SS-31 (Elamipretide): The Mitochondrial Peptide, Its Trials, and Its FDA Status

What is SS-31 (elamipretide)?

SS-31 is a synthetic four-amino-acid peptide. The name is an abbreviation: it was the 31st in a series of "Szeto-Schiller" peptides designed by Hazel Szeto and Peter Schiller at Cornell to carry antioxidant activity directly into mitochondria. Its full sequence is D-Arg-2',6'-dimethyltyrosine-Lys-Phe-NH₂. That structure gives it two useful properties: a net positive charge and alternating aromatic and basic residues, which together let it cross cell membranes and accumulate in mitochondria without needing a membrane potential to pull it in.

When it moved into drug development at Stealth BioTherapeutics, the same molecule picked up new names — MTP-131, bendavia, and finally the international nonproprietary name elamipretide. Those are all one compound. This matters because "SS-31" is the label that dominates fitness and longevity forums, while "elamipretide" is the label on the clinical trials and the regulatory file. If you only read the forum name, you would never find the actual human data.

That data is unusually deep for a compound sold in peptide circles. Unlike BPC-157 or MOTS-c, which rest largely on animal work, elamipretide has been through multiple phase 2 and phase 3 human trials and, as of September 2025, holds one FDA approval. It is one of the very few "peptides" discussed alongside research chemicals that is also a genuine, approved medicine.

How SS-31 works: cardiolipin and the mitochondrial membrane

The whole story of SS-31 runs through one molecule: cardiolipin. Cardiolipin is a distinctive phospholipid found almost exclusively in the inner mitochondrial membrane, where it helps organize the folded membrane structures called cristae and anchors the protein complexes of the electron transport chain. When cardiolipin is damaged or abnormal, mitochondria leak, lose efficiency, and generate excess reactive oxygen species.

SS-31 concentrates in the inner membrane and binds cardiolipin through electrostatic and hydrophobic interactions. Hazel Szeto's group mapped the consequences: by associating with the cytochrome c–cardiolipin complex, SS-31 helps keep cytochrome c working as an electron carrier rather than switching to a peroxidase that damages the membrane, which supports electron transport and ATP synthesis (PubMed 24134698). It protects the machinery of cellular energy at the exact spot where that machinery is anchored.

Later biophysical work sharpened the picture. A 2020 study in the Journal of Biological Chemistry showed that SS-31 binds lipid bilayers and modulates the membrane's surface electrostatics, proposing that this change in the membrane's electrical environment is central to how it acts (PubMed 32273339). The same year, a PNAS paper mapped the peptide's interaction landscape and found it associates with a set of mitochondrial proteins involved in ATP production, not cardiolipin alone — so the "cardiolipin-only" model is a useful simplification rather than the whole story (PubMed 32554501). Two 2025 reviews collect the current understanding of its structure, mechanism, and therapeutic testing (PubMed 39940712, PubMed 40294492). The mechanism is among the best-characterized in this category. The open question was never whether it does something to mitochondria — it was whether that translates into a measurable benefit in patients.

Does SS-31 actually work?

The mechanism cannot answer this, and the honest reply is "sometimes, in some conditions, and not in the ones people usually search for."

In cells and animals, elamipretide is consistently active: across models of aged muscle, injured heart, and failing kidney, it improves mitochondrial function, raises ATP output, and reduces oxidative damage. That preclinical record justified taking it into humans. But biology in a dish and a mouse does not automatically survive a placebo-controlled trial, and elamipretide's clinical history is a case study in that gap. Its most important trials — in primary mitochondrial myopathy, heart failure, and macular degeneration — largely missed their primary endpoints, while its one approval came in an ultra-rare disease where the molecular target it was designed for is exactly what goes wrong. So the question has to be broken apart by indication, which the rest of this article does.

SS-31 and primary mitochondrial myopathy: the MMPOWER-3 failure

Primary mitochondrial myopathy (PMM) is a group of genetic diseases in which faulty mitochondria leave muscles chronically short of energy, causing exercise intolerance and severe fatigue. It was elamipretide's flagship target, and the program was named MMPOWER.

An earlier randomized crossover trial in adults with PMM produced encouraging but mixed signals on exercise capacity (PubMed 32096613), which set up the pivotal test. That test, MMPOWER-3, was a phase 3 randomized, double-blind, placebo-controlled trial of daily subcutaneous elamipretide in 218 people with genetically confirmed PMM over 24 weeks. It did not meet its primary endpoints: no significant benefit over placebo on the distance walked in a six-minute walk test or the total fatigue score on a symptom scale (PubMed 37268435). The drug was well tolerated, but on efficacy it failed.

