---
title: "Fitusiran (Qfitlia): The First RNAi Therapy for Hemophilia - How Silencing Antithrombin Rebalances Clotting in Hemophilia A and B, With or Without Inhibitors (August 30, 2026) | PepTracker Pro Blog"
url: https://peptrackerpro.com/blog/fitusiran-qfitlia-sanofi-alnylam-antithrombin-serpinc1-galnac-sirna-rnai-hemophilia-a-b-inhibitors-bleeding-prophylaxis-rebalancing-hemostasis-atlas-inh-atlas-a-b-fda-approved-boxed-warning-thrombosis-august-30-2026
description: "Most hemophilia drugs replace a missing clotting factor. Fitusiran (Qfitlia) does the reverse: it is an RNAi therapy that silences the antithrombin gene in the liver to rebalance clotting - and in the Phase 3 ATLAS trials it cut bleeds by about 90% in hemophilia A and B, with or without inhibitors. FDA-approved in March 2025, it is the first siRNA therapy for hemophilia, dosed as few as six times a year, and carries a boxed warning for clots."
lang: en
---

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Research & Compounds

Research & Compounds

# Fitusiran (Qfitlia): The First RNAi Therapy for Hemophilia - How Silencing Antithrombin Rebalances Clotting in Hemophilia A and B, With or Without Inhibitors (August 30, 2026)

PepTracker Pro Research Team August 30, 2026 8 min read

## Table of Contents

- A different way to treat hemophilia: lower the brake instead of adding the accelerator
- What fitusiran is, and what an siRNA does
- Why 'rebalancing' works across hemophilia A and B - even with inhibitors
- The ATLAS trials: about 90% fewer bleeds
- As few as six injections a year
- The catch: a boxed warning for clots
- Dosing to a number: antithrombin-based dosing
- Fitusiran versus emicizumab and gene therapy
- Where fitusiran fits in the RNAi story
- The bottom line

## A different way to treat hemophilia: lower the brake instead of adding the accelerator

For as long as hemophilia has been treatable, the strategy has been replacement - infuse the clotting factor the patient is missing (factor VIII in hemophilia A, factor IX in hemophilia B). Fitusiran, sold as Qfitlia, does the opposite. Instead of adding a pro-clotting factor, it lowers antithrombin, the body's main natural anticoagulant. Blood clotting is a balance between factors that promote clots and proteins that hold them back; in hemophilia the pro-clotting side is weak, so fitusiran restores the balance by easing off the natural brake. The U.S. FDA approved it on March 28, 2025, making it the first RNA-interference (siRNA) therapy for hemophilia and the first medicine of any kind that treats bleeding by lowering antithrombin.

## What fitusiran is, and what an siRNA does

Fitusiran is a small interfering RNA (siRNA) attached to a sugar tag called GalNAc that steers it into liver cells. Inside those cells it hijacks the body's own gene-silencing machinery to destroy the messenger RNA for SERPINC1 - the gene that makes antithrombin. Less messenger RNA means less antithrombin protein, which means the clotting enzyme thrombin faces less opposition and the blood can form more stable clots. This is the same GalNAc-siRNA technology behind cholesterol drugs like inclisiran and amyloidosis drugs like vutrisiran, now pointed at a bleeding disorder.

## Why 'rebalancing' works across hemophilia A and B - even with inhibitors

Because fitusiran acts downstream of the missing clotting factor, it does not care which factor is absent. The same drug helps in hemophilia A and hemophilia B. Most importantly, it works in people who have developed inhibitors - neutralizing antibodies against factor replacement that affect roughly one in four patients and have long been the hardest group to protect from bleeds. A single, factor-independent prophylaxis that covers all of these patients is a genuinely unifying idea in a field where treatment has always been split by disease type and inhibitor status.

## The ATLAS trials: about 90% fewer bleeds

Fitusiran's approval rests on the Phase 3 ATLAS program, published in The Lancet and The Lancet Haematology in 2023. In ATLAS-INH, in patients with inhibitors, fitusiran cut the mean annualized bleeding rate to about 1.7 versus 18.1 with on-demand bypassing agents - roughly a 90.8% reduction - and 66% of fitusiran-treated participants had zero treated bleeds versus just 5% on-demand. In ATLAS-A/B, in severe hemophilia without inhibitors, 51% of fitusiran-treated participants had zero treated bleeds versus 5% on prior on-demand treatment, again with about a 90% overall reduction in bleeding rate.

## As few as six injections a year

One of fitusiran's biggest practical draws is convenience. It is a subcutaneous injection - under the skin, not into a vein - starting at 50 mg once every two months, roughly six times a year. That is a sharp contrast to the frequent intravenous factor infusions many patients otherwise manage, and it is possible because the siRNA produces deep, long-lasting antithrombin knockdown from each dose.

