ZURICH (Reuters) – Novartis scientists learnt months ago about safety concerns surrounding its gene therapy Zolgensma but delayed telling the U.S. Food and Drug Administration (FDA) due to what the Swiss drugmaker said was a “mistake”.

The FDA on Wednesday placed a partial hold on a Novartis trial of the $2.1 million-per-patient gene therapy after the company informed the agency about a primate study that raised concerns of possible nerve damage or loss.

Novartis, under fire since August for not telling regulators about separate data manipulation allegations until after Zolgensma was approved, said scientists at its AveXis unit learnt of the primate study’s findings in March.

AveXis, which is developing Zolgensma, planned to include the findings in an annual update planned for September to the FDA but never delivered the news, Novartis said.

“Unfortunately, a mistake was made, and this update was not implemented,” Novartis said in a statement on Friday. “We notified investigators and FDA at the end of last week and were notified about the partial clinical hold earlier this week.”

The delay was first reported this week by several publications, including industry website Fierce Pharma.

Zolgensma is already approved in the United States for children aged up to two with the deadly muscle wasting disease spinal muscular atrophy (SMA) and is given by infusion into the young patients’ veins.

The therapy’s approval in Europe and Japan has been pushed back to next year.

The trial now on hold, called STRONG, includes patients aged up to five who were to receive a higher dose of the gene therapy via a spinal, or intrathecal, infusion. The study is a key part of Novartis plan to expand use in older patients, where the company hopes to compete with a rival treatment from Biogen.

Novartis is hoping to quickly resume the STRONG trial, which has already dosed 32 patients.

“Our discussions with FDA are ongoing and we will work diligently with FDA to identify any additional actions necessary to resume dosing,” Novartis said.

(Reporting by John Miller; Emelia Sithole-Matarise)