A later post hoc analysis added a wrinkle. Split by the genetic origin of their disease, patients with disease-causing variants in nuclear DNA showed an improvement in the six-minute walk test, while those with mitochondrial-DNA variants did not (PubMed 39574155). A post hoc subgroup finding is a hypothesis, not proof — the kind of result that needs a fresh, prospective trial to confirm. But it is a reminder that "the trial failed" and "the drug does nothing" are not the same statement.

SS-31 and Barth syndrome: the trial behind the approval

Barth syndrome is where the SS-31 story turns. It is an ultra-rare X-linked disease — roughly 150 diagnosed individuals in the United States — caused by mutations in the TAZ (tafazzin) gene. Tafazzin is the enzyme that remodels cardiolipin into its mature form; without it, cells accumulate an abnormal precursor and lose healthy cardiolipin, producing cardiomyopathy, muscle weakness, low neutrophil counts, and growth delay. The molecular defect is a cardiolipin defect — the precise target elamipretide was engineered to stabilize, which makes the mechanistic fit tighter here than anywhere else.

The trial was TAZPOWER: a phase 2/3 randomized, double-blind, placebo-controlled crossover study of daily subcutaneous elamipretide followed by an open-label extension (PubMed 33077895). Results were again mixed. The blinded portion did not meet its primary endpoints — six-minute walk distance and a patient-reported symptom assessment did not separate significantly from placebo over the short blinded period. But in the longer open-label extension, patients showed gains in muscle strength and walking distance that grew over time, a pattern examined in follow-up analyses of who responded and how the disease-specific symptom scale performed (PubMed 37041653, PubMed 40281531).

Those durable gains — knee extensor strength improved by more than 45% in the extension and correlated with walking improvement — became the basis of the regulatory case. In an ultra-rare disease with no other treatment, a consistent, mechanism-aligned effect on muscle strength was enough to move regulators, even though the original blinded endpoints had been missed.

Is SS-31 (elamipretide) FDA approved?

Yes — for one disease, and with an important caveat. On September 19, 2025, the FDA granted accelerated approval to elamipretide, marketed as FORZINITY (elamipretide HCl), to improve muscle strength in adult and pediatric patients with Barth syndrome who weigh at least 30 kilograms. It is the first therapy ever approved for Barth syndrome and the first approved mitochondria-targeted drug of its kind.

Accelerated approval is a conditional pathway: it clears a drug for a serious disease based on an intermediate clinical measure — here, knee extensor strength — that is reasonably likely to predict real benefit, on the condition that the developer confirm that benefit in further study. It is a green light with strings attached, not a standard approval built on a trial that clearly hit its primary endpoint. The path there was not smooth; the application went through an FDA advisory committee and a complete response letter before the resubmission was cleared.

For everything else, elamipretide is not approved. It is investigational for primary mitochondrial myopathy, heart failure, and eye disease, and there is no approved indication for general energy, athletic performance, or anti-aging use. A vial labeled "SS-31" bought online is not FORZINITY and is not a regulated medicine at all — a distinction covered in our overview of peptide legality and regulatory status.

SS-31 in heart failure and eye disease

Beyond muscle disease, elamipretide was tested in two large areas where mitochondrial dysfunction is thought to contribute, and both show why the clinical picture is called mixed.

In heart failure with reduced ejection fraction, the PROGRESS-HF phase 2 trial tested whether elamipretide would improve the heart's pumping function. It did not meet its primary endpoint of change in left ventricular end-systolic volume (PubMed 32068002), and the cardiac program did not advance to approval.

In the eye, the ReCLAIM studies tested elamipretide in dry age-related macular degeneration and geographic atrophy, conditions with a strong mitochondrial component in the retina. Early phase 1 work in high-risk drusen and non-central geographic atrophy suggested effects on visual function worth pursuing (PubMed 36246187, PubMed 36246181), and a larger phase 2, ReCLAIM-2, followed (PubMed 39605874). Results across the eye program have been mixed, and elamipretide remains investigational for macular degeneration — a compound with a plausible target that keeps producing partial, inconsistent signals rather than clear wins.

SS-31 side effects reported in trials

Because elamipretide has a real trial record, its side-effect profile is actually documented — a rarity in this category. Across the MMPOWER and Barth programs, it was generally well tolerated, and the most common adverse events were consistent from study to study.

