Ultragenyx Pharmaceutical (RARE) said Friday the US Food and Drug Administration has issued a complete response letter for its biologics license application for UX111, a gene therapy targeting Sanfilippo syndrome type A, a rare, inherited genetic disorder.

The FDA's letter cited the need for additional information and improvements related to chemistry, manufacturing and controls, as well as observations from recent manufacturing facility inspections, the company said in an SEC filing.

The issues are related to facilities and processes and not the product's quality, the company said, adding that it believes the observations are "readily addressable."

The agency also requested updated clinical data from ongoing patients to be included in the resubmission.

Ultragenyx said it plans to work closely with the FDA over the coming months to resolve the concerns and expects to resubmit the biologics license application, anticipating a review period of up to six months after resubmission.