ZURICH, Sept 23 (Reuters) - Novartis's bid to expand its $2.1 million-per-patient gene therapy Zolgensma to more spinal muscular atrophy patients faces a possible delay after U.S. regulators requested another study in older children getting the drug via a spinal infusion.

Novartis, whose shares fell 1% in early Wednesday trading, said the U.S. Food and Drug Administration's (FDA) request for an additional study was not linked to an ongoing safety hold on a Zolgensma trial after possible neurological damage in primates emerged last year.

The Basel-based company also said the move to satisfy the FDA would not affect the therapy's existing approval in babies up to two years with the rare genetic condition.

Still, an analyst said the FDA's request for more data - from children aged 2 to 5 years getting Zolgensma via spinal infusion - would delay Novartis's efforts to expand approval to at least late 2023, damaging financial prospects especially after Roche's rival oral medicine Evrysdi hit the market in August.

The development "causes us to reduce our peak revenue forecast to $1.2 million from $2.3 million," Zuercher Kantonalbank analyst Michael Nawrath told investors in a note, describing the FDA's call for a new study as "negative".

Novartis, which spent $8.7 billion for U.S.-based AveXis in 2018 to get Zolgensma, which is the world's most-expensive one-time treatment, said it remained confident in the gene therapy's benefit-risk profile.

"The FDA has acknowledged the potential of (Zolgensma) in this patient population and recommends a pivotal confirmatory study to supplement" existing data and support filing for approval, Novartis said. (Reporting by John Miller; Editing by Michael Shields and Mark Potter)