IN CLINICAL TRIALS

SS-31 (Elamipretide)

A small peptide designed to do something most drugs can't: get inside mitochondria and stabilize them. The most clinically advanced investigational peptide in the longevity-and-mitochondria space.

The 30-second read

SS-31 is a tetrapeptide engineered to penetrate the inner membrane of mitochondria and bind to cardiolipin, a fat molecule that helps mitochondria produce energy. Stealth Biotherapeutics developed it (under the names elamipretide and Bendavia) for diseases where mitochondria stop working properly, like Barth syndrome and primary mitochondrial myopathy. Late-stage trials have produced mixed results, FDA reviews have gone back and forth, and as of 2026 it has FDA Fast Track designation but no full approval. Of all the peptides in the broader longevity conversation, this is the one furthest along in real clinical development. Off-label biohacker use exists too, with much weaker evidence.

Why this peptide is on people's radar

SS-31 sits at the intersection of two stories. The first is the rare-disease story: Stealth Biotherapeutics has spent years pushing elamipretide through Phase 3 trials in Barth syndrome (a rare cardiomyopathy in children) and primary mitochondrial myopathy. The FDA has reviewed it multiple times, with Complete Response Letters and resubmissions along the way. As of 2026 it carries Fast Track designation but no full approval, the most advanced clinical-development position any investigational mitochondrial peptide has reached.

The second is the longevity story. Mitochondrial dysfunction is one of the recognized "hallmarks of aging", as we get older, our mitochondria become less efficient and more prone to damage. A peptide that can localize to and stabilize mitochondria has obvious appeal in that frame, and longevity-focused researchers and biohackers have followed elamipretide's clinical journey closely. Off-label, peer-prescribed, and research-peptide use exists alongside the rare-disease development program.

The mechanism is unusual enough to make the molecule stand out. Most drugs can't easily get inside mitochondria, there's an inner membrane that blocks most things. SS-31 was designed specifically to penetrate that membrane and accumulate there at concentrations a thousand or more times higher than in surrounding cytoplasm. Once there, it binds cardiolipin, the signature lipid of the inner membrane, and helps stabilize mitochondrial energy production.

What people are usually trying to do with it

People exploring SS-31 are usually focused on:

  • Managing a rare mitochondrial disease (Barth syndrome, primary mitochondrial myopathy)
  • Supporting mitochondrial function as part of a longevity protocol
  • Improving exercise capacity, fatigue, or muscle weakness from age-related mitochondrial decline
  • Recovery from a heart-related event or treatment of cardiac mitochondrial dysfunction
  • Research on age-related macular degeneration (one of the trial indications studied)

What the science actually shows

SS-31 has more rigorous clinical-trial data than almost any other peptide in this category. The picture is mixed. Plain-English summary:

Mitochondrial localization and cardiolipin binding

The basic biology is well-established: SS-31 accumulates inside mitochondria at concentrations a thousand or more times higher than in surrounding cytoplasm and binds cardiolipin in the inner mitochondrial membrane. This is unusual and well-characterized.1

Barth syndrome (TAZPOWER and follow-up trials)

Phase 3 trial data for Barth syndrome have produced mixed signals, some endpoints positive, others not, and have led to multiple FDA review cycles. As of 2026, full approval has not been granted, though Fast Track designation remains.2

Primary mitochondrial myopathy (MMPOWER-3)

The Phase 3 MMPOWER-3 trial in adults with primary mitochondrial myopathy missed its primary endpoint, contributing to the regulatory complexity around the program.3

Age-related macular degeneration

Trials in dry AMD have explored elamipretide's effect on retinal mitochondrial function. Results have been suggestive but not yet definitive.4

Preclinical work in longevity and cardiovascular contexts

Animal and tissue studies suggest the peptide can preserve mitochondrial function in age-related and ischemic injury models. Translation to human longevity applications is the speculative end of the literature.5

The honest read

What's solid:

The molecule is a real, well-characterized drug candidate that has been through proper clinical development. The mechanism (mitochondrial penetration, cardiolipin binding, membrane stabilization) is solid biology. Stealth Biotherapeutics has earned both Fast Track designation and the harder lessons of mixed Phase 3 readouts.

