The FDA has agreed to allow Alnylam (NASDAQ:ALNY) and Sanofi (NASDAQ:SNY) to resume trials of their hemophilia therapy, fitusiran. Previously, these companies had to halt their fitusiran studies after a patient who had advanced into an extension study died because of a fatal blood clot. Is this news good for patients and investors?

Here's the background

Hemophilia A is caused by a deficiency of blood coagulation factor VIII and hemophilia B is caused by a deficiency of blood coagulation factor IX. In both cases, the low levels of clotting factor put patients at significant risk of bleeding and hemorrhages. It's estimated that there are about 150,000 patients with hemophilia A and 30,000 patients with hemophilia B, globally.

A microphone on a stand in the foreground with a businessman in the background.

Image source: Getty Images.

Historically, patients with very low factor VIII and factor IX activity are prescribed lifelong prophylactic infusions of these factors. These infusions work, but they pose a significant burden on patient quality of life and they're extremely expensive. Infusions can be given multiple times per week and they can run into the hundreds of thousands of dollars per year.

What's fitusiran?

Fitusiran is a clinical-stage RNA therapy that reduces the production of a naturally occurring enzyme that prevents clotting and thus increases thrombin, another important enzyme involved in blood clotting. Unlike factor VIII and factor IX infusions that must be taken often, fitusiran is a once-monthly injection.

In July, Alnylam and Sanofi reported post-hoc data from patients participating in its phase 2 trial. The results show that patients' median annual bleed rate fell from 20 to 1 prior to entering the fitusiran study. Furthermore, many patients were bleed free and 67% of patients experienced zero spontaneous bleeds.

What's next?

The study restart is good news for these companies because hemophilia is a blockbuster indication. Alnylam and Sanofi have previously said that they could get results from their phase 3 study of fitusiran in late 2019, suggesting its likely that it wouldn't reach the market until 2020.

If it does eventually win approval, then Alnylam and Sanofi will co-commercialize it in the U.S., Canada, and Western Europe. Sanofi has agreed to pay Alnylam up to $75 million in development and regulatory milestones, and profits in the co-marketed territories will be shared equally between them. In territories where Sanofi has exclusive rights, Alnylam can receive tiered royalties up to 20%.

Although restarting the trial is good news, investors might want to rein in some enthusiasm. Gene therapy companies Spark Therapeutics and BioMarin both recently reported intriguing data in hemophilia that suggests the future of hemophilia treatment isn't regular infusions, but one-and-done treatment. Additionally, since hemophilia treatment is relatively safe, any whiff of safety concerns could make it unlikely that fitusiran achieves meaningful market share. The stakes are undeniably high given the size of the market, but I don't think that this news alone is enough to justify buying stock in Alnylam or Sanofi.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.