Will Pfizer’s muscular dystrophy gene therapy cross the finish line?

River D'Almeida, Ph.D
4 min readJul 7, 2021

Others have tried and failed — could this one yield a happy ending for patients?

Image via Unsplash

In early 2021, a young boy in Barcelona was the first to receive Pfizer’s investigational gene therapy for Duchenne muscular dystrophy (DMD). This was part of a global Phase 3 human trial to test Pfizer’s lead candidate, PF-06939926, in a cohort of 99 patients with DMD-a debilitating genetic condition that causes progressive muscle wasting mostly in males-for which there are currently no approved treatment options.

This is the last lap for Pfizer in the onerous race towards an approved gene therapy for DMD, but the pharma giant isn’t the only competitor. Others with similar gene therapy offerings have sprinted forward with bursts of apparent success, only to end up stumbling and losing their lead.

Take Sarepta Therapeutics, for instance, whose experimental gene therapy SRP-9001 was reported to dramatically improve DMD patients’ conditions in early trials, cutting in half the time it took for one patient in the trial to rise up from the ground and climb a set of stairs. However, these triumphs were short-lived, with no significant differences observed between the placebo and treated groups in mid-stage trials, causing the company’s stock prices to plummet.

Sarepta blamed the disappointing results on an issue with patient recruitment: the placebo group had higher baseline values, which meant seeing statistically significant improvements in the boys who received SRP-9001 was “virtually impossible”. This is a very real challenge in conducting trials for rare diseases like DMD. First, it’s not easy to recruit enough patients to achieve statistical confidence. Even with these numbers, patients present with a spectrum of symptom baselines, making it difficult to judge improvements as a result of the treatment.

In spite of these hurdles, Pfizer has reached the milestone of a late-stage trial and, in doing so, has overtaken Sarepta. But does PF-06939926 have what it takes to go all the way?

Both Pfizer’s and Sarepta’s lead candidates are “one-and-done” gene therapies, which deliver a shortened version of the human DMD gene to replace the faulty one in patients. The gene encodes a mini version of the dystrophin…

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River D'Almeida, Ph.D

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