What's still being worked out:

FDA approval for any indication. The trial data has been mixed, some endpoints met, some not, and the regulatory trajectory has been complex. What the right indication, dosing, and patient population are is still being refined. Off-label longevity use is several big inferential steps from the rare-disease evidence.

What's hyped beyond the evidence:

Treating SS-31 as an established anti-aging or longevity drug. The longevity case rests on extrapolation from the mitochondrial-dysfunction story, not on direct human longevity outcomes. Claims about general energy, fatigue, or "feeling more vital" in healthy adults aren't supported by the trial program. The serious clinical evidence is in narrow rare-disease populations.

Things to know if you're looking into it

  • Investigational status: elamipretide has FDA Fast Track designation but no full FDA approval as of May 2026. It's been through multiple Phase 3 trials and FDA review cycles for Barth syndrome and primary mitochondrial myopathy.
  • How it's used in trials: typically a once-daily subcutaneous injection at trial-defined doses for the rare-disease indications.
  • Off-label and research-peptide use: exists alongside the official development program, particularly in longevity-focused contexts. The clinical evidence in healthy adults is much thinner than in the rare-disease populations.
  • Reported side effects in trials: generally favorable. Injection-site reactions are the most common. Serious adverse events have been infrequent across the development program.
  • The right population matters: the strongest case for SS-31 has always been in patients with measurable mitochondrial dysfunction. Translating that to healthy adults with normal mitochondrial function is a different question.
  • Healthcare provider involvement: recommended for any clinical use, particularly given the mitochondrial-targeting mechanism and the absence of full FDA approval.
  • Specific dosing protocols, mechanism, half-life, trial detail, and references: all in the "Want to go deeper?" section below.

Reconstitution & dose calculator

Investigational drug. FDA Fast Track designation but no full approval as of May 2026. Clinical trial doses for Barth syndrome and primary mitochondrial myopathy are 40 mg subcutaneous once daily — that's the dose the development program is built around. Research-peptide community use of SS-31 in healthy adults for "longevity" is at much lower doses (5–10 mg per injection) and is several big inferential steps from the rare-disease evidence base. The dose math below covers research use, not clinical-trial dosing. This is an educational reference, not dosing guidance.
Suggested start
5 mg/inj
Lower end of community range
Common range
5–10 mg/inj
Once daily, typically
Max community dose
20 mg/inj
Trial doses are 40 mg/day under medical supervision
Cycle
4–8 wks on
Trials use continuous dosing under supervision
mL
Defaults to 10 mg/mL (1 mL into a 10 mg vial) — SS-31 doses are large enough that more dilute concentrations push past one syringe. Standard 5 mg dose lands at exactly 50 units; 10 mg uses the full 100-unit syringe. Adjust to taste.
mg
Subcutaneous injection. Research community typically doses once daily; clinical trials for Barth syndrome use 40 mg/day under medical supervision (would require 4 of these vials per dose).
Concentration
10.0 mg/mL
Per dose
0.50 mL
50 units on insulin syringe
Doses per vial
~2
~2 days (~0.3 weeks) of daily dosing

When to stay put vs. adjust

Stay put at 5 mg daily while you have a baseline to compare against. SS-31's effects, like other mitochondrial-targeting compounds, are subtle and metabolic, not acutely felt. The signal to track is improvement on objective markers (exercise capacity, fatigue scores, recovery, mitochondrial-function biomarkers if you have access to them) over a full cycle, not day-to-day sensation.

Consider stepping to 10 mg daily only after at least four weeks at 5 mg with clear tolerability and limited progress on the markers you're tracking. Above 10 mg per injection in research use, you're moving toward clinical-trial dose territory without the clinical-trial monitoring or patient population the dose was designed for.

Don't push toward the 40 mg trial dose without clinical context. The Phase 3 dose was designed for patients with primary mitochondrial myopathy or Barth syndrome — populations with measurable, significant mitochondrial dysfunction. Translating that dose to a healthy adult with normal mitochondrial function isn't supported by the trial data and isn't what the development program is investigating.

