Fitusiran treatment modulates the ratio between alpha- and beta-antithrombin isoforms

Area of Science:

• Biochemistry and Molecular Biology
• Hematology
• Pharmacology

Background:

• Antithrombin (AT) exists as alpha-AT and beta-AT isoforms, differing in glycosylation and inhibitory activity.
• Beta-AT exhibits higher affinity for heparin and stronger anticoagulant potential compared to alpha-AT.
• Fitusiran, a therapeutic agent, targets post-transcriptional silencing of AT for hemophilia treatment.

Purpose of the Study:

• To investigate the impact of fitusiran on the distribution of alpha-AT and beta-AT isoforms.
• To determine if reduced AT production influences the ratio of beta-AT to alpha-AT.
• To assess the anticoagulant potential associated with changes in AT isoform ratios.

Main Methods:

• Utilized isoform-specific activity, antigen, and immunoprecipitation assays to differentiate alpha-AT and beta-AT.
• Analyzed AT isoform distribution in F8-/- mice and hemophilia A patients treated with fitusiran.
• Measured beta-AT/total AT ratios in patients with congenital AT deficiency and liver cirrhosis.

Main Results:

• Fitusiran treatment significantly reduced total AT activity (<20% of normal) in mice and patients.
• A marked increase in beta-AT activity relative to total AT activity was observed (3.8-fold in mice, 3.3-fold in patients).
• The ratio of beta-AT/total AT antigen levels increased 1.8-fold in human patients, enhancing anticoagulant potential.

Conclusions:

• Reduced antithrombin production, induced by fitusiran, significantly modulates the ratio of beta-AT to alpha-AT.
• The increased proportion of beta-AT contributes to enhanced anticoagulant potential.
• Elevated beta-AT/total AT ratios are also observed in other conditions like AT deficiency and liver cirrhosis.