– Design of Confirmatory Study in Barth Syndrome to Be Presented –

– Phase 3 NuPOWER Results Support Development of Elamipretide in POLG1 Subgroup –

NEEDHAM, Mass., May 26, 2026 /PRNewswire/ — Stealth BioTherapeutics (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 its planned presentations at the International Meeting on Mitochondrial Pathology (Euromit), which is being held May 31-June 4, 2026, in Angers, France. The Company will present the design of its confirmatory study of elamipretide in subjects with Barth syndrome. In addition, the Company and its research partners will present clinical and preclinical findings supporting the evaluation of elamipretide as a treatment for mitochondrial myopathy due to polymerase gamma (POLG) mutations.

“We are committed to advancing a new class of medicines that we believe will redefine what is possible for people living with mitochondrial disease,” said Reenie McCarthy, Chief Executive Officer of Stealth BioTherapeutics. “We are pleased to partner with scientific thought-leaders [and patients] during Euromit to align on future development pathways to address the unmet needs of patients worldwide.”

“POLG disease is a devastating genetic mitochondrial disorder that can lead to the dysfunction and failure of multiple organ systems, with symptoms ranging from vision problems and muscle weakness to epilepsy and liver failure. With no treatments beyond symptom management, this community desperately needs more options,” said Mary Kay Koenig, M.D., pediatric neurologist, Endowed Chair of Mitochondrial Medicine, and Associate Vice Chair for Clinical Research, Department of Pediatrics, at the University of Texas McGovern Medical School. “The Phase 3 NuPOWER data support the potential of elamipretide for individuals with POLG1 mutations, and I am enthusiastic about partnering with the Stealth team to advance a more targeted study in this genetically defined population.”

Stealth’s Euromit presentations are as follows:

Flash Talk and Poster: “Elamipretide: Clinical Manifestations and Phase 3b/4 Confirmatory Trial”

• Abstract Number: 00276
• Presenter: Anthony Abbruscato, Pharm.D., Senior Vice President of Clinical Development, Stealth BioTherapeutics
• Presentation Time: June 4, 2026 1:25-3:00pm
• Location: Angers Convention Centre
• Session 7: Therapeutic and Diagnostic Developments

Poster: “NuPOWER – Phase 3 Study of Elamipretide in Subjects with Primary Mitochondrial Disease: Overall Outcomes and Emerging Evidence of Genotype-Dependent Response in the POLG1 Subgroup”

• Abstract Number: 00302
• Presenter: Mary Kay Koenig, pediatric neurologist, Endowed Chair of Mitochondrial Medicine, and Associate Vice Chair for Clinical Research, Department of Pediatrics, at the University of Texas McGovern Medical School
• Presentation Time: June 4, 2026 1:25-3:00pm
• Location: Angers Convention Centre
• Session 7: Therapeutic and Diagnostic Developments

Flash Talk and Poster: “Beneficial Effects of Elamipretide in Preclinical Models of POLG Disease”

• Abstract Number: 00259
• Presenter: David A. Brown, Ph.D., Chief Scientific Officer, Stealth BioTherapeutics
• Presentation Time: June 4, 2026 1:25-3:00pm
• Location: Angers Convention Centre
• Session 7: Therapeutic and Diagnostic Developments

Euromit is the world’s leading international congress dedicated to mitochondrial disease research. Held every three years, Euromit brings together hundreds of scientists, clinicians and stakeholders from across the globe who are passionate about advancing mitochondrial medicine.

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
The Company’s mission is to develop novel therapies to improve the lives of patients living with diseases of mitochondrial dysfunction. The Company’s first commercial product was granted accelerated approval by the U.S. Food & Drug Administration (FDA) in September 2025. The Company is studying elamipretide in additional indications, including dry age-related macular degeneration and POLG-related myopathy, and is developing its next-in-class clinical-stage candidate, bevemipretide, for ophthalmic and neurological disease indications. The Company’s preclinical pipeline candidates include SBT-589 and SBT-255, which are both progressing through developmental milestones.

Stealth Media Contact
Ascent Strategic Communications
Anna Stallmann
anna@ascentcomms.com

Stealth Investor Contact
Precision AQ
Austin Murtagh
Austin.Murtagh@precisionaq.com

Stealth Patient Advocacy
Patient.Advocacy@stealthbt.com

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SOURCE Stealth BioTherapeutics Inc.