Spark hemophilia B gene therapy clears test en route to Pfizer-sponsored phase 3

Spark Therapeutics logo on labcoat
The next steps are for Spark to transfer the clinical program to Pfizer and provide it with study drug for the phase 3. (Spark Therapeutics)

Spark Therapeutics has presented data on hemophilia B patients who received a version of its gene therapy manufactured using a modified process. Early data on the patients suggest the new batch of therapies is at least as effective as its predecessor, clearing away a potential pitfall on Spark’s path to market.

Philadelphia-based Spark switched to the new vector manufacturing process last year in anticipation of producing the gene therapy, SPK-9001, at commercial scale. The mid-development switch created a risk that the gene therapy Spark and partner Pfizer moved into phase 3 would be less effective than the one that shone in phase 1/2. Data shared this week allay that fear.

Presenting at the World Federation of Hemophilia World Congress, Spark provided data on the first three patients to receive the gene therapy made under the new process. Each of these patients had 12 weeks of follow up, the length of time needed for factor IX activity levels to reach steady state.


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Factor IX activity levels in the three patients ranged from 38.1% to 54.5%. Going into the trial, the patients had factor IX activity levels of less than 2%. The readout links the latest gene therapy to big increases in factor IX activity that are in line with those seen in patients treated with product made under the old process. Factor IX levels in these 10 patients range from 14.3% to 76.8%.

The factor IX activity of all 13 patients with 12 weeks of follow up is well above the levels needed to improve outcomes. Across all 15 patients—two of whom lack 12 weeks of follow up—the rate of annualized bleeding is down 98%. The annualized factor IX infusion rate is down 99%. People who used to suffer multiple bleeds a year and take repeated infusions are largely free of these burdens.

Spark is now working with Pfizer to generate the phase 3 data needed to validate the potential of the gene therapy and bring it to market. The next steps are for Spark to transfer the clinical program to Pfizer—something it expects to happen this summer—and provide it with study drug for the phase 3. 

If all goes to plan, Pfizer will ace the phase 3 and the phase 1/2 will continue to show the long-term efficacy of the one-shot treatment. That will give Pfizer and Spark a shot at disrupting the existing hemophilia B market while holding off rival gene therapies from companies including uniQure.  

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