OC 73.2 - Fitusiran displays efficient pro-hemostatic activity in a mouse model of inducible factor X-deficiency

Background: Factor X (FX)-deficiency is a rare bleeding disorder, manifesting a variable bleeding tendency dependent on residual FX levels. Current treatments rely on replacement therapies.

Aims: We aimed to explore the use of fitusiran (an investigational siRNA that silences antithrombin expression) to increase thrombin generation (TG) and the in vivo hemostatic potential under conditions of FX-deficiency.

Methods: Inducible FX-deficient mice were generated using the LoxP/MX1-Cre-recombinase system. Fitusiran was kindly provided by Sanofi. TG-assays were initiated using tissue factor (TF; 1 pM). Saphenous vein bleeding (SVB)-model was performed in accordance with ethic guidelines.

Results: We developed a novel mouse model for FX-deficiency, expressing FX-levels below 0.5% of normal plasma (termed F10low-mice, controls were F10WT-mice). PT and aPTT were increased from 19±1s to 112±39s and 42±3s to 170±42s (p=0.0002), respectively. TG was severely reduced, with the Endogenous Thrombin Potential (ETP) being reduced from 894±58 nM•min to 149±126 nM•min. In the SVB-model, the number of generated clots was reduced from 19±5 clots/30 min for F10WT-mice, to 2±2 clots/30 min (p < 0.0001) for F10low-mice.
F10low-mice received fitusiran (2x10 mg/kg at one-week interval), resulting in antithrombin activity being 17±6%. TG was increased, with the ETP reaching 616±545 nM•min (p=0.026 compared to untreated). A low dose of FX (2%) further enhanced this effect: ETP=783±463 nM•min and 1724±260 nM•min for F10low-mice/2% FX and fitusiran-treated F10low-mice/2% FX (p=0.0179). Fitusiran treatment was associated with a reduced bleeding tendency in the SVB-model: F10low-mice treated with fitusiran generated more clots than untreated F10low-mice (8±6 versus 2±2 clots/30 min; p< 0.0001). Infusion of a low FX dose (0.1 U/kg) further increased this number to 17±4 clots/30 min, significantly higher than FX alone (12±5 clots/30 min; p=0.047).

Conclusion(s): We demonstrate by using a novel mouse model for FX-deficiency, that fitusiran significantly increases TG and reduces the bleeding tendency, even at FX levels below 1%.