A young patient died due to cardiac arrest after receiving Pfizer Inc’s (NYSE:PFE) experimental gene therapy in a mid-stage trial for Duchenne muscular dystrophy (DMD), the company told in a letter to Parent Project Muscular Dystrophy.

The boy was enrolled in Phase 2 DAYLIGHT study studying the treatment in boys at least two years old and under four years old. 

The patient received the investigational gene therapy, fordadistrogene movaparvovec (PF-06939926), in early 2023.

Citing Pfizer, Reuters highlighted that the boy died of cardiac arrest but that it was still working to understand what happened and the exact cause.

All participants will be followed in the study for five years after treatment with gene therapy, initiated in August 2022 and estimated to be completed in early 2029, as per the data from Clinicaltrial.gov.

Pfizer and the independent external data monitoring committee said that the data is being reviewed to understand the potential cause.

The gene therapy candidate is also being tested in another late-stage DMD study, CIFFREO Phase 3, in patients in boys at least 4 years old and less than eight years old.

There is not an impact to our expectation of having late-stage results, the company told Reuters in its email.

“We anticipate potentially beginning the primary analysis of the Phase 3 CIFFREO trial of fordadistrogene movaparvovec at the end of this month and sharing top-line results relatively soon,” it added.

Pfizer has decided to pause dosing associated with the cross-over portion of the CIFFREO study. 

Beyond CIFFREO, the dosing pause does not apply to other ongoing trials in the fordadistrogene movaparvovec program as dosing has been completed in those studies.

It is the second death to be associated with fordadistrogene movaparvovec during a clinical study.

In December 2021, Pfizer paused screening and dosing in a Phase 1b trial assessing fordadistrogene movaparvovec after the death of a young man.

The FDA lifted the clinical hold in April 2022.

Price Action: PFE shares are up 1.31% at $28.14 at last check Wednesday.

Photo via Shutterstock