The following article features coverage from the American Society of Hematology 2021 meeting. Click here to read more of Hematology Advisor‘s conference coverage.

Compared with on-demand treatment using bypassing agents, fitusiran prophylaxis appears to reduce the rate of bleeding events among patients with hemophilia A or B with inhibitors (PwHI), according to research presented at the 2021 American Society of Hematology (ASH) Annual Meeting.

Hemophilia A and B, rare bleeding disorders resulting from respective factor (F) VIII and FIX deficiencies, are linked with ineffective clot formation. Fitusiran, an siRNA drug that targets antithrombin in patients with hemophilia A or B regardless of inhibitor development, has previously shown promise in this patient population.

For the randomized phase 3 ATLAS-INH study ( Identifier: NCT03417102), researchers investigated the safety and efficacy of fitusiran prophylaxis among PwHI. Eligible patients were receiving on-demand BPA; researchers evaluated whether the experimental drug improved the annualized bleeding rate (ABR) compared with those who continued their normal treatment.

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Overall, 57 patients were enrolled and randomly assigned 2:1 to receive fitusiran (38 patients) or BPA on demand (19 patients); the mean age among all study participants was 28.4 years (range, 13-63).

In the BPA on demand group, the estimated ABR overall was 18.07, the estimated ABR for treated spontaneous bleeds was 15.68, and the estimated ABR for treated joint bleeds was 13.76. By comparison, in the fitusiran prophylaxis group, the estimated ABR overall was 1.67 (rate ratio [RR], 0.09; P <.0001), the estimated ABR for treated spontaneous bleeds was 0.87 (RR, 0.87; P <.0001), and the estimated ABR for treated joint bleeds was 1.35 (RR, 0.1; P <.0001).

There was also an improvement in physical health domain score in the fitusiran group (difference, -28.72; P <.0001).

In the fitusiran and on-demand BPA arms, 92.7% and 57.9% of patients, respectively, had at least 1 treatment-related adverse event. Seven patients in the fitusiran arm, furthermore, had a serious treatment-related adverse event, though none were fatal.

Disclosure: The study author(s) declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures.

Read more of Hematology Advisor‘s coverage of the ASH 2021 meeting by visiting the conference page.


Young G, Srivastava A, Kavakli K, et al. 4 Efficacy and safety of fitusiran prophylaxis, an siRNA therapeutic, in a multicenter phase 3 study (ATLAS-INH) in people with hemophilia A or B, with inhibitors (PwHI). Presented at ASH 2021; December 11-14, 2021. Abstract 4.