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SS-31

SS-31 (elamipretide) is a small four-amino-acid peptide that binds cardiolipin, a fat in the inner membrane of mitochondria, and is meant to improve how mitochondria work. In September 2025 the FDA approved it as Forzinity, an injection for Barth syndrome, a rare inherited disease. Trials in other conditions have mostly missed their main goals.

ApprovedChecked October 2026
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Approval status

On September 19, 2025, FDA granted accelerated approval to Forzinity (elamipretide) injection from Stealth BioTherapeutics, to improve muscle strength in adults and children with Barth syndrome who weigh at least 30 kg. Approval rests on knee-extensor strength, and continued approval may depend on a confirmatory trial. In the EU it has orphan designation for Barth syndrome, which is not a marketing authorization.12109

What it’s used or studied for

  • Approved use

    FDA accelerated approval (September 2025) to improve muscle strength in Barth syndrome, for patients weighing at least 30 kg; the benefit must be confirmed in a further trial.12

  • Human studies

    In the main Barth syndrome trial, the placebo-controlled phase did not improve walking distance or symptoms; improvements appeared in the open-label extension.45

  • Human studies

    A phase 3 trial in primary mitochondrial myopathy found no improvement in walking distance or fatigue compared with placebo.6

  • Human studies

    A phase 2 trial in dry age-related macular degeneration with geographic atrophy missed its main goals; some retinal imaging measures favored the drug.7

  • Human studies

    A phase 2 trial in heart failure with reduced ejection fraction found no improvement in heart volume or pumping function compared with placebo.8

Each label shows how strong the evidence is: an approved use, human studies, animal or lab studies only, or a popular claim with no published studies. Being studied is not the same as being proven safe or effective.

Side effects

Common

  • Injection site redness
  • Injection site pain
  • Injection site hardening
  • Injection site itching
  • Injection site bruising or hives
  • Raised eosinophils (a type of white blood cell)

Serious or rare

  • Serious allergic reactions, including rash, eczema-like skin reactions and cough
  • Benzyl alcohol toxicity (gasping syndrome) in newborns; not approved for neonates

From the FDA label, based mainly on 12 patients with Barth syndrome. Injection site reactions were also the most common events in larger trials in other conditions. The rise in eosinophils was not linked to symptoms and settled over time.27

Cautions

  • Do not use after a serious allergic reaction to elamipretide or any ingredient of the product.2
  • Forzinity contains benzyl alcohol as a preservative; it is not approved for newborns or for IV use.2
  • Kidney impairment raises blood levels of the drug and its breakdown products.2
  • Signs of overdose are likely histamine-related, such as low blood pressure or feeling faint.2

Sources

  1. FDA news release: FDA grants accelerated approval for first treatment for Barth syndrome (September 19, 2025)
  2. FDA label: Forzinity (elamipretide) injection, for subcutaneous use
  3. PubChem: Elamipretide (CID 11764719)
  4. Reid Thompson W et al. A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome. Genet Med. 2021
  5. Thompson WR et al. Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER. Genet Med. 2024
  6. Karaa A et al. Efficacy and safety of elamipretide in individuals with primary mitochondrial myopathy: the MMPOWER-3 randomized clinical trial. Neurology. 2023
  7. Ehlers JP et al. ReCLAIM-2: a randomized phase II clinical trial evaluating elamipretide in age-related macular degeneration. Ophthalmol Sci. 2025
  8. Butler J et al. Effects of elamipretide on left ventricular function in patients with heart failure with reduced ejection fraction: the PROGRESS-HF phase 2 trial. J Card Fail. 2020
  9. EMA: Orphan designation EU/3/21/2430, elamipretide for Barth syndrome
  10. Drugs@FDA: NDA 215244, Forzinity (elamipretide hydrochloride)

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