Cycle off at the 4–8 week mark for 2–4 weeks. Clinical trials use continuous dosing because the patient populations have ongoing mitochondrial dysfunction that needs continuous support. Healthy-adult research use doesn't have the same rationale, and cycling errs on the side of caution given how thin the long-term safety data in healthy adults is.

Watch for injection-site reactions. The most common side effect across the SS-31 development program. Generally mild and resolving; rotate sites and reduce frequency or dose if persistent.

The honest read. SS-31 is the most clinically-developed peptide in the longevity-adjacent category — real Phase 3 trials, real FDA review cycles, real Fast Track designation. That's important context. It's also a drug developed for rare mitochondrial diseases, where the patient population has measurable dysfunction the drug is designed to address. The trial data has been mixed even in those populations: some endpoints met, others not, multiple Complete Response Letters from FDA, no full approval as of May 2026. Off-label use in healthy adults for "longevity" or "mitochondrial optimization" is several big inferential steps from the rare-disease evidence. The biology is striking; the case for use in healthy adults is genuinely thin. If you're using SS-31 for a measurable issue, track it; if you're using it because mitochondrial health sounds important in the abstract, the value-for-cost is harder to defend.

For educational and research purposes only. This is not medical advice. SS-31 (Elamipretide) is investigational and not FDA-approved as of May 2026. Off-label use exists outside the rare-disease populations the drug is being developed for. Consult a licensed healthcare provider before any health decision.

What people often ask

Is SS-31 the same as elamipretide?

Yes. "SS-31" is the original research-chemistry designation. "Elamipretide" is the international nonproprietary name (INN). "Bendavia" was an earlier brand name used by Stealth Biotherapeutics. "MTP-131" is another designation used in the literature. All refer to the same molecule.

Is it FDA-approved?

Not as of May 2026. Stealth Biotherapeutics has submitted multiple applications for Barth syndrome; the FDA has issued Complete Response Letters and the company has resubmitted. Fast Track designation remains. Full approval status changes over time, check the FDA's tracker for the latest.

Will it help with general fatigue or aging?

The case in healthy adults is speculative. The strongest clinical signal is in patients with established mitochondrial disease. Some longevity-clinic protocols use it off-label for adults concerned about age-related mitochondrial decline; the evidence for benefit in those populations is much thinner.

How does it actually get into mitochondria?

The peptide is engineered with a specific charge and structure that lets it cross the outer mitochondrial membrane and accumulate in the inner membrane, where it binds cardiolipin. Most drugs can't do this, getting compounds into mitochondria specifically is a hard problem in pharmacology.

What about the mixed Phase 3 results?

The trial program has been a real, hard-won clinical-development story. Some endpoints met, some not. Some indications more responsive than others. That's normal for a first-in-class drug in rare diseases. The regulatory back-and-forth is part of how science gets done, not a sign the molecule doesn't work, and not proof that it does either.

Is there a longevity use case that's been studied?

Animal data is suggestive. Small clinical studies in older adults have reported improvements in mitochondrial function in muscle and other tissues. None of that has reached the rigor or scale needed to support a longevity-specific FDA approval.

FDA and regulatory status

Status as of May 5, 2026: Investigational. FDA Fast Track designation for primary mitochondrial myopathy and Barth syndrome. Stealth Biotherapeutics has submitted New Drug Applications for Barth syndrome that have received Complete Response Letters from the FDA, with subsequent resubmissions. Full FDA approval has not been granted as of this date. Status updates land here when they happen.

Want to go deeper? Mechanism, mitochondrial-targeting design, dosing, the MMPOWER and TAZPOWER trial detail, side effects, and references. Click to expand.

Background and development

SS-31 (D-Arg-2',6'-dimethylTyr-Lys-Phe-NH₂) is a tetrapeptide originally developed in the laboratory of Hazel Szeto and Peter Schiller at Cornell University. It belongs to a class known as "Szeto-Schiller (SS)" peptides, designed specifically to target the inner mitochondrial membrane via a combination of charge characteristics and aromatic residues. Stealth Biotherapeutics licensed the molecule and developed it under the names Bendavia, MTP-131, and finally elamipretide.

