SS-31 (Elamipretide): What the Research Actually Shows

What It Is

A Peptide That Targets a Membrane Lipid, Not a Receptor

Elamipretide, also known as SS-31 or by its earlier research name MTP-131, is a small, cell-permeable peptide built from alternating positively charged and aromatic amino acids. Unlike most peptides in this category, it doesn't act through a cell-surface receptor. It binds directly and selectively to cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane.

Cardiolipin's job is structural. It organizes the individual protein complexes of the electron transport chain into larger, more efficient assemblies called supercomplexes, and it helps hold the membrane's characteristic folded cristae in shape. When cardiolipin is damaged by oxidation, the same reactive oxygen species the electron transport chain generates as a normal byproduct, that structure degrades: cristae flatten, supercomplexes come apart, electron transport becomes leaky, and the cell ends up with less ATP and more oxidative stress at the same time. Elamipretide's binding to cardiolipin is thought to protect it from that oxidative damage, preserve cristae architecture, and support more efficient ATP production with less electron leak (Tung et al., 2025).

The Evidence

The FDA Approval, and What the Broader Trial Record Shows

The FDA Approval: Barth Syndrome

In September 2025, the FDA granted accelerated approval to Forzinity (elamipretide HCl) to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg, based on improvement in knee extensor muscle strength, an intermediate clinical endpoint. It is the first approved treatment for Barth syndrome (FDA/Stealth BioTherapeutics, 2025). Barth syndrome is a rare, X-linked genetic disorder caused by mutations in the tafazzin gene, which is directly responsible for remodeling cardiolipin in the inner mitochondrial membrane. It affects an estimated 150 people in the United States, almost all male, and causes cardiomyopathy, skeletal muscle weakness, and growth delay. Mechanistically, this is about as direct a match between drug and disease as exists anywhere in the elamipretide evidence base: a drug that stabilizes cardiolipin, approved for a disease caused by a defect in cardiolipin itself.

The approval rests substantially on the TAZPOWER trial's 168-week open-label extension, in which participants who continued elamipretide long-term showed durable improvements in six-minute walk distance, handheld-dynamometry muscle strength, and balance and mobility measures, alongside clinically meaningful improvements in left ventricular volumes and stroke volume. Heart rate, blood pressure, ejection fraction, and ECG findings showed no statistically significant changes across the extension period, and the most commonly reported adverse event was injection site reactions (Thompson et al., 2024).

Open-Label Extension, 168 Weeks · Barth Syndrome, Cardiolipin Remodeling Defect · Basis for FDA Accelerated Approval

Acute Heart Attack: EMBRACE-STEMI

Gibson CM, Giugliano RP, Kloner RA, et al. "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. 2016;37(16):1296–1303. In 118 patients undergoing emergency angioplasty for a large heart attack, intravenous elamipretide given at the time of reperfusion did not reduce infarct size compared with placebo, the trial's primary endpoint.

Phase 2a, Randomized, Placebo-Controlled · N=118 · Primary Endpoint: Infarct Size · Missed Primary Endpoint

Heart Failure: PROGRESS-HF

Butler J, Khan MS, Anker SD, et al. "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. 2020;26(5):429–437. Four weeks of elamipretide did not reduce left ventricular end-systolic volume, the trial's primary endpoint, in patients with heart failure and reduced ejection fraction. Secondary measures of ejection fraction and end-diastolic volume also showed no statistically significant improvement. The authors noted that a shorter treatment window than the three months used in supporting preclinical work may partly explain the result.

Phase 2, Randomized, Placebo-Controlled · Primary Endpoint: LVESV · Missed Primary Endpoint

Primary Mitochondrial Myopathy: MMPOWER-3

Karaa A, Bertini E, Carelli V, et al. "Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial." Neurology. 2023;101(3):e238–e252. In 218 patients with primary mitochondrial myopathy from a range of underlying genetic causes, elamipretide did not improve six-minute walk distance or a validated fatigue score, its two co-primary endpoints. A post hoc analysis found that the subgroup of participants with nuclear DNA-related mitochondrial defects, as opposed to mitochondrial DNA defects, showed a 25.2-meter improvement in six-minute walk distance favoring elamipretide, a genotype-specific signal worth confirming in a prospective trial.

Phase 3, Randomized, Placebo-Controlled · N=218 · Co-Primary Endpoints: 6MWT and Fatigue Score · Missed Primary Endpoints, Positive Post Hoc Subgroup

Dry Macular Degeneration: ReCLAIM-2

Ehlers JP, Hu A, Boyer D, Cousins SW, Waheed NK, Rosenfeld PJ, et al. "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. 2025;5(1):100628. In patients with geographic atrophy from age-related macular degeneration, elamipretide did not slow geographic atrophy growth or preserve best-corrected visual acuity, its co-primary endpoints. Secondary analyses did find better preservation of the retinal ellipsoid zone and a higher proportion of visual-function responders in the treatment group.

Phase 2, Randomized, Placebo-Controlled · Co-Primary Endpoints: GA Growth and BCVA · Missed Primary Endpoints, Positive Secondary Findings

Put plainly: the cardiolipin-stabilization mechanism is real, and it has already produced one validated clinical benefit, in a disease where the drug's molecular target and the disease's genetic cause are literally the same molecule. In four other conditions where mitochondrial dysfunction plays a contributing rather than causative role, the primary endpoint hasn't yet been met, though multiple trials turned up secondary or subgroup findings worth building on. Every one of those four trials tested elamipretide against an active disease process, not against aging itself. That distinction matters, because the research aimed specifically at aging tells a considerably more encouraging story.

Honest Limits

What Hasn't Been Tested in Humans Yet

Every completed trial in a specific disease, Barth syndrome and the four conditions above, enrolled patients with that diagnosed disease. One small trial has now tested elamipretide in older adults without a diagnosed disease (Roshanravan et al., 2021 above), but it used a single infusion, measured mitochondrial function directly rather than energy, cognitive performance, or longevity, and its effect faded within a week. No trial of any duration has tested elamipretide for sustained improvements in general energy, cognitive performance, or longevity in people without a diagnosed condition, which is how it is most often marketed outside its approved use. The aging research above is a genuinely encouraging reason to expect that gap gets tested further, but animal results in this space haven't always translated cleanly to humans, as the four disease trials above show, and the aging findings so far come substantially from one research program rather than independent replication.

Outside Barth syndrome, elamipretide is investigational and not FDA-approved for any other indication.

Sources

References