After a rocky regulatory pathway filled with delays and a previous rejection, the US Food and Drug Administration (FDA) has approved Stealth BioTherapeutics’ Forzinity (elamipretide HCl) for the treatment of Barth syndrome.
Adult and paediatric patients with the genetic disorder who weigh at least 30kg will be eligible for the mitochondria-targeting subcutaneous injection. Stealth said it will work with the FDA on a potential label expansion to include patients weighing less than 30kg.
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The FDA approved Forzinity via its accelerated approval pathway, a regulatory framework that allows expedited approval of drugs for more serious conditions. Stealth Bio’s drug becomes the first therapy approved to treat Barth syndrome.
Primarily affecting boys due to its X chromosome mutation pathogenesis, Barth syndrome typically starts with severe heart failure in infancy. Patients who survive into adolescence and adulthood often have fatigue, poor stamina and a shortened life expectancy.
Set against a backdrop of wider accelerated approval sentiment shifts at the FDA, the agency said that Forzinity was approved on evidence “considered reasonably likely to predict patient benefit but does not directly assess the benefit to the patient”.
Evidence came from the Phase II/III TAZPOWER clinical trial (NCT03098797). During the open-label portion of the trial, the drug improved the strength of the muscle used to straighten the leg at the knee. The agency considered this improvement reasonably likely to predict patient benefit, such as the ability to stand more easily or walk farther.
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By GlobalDataAs a condition of the accelerated approval, the FDA has tasked Stealth with conducting a post-approval randomised, double-blind, placebo-controlled trial to confirm that the changes seen in knee muscle strength translate into patient benefit.
The FDA greenlight brings a close to a tumultuous regulatory journey for Stealth Bio’s product. Lacking an “adequate and well-controlled” trial, Stealth Bio’s first NDA filing was not even accepted by the agency in 2021.
A new NDA was submitted in January 2024, which the FDA accepted, and that was later converted into a priority review. However, a rejection came in May 2025 following a 16.5-month priority review cycle that contained delays. The rejection also came despite a positive vote by an advisory committee (AdComm) meeting in October 2024. It was a double blow to Stealth Bio, which had to cut 30% of its workforce to conserve resources amid the regulatory wringer.
The to-and-fro between the FDA and Stealth Bio even caused seven members of Congress to ask for clarity from the agency’s commissioner, Marty Makary, about the drug’s regulatory path. A July 2025 letter from the members asked Makary several questions, including why the AdComm’s vote was not heeded, the rationale behind denying traditional approval for Forzinity, and wider concerns about the future access of drugs placed on accelerated approval.
The FDA’s Center for Drug Evaluation and Research (CDER) director George Tidmarsh said: “The FDA remains committed to facilitating the development of effective and safe therapies for rare diseases and will continue to work diligently to help ensure patients with rare diseases have access to innovative treatments.”
“We are grateful that FDA leadership has listened to our community and approved Forzinity for some of our population. Barth syndrome patients live every day with progressively diminishing quality of life,” said Kate McCurdy, board chair of the Barth Syndrome Foundation, whose son passed away from the disease at age 28.
McCurdy, however, said that more work needs to be done, highlighting that only around half of patients survive long enough to weigh the 30kg needed to qualify for treatment.
She concluded: “We deeply appreciate Stealth’s pledge to work closely with the FDA on prompt and broad label expansion, so that our youngest and most vulnerable patients can also gain access to this therapy.”