## The catch: a boxed warning for clots

Lowering a natural anticoagulant is powerful but double-edged. Push antithrombin too low and the balance tips the other way, toward dangerous clots. In earlier development the original fixed 80 mg once-monthly dose caused serious and even fatal thrombotic events, including a fatal clot in the veins of the brain (cerebral venous sinus thrombosis); thrombotic events occurred in about 2.6% of patients on that regimen. Qfitlia's label carries a boxed warning for thrombotic events and for gallbladder disease (gallstones and cholecystitis, sometimes needing surgery), plus a warning for liver enzyme elevations.

## Dosing to a number: antithrombin-based dosing

The program's response to the clot risk is a lesson in how to handle a drug with a narrow safety window. Rather than a one-size dose, fitusiran is now dosed to a target: antithrombin activity is measured with an FDA-cleared companion test and the dose or interval is adjusted to keep it between 15% and 35%. On this antithrombin-based regimen, the thrombotic-event rate fell to about 1.4%. Liver blood tests are also monitored at baseline and periodically after starting.

## Fitusiran versus emicizumab and gene therapy

Fitusiran is not the only non-factor option. Emicizumab (Hemlibra) is an antibody that mimics factor VIII and is widely used in hemophilia A - but it does not treat hemophilia B. Gene therapy offers a potential one-time fix but is limited by eligibility, cost, and durability questions. Fitusiran's niche is breadth: one subcutaneous medicine that covers hemophilia A and B, with or without inhibitors. The trade-off is the monitoring and thrombosis risk that come with manipulating the clotting balance.

## Where fitusiran fits in the RNAi story

Fitusiran extends RNA interference beyond the cholesterol, amyloidosis, and rare-metabolic diseases where GalNAc-siRNAs first proved themselves, into coagulation. It sits alongside inclisiran (PCSK9), vutrisiran (transthyretin), zilebesiran (blood pressure), olpasiran (Lp(a)) and plozasiran (triglycerides) as evidence that silencing a single liver gene can reshape a chronic disease - here, turning a lifelong bleeding disorder into something managed with a handful of injections a year.

## The bottom line

Fitusiran (Qfitlia) is a first-in-class, FDA-approved RNAi therapy that treats hemophilia A and B - with or without inhibitors - by lowering antithrombin to rebalance clotting, cutting bleeds by about 90% in pivotal trials with dosing as infrequent as every two months. Its power comes with a real, monitorable trade-off: a boxed warning for clots and gallbladder disease, and dosing that must be titrated to a measured antithrombin target. It is a prescription biologic used under hematology care - not a supplement or research chemical, and not something to source outside a licensed pharmacy.

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## Citations

1. [1] FDA Approves Novel Treatment for Hemophilia A or B, with or without Factor Inhibitors - U.S. Food & Drug Administration (March 28, 2025) Source (https://www.fda.gov/news-events/press-announcements/fda-approves-novel-treatment-hemophilia-or-b-or-without-factor-inhibitors)
2. [2] Qfitlia approved as the first therapy in the US to treat hemophilia A or B with or without inhibitors - Sanofi (March 28, 2025) Source (https://www.sanofi.com/en/media-room/press-releases/2025/2025-03-28-20-07-38-3051637)
3. [3] FDA Approves Qfitlia (fitusiran), the First siRNA (RNAi Therapeutic) for the Treatment of Hemophilia A or B - Alnylam Pharmaceuticals (March 28, 2025) Source (https://investors.alnylam.com/press-release?id=28901)
4. [4] QFITLIA (fitusiran) injection - full U.S. Prescribing Information (boxed warning), DailyMed / NIH Source (https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=6dd2f8ac-6f90-4cbf-b197-97d74964135c)
5. [5] Young G, et al. Efficacy and safety of fitusiran prophylaxis in people with haemophilia A or B with inhibitors (ATLAS-INH): a phase 3 trial - The Lancet (2023) Source (https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(23)00284-2/abstract)
6. [6] Srivastava A, et al. Fitusiran prophylaxis in people with severe haemophilia A or B without inhibitors (ATLAS-A/B): a phase 3 trial - The Lancet Haematology (2023) Source (https://www.thelancet.com/journals/lanhae/article/PIIS2352-3026(23)00037-6/abstract)

**Disclaimer:** This article is for educational purposes only and does not constitute medical advice. Always consult a licensed healthcare provider. Read full research disclaimer → (https://peptrackerpro.com/research-disclaimer)

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