The drug company Novartis reported yesterday (August 11) that two children suffering from spinal muscular atrophy—a rare, frequently fatal muscle-wasting disease—died of acute liver failure within five to six weeks of taking the gene therapy Zolgensma, several outlets report. Novartis has notified health authorities, including the US Food and Drug Administration (FDA), and other healthcare professionals about the deaths, which occurred in Russia and Kazakhstan, according to Reuters.

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“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 wrote in an emailed statement, reports Bloomberg.

Zolgensma, which the FDA approved in 2019, is an expensive, one-time gene therapy that replaces the faulty gene responsible for spinal muscular atrophy with a functional copy. No deaths occurred in the clinical trials evaluating the safety and efficacy of the treatment, but a subsequent analysis published in Science Translational Medicine raised concerns that elevated liver enzymes noted in the trial data could represent a safety hazard.   

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“We will be updating the labeling to specify that fatal acute liver failure has been reported,” a Novartis spokesperson tells STAT.