What it is
SS-31 is a small tetrapeptide (four amino acids) engineered to concentrate specifically in the inner mitochondrial membrane, accumulating there at roughly 1,000 to 5,000 times the concentration found elsewhere in the cell. It is developed by Stealth BioTherapeutics under the name Elamipretide, and has been studied across a genuine clinical trial programme spanning Barth syndrome, primary mitochondrial myopathy, heart failure and age-related macular degeneration.
How it works
SS-31 binds and stabilises cardiolipin, a phospholipid in the inner mitochondrial membrane that holds the electron transport chain together. As cardiolipin degrades with age or disease, ATP production falls and reactive oxygen species rise. By stabilising cardiolipin, SS-31 is proposed to improve mitochondrial bioenergetics, reduce oxidative stress, and limit the release of cytochrome c that triggers cell death.
What the evidence shows
SS-31's distinguishing feature among "mitochondrial" peptides is that it actually has meaningful human trial data, not just animal and cell studies. That said, results across its trial programme have been mixed.
- In Barth syndrome, a rare genetic disorder of mitochondrial cardiolipin metabolism, a phase 2/3 randomised trial followed by open-label extension found improvements in the 6-Minute Walk Test and symptom scores after 48 weeks of treatment.
- It has FDA Rare Pediatric Disease designation for Barth syndrome, reflecting genuine regulatory engagement, though designation is not the same as approval.
- Trials in heart failure and age-related macular degeneration have not uniformly succeeded; some trial endpoints were not met, which is part of why approval has not followed despite years of study.
- One review summarising 18 human clinical trials of SS-31 describes "both successful and unsuccessful results," underlining that the picture is genuinely mixed rather than uniformly positive.
Reported side effects
Across trials, SS-31 has generally been described as well tolerated. Injection site reactions are the most commonly reported adverse effect.
Where it stands
SS-31 occupies an unusual middle ground: more rigorously tested in humans than most research peptides, with a credible mechanism and genuine clinical trial infrastructure behind it, but without an approval to show for it yet. It remains a research compound, not an approved therapy.