surrounding individual muscle fibres (Fig.1). Results of the treadmill study demonstrate a lower bleeding score in the treated group compared to the control group despite undetectable FIX plasma activity at the start of the experiment (Fig.2).

    Conclusion(s): These preliminary data show the presence of Col IV-bound FIX in muscle tissues even when it is completely cleared from the circulation. This discrepancy might explain the hemostatic protection noted in FIX-KO mice that were exposed to the hemostatic challenge 24 hours after the FIX injection, when FIX activity was already below the detection limit ( < 1%).

    Learning Objectives:

    • Upon completion, participant will be able to understand the necessity to further explore the distribution of factor IX.
    • Upon completion, participant will be able to identify the potential drawbacks of existing extended-half life products.
    • Upon completion, participant will be able to recognize the potential of the vast distribution of factor IX in informing future drug development to improve replacement therapy.