Mechanism of action

Mitochondrial targeting

SS-31 accumulates in the inner mitochondrial membrane at concentrations approximately 1,000 to 5,000 times higher than in surrounding cytoplasm, due to its charge profile and aromatic residues. The targeting is intrinsic, no separate transport mechanism required.

Cardiolipin binding

Once concentrated in the inner membrane, SS-31 binds to cardiolipin, the signature phospholipid of the inner mitochondrial membrane and a key cofactor for proper electron-transport-chain organization. Cardiolipin damage is associated with mitochondrial dysfunction in aging, cardiovascular disease, and inherited mitochondrial disorders.

Stabilization of energy production

By preserving cardiolipin integrity, SS-31 stabilizes the supercomplexes that produce ATP and reduces the leak of reactive oxygen species. The downstream effect is more efficient mitochondrial ATP production with less oxidative damage.

Investigational dosing

Investigational only. The doses described below are from clinical trials and do not constitute prescribing recommendations.

Trials have used once-daily subcutaneous administration at doses around 40 mg for adults with primary mitochondrial myopathy or Barth syndrome. Pediatric Barth syndrome trials have used weight-based dosing.

The major clinical trials

MMPOWER series, primary mitochondrial myopathy

MMPOWER (Phase 1/2) and MMPOWER-2 (Phase 2) supported continued development. MMPOWER-3 (Phase 3) tested elamipretide in adults with primary mitochondrial myopathy and missed its primary endpoint of change in 6-minute walk distance.

TAZPOWER and follow-up trials. Barth syndrome

TAZPOWER (Phase 2/3) in Barth syndrome (a rare X-linked cardiomyopathy in children) produced mixed results across endpoints. Subsequent open-label extensions and resubmissions have continued the regulatory dialogue.

ReNEW and ReCLAIM, dry age-related macular degeneration

Trials of elamipretide in dry AMD have explored visual-function endpoints associated with retinal mitochondrial dysfunction. Results have been suggestive of benefit on some endpoints.

Heart-failure trials

Earlier development included trials in cardiac contexts (acute myocardial infarction, heart failure with preserved ejection fraction). These programs were de-prioritized as the rare-disease indications advanced.

Side effects and safety profile

Across the development program, the safety profile has been generally favorable:

  • Injection-site reactions are the most common adverse event
  • Mild GI complaints in some patients
  • Serious adverse events have been infrequent
  • Long-term safety data exists from open-label extension studies

The relatively clean safety profile is one reason the program has continued through mixed efficacy readouts, the drug has not raised the safety concerns that often end clinical development.

References

  1. Birk AV, Liu S, Soong Y, et al. (2013). "The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin." J Am Soc Nephrol, 24(8), 1250–1261. PubMed
  2. Reid Thompson W, Hornby B, Manuel R, et al. (2021). "A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome." Genet Med, 23(3), 471–478. PubMed
  3. Karaa A, Haas R, Goldstein A, et al. (2020). "A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy." J Inherit Metab Dis, 43(5), 1091–1101. PubMed
  4. Allingham MJ, Mettu PS, Cousins SW. (2022). "Elamipretide, a mitochondrial-targeted drug, for the treatment of vision loss in dry AMD." Ophthalmol Sci, 2(1), 100097. PubMed
  5. Siegel MP, Kruse SE, Percival JM, et al. (2013). "Mitochondrial-targeted peptide rapidly improves mitochondrial energetics and skeletal muscle performance in aged mice." Aging Cell, 12(5), 763–771. PubMed
  6. Stealth Biotherapeutics. "Elamipretide development program, clinical trial registry." ClinicalTrials.gov
  7. U.S. Food and Drug Administration. "Elamipretide regulatory history." FDA.gov
For educational and research purposes only. This is not medical advice. SS-31 (elamipretide) is investigational, not FDA-approved as of May 2026. Consult a licensed healthcare provider before considering any peptide. PeptideLibraryHub is independent and does not sell peptides or accept money from anyone who does.