Regenxbio (RGNX) said late Monday the US Food and Drug Administration did not approve its biologics license application for RGX-121 to treat Mucopolysaccharidosis II, an ultra-rare neurodegenerative disease known as Hunter syndrome.

The firm said it received a Complete Response Letter from the FDA, citing several reasons for not approving the gene therapy, including uncertainty over the study eligibility criteria to adequately define a population with neuronopathic disease.

"This decision is devastating for the families of boys living with this progressive, life-threatening disease," said Chief Executive Officer Curran Simpson.

"We are concerned about FDA's feedback regarding the overall development path and evaluation of the data in the context of the urgent need for this irreversible ultra-rare disease."

The letter outlined potential measures, including conducting a new study, treating more patients, the company said.

Regenxbio said it intends to request a Type A meeting to discuss the letter and a planned resubmission of the application.

Shares were about 17% lower in after-hours trading.