Novartis has recorded two deaths after treatment with its spinal muscular atrophy gene therapy Zolgensma, once again bringing gene therapy’s safety into attention.

Two children in Russia and Kazakhstan died about five to six weeks after receiving Zolgensma, Novartis confirmed to Fierce Pharma. Both patients died of acute liver failure, a known side effect of Zolgensma that’s included in a boxed warning on the one-time therapy’s label. Both had received corticosteroids to restore liver function.

While serious liver injury is a known risk of Zolgensma, these two are the first reports of fatal cases, Novartis said. The deaths were first reported by Stat. Novartis will ask regulators, including the FDA, to update Zolgensma’s labeling to specify that fatal acute liver failure has been reported, Novartis said in a statement.

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