What is SS-31?
SS-31 is a four-amino-acid peptide. The SS stands for Szeto-Schiller, after Hazel Szeto and Peter Schiller, who designed a family of small peptides that alternate positively charged and aromatic amino acids. That pattern lets SS-31 cross cell membranes and collect at the inner membrane of mitochondria. It was later developed as a drug candidate by Stealth BioTherapeutics under the name elamipretide.
In September 2025, the US FDA granted elamipretide accelerated approval, as Forzinity, to improve muscle strength in people with Barth syndrome, a rare genetic condition that disrupts cardiolipin. Accelerated approval rests on an early measure of benefit and depends on further confirmation. It is given as an injection in its approved use.
How does SS-31 work?
The inner membrane of mitochondria is where cells run the chain of reactions that produces ATP, their main energy molecule. A fat called cardiolipin holds the proteins of that chain together and keeps the membrane folded correctly. SS-31 binds to cardiolipin. Researchers think this helps stabilize the membrane, protects cardiolipin from oxidative damage and lets the energy-making machinery work more efficiently, with less leakage of reactive oxygen species. Unlike general antioxidants, it is aimed at the site where much of this damage starts.
What has research looked at?
- Barth syndrome
- In this rare disease of faulty cardiolipin, a controlled trial missed its main goal, but longer open-label treatment was linked to better muscle strength and walking distance. The muscle strength results supported the 2025 approval.
- Primary mitochondrial myopathy
- A large phase 3 trial in adults with inherited mitochondrial muscle disease did not meet its main goals of improving walking distance and fatigue.
- Heart failure
- Small human trials in heart failure, and one in heart attack, mostly did not show a clear benefit, while animal studies more consistently showed better heart function.
- Aging in animals
- In aged mice, researchers reported improved heart and muscle function after treatment.
- Eye and kidney disease
- Trials in dry age-related macular degeneration and small studies in kidney disease have been run, with early or mixed results.
SS-31 has more human data than most mitochondrial peptides, including large controlled trials. Those trials suggest it is generally tolerated, but strong animal results have often not carried over into clear benefit in people. Its 2025 approval covers a single rare disease.
Side effects and cautions
Across clinical trials, elamipretide has generally been well tolerated. The most common side effects are injection site reactions, such as redness, itching, pain or swelling, which occasionally led people to stop treatment. Headache and dizziness have also been reported. Most human safety data come from people with serious diseases, so effects in healthy people over the long term are less well known. Anyone with a heart, kidney or mitochondrial condition should talk to a qualified clinician.
- Injection site reactions
- Redness, itching, pain or swelling where it is injected are the most frequently reported effects.
- Headache and dizziness
- These were reported in some trials and were usually mild.
- Allergic reactions
- Allergic reactions have been reported, in some cases serious enough to need emergency care.
- Limited data in healthy people
- Trials focused on people with mitochondrial, heart, eye or kidney disease, so effects in healthy people are largely unknown.
Further reading
This page summarizes the research in plain words. To read the studies yourself:
- Published studies on SS-31 PubMed, US National Library of Medicine
- Registered human trials of SS-31 ClinicalTrials.gov
