FDA grants accelerated approval to elamipretide (SS-31) as Forzinity for Barth syndrome

The FDA's approval letter to Stealth BioTherapeutics, signed on 19 September 2025, approves Forzinity (elamipretide) injection under accelerated approval, section 506(c) of the Federal Food, Drug, and Cosmetic Act and 21 CFR 314.510. The letter says the application provides for use of Forzinity "to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg" ^[1]. The same letter granted the company a rare pediatric disease priority review voucher ^[1].

The prescribing information describes elamipretide as a mitochondrial cardiolipin binder that localises to the inner mitochondrial membrane ^[2]. Barth syndrome is a rare, X-linked genetic disorder, and the label notes that it is not likely to affect females ^[2]. A review in the journal Drugs described the decision as the first approval of a disease-specific treatment for Barth syndrome, and the first approval of elamipretide for any use ^[3].

This is the event behind searches for elamipretide FDA approval and SS-31 FDA approval. The approval applies to Forzinity, a sterile product made under an approved application, for one rare disease. It does not approve the SS-31 peptide sold online by research suppliers, and it does not cover the other uses discussed in our SS-31 guide.

The label's clinical studies section is unusually frank. The randomised part of the programme, TAZPOWER ^[4], was a double-blind, placebo-controlled crossover trial in 12 patients aged 12 or over, testing elamipretide 40 mg once daily under the skin for 12 weeks. Its primary end points were the six-minute walk test and a fatigue score, and the label states that elamipretide "was not superior to placebo" on either ^[2].

Knee extensor muscle strength, measured with a handheld dynamometer, was a secondary end point. The label says strength did not improve during the randomised trial but did improve during the open-label extension. Median strength rose by 34 newtons at extension week 12 and by 63 newtons at week 168 among the 8 patients still taking part, from a median baseline of 124 newtons ^[2]. The FDA treated this strength gain as an intermediate clinical end point, which is what accelerated approval allows ^{[1] [2]}.

A peer-reviewed report of the 168-week open-label extension found the drug well tolerated, with injection-site reactions the most common adverse events, and reported improvements from extension baseline in walking distance and some cardiac measures ^[4]. Open-label extensions have no control group, so these results carry less weight than a randomised comparison.

The label dose is 40 mg injected under the skin once daily for patients weighing at least 30 kg, reduced to 20 mg once daily in adults with an eGFR below 30 mL/minute who are not on dialysis ^[2]. It is supplied as 280 mg/3.5 mL (80 mg/mL) single-patient-use vials, and opened vials are discarded after 8 days ^[2]. These are label doses for a diagnosed rare disease, cited as information only.

In the placebo-controlled crossover, injection-site reactions occurred in all 12 patients on elamipretide and 8 of 12 on placebo. Erythema, induration, itching and pain were the most frequent ^[2]. The label warns of hypersensitivity reactions, including serious allergic reactions needing emergency treatment, and of benzyl alcohol toxicity in neonates, because the solution contains benzyl alcohol as a preservative. It also reports raised eosinophil counts that peaked around 90 days and then returned to baseline ^[2].

Accelerated approval is conditional. The letter requires Stealth to run a randomised, double-blind, placebo-controlled trial in patients aged 5 and over with Barth syndrome to verify the clinical benefit predicted by the muscle-strength results. The timetable in the letter puts study completion in September 2029 and the final report in March 2030. The letter states that the FDA may withdraw the approval if the trial fails to verify benefit or is not run with due diligence ^[1].

A phase 3b/4 study in Barth syndrome, 4TAZPower, is listed on ClinicalTrials.gov as recruiting, with an actual start date of 2 July 2026 and an estimated 48 participants ^[5]. The Drugs review also notes phase 3 development in dry age-related macular degeneration and mitochondrial myopathies ^[3]. None of those uses is approved.

Before September 2025, SS-31 had no approved form. It now has one, for one narrow indication, in the US ^[3]. Products listed online as SS-31 are not Forzinity, and the FDA's evidence for Forzinity does not transfer to them. Regulators have acted on this. An FDA warning letter dated 24 August 2026 listed a seller's "SS-31 (Elamipretide)" among unapproved new drugs ^[6], and a Health Canada advisory in April 2026 named SS-31 among unauthorised peptide products seized ^[7]. Our US legal status page and legal status overview summarise the rules by country.

If you are comparing listings, our SS-31 price page shows what suppliers claim and which publish third-party test certificates, and the guide to what research peptides are explains why a research-use label is not an approval. Elamipretide is sometimes discussed alongside MOTS-c, a different mitochondria-related peptide that this approval does not cover.