SS-31 (Elamipretide): Approved in 2025 for One Ultra-Rare Disease After Every Broad Trial Missed

On 19 September 2025 FDA approved elamipretide as FORZINITY. SS-31 is a real, approved medicine — for Barth syndrome, in patients weighing at least 30 kg, for muscle strength only. Before that, every broad trial it ran had missed its primary endpoint.

Both halves of that matter, and most writing about this compound carries only the first. SS-31 sits in the immune support and longevity peptides category and in our overview of peptides for energy and mitochondrial function, where it is the only member that became a licensed drug and the only one with documented failures on its record.

Is SS-31 FDA-approved?

Yes. FDA approved elamipretide as FORZINITY under NDA 215244 on 19 September 2025, making SS-31 a real, approved medicine. The approval is narrow: Barth syndrome only, in patients weighing at least 30 kg, for muscle strength only, granted under accelerated approval on a surrogate endpoint. Almost nothing written about this compound has caught up with that fact — including pages on this site, which still describe SS-31 as unapproved.

We are flagging our own error deliberately. A site that tracks regulatory status has to say when its own pages are out of date, and this is one of those cases: the SS-31 compound page and related listings predate the September 2025 approval.

Elamipretide is also sold and discussed as MTP-131, Bendavia and SS-31. Those are the same molecule, and one of them is now on a label.

What evidence supported the FORZINITY approval?

The FORZINITY approval rests on the open-label extension of TAZPOWER, a crossover trial in twelve patients, showing a median increase of 63 newtons in knee extensor muscle strength at Week 168. That is a surrogate endpoint, not a clinical outcome like survival or hospitalisation. The randomised, controlled portion of the same trial missed both of its primary endpoints: the 6-minute walk test returned p = 0.97 and the fatigue endpoint returned p = 0.89.

Those two p-values are worth reading twice. They are not near-misses. They are as null as trial results get, and they came from the randomised portion of the very study whose extension supported the approval.

Accelerated approval exists precisely for this situation: an ultra-rare disease with no approved therapy, where a conventional trial is not feasible and a surrogate endpoint reasonably likely to predict benefit is accepted, with confirmatory evidence required afterward. Whether that was the right call for Barth syndrome is a reasonable policy question. What it is not is evidence that elamipretide improves mitochondrial function generally.

Which elamipretide trials missed their primary endpoints?

Four, including the pivotal one. Elamipretide missed its primary endpoint in EMBRACE-STEMI (ST-elevation myocardial infarction), in PROGRESS-HF (heart failure with reduced ejection fraction, n=71) and in MMPOWER-3 (primary mitochondrial myopathy, n=218). TAZPOWER's randomised phase missed both of its primaries in 12 Barth syndrome patients. MMPOWER-3 is the one that matters most to anyone buying SS-31 as a general mitochondrial agent, and it is the largest trial of the four.

MMPOWER-3 enrolled 218 patients with primary mitochondrial myopathy — genetically confirmed mitochondrial disease, the population where a mitochondria-targeted drug has the strongest theoretical case there is. It missed its primary endpoint.

If elamipretide does not produce a measurable functional benefit in people whose mitochondria are demonstrably broken, the prior on it improving mitochondrial function in a healthy adult who feels tired is low.

None of this is a criticism of Stealth BioTherapeutics, which ran real trials in hard populations and published the results. It is a criticism of how that programme gets summarised in the grey market, where a run of failed phase 2 and phase 3 trials followed by a narrow accelerated approval becomes "FDA-approved mitochondrial peptide".

What is SS-31, and why does its structure matter?

SS-31 is the tetrapeptide D-Arg-Dmt-Lys-Phe-NH₂, CAS 736992-21-5, with a free-base molecular weight of 639.79 and a trihydrochloride weight of 749.2. It is also called elamipretide, MTP-131 and Bendavia. Two residues carry the whole identity problem: D-Arg is the D-enantiomer of arginine, and Dmt is 2,6-dimethyltyrosine, a non-standard residue. The mechanism is cardiolipin binding in the inner mitochondrial membrane, which stabilises cristae structure and holds the electron transport chain complexes in place.

The mechanism is genuinely distinctive. SS-31 does not scavenge free radicals in the general sense; it concentrates in the inner mitochondrial membrane and binds cardiolipin, the phospholipid that organises cristae architecture and holds the electron transport chain complexes in place.

Barth syndrome is a cardiolipin remodelling disorder. The approval and the mechanism line up, which is exactly why the approval landed where it did and nowhere else.

