SS-31 (elamipretide): what the trials show, status and safety
Summary
Elamipretide, known in laboratories as SS-31, is a tiny peptide of four building blocks that attaches to a particular fat inside the mitochondria, the power plants of our cells. In September 2025 it became an approved medicine in the United States for one rare inherited disease, on data from twelve patients. Every larger properly controlled trial of it has failed to show what it set out to show. No other country has approved it for anything.
Key findings at a glance
- Approved in one country, for one disease. The American regulator granted a fast-track approval on 19 September 2025, for the rare inherited condition called Barth syndrome [1].
- The whole approval rests on twelve patients. The TAZPOWER trial randomly assigned twelve people, and the muscle strength finding that carried the approval comes from the follow-on phase afterwards, by which time only eight patients were left at week 168 [2], [3].
- Both of the trial's own main questions came out negative. Neither the distance patients could walk in six minutes nor their fatigue score improved compared with the dummy treatment [3].
- The large trial in a muscle disease of the mitochondria failed. In 218 adults, patients on the drug walked 3.2 metres less far, and the range compatible with the data ran from 18.7 metres worse to 12.3 metres better [4].
- No effect on the heart. The size of the damaged area was unchanged in 300 patients after a heart attack, and so was heart chamber size in 71 patients with heart failure [5], [6].
- The eye trial failed on both of its main questions. It ran for 48 weeks in 176 patients with the dry form of age-related macular degeneration [7].
- The trial that has to confirm the benefit reports in 2030. The regulator imposed it as a condition of approval, and it is due to finish in September 2029 [1].
What it is
SS-31 is the laboratory code for the drug elamipretide. The molecule is unusually small, a chain of just four building blocks [8], [9]. Hazel Szeto and Peter Schiller created the family of peptides it belongs to, and the company Stealth BioTherapeutics developed this particular member [10]. Older records call it MTP-131, RX-31 or SBT-131, and the heart trials used the name Bendavia.
The finished American medicine is sold as Forzinity, and that is the only approved brand name this substance carries anywhere in the world [1]. An approval applies to a manufactured product with a defined content and a specified purity, never to a chemical name as such. Powder circulating under the laboratory code is therefore a different thing in the eyes of the law.
Quick facts
| Field | Value | Ref |
|---|---|---|
| Laboratory and development codes | SS-31, MTP-131, RX-31, SBT-131, Bendavia | [8], [9] |
| Class | Mitochondrial cardiolipin binder | [2] |
| Sequence | D-Arg-Dmt-Lys-Phe-NH2 | [8] |
| Structure | Szeto-Schiller tetrapeptide, amidated C terminus | [8] |
| Formula and mass, free base | C32H49N9O5, 639.8 g/mol | [8] |
| Mass, hydrochloride in the approved product | 749.2 g/mol | [2] |
| Half-life | Not published | — |
| Route in trials | Subcutaneous, intravenous and ophthalmic | [2], [5] |
| Status | Approved in the United States only; investigational elsewhere | [1], [11] |
| Developer | Stealth BioTherapeutics | [10] |
| CAS registry number | 736992-21-5 | [8] |
| UNII | 87GWG91S09 | [9] |
| PubChem CID | 11764719 | [8] |
| DrugBank | DB11981 | [12] |
| ChEMBL | CHEMBL3833370 | [8] |
How it works
The approved package insert says no more than that elamipretide gathers in the inner membrane of the mitochondria and improves their shape and function [2]. Everything more detailed below comes from cells in a dish and from animals.
- It sticks to a particular fat. The peptide binds cardiolipin, the fat that gives the inner membrane its folds and holds the cell's energy machinery in place [13].
- It blocks a damaging reaction. Once bound, the peptide is reported to stop a chemical attack that would otherwise oxidise that fat and cause the folds to collapse [13].
- It gets in without a ticket. The peptide's chemical make-up is said to let it slip across membranes on its own, and one particular building block is essential for its protective effect against oxidation [10].
- It is not a hormone-style drug. There is no receptor and no hormone system involved. The proposed effect comes down to where the molecule settles and which fat it binds.
