In March, the FDA approved Sanofi’s Qfitlia (fitusiran) — a first-in-class RNA interference (RNAi) therapy to treat hemophilia A or B. Since hemophilia is caused by genetic mutations that reduce clotting factors in the blood, traditional therapies have focused on either replacing or mimicking these missing factors. Instead, Qfitlia’s mechanism of action relies on balancing out clotting proteins to return the system to homeostasis and prevent bleeding episodes. As the 7th FDA-approved RNAi therapeutic since 2018, Qfitila signals a future of sustainable growth for RNAi medicines.
How Qfitlia works to restore clotting
Qfitlia is the first medication that targets antithrombin, a protein that prevents blood coagulation by inhibiting the clotting protein thrombin. When the body starts producing more thrombin as a result, it balances out the patient’s missing clotting factor and prevents bleeding episodes.
As an RNAi therapy, Qfitlia achieves this balancing act by silencing the SERPINC1 gene (serpin family C member 1) that codes for antithrombin. The drug became the 6th FDA-approved RNAi therapeutic discovered by Alnylam Pharmaceuticals.
The FDA’s approval of Qfitlia was based on positive results from three Phase 3 ATLAS trials showing bleed reductions of around 70 percent in patients with and without inhibitors, which are antibodies produced by the immune system in response to factor replacement therapies. Qfitlia did raise the risk of thrombotic events, gallbladder disease, and hepatotoxicity, but the most common side effects were viral and bacterial infections and nasopharyngitis.
RNAi momentum
The RNAi market is poised to continue growing as more companies invest in RNAi technology, with the most recent example coming from Biogen’s May 2025 announcement of their collaboration with City Therapeutics, which could pay out up to $1 billion. The overall market value of antisense and RNAi therapeutics is predicted to reach $3.3 billion by 2033.
The upward trend comes as safety and cost issues are rising for gene therapies. Sarepta Therapeutics’ Duchenne program has been linked to patient deaths, while major companies are scaling back, with Pfizer phasing out all gene therapy efforts.
The last drug in Pfizer’s portfolio, Beqvez, was already FDA-approved to treat hemophilia B, but they dropped it after patient uptake of other hemophilia gene therapies remained low. Patients with hemophilia and sickle cell disease are already rejecting these therapies, pointing to a future where other patient groups may also pass on approved gene therapies. Alternatives that offer a good quality of life without the risk of permanent off-target effects leave little incentive for patients to accept a one-time gene therapy with variable efficacy, as hemophilia A trials have shown.
Fewer doses, proven results
Qfitlia, like other RNAi medications, silences the SERPINC1 gene only temporarily. Plus, it only has to be given subcutaneously every other month, which is a drastic improvement for a disease like hemophilia, where patients are often used to weekly infusions.
“Qfitlia delivers the fewest doses of any prophylactic therapy in hemophilia, and its unique mechanism allows it to be used to treat all types of hemophilia, including with inhibitors and hemophilia B, where unmet medical needs remain,” said Guy Young, Director of the Hemostasis and Thrombosis Center at the Children's Hospital, Los Angeles in a press release.
This article is part of our 2025 Novel FDA Approvals series, highlighting groundbreaking therapies that received FDA approval this year. Each story explores the science, clinical impact, and future potential of these innovative treatments shaping the next era of medicine.
