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

How PF-06939926 works

PF-06939926 uses a modified virus called an adeno-associated virus serotype 9 (AAV9) capsid to deliver a shortened version of the human DMD gene that encodes for a so-called mini-dystrophin protein. DMD is caused by a mutation in the DMD gene, which is the largest human gene. Because the gene is too long to fit into the adenoviral vector, a shorter but still functional version of the gene is used for the therapy. The AAV9 capsid lets the therapy specifically target the muscles.

PF-06939926 in clinical trials

A Phase 1b open-label clinical trial (NCT03362502) assessing the safety and tolerability of a single intravenous infusion of PF-06939926 in boys ages 5–12 with DMD who can still walk is ongoing at sites across the United States. One-year data from the first nine treated boys (mean age of 8) showed treatment was largely well-tolerated, and resulted in sustained production of the mini-dystrophin protein in muscle tissues and in motor function improvements.

Three boys were given a predefined low dose of PF-06939926 (1E14 vector genomes (vg)/kg), while the other six received a higher dose of 3E14 vg/kg.

Patients showed a median improvement of 3.5 points in the NorthStar Ambulatory Assessment Score, which assesses motor function, at one year, which significantly differed from the 4-point loss reported for age-, weight-, and function-matched boys with DMD given a placebo in previous clinical trials. Greater benefits were seen among the boys treated at high dose.

The most common adverse events thought to be associated with treatment were vomiting, nausea, decreased appetite, and fever. Three serious adverse events were reported in the first 14 days following treatment; all were fully resolved at the latest clinic visits.

A randomized, placebo-controlled Phase 3 trial, called CIFFREO (NCT04281485), is now enrolling up to 99 DMD boys, ages 4 to 7, at sites in Italy, Israel, and Spain, and is expected to expand to 55 sites in 15 countries.

Patients will be randomly assigned to either a single intravenous (IV) infusion of PF-06939926 in the study’s first year or to a placebo infusion. These groups with switch in its second year, with those treated now getting a single placebo infusion, and those initially given a placebo now receiving 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.

Additional information

PF-06939926 received orphan drug designation by the U.S. Food and Drug Administration (FDA) and an orphan medicinal product designation by the European Medicines Agency (EMA) in May 2017.


Last updated: Jan. 14, 2021


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