Every step from a protected fat to a patient feeling better is supported in animals and confirmed in nobody. That sets elamipretide apart from peptides such as MOTS-c, which the body produces from its own genes rather than being built in a laboratory to stick to a membrane [14].
What the trials found
Barth syndrome
Phase 2/3 RCT TAZPOWER enrolled twelve male patients aged twelve and over, each of whom received both the drug and a dummy treatment in turn. Both of the questions the trial was built to answer came out negative: neither the distance walked in six minutes nor the fatigue score improved [3]. The strength of the muscle that straightens the knee shifted by a median gain of 4 newtons on the drug, against a loss of 5 newtons on the dummy treatment [2].
Open-label extension The strength finding that carried the approval only appeared once patients and doctors knew what was being given. Measured against the starting point, the typical gain was 34 newtons at week 12 in ten patients, 68 newtons at week 24 in nine, and 63 newtons at week 168 in eight [2], [15]. Walking distance rose by 95.9 metres at week 36 and 96.1 metres at week 168 [15].
TAZPOWER, in which 12 patients received drug and dummy treatment in turn, followed by an open continuation with no comparison group (10 patients at week 12, 9 at week 24, 8 at week 168). Both questions the randomised part was built to answer came out negative. Sources [2], [3], [15].
A continuation without a comparison group cannot tell a drug effect apart from the natural course of the illness, from patients simply getting better at the test through repetition, or from the fact that those who were doing well are the ones who stayed in the study. That is precisely why the regulator demanded a proper confirmatory trial [1].
Mitochondrial myopathy
Phase 3 RCT MMPOWER-3 followed 218 adults for 24 weeks who had a genetically confirmed muscle disease caused by faulty mitochondria. Patients on the drug walked 3.2 metres less far than those on the dummy treatment, and the range compatible with the data ran from 18.7 metres worse to 12.3 metres better, nowhere near a real difference (p = 0.69). Fatigue was no different either. The authors classified the result as strong evidence that elamipretide does not help with these things [4].
Phase 1/2 and phase 2 RCT The earlier, more hopeful signal did not hold up. In 36 adults the highest dose produced 64.5 metres against 20.4 metres on the dummy treatment, which just missed the mark statistically (p = 0.053) [16]. A trial in 30 adults who received both treatments in turn produced 19.8 metres, and missed the mark again (p = 0.0833) [17].
Three randomised trials: MMPOWER-3 in 218 adults over 24 weeks, a trial in 30 adults who received both treatments in turn, and an early trial in 36 adults. Every one of them failed on the question it was built to answer. The first two bars show the gap against dummy treatment, the last two the change within each group. Sources [4], [16], [17].
Digging through the MMPOWER-3 data afterwards turned up an apparent benefit in patients whose genetic fault lies in the nucleus rather than in the mitochondria themselves. Such an analysis after the fact proves nothing, but it became the reason for the next trial [18]. That follow-on trial in 102 patients has finished without posting any results [19].
Heart failure and heart attack
Phase 2a RCT EMBRACE STEMI gave the drug through a drip to 300 patients who had just had their first heart attack of a particular type. The amount of heart muscle destroyed, judged from an enzyme released by dying muscle, came out almost identical in the two groups, at 5570 against 5785 ng·h/mL. Imaging, X-ray studies of the arteries and how the patients actually fared showed no benefit either [5].
Phase 2 RCT PROGRESS-HF followed 71 patients over 28 days whose hearts pumped out too little blood with each beat. The amount of blood left in the main chamber after a beat differed from the dummy treatment by less than 3 millilitres at both doses tested (−0.3 mL and 2.3 mL), which is nothing [6].
An earlier study in 36 patients given a single drip reported that this volume shrank by 14 to 18 millilitres by the end of the infusion. Giving the drug repeatedly did not reproduce it [20].
