Sanofi is trumpeting new clinical data for a hemophilia A drug developed by Bioverativ that it contends could transform treatment of the bleeding disorder.

The phase 1/2 trial of BIVV001 (rFVIIIFc-VWF-XTEN) suggests that a single dose of the replacement drug almost tripled the half-life of recombinant factor VIII (rFVIII) in the blood to 37 hours from 13 hours with regular rFVIII, potentially making it suitable for less frequent dosing than current long-acting hemophilia A products.

BIVV-001 was one of the main draws for Sanofi’s $11.6 billion takeover of Bioverativ earlier this year as part of a push into the rare disease arena. The drug is designed to overcome a natural half-life cap on rFVIII, namely a glycoprotein found in the blood called von Willebrand factor (vWF) that binds to the clotting factor and is thought to be responsible for limiting the amount of time rFVIII replacement therapies stay active in the body.

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BIV001 has been re-engineered so it has a vWF component along with other polypeptides designed to extend its time in circulation, and it seems the performance in trials has outdone even Bioverativ’s expectations. Earlier, the company told us that it was hoping that the drug would have double the half-life of long-acting rFVIII drugs like Bioverativ’s own Eloctate brand and Shire’s Adynovate, which are given two to three times a week, and that this could support weekly administration.

“For decades, scientists have been trying to overcome the [vWF] ceiling…and these data demonstrate that BIVV001 has finally broken through that ceiling,” commented Joachim Fruebis, senior VP of development at Bioverativ.

He also said the factor levels seen in this study—which came in at an average of 13% at five days and 5.6% seven days after infusion of the drug—are “unparalleled” in hemophilia A. Those levels put the patients, who all had severe hemophilia at enrollment with levels less than 1%, into the bracket of mild disease.

“We are excited about the potential for BIVV001 to transform the treatment paradigm for patients and physicians,” continued Fruebis.

The data suggest BIV001 could stack up well against rFVIII drugs, but the bigger question for Sanofi and Bioverativ is how a biweekly factor VIII product will stack up in the marketplace against Roche/Genentech’s Hemlibra (emicizumab), which looks like it could be dosed just once a month on the back of data from the HAVEN-4 trial released this week.

Roche/Genentech has also just revealed figures from the HAVEN-3 trial that support extending the use of the drug to all hemophilia A patients, not just those with rFVIII inhibitors as at present. Analysts at Jefferies said the data “sets Hemlibra up to become the new standard of care for hemophilia A, which we view as a $5 billion peak sales opportunity.”

Another consideration is how quickly progress will be made with gene therapies for hemophilia, which offer the potential of very long-term responses with a single treatment.

Bioverativ has previously said it thinks there is still a long way to go before the safety, efficacy and durability of gene therapies is established, and there will continue to be a role for rFVIII products for many years, particularly as people with hemophilia tend to be a highly risk-averse population.