What is SS-31? A Mitochondria-Targeted Peptide Breakdown

Compound Spotlight

SS-31 is, as of 2025, one of the few compounds in this catalog to have earned FDA approval — and the story of how it got there is more instructive than the approval itself. It is a compound with an elegant, well-validated mechanism, a string of failed efficacy trials in common conditions, and a narrow approval in an ultra-rare disease. All three of those facts matter.

Cardiolipin, and why it is the target

Cardiolipin is an unusual phospholipid found almost exclusively in the inner mitochondrial membrane. Unlike typical phospholipids with two fatty acid tails, cardiolipin has four, giving it a conical shape that drives membrane curvature. That curvature is what forms cristae — the folded invaginations that pack respiratory machinery into a small volume.

Cardiolipin does more than shape the membrane. It physically organizes the electron transport chain complexes into supercomplexes, holding them in the arrangement that lets electrons pass efficiently from one to the next. When cardiolipin is damaged or depleted, that organization degrades, electrons leak, and leaked electrons generate reactive oxygen species — which damage more cardiolipin. It is a self-reinforcing failure loop, and it appears in aging tissue, heart failure, and inherited mitochondrial disease alike.

A structural mechanism, not an antioxidant

This is the distinction that makes SS-31 mechanistically interesting. Conventional antioxidants neutralize reactive oxygen species after they form. SS-31 works upstream: by binding cardiolipin and stabilizing membrane architecture, it reduces the electron leak that produces ROS in the first place.

The targeting is itself notable. SS-31 carries alternating aromatic and cationic residues, a structure that drives accumulation in the inner mitochondrial membrane at concentrations roughly a thousand times higher than in cytoplasm — and it does so without depending on membrane potential, which matters because damaged mitochondria have reduced membrane potential and would otherwise be hardest to reach. A mitochondrial therapeutic that fails at exactly the mitochondria most in need of help would be useless; SS-31 avoids that trap.

Reported downstream effects include restored cristae structure, improved supercomplex assembly, increased ATP production, reduced ROS, and inhibition of the mitochondrial permeability transition pore. It was co-discovered by Hazel Szeto at Cornell and Peter Schiller at the IRCM in Montreal.

The clinical record — honestly

Stealth BioTherapeutics developed SS-31 for roughly two decades across multiple indications. The record is mixed, and the failures are as informative as the success.

In primary mitochondrial myopathy, the MMPOWER-3 trial enrolled 218 participants and missed its primary endpoints. Work in heart failure and ischemia-reperfusion also failed to demonstrate the hoped-for efficacy. These were not small pilot studies — they were adequately powered trials in conditions where the mechanistic rationale seemed strong, and the compound did not deliver.

Barth syndrome is the exception. It is an ultra-rare X-linked disease affecting roughly 150 people in the United States, caused by TAZ gene mutations that directly disrupt cardiolipin remodeling. That is an unusually precise mechanistic match: a disease of cardiolipin, treated with a cardiolipin-binding compound. On 19 September 2025 the FDA granted accelerated approval to elamipretide as FORZINITY for improving muscle strength in Barth syndrome patients weighing at least 30 kg. It is the first FDA-approved mitochondria-targeted therapeutic.

Worth noting even here: the approval came via accelerated pathway after an advisory committee vote of 10 to 6, drawing on open-label extension data and a natural-history control study. The randomized portion of the Barth trial did not meet its endpoints either. This was a hard-won approval in a desperate rare-disease setting, not a decisive efficacy demonstration.

What the approval does and does not mean

The approval covers one ultra-rare genetic disease. It is not an endorsement of SS-31 for aging, athletic performance, general fatigue, heart health, or kidney function. The pattern — strong mechanism, strong animal data, disappointing results in large trials of common conditions, approval only where the genetic defect matches the mechanism exactly — is a recurring one in mitochondrial medicine, and it is the most useful thing to take from this compound’s history.

Across trials, the most consistently reported adverse effect was injection-site reactions: redness, pain, and induration. Systemic safety signals were generally minimal, which is part of why development continued as long as it did.

Frequently asked questions

Is SS-31 an antioxidant?

Not in the conventional sense. It does not primarily scavenge free radicals; it stabilizes cardiolipin and mitochondrial membrane structure, reducing the electron leak that generates reactive oxygen species upstream.

Since it is FDA approved, is it proven for general use?

No. The approval is specific to Barth syndrome, an ultra-rare disease with a genetic defect in cardiolipin remodeling. Trials in mitochondrial myopathy and heart failure failed their primary endpoints.

How does SS-31 differ from MOTS-c?

Entirely different mechanisms. SS-31 is a synthetic peptide that physically stabilizes the inner mitochondrial membrane. MOTS-c is a naturally occurring mitochondrially encoded signaling peptide that acts on cellular metabolic pathways such as AMPK.

How do I know what is actually in the vial?

Every lot we sell has a published Certificate of Analysis from an independent, third-party lab confirming identity and purity. Lot numbers on the vial should match the COA you are referencing.

References

• Szeto HH. First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics. Br J Pharmacol. 2014;171(8):2029–2050.
• Birk AV, et al. The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin. J Am Soc Nephrol. 2013;24(8):1250–1261.
• Karaa A, et al. Randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy. Neurology. 2018.
• Stealth BioTherapeutics. FDA accelerated approval of FORZINITY (elamipretide HCl), 19 September 2025.

View the SS-31 product listing →

Read our mitochondrial peptides guide →

Look up your lot Certificate of Analysis →

For laboratory and research use only. Not for human consumption. Elamipretide is FDA approved only for Barth syndrome under the brand name FORZINITY; material sold for research use is not an approved medicine. This article summarizes published research for informational purposes and is not medical advice, nor a recommendation or protocol for use.

Related research compounds

Arc Peptides supplies these compounds, each third-party tested for purity and identity with a lot-specific Certificate of Analysis:

• SS-31

For laboratory research use only. Not for human consumption.