Eye disease
Phase 2 RCT ReCLAIM-2 followed 176 patients aged 55 and over with the dry form of age-related macular degeneration for 48 weeks. Both of the trial's main questions came out negative: how well patients could see in dim light, and how fast the damaged patch of retina grew. Some side measurements looked better, but so many things were measured that such findings can easily arise by chance [7]. A larger trial in 313 patients has not reported [21].
Ageing and muscle function
Phase 2 RCT, single dose This trial looked at 39 adults aged 60 to 85 whose mitochondria worked poorly. After a single drip, the maximum amount of cellular fuel their muscles could produce rose by enough to count as more than chance (p = 0.045 for the percentage change). By day seven the difference had vanished. How long the muscle resisted fatigue, and how efficiently the mitochondria worked at rest, did not change at all [22]. This is the sole piece of human evidence behind the claims about ageing, and it describes an effect that disappeared within a week.
What is still unknown
- Whether the approved benefit is real at all. The trial that has to confirm it finishes in September 2029, and its final report is due in March 2030 [1].
- Cancer risk. Nobody had established whether the drug can cause cancer by the time it was approved. Two animal studies were made a condition, one reporting in 2026 and one in 2030 [1].
- Everyone unlike the twelve patients studied. They were all male, aged between 12 and 35, and almost all of the same ethnic background [2].
- The idea behind the eye trials. The large imaging trial has enrolled everyone it needs, and results are expected at the end of 2027 [21], [11].
- How long it stays in the body. None of the cited records gives a figure for how quickly the body clears it.
Side effects and safety
Everything known about safety at the time of approval comes from twelve male patients aged between 12 and 35, eleven of whom were recorded as white [2]. The figures below come from that group, comparing the drug with the dummy treatment.
| Event | Elamipretide | Placebo |
|---|---|---|
| Any injection site reaction | 12/12 | 8/12 |
| Injection site erythema | 12/12 | 3/12 |
| Injection site pain | 9/12 | 5/12 |
| Injection site induration | 8/12 | 2/12 |
| Injection site pruritus | 8/12 | 2/12 |
| Injection site bruising | 3/12 | 0/12 |
| Injection site urticaria | 3/12 | 0/12 |
Further findings across the programme:
- One type of white blood cell increases. The count of eosinophils, the cells involved in allergy, starts climbing around day 30 and peaks near day 90 at roughly 0.5 to 0.6 × 10³/µL above the starting level, before settling back over six to twelve months. No ill effects from it are described [2].
- One absolute bar to using it. A severe allergic reaction to any ingredient. The package insert tells patients to seek medical help at the first sign of a reaction [2].
- Not for newborns. The preserved form contains benzyl alcohol, which newborn babies cannot cope with [2].
- Gaps around the kidneys. The package insert records that there is too little information for adults on dialysis, and none at all for children with damaged kidneys [2].
- Injection site reactions in the other trials too. They affected 80% of participants in a trial in mitochondrial muscle disease, mostly mildly [17]. In the eye trial, side effects of any kind reached 86% against 71% on the dummy treatment [7].
Twelve patients cannot reveal rare risks, and they say nothing about women, young children or use beyond 192 weeks. The several hundred patients in the failed trials add some reassurance about the short term, but those studies were never designed to catch uncommon harms [4], [5], [6], [7].
Why this page lists no doses
Elamipretide is available only on prescription in the one country that has approved it, and it is still experimental everywhere else. Either way, the amounts and the practical details belong in the official product information or the trial protocol, and in the hands of the doctor working from them.
Development and approval status
Programme timeline
- 2004The Szeto-Schiller peptide series is publishedRef [10]
- 2016EMBRACE STEMI reports a negative resultn = 300 after a heart attack, ref [5]
- 2018 to 2021The myopathy programme ends negative; TAZPOWER is publishedn = 218 and n = 12, refs [4], [3]
- May 2025Complete response letter, then resubmission in AugustRef [1]
- 19 September 2025FDA accelerated approval for Barth syndromeUnited States only, ref [1]
- July 2026Phase 4 Barth syndrome study starts recruitingRef [24]
- March 2030Final report of the confirmatory trial dueCompletion due September 2029, ref [1]
- 2004 — the founding work on this family of peptides is published [10].
