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BERLIN—Researchers have reported positive results from an ongoing phase 2 trial of fitusiran in patients with hemophilia A or B, with or without inhibitors.

Once-monthly treatment with fitusiran reduced the median annualized bleeding rate (ABR) from 20 to 1 in all patients. In patients with inhibitors, the median ABR fell from 38 to 0.

Most adverse events (AEs) were mild or moderate in severity, with the most common AEs being alanine aminotransferase (ALT) increases and injection-site reactions.

These results were presented* at the International Society on Thrombosis and Haemostasis (ISTH) 2017 Congress.

The research was sponsored by Alnylam Pharmaceuticals, Inc., the company developing fitusiran in collaboration with Sanofi Genzyme.

Fitusiran is an RNAi therapeutic targeting antithrombin for the treatment of patients with hemophilia A and B. Fitusiran is designed to lower levels of antithrombin and promote sufficient thrombin generation upon activation of the clotting cascade to restore hemostasis and prevent bleeding.

John Pasi, PhD, of Barts and the London School of Medicine and Dentistry in London, UK, and his colleagues tested fitusiran in 33 patients, ages 19 to 61, with hemophilia A (n=27) or hemophilia B (n=6), with inhibitors (n=14) or without (n=19).

Fitusiran was administered as a monthly, subcutaneous, fixed dose of 50 mg (n=13) or 80 mg (n=20).

Patients were treated for up to 20 months, with a median of 11 months on study. Five patients discontinued treatment—4 due to withdrawn consent and 1 due to an AE.


The incidence of AEs was 70%. Most were mild or moderate in severity and unrelated to fitusiran.

Non-laboratory AEs included injection site reactions (18%, n=6), abdominal pain (9%, n=3), diarrhea (9%, n=3), and headache (9%, n=3).

Eleven patients had asymptomatic ALT increases greater than 3 times the upper limit of normal, without concurrent elevations in bilirubin greater than 2 times the upper limit of normal. All of these patients were hepatitis C antibody-positive.

At last follow-up, all ALT elevations were resolved (n=10) or resolving (n=1).

There were serious AEs in 6 patients, and 2 of these events were considered possibly related to fitusiran. One event was seizure with confusion in a patient with a prior history of seizure disorder.

The other serious AE was an asymptomatic ALT elevation in a patient with chronic hepatitis C virus infection. This patient discontinued treatment due to the event.

There were no thromboembolic events, no laboratory evidence for pathological clot formation, and no instances of anti-drug antibody formation.


Fitusiran resulted in approximately 80% lowering of antithrombin, with corresponding increases in thrombin generation.

In all patients, fitusiran reduced the median ABR from 20 (interquartile range [IQR]: 4-36) to 1 (IQR: 0-3).

In patients with inhibitors, fitusiran reduced the median ABR from 38 (IQR: 20-48) to 0 (IQR: 0-3).

Forty-eight percent of all patients (n=16) remained bleed-free during the observation period, and 67% (n=22) did not experience any spontaneous bleeds.

All breakthrough bleeds were successfully managed with replacement factor (recombinant factor VIII or recombinant factor IX) or bypassing agents (recombinant factor VIIa or activated prothrombin complex concentrate).

Based on these results, Sanofi and Alnylam initiated the ATLAS phase 3 program for fitusiran in patients with hemophilia A and B, with or without inhibitors.

*Data in the abstract differ from data presented at the meeting.

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