NEEDHAM, Mass., September 19, 2025 – Stealth BioTherapeutics Inc. today announced that the U.S. Food and Drug Administration (FDA) has granted accelerated approval to FORZINITY™ (elamipretide HCl), marking the first approved treatment for Barth syndrome, a progressive and life-limiting ultra-rare genetic disease caused by mitochondrial dysfunction. This groundbreaking decision provides new hope to a patient population of fewer than 150 individuals in the United States, most of whom are children and adolescents facing severe muscle weakness, heart complications, and shortened life expectancy.
Science Significance
Barth syndrome is an ultra-rare, X-linked genetic disorder characterized by mitochondrial abnormalities that impair energy production and lead to exercise intolerance, debilitating fatigue, skeletal muscle weakness, heart failure, recurrent infections, and delayed growth. FORZINITY is the first mitochondria-targeted therapeutic ever approved by the FDA, representing a critical milestone for the field of mitochondrial medicine.
The approval is supported by data from the TAZPOWER clinical trial, where patients experienced improvements in knee extensor muscle strength, an intermediate endpoint reasonably likely to predict clinical benefit. The most common adverse reactions were injection-site reactions, which can be managed with oral antihistamines or topical corticosteroids. Continued approval will depend on verification of clinical benefit in a confirmatory trial.
Regulatory Significance
This decision follows months of close engagement between Stealth and the FDA after the company received a complete response letter in May 2025. By granting accelerated approval, the FDA enabled earlier patient access while requiring ongoing data collection. FORZINITY carries Orphan Drug, Fast Track, Priority Review, and Rare Pediatric Disease designations, highlighting the agency’s commitment to facilitating treatments for underserved patient populations.
As part of the approval, Stealth received a Rare Pediatric Disease Priority Review Voucher, a valuable incentive that can be used to expedite future FDA reviews or sold to other companies.
Business Significance
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The approval of FORZINITY transforms Stealth BioTherapeutics into a commercial-stage biopharmaceutical company with a marketed product, strengthening its position in the rare-disease space and validating its mitochondria-targeted platform. The company plans to make FORZINITY available through a specialty pharmacy in the United States by year-end and will work with payers and providers to secure coverage and equitable patient access.
Beyond Barth syndrome, Stealth is advancing a pipeline of mitochondrial therapies, including candidates for dry age-related macular degeneration, primary mitochondrial myopathy, and neurological disorders. The accelerated approval and accompanying pediatric voucher provide both strategic and financial advantages, positioning the company for future growth.
Patients’ Significance
For patients and families confronting Barth syndrome, FORZINITY represents the first approved treatment and a critical lifeline. The therapy is indicated for adult and pediatric patients weighing at least 30 kilograms (≈66 pounds). Many patients, however, are too young or small to meet this threshold.
Stealth has pledged to maintain expanded access programs and compassionate use for children under 30 kg while working with the FDA to gather data for potential label expansion. Patient advocates, including the Barth Syndrome Foundation, hailed the approval as a “pivotal victory” but emphasized the urgent need for broader access to reach the most vulnerable patients.
“Half of our patients may not survive long enough to meet the current weight requirement,” noted Kate McCurdy, Board Chair of the Foundation, highlighting the importance of continued regulatory engagement.
Policy Significance
The FDA’s decision underscores the value of regulatory flexibility and orphan incentives in addressing diseases with extremely small patient populations. The agency’s use of the accelerated approval pathway reflects a growing recognition that traditional clinical trial frameworks are often impractical for ultra-rare diseases.
This case also demonstrates how programs such as Priority Review Vouchers and Orphan Drug Designation can de-risk investment and encourage small biotechnology companies to develop transformative therapies despite limited commercial markets. The collaborative approach between Stealth, patient advocates, and the FDA sets a precedent for future therapies targeting other mitochondrial disorders and ultra-rare conditions.
Transaction Highlights
Under the FDA’s accelerated approval, FORZINITY™ (elamipretide HCl) is indicated to improve muscle strength in adult and pediatric patients with Barth syndrome who weigh at least 30 kg. Approval was based on data from the TAZPOWER trial, which showed meaningful gains in knee extensor muscle strength. Stealth will continue to provide expanded access for children below 30 kg and will work with the FDA to gather additional evidence for potential label expansion.
The company also received a Rare Pediatric Disease Priority Review Voucher, a valuable regulatory asset that can be used to speed future product reviews or monetized to support additional R&D efforts. FORZINITY is expected to be available for prescription in the United States by the end of 2025 through a specialty pharmacy distribution model.
Source: Stealth BioTherapeutics Inc. Press Release
