Last updated March 7, 2022, by Teresa Carvalho, MS

✅ Fact-checked by José Lopes, PhD

PF-06939926 (fordadistrogene movaparvovec), developed by Pfizer, is an investigational gene therapy for Duchenne muscular dystrophy (DMD).

How PF-06939926 works

DMD, the most common type of muscular dystrophy, is caused by mutations in the DMD gene, which provides instructions to make a protein called dystrophin. Dystrophin is a structural protein that provides support for muscle cells as they stretch and contract with motion.

Most mutations that cause Duchenne lead to a complete lack of functional dystrophin protein, resulting in muscle cell damage and death.

PF-06939926 uses a modified virus, known as adeno-associated virus serotype 9 (AAV9). Because the DMD gene is too long to fit into the adenoviral vector — DMD is the largest known human gene — a shorter but still functional version is used for the therapy, coding for a so-called mini-dystrophin protein to be produced specifically in muscles.

By providing a source of a working mini-dystrophin to tissues, this treatment is expected to slow or stop muscle degeneration in people with DMD.

PF-06939926 in clinical trials

A Phase 1b open-label clinical trial (NCT03362502) has been testing PF-06939926 in males with DMD, ages 4 and older, at 21 sites in the U.S. Two doses of PF-06939926 have been used: a low dose of 1E14 vector genomes (vg)/kg) and a higher dose of 2E14 vg/kg.

The main aim is to assess the safety and tolerability of a single intravenous (IV or into-the-vein) infusion of PF-06939926, by evaluating treatment-related side effects. Secondary goals include measuring the levels and distribution of the mini-dystrophin protein in muscle biopsies.

Early data from five of six boys showed an improvement (or at least no decline) in the NorthStar Ambulatory Assessment (NSAA), compared with participants given a placebo during previous clinical trials. NSAA is a 17-item test of motor function, used as a standard measure in children with DMD who can walk.

Side effects included vomiting, fever, nausea, decreased appetite, fatigue, and headache. Serious adverse events, reported in three participants, included dehydration, acute kidney injury, and low levels of platelets. All of these were resolved within three weeks. The third serious adverse event prompted changes to the study protocol, to include close monitoring of patients after dosing and higher doses of glucocorticoids post-infusion.

However, the death of a participant, a young man who was not able to walk, was recently reported. As such, the trial has been paused and a clinical hold has been issued by the U.S. Food and Drug Administration (FDA) on the therapy’s investigational new drug application.

A randomized, placebo-controlled Phase 3 trial, called CIFFREO (NCT04281485), is planned to enroll 99 DMD boys, ages 4 to 7, in multiple countries. The trial was delayed in the U.S. by the FDA, due to questions on Pfizer’s potency tests, which are used to measure the potential therapy’s ability to induce a specific response at a selected dose.

Patients have been randomly assigned to either a single IV infusion of PF-06939926 or to a placebo infusion. In the second year, these groups switch: those treated with PF-06939926 will get a single placebo infusion, and those initially given a placebo will receive the gene therapy. Nearly two-thirds of the boys will be given the gene therapy at the study’s start, and all will be monitored for five years after treatment with PF-06939926. The study’s main outcome measure is changes in NSAA scores at one year after treatment. 

Other information

PF-06939926 was named an orphan drug by the FDA and the European Medicines Agency in 2017, and also awarded the FDA’s rare pediatric disease designation.  These designations support the therapy’s development and regulatory review, while ensuring marketing exclusivity for a period of time after approval, if that is granted.

The FDA in 2020 granted fast track designation to this investigational therapy. This designation also is intended to facilitate the development and accelerate the review of treatments that address serious conditions for which available therapies are lacking.


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