- 2013 — the idea that the peptide binds a specific fat in the mitochondrial membrane is described [13].
- 2016 — EMBRACE STEMI reports a negative result in 300 patients [5].
- 2018 to 2021 — the programme in mitochondrial muscle disease runs from an early hopeful signal to a large trial that fails [16], [4].
- 2021 — TAZPOWER is published, and Europe grants the substance rare disease status for Barth syndrome [3], [23].
- 2024 — the application goes to the American regulator on 29 January [1].
- 2025 — the regulator sends the application back in May, the company resubmits in August, and fast-track approval follows on 19 September [1].
- 2026 — the medicine reaches the market and the first prescriptions are written, and a follow-up study in Barth syndrome starts in July [11], [24].
| Market | Status | Since or note | Ref |
|---|---|---|---|
| United States | Approved | 19 September 2025 | [1] |
| European Union | Investigational | Orphan designations only | [23], [25] |
| Germany | Investigational | Follows EU route | [23] |
| United Kingdom | Investigational | No authorisation | [26] |
| Australia | Investigational | No registration | [26] |
| Canada | Investigational | No identification number | [26] |
| Switzerland | Investigational | No authorisation | [26] |
Every official register behind this table was queried on 6 September 2026.
What kind of approval this is matters as much as the fact that it exists. It came through a fast-track route, which lets a medicine onto the market on the basis of a measurement that is merely thought likely to predict a real benefit [1], [27]. The regulator attached four conditions: the confirmatory trial, two animal studies on cancer risk and a study of interactions with other medicines [1]. Independent reporting and a later commentary describe the decision as disputed [28], [29].
Anti-doping
The 2026 prohibited list names neither elamipretide nor the codes SS-31 and MTP-131 [30]. Category S0 bans at all times "any pharmacological substance which is not addressed by any of the subsequent sections of the List and with no current approval by any governmental regulatory health authority for human therapeutic use" [30]. The approval granted in 2025 removes the basis for that ban, but only for the approved medicine and only in one country. Material sold under the laboratory code is not an approved medicine, so the wording still covers it. Athletes should put the question to their national anti-doping body rather than rely on any general reading, including this one. The wider picture is at peptides banned in sport.
Compared with related peptides
| Peptide | What it is | Status | Strongest result in its own study |
|---|---|---|---|
| SS-31 (elamipretide) | Synthetic tetrapeptide, cardiolipin binder | Approved in the US for Barth syndrome only | Knee extensor strength, median +63 N at week 168, uncontrolled extension, n = 8 [2] |
| MOTS-c | Peptide encoded in mitochondrial DNA | Approved nowhere; first trial of the native peptide enrolling since 2026 | Reduced obesity and insulin resistance in mice; no completed human trial [31] |
| Humanin | Peptide encoded in mitochondrial DNA | Approved nowhere; no interventional trial identified | Measured as the body's own marker only [14] |
| SS-20 | Szeto-Schiller tetrapeptide without dimethyltyrosine | No clinical development | Differs from SS-31 in membrane binding and cell activity [32] |
These four appear in one table only because mitochondria come into all of them. SS-31 is a laboratory-built drug candidate that made it to an approval; the other three have never completed a controlled trial in people showing that they work. MOTS-c and humanin are made by the body from its own genes, and they differ from each other in sequence and in origin [14].
SS-20 is the clearest warning against carrying findings from one member of this peptide family to another. Studies comparing structure with effect found sizeable differences between family members, both in how firmly they bind to the membrane and in how well they restore mitochondrial function [32]. Bevemipretide, another compound from the same company, is in early testing as an eye drop and is approved nowhere [11].
Common misconceptions
- "The approval vouches for powder sold as SS-31." They are the same molecule but two entirely different things in law. The approved product has a checked content, a purity specification, evidence that it keeps, and a use-by date. None of that has been verified for the powder [1], [2].
- "You can buy the approved medicine as a research chemical." No. It requires a prescription in the United States and is unapproved everywhere else. Anyone selling it under the laboratory code is not supplying the medicine that was examined [1], [26].
