Fitusiran: Great 'Leap Forward' in Hemophilia Treatment

M. Alexander Otto

December 21, 2021

Remarkable results were reported at the annual meeting of the American Society of Hematology for the investigational drug fitusiran to prevent bleeding in hemophilia patients.

Fitusiran is a small interfering RNA molecule that blocks antithrombin production in liver cells. Instead of taking the traditional approach in hemophilia treatment of boosting the coagulation cascade by replacing what's missing, the idea of fitusiran is to short circuit the body's anticoagulation system by targeting antithrombin.

Patients in two trials presented at the meeting, ATLAS-A/B and ATLAS-INH, had about a 90% reduction in their annualized bleeding rates when treated with prophylactic fitusiran, with half or more having no bleeds that required treatment during the 9-month trials. The median annualized bleeding rate fell to 0, trial investigators reported at the meeting.

These findings held in both hemophilia A and B with and without inhibitors, which are antibodies formed against exogenous clotting factors, and on subanalysis of spontaneous and joint bleeding rates. Reduced bleeding was associated with substantial improvements in health-care related quality of life, particularly in the physical health domain.

A Question About Study Design

An audience member at ASH noted that the trials didn't compare fitusiran against prophylactic treatment, which is standard of care for hemophilia, but rather against episodic treatment – concentrated factors or bypassing agents in subjects with inhibitors – once subjects in the control groups started to bleed.

Still, the numbers reported in the studies "have never been achieved with standard prophylaxis in the past." Furthermore, standard prophylaxis requires lifelong intravenous infusions, sometimes several a week, said lead ATLAS-A/B investigator Alok Srivastava, MD, a hematologist at the Christian Medical College in Vellore, India.

Fitusiran was dosed in the studies as a once-a-month 80 mg subcutaneous injection, so is much less bothersome. Also, it seems likely that some patients will only need dosing every other month. Maker Sanofi Genzyme is exploring lower and less frequent dosing to reduce thrombotic event risks that emerged in earlier studies, said Steven Pipe, MD, a pediatric hematologist at the University of Michigan, Ann Arbor, and the senior investigator on ATLAS-A/B, which assessed fitusiran in patients without inhibitors.

Serious thrombotic events occurred in two fitusiran patients in the trials, one of which led to discontinuation.

No Pricing Information

Overall, "I think [fitusiran] is a tremendous leap forward" with "the opportunity to transform the day-to-day lives of patients," particularly those with hemophilia B, who have limited treatment options, Pipe said.

If approved for the U.S. market, fitusiran will go up against the monoclonal antibody emicizumab (Hemlibra), a subcutaneous injection dosed weekly to monthly that mimics the function of factor VIII, so it's approved only for hemophilia A with or without inhibitors.

Several audience members at ASH noted that a major consideration for fitusiran, if approved, will be its cost. There's no pricing information yet, but annual list price for emicizumab is reported to be in the $500,000 range.

For hemophilia A, "it will come to what proves to be the most efficacious and safe, with also consideration given to pricing," Nigel Key, MD, a hematologist at the University of North Carolina at Chapel Hill, said in a comment.

Trial Details

In ATLAS-A/B, 80 male patients were randomized to prophylactic fitusiran once monthly and 40 to continue with clotting factors as needed for bleeding. Just over 20% had hemophilia B, the rest hemophilia A. The mean age was 34 years, and subjects had a mean of about 12 bleeds in the 6 months leading up to the study.

Half of the fitusiran group had no treated bleeds during the study period versus only 5% in the control arm.

The five treatment emergent serious adverse events in the fitusiran arm included cholelithiasis in two subjects, plus cholecystitis, lower respiratory tract infection, and asthma in one each. Two fitusiran patients discontinued treatment because of cholecystitis and increased alanine aminotransferase.

ATLAS-INH had the same study design, and investigated patients with inhibitors; again, just over 20% had hemophilia B, the rest A. Mean age was 28 years, and patients had a mean of about 13 bleeds over the 6 months before the study. A total of 38 subjects were randomized to fitusiran, and 19 to bypassing agents as needed.

Almost 66% of fitusiran patients had no treated bleeds versus about 5% in the control arm.

There "was far less bleeding both for hemophilia A and B. It's definitely a clinically meaningful and important" effect, said ATLAS-INH lead investigator Guy Young, MD, director of the hemostasis and thrombosis program at Children's Hospital Los Angeles.

He said the improved quality of life with fitusiran noted in both trials was "not surprising. If you only have to dose once a month subcutaneously, and you are not bleeding, of course your quality of life is going to improve."

Both fitusiran thrombotic events occurred in ATLAS-INH. One patient developed deep vein thrombosis, subclavian vein thrombosis, and superficial thrombophlebitis but stayed in the study. Another discontinued after developing suspected spinal vessel thrombosis. Serious adverse events among five other subjects included acute cholecystitis and hematuria.

Fitusiran was associated with liver enzyme elevations in both trials, but they were generally mild to moderate.

The studies were funded by fitusiran maker Sanofi Genzyme. Several investigators were employees. Pipe is a consultant, Srivastava is a researcher and adviser, and Young is a speaker and consultant for the company and disclosed honoraria from it. Key had no relevant disclosures.

This article originally appeared on, part of the Medscape Professional Network.


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