NEEDHAM, Mass., Sept. 19, 2025 /PRNewswire/ — Stealth BioTherapeutics Inc. (the “Company” or “Stealth”), a commercial-stage biotechnology company focused on the discovery, development and commercialization of novel therapies for diseases involving mitochondrial dysfunction, today announced that the U.S. Food and Drug Administration (FDA) has granted accelerated approval to FORZINITY™ (elamipretide HCl) to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kilograms (kg) (approximately 66 pounds). Barth syndrome is a life-limiting pediatric mitochondrial cardioskeletal disease that affects approximately 150 individuals in the United States.
“The approval of FORZINITY, the first treatment option for Barth syndrome and the first approved mitochondria-targeted therapeutic, is a pivotal victory for the Barth syndrome community and offers hope for expedited regulatory attention to other ultra-rare diseases,” said Reenie McCarthy, Stealth’s Chief Executive Officer. “We appreciate the FDA’s close engagement in recent months and are grateful to the trial participants, caregivers, advocates, researchers and healthcare providers who persevered in partnership with us over this decade-long journey. We plan to continue providing expanded access to children weighing less than 30 kilograms who are currently receiving treatment or require emergency access, while we work with the FDA to generate data needed to expand the indication to include these children. We are committed to the continued development of therapies to treat all patients with Barth syndrome and other devastating diseases of mitochondrial dysfunction.”
The approval of FORZINITY is supported by the efficacy and safety data from the TAZPOWER clinical trial. During the open-label portion of the TAZPOWER trial, knee extensor muscle strength improved from study baseline. The most common adverse reactions were injection site reactions which can be treated with oral antihistamines or topical corticosteroids. Continued approval for this indication may be contingent upon verification of clinical benefit in a confirmatory trial.
This decision follows months of collaborative dialogue with the FDA to resolve the final regulatory milestones following Stealth’s May 2025 receipt of a complete response letter from the FDA. The approval reflects a shared commitment to ensuring timely access for patients facing this devastating disease. Many of these patients and their families, along with their healthcare providers, have worked tirelessly to educate the FDA about the significant burden of this ultra-rare disease.
“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. “I witnessed the terrible toll this disease took every day of my son’s life and the many serious medical challenges that ensued. Our patients and their physicians have seen the truly positive impact FORZINITY can have on the devastating muscle weakness that restricts daily activities. While we celebrate this critical milestone, we are deeply aware that only half of our patients survive long enough to weigh the 30 kilograms they must in order to be eligible for this treatment now. Therefore, 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.”
“I am thrilled to have an FDA-approved treatment to offer to patients with Barth syndrome, who often face serious manifestations including severe muscle weakness,” said Hilary Vernon, M.D., Ph.D., Professor of Genetic Medicine at Johns Hopkins University School of Medicine and Founder and Director of the Barth Syndrome Clinic at the Kennedy Krieger Institute. “As the director of one of only two interdisciplinary Barth syndrome clinics worldwide, I have the privilege of interacting with a large percentage of the Barth community in the U.S. and around the world, and I am grateful to have a new therapeutic option available for patients living with Barth syndrome. I look forward to chairing the Trial Scientific Review Committee for the post-marketing trial as I know there is tremendous interest in participation from patients around the world.”
FORZINITY received Orphan Drug, Fast Track, Priority Review, and Rare Pediatric Designations from the FDA and Orphan Drug Designation from the European Medicines Agency (EMA) for the treatment of Barth syndrome. In connection with the approval, Stealth has been granted a Rare Pediatric Disease Priority Review Voucher from the FDA.
The approval is limited to children and adults weighing at least 30 kg. Stealth intends to work with the FDA on a plan to collect additional data in children weighing less than 30 kg and to qualify its preservative-free formulation used in expanded access for newborns. Pending potential label expansion and qualification of the preservative-free formulation, Stealth will continue to provide compassionate use access for patients weighing less than 30 kg currently enrolled in its expanded access program or for whom emergency access is necessary.
Stealth is committed to ensuring uninterrupted access for all current patients living with Barth syndrome. Stealth plans to work with payers and providers to ensure timely and equitable access to FORZINITY, which it expects will be available for prescriptions in the United States through a specialty pharmacy by year-end. Stealth expects to announce its patient support and access initiatives in the coming weeks.
About FORZINITY™ (elamipretide HCl)
INDICATION
FORZINITY™ is indicated to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg.
This indication is approved under accelerated approval based on an improvement in knee extensor muscle strength, an intermediate clinical endpoint. Continued approval for this indication may be contingent upon verification and description of clinical benefit in a confirmatory trial(s).
CONTRAINDICATIONS
Serious hypersensitivity to any of the ingredients.
WARNINGS AND PRECAUTIONS
Benzyl Alcohol Toxicity: Do not use in neonates
ADVERSE REACTIONS
Most common adverse reactions are injection site reactions.
To report SUSPECTED ADVERSE REACTIONS, contact Stealth BioTherapeutics Inc. at 1-844-444-6486 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.
For more information about FORZINITY, please see the full US Prescribing Information.
About Barth Syndrome
Barth syndrome is an ultra-rare genetic condition characterized by mitochondrial abnormalities leading to exercise intolerance, muscle weakness, debilitating fatigue, heart failure, recurrent infections, and delayed growth. The disease is associated with reduced life expectancy, with 85% of early deaths occurring by age 5. Barth syndrome occurs primarily in males and is estimated to affect one in 1,000,000 males births or around 150 individuals in the United States. There are no EMA-approved therapies for patients with Barth syndrome.
About Stealth BioTherapeutics
Stealth BioTherapeutics’ mission is to develop novel therapies to improve the lives of patients living with diseases of mitochondrial dysfunction. Stealth’s commercial product, FORZINITY, was granted accelerated approval by the U.S. Food & Drug Administration (FDA) in September 2025 as the first FDA-approved treatment for Barth syndrome, as well as the first FDA-approved mitochondria-targeted therapeutic. Continued approval for this indication may be contingent upon verification of clinical benefit in confirmatory trial(s). Stealth is studying elamipretide in additional indications, including dry age-related macular degeneration and primary mitochondrial myopathy, and is developing its second-generation clinical-stage candidate, bevemipretide (SBT-272), for ophthalmic and neurological disease indications.
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