(Bloomberg) -- Two children who received Zolgensma, Novartis AG’s gene therapy for a rare condition called spinal muscular atrophy, died from liver complications.

The children died from acute liver failure about six weeks after receiving the infusion of Zolgensma and one to 10 days after tapered doses of a steroid that’s meant to combat liver complications, Novartis said in an emailed statement Thursday. The deaths occurred in Russia and Kazakhstan. 

Liver toxicities have dogged genetic treatments, and regulators had warned that Zolgensma could cause serious and potentially lethal liver complications. These are the first two instances of death from acute liver failure in patients who received Zolgensma, Novartis said. As a result, the company will update the drug’s label to specify that the side effect has resulted in death.

“While this is clinically important safety information, it is not a new safety signal and we firmly believe in the overall favorable risk/benefit profile of Zolgensma, which to date has been used to treat more than 2,300 patients worldwide across clinical trials, managed access programs, and in the commercial setting,” a Novartis spokesperson said. 

Zolgensma corrects a genetic defect that causes spinal muscular atrophy, and may prove to be a one-time treatment for babies and toddlers with the disease, in which muscles weaken and eventually deteriorate. Babies born with severe forms of the disease typically die by their second birthday. Zolgensma was approved in the US in 2019. 

Stat reported the deaths earlier Thursday. 

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