Can a lab test confirm you received real SS-31?

A certificate of analysis can confirm most of SS-31, but not the feature the molecule depends on. Mass spectrometry confirms the 639.79 Da free-base backbone and rules out tyrosine substituting for 2,6-dimethyltyrosine, a −28 Da shift. It cannot confirm the D-arginine: D-Arg and L-Arg have identical mass and identical formula, co-elute on standard reversed-phase HPLC, and are invisible to routine mass spectrometry. Only chiral analysis reaches that question.

Why does the stereochemistry matter enough to build a section around? Because the D-configuration is not decoration. All-L peptides are substrates for the body's proteases; D-amino acids resist them. The D-Arg is part of why elamipretide survives long enough to reach mitochondria at all. A well-made L-Arg analogue would look identical on every test in a standard certificate of analysis and behave differently in the body.

The Dmt residue is the opposite case, and the contrast is the practical reading rule. 2,6-dimethyltyrosine → tyrosine is a −28 Da shift, immediately visible to mass spectrometry, so any vendor providing MS data has effectively ruled that substitution out. One possible substitution is trivially caught; the other is invisible to everything a peptide vendor routinely performs. See mass spectrometry for peptides and what HPLC testing can and cannot tell you.

The salt point is not minor either: 639.79 versus 749.2 is a 17% mass difference. A certificate that does not state the salt form leaves a sixth of your dose calculation undefined. See why 10 mg isn't 10 mg and red flags in peptide certificates of analysis.

Which SS-31 claims does the trial record support?

Two of ten. Of the ten claims routinely made for SS-31, only the FDA approval itself and the cardiolipin-binding mechanism hold up. Four are refuted by trials that ran and missed — mitochondrial myopathy, post-infarction cardioprotection, heart failure and exercise capacity. Three are false as stated, including the claim that purity testing confirms you received SS-31. One, anti-aging and general energy, has no trial behind it at all — the only row where nothing has been measured.

Four refuted rows is unusual on this site. Most compounds we cover have empty evidence columns, not negative ones, and a negative result is a stronger fact than an absent one.

Is it legal to buy SS-31 as a research chemical now that it is an approved drug?

No — and the approval makes the position clearer, not murkier. In the United States elamipretide is a prescription drug: FORZINITY is approved and marketed for Barth syndrome. Selling an approved drug's active ingredient as a research chemical for human use is the sale of an unapproved new drug, and "for research use only" labelling is not a defence, as FDA has established repeatedly in warning letters across this market.

The approval also changed the compounding analysis. As a component of an FDA-approved drug, elamipretide satisfies one of the three statutory routes under 21 U.S.C. § 353a(b)(1)(A) for a bulk substance in 503A compounding, and it does not appear on FDA's list of bulk substances that may present significant safety risks. That is a different question from whether a research vial is legitimate, and the answer to the second question did not improve in September 2025 — if anything, grey-market SS-31 became a clearer violation, because there is now an approved product with an approved label. See compounding pharmacy versus research peptide and our FDA peptide regulation timeline.

WADA: elamipretide is not named on the 2026 Prohibited List. Because it now holds approval from a governmental regulatory health authority for human therapeutic use, the S0 catch-all does not obviously capture it either. That is a change from its pre-approval status, and it is worth obtaining a ruling rather than an assumption if you compete.

What do 194 lab tests tell you about SS-31, and what can't they?

SS-31's 194 lab tests tell you the material is homogeneous and the mass is right. The Purity Index records SS-31 at 99.68 across 194 recency-weighted tests from 16 laboratories (verified August 2026), against a platform composite of 99.50 — one of the higher scores we track. What that figure cannot tell you is whether the D-amino acid content is correct, because no routine test in this market verifies stereochemistry. The Purity Index answers a real question well here and simply cannot reach the one that matters most.

SS-31 price data shows a median of $4.00/mg (verified August 2026). Those figures exclude shipping, taxes and customs, coupon codes, bulk tiers, multi-vial kits and account-gated pricing. Peptigrity tracks 530 shops and 11,852 independent lab tests across 118 peptides, with 1,283 community reviews (verified August 2026), weighting community reviews and independently verified purity equally, at 50% each, in every trust score.

For turning a vial into an injection volume, the peptide dosing calculator and the reconstitution calculator handle the arithmetic. Use the free-base or trihydrochloride mass consistently, because the 17% gap between them propagates straight into the result.

