Bluebird slips as new data paints a cloudy picture for its gene therapy

Bluebird bio ($BLUE) has seen its share price soar over the past year on the promise that its in-development gene therapy could be a functional cure for a rare blood disease. But new clinical data reveal that not all patients respond the same way, possibly dimming the potential of the company's lead treatment.

bluebird CEO Nick Leschly

The company is testing a therapy for beta-thalassemia major, which results from a defective beta-globin gene that stops patients from producing the hemoglobin they need, often leading to severe anemia. Bluebird has been posting regular updates on the progress of 9 patients given a one-time infusion of its treatment, LentiGlobin BB305, which is designed to free beta-thalassemia patients from the frequent blood transfusions they need to survive.

But in results from two trials to be presented at a medical conference next month, bluebird discovered that its gene therapy is unlikely to be a one-size-fits-all solution for the disease. Of the 9 patients, 6 remain free of the need for transfusions after between 6 and 18 months, bluebird said, but another three with a more severe gene mutation haven't responded as well. Of those patients, who produce no beta-globin, two have received a single transfusion and one remains entirely dependent on new blood.

To date, all of bluebird's BB305 disclosures had been upbeat, keeping beta-thalassemia patients free of transfusions at each step. The latest results complicate that narrative, sending bluebird's shares down more than 20%.

Bluebird is planning to enroll two more beta-thalassemia studies for BB305 with 15 patients each, following subjects for two years with a primary endpoint of transfusion independence. The ongoing results will inform the company's chosen regulatory paths in the U.S. and Europe, management has said.

Outside of its lead indication, BB305 is in development as a potential one-time treatment for sickle cell disease. In the most recent data, a sickle cell patient dosed with BB305 remained transfusion-free after 9 months and has not been hospitalized, the company said. In a second Phase I study, bluebird has dosed one of two enrolled patients but doesn't yet have efficacy data to report.

Bluebird's lead treatment differs from many of the other gene therapies in development around the world, which involve inserting corrective genes directly into a patient. Instead, bluebird's method works by removing a patient's hematopoietic stem cells, equipping them with a functional beta-globin gene and then reinserting them through infusion.

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