ZURICH: Novartis will launch a new phase three study to expand the use of Zolgensma - the world's most expensive one time therapy at $2.1 million per patient treatment - after the US regulator lifted its restrictions.

The Swiss company will study the use of the drug for patients aged between 2 and 18 for the treatment of spinal muscular atrophy, it said on Tuesday.

The decision comes after US Food and Drugs Administration said existing trials may proceed, thereby lifting the partial clinical trial hold imposed in October 2019, after Novartis supplied new data.

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Novartis said it March it was laying off 400 people and closing a US gene therapy location as sales momentum for Zolgensma stalled last year.

Zolgensma's sales have been hurt, in part, by blocked efforts to expand its use in older patients after safety concerns prompted Novartis to pause a trial in order to collect more data for the US Food and Drug Administration.