The FDA has paused Rocket Pharmaceuticals' mid-stage gene therapy trial after a patient died from serious complications.

The decision was taken after the patient, who was treated in early May, faced difficulties due to capillary leak syndrome, in which blood vessels leak fluids and proteins, causing swelling and low blood pressure, the company said.

Rocket was testing its experimental gene therapy, RP-A501, for the treatment of Danon disease, a rare genetic disorder that causes heart muscle damage and progressive muscle weakness.

The company said it was investigating the cause of death, with a focus on a new immune suppression agent that was recently added to the treatment regimen to enhance patient safety.

The immune suppressor was administered to two patients and both of them showed capillary leak syndrome, the company said.

CEO Gaurav Shah said Rocket is "carefully evaluating whether a mechanism related to the new agent may have influenced immune responses in an unexpected or paradoxical way."

The incident supports the notion that serious safety events are unpredictable with gene therapies, making it challenging to invest in them broadly, Jefferies analyst Andrew Tsai said.

In March, Sarepta Therapeutics had reported a teen's death after receiving its gene therapy for muscular dystrophy.

Rocket declined to provide a timeline for the completion of the trial until the clinical hold is lifted and said it is working with the FDA and experts. The FDA did not respond for comment.