How does SS-31 compare with the other mitochondrial compounds?

SS-31 is the most tested compound in the mitochondrial group and the one whose testing produced the least encouraging picture. It ran four or more phase 2 and phase 3 trials, missed most primaries, and finished with one accelerated approval in an ultra-rare disease. MOTS-c has one recruiting trial and no results. NAD+ boosters have many randomised trials in which the biomarker rises and function does not. FOXO4-DRI has none, ever.

That ordering is the most useful lesson the group offers: the compounds that look best on paper are usually the ones nobody has tested. MOTS-c and FOXO4-DRI have no failed trials because they have no trials. SS-31 has four failures on its record because it went and asked.

What trial would settle this?

The trial that would settle this is a randomised, double-blind, placebo-controlled study of SS-31 in the population that actually buys it — healthy or subclinically fatigued adults — which nobody has run. It would need a functional primary endpoint rather than a strength surrogate, would need to account for MMPOWER-3's null result in 218 patients with confirmed mitochondrial disease, and would need to run well beyond 12 weeks.

The last row is the one to watch. Accelerated approval is conditional. FDA requires confirmatory evidence of clinical benefit, and approvals granted this way have been withdrawn when that evidence did not arrive.

Frequently Asked Questions

Is SS-31 FDA-approved?

Yes, as of 19 September 2025. Elamipretide is approved as FORZINITY, NDA 215244, from Stealth BioTherapeutics — for Barth syndrome, in patients weighing at least 30 kg, for muscle strength only, under accelerated approval on a surrogate endpoint. Any page stating SS-31 is not FDA-approved is out of date, including pages on this site that have not yet been corrected.

Does that approval mean SS-31 works for mitochondrial dysfunction generally?

No. It means FDA accepted a surrogate strength endpoint in one ultra-rare cardiolipin disorder. Elamipretide missed its primary endpoint in heart attack (EMBRACE-STEMI), heart failure (PROGRESS-HF, n=71) and primary mitochondrial myopathy (MMPOWER-3, n=218).

What evidence supported the approval?

The open-label extension of TAZPOWER, a crossover trial in 12 patients, showing a median increase of 63 newtons in knee extensor strength at Week 168. The randomised portion of that same trial missed both primary endpoints — the 6-minute walk test at p=0.97 and fatigue at p=0.89.

Can a lab test confirm I received real SS-31?

Partially. Mass spectrometry confirms the 639.79 Da free-base mass and rules out tyrosine substituting for 2,6-dimethyltyrosine, which would show as −28 Da. It cannot confirm the D-arginine, because D-Arg and L-Arg are identical in mass and co-elute on standard columns. Verifying that requires chiral analysis, which essentially no vendor provides.

Why does the D-amino acid matter?

D-amino acids resist the proteases that degrade ordinary peptides. The D-configuration is part of why elamipretide survives in the body long enough to reach mitochondria. An all-L version would pass every routine test and behave differently.

Is SS-31 banned in sport?

It is not named on the WADA 2026 Prohibited List, and since September 2025 it holds a governmental approval for human therapeutic use, which complicates the S0 catch-all analysis. No anti-doping body has published a ruling. Seek one rather than assuming.

SS-31 is the rare grey-market peptide that became a real drug, and the details of how it got there are more instructive than the fact itself.

Stealth BioTherapeutics did the work: four substantial trials across cardiology and mitochondrial medicine, published results, and an approval at the end of it. Three of those trials missed. The fourth missed too, in its randomised phase, and the approval came from its open-label extension in twelve patients on a strength surrogate, under a pathway designed for exactly this situation — an ultra-rare disease with nothing else available.

Read honestly, that record says elamipretide is a real drug for Barth syndrome and an unproven one for everything else, including everything it is sold for. The compound that has been tested most in this category has the weakest efficacy record in it, and that is not an accident of quality — it is what happens when a compound is actually asked the question.

Browse the immune support and longevity peptides category, or our complete peptide guide with 118 compounds (verified August 2026). For per-injection volume, use the peptide dosing calculator alongside the reconstitution calculator. Compare shops through independent lab tests and community-verified shop reviews.

This article is for educational and informational purposes only and does not constitute medical advice. Peptides discussed may be investigational compounds not approved by the FDA (or equivalent regulators in your jurisdiction) for human use. Always consult a qualified healthcare provider before using any peptide or research compound. Peptigrity is an independent review platform and does not sell, endorse, or recommend specific products or vendors.