Stealth BioTherapeutics Provides Commercial Launch Update and Pipeline Progress Across Mitochondrial Disease Portfolio

– FORZINITY™ (elamipretide) injection early commercial momentum demonstrates strong patient uptake and rapid access timelines –

– Advancing differentiated mitochondrial-targeted pipeline across rare and age-related indications –

NEEDHAM, Mass., April 30, 2026 /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 provided an update on the early commercial demand for FORZINITY and highlighted continued progress across its clinical and research pipeline.

"We are encouraged that the strong early momentum of the FORZINITY launch is facilitating prompt access for patients living with Barth syndrome," said Reenie McCarthy, Chief Executive Officer of Stealth BioTherapeutics. "We continue to advance our pipeline in rare and age-related diseases of mitochondrial dysfunction, including dry age-related macular degeneration, positioning Stealth at the forefront of the emerging therapeutic modality of mitochondrial medicine."

FORZINITY is currently commercially available for eligible U.S. patients weighing at least 30kg who are living with Barth syndrome (BTHS), a rare, serious, and life-threatening mitochondrial disease, with early launch performance demonstrating strong uptake and efficient patient onboarding to therapy.

• 33 patients have initiated therapy with 85% of patients fully covered, reflecting meaningful early adoption across treatment centers.
• Across the payer universe, we are seeing broad reimbursement access and strong coverage.
• Average time to therapy initiation is less than 30 days, from start form submission to prescription fulfillment, highlighting effective coordination across access channels.
• The Mito Assist™ patient support program has been broadly adopted, with 100% of eligible patients with FORZINITY prescriptions enrolled, providing support throughout the treatment journey.
• The Company's field medical team deployed in early April, and the sales team, with extensive pharma and biotech experience including rare disease, will launch in early May.
• Medical and commercial organization buildout is nearing completion, including hiring across sales, market access, and promotional functions to support continued launch execution.

In parallel with commercial execution, the Company continues to advance its development of elamipretide and other mitochondria-targeted pipeline compounds for diseases of mitochondrial dysfunction.

• BTHS. The Company met and aligned with the U.S. Food and Drug Administration (FDA) on a pathway toward potential label expansion for individuals living with BTHS who do not currently meet the approved weight threshold for FORZINITY. This entails a 1-month pharmacokinetic study to inform dosing for children weighing less than 30 kilograms, which the Company plans to initiate this year to inform a potential 2027 sNDA submission for children as young as age 5.  For infants and younger children, the FDA encouraged continued provision of access through the Company's expanded access program (EAP), with a plan to initiate a registry to standardize data collection to better inform FDA review. The Company has initiated the first site in its Phase 4 trial, which is in Bristol, UK, and is meeting all FDA-required post-marketing commitments to further confirm clinical benefit.
• POLG-related myopathy. The Company had a constructive engagement with the FDA in mid-April 2026 to discuss the regulatory pathway for elamipretide in mitochondrial myopathy due to polymerase gamma (POLG) mutations. At the FDA's request, the Company will submit a synopsis for its proposed new pivotal trial for further discussion with FDA during the second half of the year.
• Dry AMD. The Company's Phase 3 ReNEW clinical trial of elamipretide for dry age-related macular degeneration (AMD) remains on track, with recent FDA guidance for drug development supporting the potential for a single trial to serve as the basis for approval. The Company has also completed enrollment in its Phase 1 trial of bevemipretide eye drops, with preliminary data expected at the 2026 ARVO Annual Meeting and Phase 2 initiation planned for late 2026.
• Pipeline. The Company continues to advance its broader pipeline of investigational mitochondria-targeted therapies, including participation in the XPRIZE Healthspan competition evaluating elamipretide in age-related functional decline, ongoing scientific presentations in aging research, and newly funded preclinical work on SBT-589 in Leigh syndrome through a collaboration supported by the UK Medical Research Council.

About FORZINITY™ (elamipretide) injection

INDICATION

FORZINITY™ is a mitochondrial cardiolipin binder 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).

IMPORTANT SAFETY INFORMATION

Contraindications
FORZINITY is contraindicated in patients with serious hypersensitivity to elamipretide or any of the excipients in FORZINITY.
Warnings and Precautions
Benzyl Alcohol Toxicity – Do Not Use in Neonates
Serious and fatal reactions (including metabolic acidosis progressing to neurotoxicity and gasping syndrome) have occurred in preterm and low–birth weight neonates receiving benzyl alcohol (BA)-containing drugs.
FORZINITY contains 20 mg BA/mL and is not approved for use in neonates or for IV administration.

Hypersensitivity
Reported reactions include rash, papular lesions, eczema/dermatitis, cough, and serious allergic reactions requiring emergency medical intervention.
Reactions may occur minutes to months after starting treatment. Monitor patients for signs and symptoms during treatment.
Discontinue FORZINITY permanently if a serious hypersensitivity reaction occurs.

Adverse Reactions
Adverse reactions occurring more commonly on FORZINITY than on placebo included injection site reactions such as injection site erythema, pain, induration, pruritus, bruising, and urticaria.

Eosinophilia
Modest elevations in eosinophil counts (peak ~90 days) occurred but were not associated with clinical symptoms.

To report SUSPECTED ADVERSE REACTIONS, contact FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.

Please see Full Prescribing Information for FORZINITY.

About Barth Syndrome
BTHS is an ultra-rare genetic condition characterized by mitochondrial abnormalities leading to muscle weakness, exercise intolerance, 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. BTHS occurs primarily in males and is estimated to affect one in 1,000,000 male births. There are no EMA-approved therapies for patients with BTHS.

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.

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SOURCE Stealth BioTherapeutics Inc.