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Pfizer cleared to restart hemophilia gene therapy trial

With the clearance to restart the hemophilia A trial, Pfizer has now successfully ticked off two safety boxes for its gene therapy research pipeline. Last week, the company announced the FDA OK’d resumption of a trial of its experimental treatment for Duchenne muscular dystrophy, which had been stopped because of side effects and the death of a patient.

In the case of the hemophilia trial, the worries over thromboses were triggered by findings that patients’ levels of clotting factor rose to 150% of normal after an infusion of the gene therapy. The protein is largely missing in hemophilia A patients, and the gene therapy stimulates its production by replacing a faulty gene responsible for making it.

While the FDA has lifted the hold, a “voluntary” pause in dosing new patients will continue as Pfizer continues to talk to the regulator about the necessary conditions for a restart. That will include “updated study protocols,” but executives didn’t specify what those changes would entail.

In restarting the muscular dystrophy trial, Pfizer said patients will now need to stay in a hospital for a week following their infusion to monitor for any dangerous side effects and treat them if necessary.

After approving two gene therapies for rare diseases in 2017 and 2019, meanwhile, the FDA looks to be taking a more cautious stance recently. BioMarin had appeared on track to win clearance for its hemophilia A candidate in 2020, but the FDA asked the drugmaker to provide longer-term follow-up data.

The increased scrutiny on safety, which has also included other clinical holds, has dampened investor expectations for the field, although BioMarin’s submission and the potential approval this year of two gene therapies from Bluebird bio could boost confidence.

Published May 3, 2022

Jonathan Gardner, Senior Reporter

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