The standout is injection-site reactions. Because the drug is given as a daily subcutaneous injection, redness, itching, pain, firmness, or bruising at the injection site were the most frequently reported events. Headache was the other common complaint. Most events were mild to moderate, and serious reactions requiring dose changes were uncommon in the published trials (PubMed 37268435). Trials managed the local reactions with routine measures and rotated injection sites across the abdomen, thigh, and upper arm.

Two caveats belong here. This safety profile describes the pharmaceutical-grade drug given under medical supervision in monitored trials — not a research-chemical vial reconstituted at home, where purity, sterility, and actual contents are unverified and no adverse-event dataset exists. And the long-term safety of chronic use outside the studied populations is unknown; "well tolerated in a 24-week trial for a specific disease" does not generalize to years of self-administration by a healthy adult.

SS-31 vs MOTS-c, NAD+ and other cellular-energy compounds

SS-31 is often lumped together with other "mitochondrial" or "longevity" compounds. They share a theme — supporting cellular energy — but differ sharply in how they act and how strong the human evidence is. The most useful comparison is not potency but what has actually been tested in people.

The pattern is clear. SS-31 sits at the top of this list for evidence quality — it is the only one with a real regulatory approval — precisely because it went through the expensive, failure-prone process of controlled human trials. The mitochondrial-derived peptide MOTS-c and the NAD+ precursors NMN and NR have compelling biology but far thinner clinical proof, and longevity peptides like epitalon rest largely on small, older studies. Sharing a "mitochondrial energy" story is not the same as sharing an evidence base.

Is SS-31 legal to buy?

There are two very different products hiding under one name. FORZINITY, the approved elamipretide, is a prescription medicine for Barth syndrome dispensed through the healthcare system. Getting it means a diagnosis and a clinician, not a checkout button.

The "SS-31" sold by research-chemical suppliers is a separate thing entirely. It is not the approved drug, is not manufactured to pharmaceutical standards, and is typically labeled "for research use only" or "not for human consumption." Marketing an unapproved compound for human use is not lawful in most jurisdictions, and the vials carry the ordinary hazards of unregulated products: uncertain identity, uncertain purity, and no sterility guarantee. Enforcement and the exact legal line vary by country. Our guide to whether peptides are legal covers how the approved-drug, prescription, and research-chemical categories differ, and why the label on the vial matters as much as the molecule inside it — the same gray market that surrounds compounds like thymosin alpha-1.

Frequently asked questions

Sources

Every claim above traces to peer-reviewed literature indexed on PubMed:

1. Targeting mitochondrial cardiolipin and the cytochrome c/cardiolipin complex to promote electron transport and optimize mitochondrial ATP synthesis — PubMed 24134698
2. The mitochondria-targeted peptide SS-31 binds lipid bilayers and modulates surface electrostatics as a key component of its mechanism of action — PubMed 32273339
3. Mitochondrial protein interaction landscape of SS-31 — PubMed 32554501
4. Elamipretide: A Review of Its Structure, Mechanism of Action, and Therapeutic Potential — PubMed 39940712
5. Contemporary insights into elamipretide's mitochondrial mechanism of action and therapeutic effects — PubMed 40294492
6. A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy — PubMed 32096613
7. Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial — PubMed 37268435
8. Genotype-specific effects of elamipretide in patients with primary mitochondrial myopathy: a post hoc analysis of the MMPOWER-3 trial — PubMed 39574155
9. A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome — PubMed 33077895
10. Identifying responders to elamipretide in Barth syndrome: Hierarchical clustering for time series data — PubMed 37041653
11. Initial Psychometric Evaluation of the Barth Syndrome Symptom Assessment (BTHS-SA) for Adolescents and Adults in a Phase 2 Clinical Study — PubMed 40281531
12. Effects of Elamipretide on Left Ventricular Function in Patients With Heart Failure With Reduced Ejection Fraction: The PROGRESS-HF Phase 2 Trial — PubMed 32068002
13. ReCLAIM-2: A Randomized Phase II Clinical Trial Evaluating Elamipretide in Age-related Macular Degeneration — PubMed 39605874
14. Phase 1 Clinical Trial of Elamipretide in Intermediate Age-Related Macular Degeneration and High-Risk Drusen: ReCLAIM High-Risk Drusen Study — PubMed 36246187
15. Phase 1 Clinical Trial of Elamipretide in Dry Age-Related Macular Degeneration and Noncentral Geographic Atrophy: ReCLAIM NCGA Study — PubMed 36246181

Regulatory status reflects the FDA's September 2025 accelerated approval of FORZINITY (elamipretide HCl) for Barth syndrome, the first approved therapy for that disease.

Keep reading