SS-31 (elamipretide): what the approved drug does, and what the research vials are

Elamipretide is a tetrapeptide: D-arginine, 2,6-dimethyltyrosine, lysine and phenylalanine, with the end capped as an amide ^[1]. Two of the four building blocks are not the standard forms found in human proteins: the arginine is the mirror-image D-form, and the tyrosine carries two extra methyl groups. The molecule was designed to enter cells and reach the inner membrane of mitochondria ^[8].

It belongs to the Szeto-Schiller peptides, named after the two researchers who developed them; "SS-31" is its number in that series ^[8]. During development it was also called MTP-131 and Bendavia. Stealth BioTherapeutics developed it and holds the approval ^[1].

Inside each mitochondrion, the inner membrane is folded into ridges where the chain of proteins that makes the cell's energy sits. Cardiolipin holds that chain together and keeps the folds in shape. SS-31 carries positive charges and a water-avoiding side, which together draw it to cardiolipin. By binding there it is thought to keep the energy chain intact and to stop a protein called cytochrome c from turning into an enzyme that damages the membrane ^[8].

Barth syndrome is a direct test of that idea. A faulty gene, TAZ, leaves people with abnormal cardiolipin, and the result is weak heart and skeletal muscle, fatigue and low white blood cell counts. If a drug can stabilize cardiolipin, these patients are where it should show.

The approval rests on one small program.

The randomized part. Twelve males aged 12 to 35 took elamipretide 40 mg a day for 12 weeks and placebo for 12 weeks, in random order, with a four-week break between. Neither main goal was met: the drug did not beat placebo on the six-minute walk or on a fatigue score ^[3]. Knee muscle strength did not improve during this part either ^[1].

The open-label extension. Ten of the twelve continued on the drug, with everyone knowing what they were taking. Eight reached week 168. Over that time, walking distance rose by about 96 meters and knee strength by a median of 63 newtons, from about 124 at the start ^[1][4]. Without a placebo group, gains over three years cannot be separated from growth, practice or other care; most of the participants were teenagers or young adults.

FDA granted accelerated approval on the knee-strength results, which it treats as an intermediate measure likely to predict real benefit. The label says continued approval may depend on a confirmatory trial ^[1]. A Phase 4 study in Barth syndrome began recruiting in July 2026 ^[9].

The approved indication is muscle strength in patients who weigh at least 30 kg. It is not approved for the heart problems of Barth syndrome, or for anything else ^[1].

Stealth tested elamipretide in several conditions before Barth syndrome. ClinicalTrials.gov lists 31 registered studies, most of them sponsored by Stealth ^[9]. The published results:

The muscle-disease result matters most for how SS-31 is marketed. MMPOWER-3 was the largest trial, in a disease defined by weak mitochondria, and the drug did no better than placebo on walking distance: the gap was 3.2 meters, with a p-value of 0.69 ^[5]. A Phase 3 trial in dry age-related macular degeneration, ReNEW, had enrolled 313 people and was still running in November 2026, with completion listed for September 2027 ^[10].

There is no trial of SS-31 in healthy people for energy, endurance or aging. One academic study, started in November 2025, is testing it for physical function in older adults ^[9].

The label's safety data come from the 12 patients with Barth syndrome ^[1]. Injection-site reactions were near universal: redness at the site in all 12 on the drug against 3 of 12 on placebo, hardening in 8 against 2, itching in 8 against 2 and pain in 9 against 5. Raised eosinophils, a type of white blood cell, were common after a month or more and peaked around 90 days, then settled; they caused no symptoms ^[1].

The label warns of allergic reactions, some needing emergency treatment, that can start minutes to months after treatment begins. It also warns that the preservative, benzyl alcohol, is dangerous to newborns. The dose is halved for adults with severely reduced kidney function ^[1]. The larger trials in other conditions reported the drug as generally well tolerated ^[5][6].

None of this tells you about a research-peptide vial, which is made by an unknown manufacturer at an unknown purity.

SS-31 is sold online in vials labeled "for research use only." In August 2026, FDA sent warning letters to five research-peptide sellers on one day, and four of the five letters named SS-31 among the unapproved drugs being sold ^[11].

Now that elamipretide is part of an approved drug, the law gives a 503A pharmacy a route to compound with it: a bulk substance that is a component of an FDA-approved drug may be used ^[12]. That does not make every compounded product lawful. The pharmacy needs a patient-specific prescription and a supplier registered with FDA, and it may not regularly make what is essentially a copy of Forzinity. At least one telehealth seller we review, RxPepsDirect, lists SS-31 in a blend with MOTS-c and tesamorelin; a blend gets none of the evidence behind the approved product.

Forzinity itself is a prescription drug for one rare disease. For anyone without Barth syndrome, there is no approved use.

SS-31 is one of the few peptides sold in the research market that has reached FDA approval, and that is worth knowing. The approval is narrow, provisional and built on very few patients. Where it was tested in larger trials, in muscle disease and heart failure, it did not work better than placebo. The case for taking it for energy or aging is a mechanism, not a result.

For the wider mitochondrial-peptide picture see MOTS-c and NAD+; for the list of approved peptide drugs, FDA-approved peptide drugs.

Counted from PubMed and ClinicalTrials.gov on October 9, 2026. A study counts only if participants were given SS-31. How we count, and the same figures for every peptide we cover.

Most recent published trials

• Initial Psychometric Evaluation of the Barth Syndrome Symptom Assessment (BTHS-SA) for Adolescents and Adults in a Phase 2 Clinical Study. Orphanet J Rare Dis, 2025.
• Genotype-specific effects of elamipretide in patients with primary mitochondrial myopathy: a post hoc analysis of the MMPOWER-3 trial. Orphanet J Rare Dis, 2024.
• Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER. Genet Med, 2024.
• Elamipretide Topical Ophthalmic Solution for the Treatment of Subjects with Leber Hereditary Optic Neuropathy: A Randomized Trial. Ophthalmology, 2024.
• Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial. Neurology, 2023.

Most recent registrations

• Study to Evaluate Pharmacokinetics, Safety, and Tolerability of Subcutaneous Injections of Elamipretide in Pediatric Subjects With Barth Syndrome NCT07927972, not yet recruiting, planned start 2026.
• Clinical Trial in Patients With Barth Syndrome- 4TAZPower NCT07531251, recruiting, started 2026.
• Study of Healthy Aging and Physical Function With Elamipretide NCT07275424, recruiting, started 2025.
• ReNEW:Phase 3 Study of Efficacy, Safety & Pharmacokinetics of Subcutaneous Injections of Elamipretide in Subjects With Dry Age-Related Macular Degeneration (Dry AMD) NCT06373731, active, not recruiting, started 2024.
• Study to Evaluate Efficacy and Safety of Elamipretide in Subjects With Primary Mitochondrial Disease From Nuclear DNA Mutations (nPMD) NCT05162768, completed, started 2022.

A registration is a plan filed before a study starts, not a result.