- "Its status in sport is settled." Shop pages state a clear position for 2026. In fact the list names none of the three identifiers, and the general ban rested on the absence of any approval anywhere, which has now changed in one country [30].
- "It is the first mitochondrial therapy, so it must help mitochondria in general." The approval covers one disease and one measurement. Trials in mitochondrial muscle disease, heart attack, heart failure and macular degeneration all failed [4], [5], [6], [7].
- "It works for ageing and fitness." A single trial with one dose raised a measure of cellular fuel that had vanished by day 7, with no change in what people could actually do [22].
- "Bendavia was a different drug." Bendavia was simply the name this same substance carried during the heart programme, and it was never approved [5], [8].
Frequently asked questions
Is SS-31 the same substance as elamipretide?
Yes. SS-31 is the laboratory code for the substance whose official international name is elamipretide. The same molecule also turns up as MTP-131, RX-31, SBT-131 and Bendavia. Being chemically identical does not make powder sold under the code the same thing as an approved medicine.
Is SS-31 approved by the FDA?
Elamipretide was given a fast-track approval on 19 September 2025 for Barth syndrome, in patients weighing at least 30 kg. The approval covers that one disease and nothing else. It rests on a measurement of muscle strength rather than on a proven benefit to patients, and the regulator made a confirmatory trial a condition of granting it.
Is elamipretide approved in Europe?
No. There is no European approval, and none in the United Kingdom, Australia, Canada or Switzerland either. Europe has granted it rare disease status twice, in 2021 and 2022. That status is an incentive to keep developing a drug for a small group of patients, not permission to sell it.
What did the elamipretide trials outside Barth syndrome show?
They all failed to show what they set out to show. In 218 adults with a mitochondrial muscle disease, patients on the drug walked 3.2 metres less far than those on the dummy treatment. In 300 patients after a heart attack the damaged area of heart muscle was no smaller, and in 71 patients with heart failure the heart chambers did not change. A trial in 176 patients with dry macular degeneration came out negative on both of its main questions.
What is the evidence behind the elamipretide Barth syndrome approval?
A single trial and the phase that followed it. TAZPOWER randomly assigned twelve patients and came out negative on both of its main questions. The finding on knee strength comes from the open continuation afterwards, which had no comparison group and was down to eight patients by week 168. A continuation like that cannot separate the drug from the natural course of the illness or from patients getting better at the test through practice.
Is SS-31 banned in sport?
The 2026 prohibited list names neither elamipretide nor SS-31 nor MTP-131. Before September 2025 the substance was caught by category S0, which covers everything that no health authority anywhere has approved. The American approval removes that basis, but only for the approved medicine itself. Material sold under the laboratory code is not an approved medicine, so the category still covers it.
What are the reported SS-31 side effects?
Reactions where the needle goes in dominate. All twelve treated patients in the approval data had one, against eight of the twelve on dummy treatment. One type of white blood cell involved in allergy rises from around day 30, peaks near day 90 and then falls back. The only absolute bar to using the drug is a severe allergic reaction to it, and whether it can cause cancer has never been established.
Why does this page list no SS-31 doses?
Elamipretide requires a prescription in the one country that has approved it, and it is still experimental everywhere else. Either way, the amounts belong in the official product information or the trial protocol, and in the hands of the doctor working from them.
How should material sold as SS-31 be understood?
As something quite different from the approved medicine, even though the chemical name is the same. The approved product has a checked content, a specified purity, evidence that it keeps and a use-by date. None of that has been verified for powder sold under the laboratory code, and the approval granted in 2025 says nothing whatsoever about it.
Sources
- US Food and Drug Administration (2025). Approval letter, NDA 215244, accelerated approval, elamipretide injection, 19 September 2025. https://www.accessdata.fda.gov/drugsatfda_docs/appletter/2025/215244Orig1s000ltr.pdf
- US Food and Drug Administration (2025). Elamipretide injection, prescribing information, revised 9/2025, NDA 215244. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/215244s000lbl.pdf
- Reid Thompson W, Hornby B, Manuel R, Bradley E, Laux J, Carr J, Vernon HJ (2021). A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome. Genetics in Medicine 23(3):471-478. PMID 33077895. DOI 10.1038/s41436-020-01006-8
- Karaa A, Bertini E, Carelli V, et al. (2023). Efficacy and safety of elamipretide in individuals with primary mitochondrial myopathy: the MMPOWER-3 randomized clinical trial. Neurology 101(3):e238-e252. PMID 37268435. DOI 10.1212/WNL.0000000000207402
- Gibson CM, et al. (2016). EMBRACE STEMI study: a phase 2a trial to evaluate the safety, tolerability, and efficacy of intravenous MTP-131 on reperfusion injury in patients undergoing primary percutaneous coronary intervention. European Heart Journal 37(16):1296-1303. PMID 26586786. DOI 10.1093/eurheartj/ehv597
- Butler J, et al. (2020). Effects of elamipretide on left ventricular function in patients with heart failure with reduced ejection fraction: the PROGRESS-HF phase 2 trial. Journal of Cardiac Failure 26(5):429-437. PMID 32068002. DOI 10.1016/j.cardfail.2020.02.001
- Ehlers JP, et al. (2025). ReCLAIM-2: a randomized phase II clinical trial evaluating elamipretide in age-related macular degeneration, geographic atrophy growth, visual function, and ellipsoid zone preservation. Ophthalmology Science 5(1):100628. PMID 39605874. DOI 10.1016/j.xops.2024.100628
- PubChem. Compound summary CID 11764719, elamipretide. Retrieved 6 September 2026. https://pubchem.ncbi.nlm.nih.gov/compound/11764719
- FDA Global Substance Registration System. Elamipretide, UNII 87GWG91S09. Retrieved 6 September 2026. https://gsrs.ncats.nih.gov/ginas/app/beta/substances/87GWG91S09
- Zhao K, Zhao GM, Wu D, Soong Y, Birk AV, Schiller PW, Szeto HH (2004). Cell-permeable peptide antioxidants targeted to inner mitochondrial membrane inhibit mitochondrial swelling, oxidative cell death, and reperfusion injury. Journal of Biological Chemistry 279(33):34682-34690. PMID 15178689. DOI 10.1074/jbc.M402999200
- Stealth BioTherapeutics (2026). Commercial and R&D pipeline update and 2026 strategic priorities, 6 January 2026. https://www.biospace.com/press-releases/stealth-biotherapeutics-provides-commercial-and-r-d-pipeline-update-and-outlines-2026-strategic-priorities
- DrugBank. Elamipretide, DB11981. Retrieved 6 September 2026. https://go.drugbank.com/drugs/DB11981
- Birk AV, Liu S, Soong Y, Mills W, Singh P, Warren JD, Seshan SV, Pardee JD, Szeto HH (2013). The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin. Journal of the American Society of Nephrology 24(8):1250-1261. PMID 23813215. DOI 10.1681/ASN.2012121216
- Merry TL, Kim SJ, Miller B, Kumagai H, Yen K, Cohen P (2020). Mitochondrial-derived peptides in energy metabolism. American Journal of Physiology: Endocrinology and Metabolism 319(4):E659-E666. PMID 32776825. DOI 10.1152/ajpendo.00249.2020
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Cite this page
Facts on this page were verified on 6 September 2026, and every register query behind the status table was run on that date. A substance with one recent approval and an open confirmatory trial changes quickly, so the version and date matter.
myPeptides Research & Editing. (2026). SS-31 (elamipretide): what the trials show, status and safety. Version 1.0, 6 September 2026. myPeptides Peptide Register. Retrieved from https://mypep.app/peptides/ss-31
How pages in this register are compiled and graded is described under methodology; the full register is at peptides.
| Version | Date | Change |
|---|---|---|
| 1.0 | 2026-09-06 | Initial publication |
Last verified: 6 September 2026. Next review: on results from the confirmatory Barth syndrome trial, on any new marketing authorisation, or on any change in the anti-doping classification.