What it is
SS-31 is a synthetic tetrapeptide belonging to the Szeto-Schiller (SS) family of mitochondria-targeted compounds, named for its developers Hazel Szeto and Peter Schiller. Its sequence is D-Arg-2',6'-Dmt-Lys-Phe-NH2, where 2',6'-Dmt denotes 2',6'-dimethyltyrosine. The alternating aromatic and cationic residues allow it to penetrate cell membranes and concentrate selectively in the inner mitochondrial membrane, driven by the large electrochemical gradient across that membrane. Once there, it binds cardiolipin, a phospholipid unique to the inner mitochondrial membrane that is essential for organizing the electron transport chain complexes. [1]
Cardiolipin stabilization is the proposed central mechanism. When cardiolipin is oxidized (a consequence of mitochondrial oxidative stress), electron transport chain efficiency falls and reactive oxygen species production increases in a self-amplifying cycle. SS-31 interrupts this cycle by scavenging electrons before they can react with oxygen and by preserving cardiolipin's structural role. The compound is also known as elamipretide in clinical development (where it was studied under the name MTP-131 or Bendavia). Under the brand name Forzinity, elamipretide received FDA accelerated approval in September 2025 for improving muscle strength in patients with Barth syndrome, a rare genetic mitochondrial cardiomyopathy, making it the first FDA-approved mitochondria-targeted therapeutic. [6] Outside Barth syndrome, elamipretide remains investigational: Phase 2 trials in heart failure with preserved and reduced ejection fraction did not meet their primary endpoints, and it is not approved for any indication beyond Barth syndrome.
What researchers study it for
- Mitochondrial dysfunction and cardiolipin stabilization The foundational mechanism of SS-31 was characterized by Szeto and Birk, who showed that SS peptides restore mitochondrial cristae architecture, improve electron transport chain coupling, and reduce mitochondrial reactive oxygen species without directly scavenging them. [1] The selectivity for cardiolipin explains why the compound concentrates in mitochondria rather than distributing nonspecifically.
- Hypertensive cardiomyopathy and heart protection In mouse models of pressure-overload hypertrophy and Gaq-overexpression cardiomyopathy, SS-31 treatment was associated with attenuation of cardiac fibrosis and mitochondrial ultrastructural damage. [3] The JACC study by Dai et al. positioned SS-31 as a potential tool for studying mitochondrial contributions to target organ damage in chronic hypertension.
- Cardiac aging and age-related protein modifications In aged mouse hearts, elamipretide treatment reduced the accumulation of oxidative post-translational modifications on key cardiac proteins including those involved in contractile function and energy metabolism. [4] This work from the University of Washington identified specific protein targets whose modification patterns change with SS-31 treatment, providing mechanistic insight into the age-cardiac phenotype.
- Skeletal muscle aging and mitochondrial ADP sensitivity Aged skeletal muscle shows impaired mitochondrial responsiveness to ADP, a key driver of the energy deficit underlying age-related functional decline. A GeroScience study found that elamipretide rapidly improved mitochondrial ADP sensitivity in aged muscle mitochondria, with effects detectable at the physiological level in both rodent and human muscle samples. [5]
- Neuroinflammation and memory in preclinical models In LPS-induced neuroinflammation mouse models, elamipretide treatment was associated with improved mitochondrial function in hippocampal tissue, reductions in synaptic protein loss, and attenuation of memory impairment on behavioral testing. [2] The study positioned mitochondrial dysfunction as a mediator of neuroinflammation-driven cognitive effects.
Research context
SS-31's evidence base spans a wide range of preclinical models and has progressed further into human trials than most research peptides, though it remains without an approved indication. The EMPOWER-HF Phase II trial and related studies examined elamipretide in heart failure with preserved ejection fraction (HFpEF), a condition where mitochondrial dysfunction is increasingly recognized as a driver. Results from those trials informed the mechanistic picture but did not yield an FDA approval. [1] [2]
The strongest mechanistic data come from rodent cardiac models and isolated mitochondria preparations from both rodents and human samples. The skeletal muscle ADP sensitivity findings are notable because they used human tissue, lending translational weight to the preclinical cardiac and neurological data. [5] Researchers reviewing this compound should distinguish between the rich in vitro and animal literature and the smaller, still-developing human clinical dataset. The research-grade compound sold by peptide vendors is not equivalent to the pharmaceutical-grade formulations used in clinical trials.
Clinical trial summary
Elamipretide has been tested across a wider range of human trials than most research peptides. Results are mixed: one clear success, several endpoint misses, and a couple of positive early-phase signals. Researchers should read this table alongside the mechanistic literature above, not instead of it.
| Trial / Indication | Phase | Outcome |
|---|---|---|
| TAZPOWER (Barth syndrome) | II/III | Positive. Significant improvement in knee extensor muscle strength and 6-minute walk test. Basis for FDA accelerated approval of Forzinity in September 2025. [6] |
| MMPOWER-2 (primary mitochondrial myopathy) | II | Signal of benefit on functional and patient-reported measures in a 30-person crossover trial; supported moving to Phase 3. [7] |
| MMPOWER-3 (primary mitochondrial myopathy) | III | Failed primary endpoints (6-minute walk test, Total Fatigue Score) in 218 participants. A pre-specified subgroup with nuclear DNA pathogenic variants showed improvement; those with mitochondrial DNA variants did not. [8] |
| PROGRESS-HF (heart failure, reduced ejection fraction) | II | Failed. No significant improvement in left ventricular end-systolic volume at 4 weeks versus placebo, at either 4 mg or 40 mg daily dosing. [9] |
| EMBRACE-STEMI (acute heart attack) | IIa | Failed on primary endpoint (infarct size). Reduced incidence of heart failure within 24 hours post-procedure was a secondary finding. [10] |
| Renal artery stenosis (stent revascularization) | IIa | Positive in a small pilot (14 participants). Attenuated post-procedural hypoxia and improved renal blood flow and kidney function versus placebo. [11] |
The pattern across trials: elamipretide's clearest human benefit is in Barth syndrome, a rare disease where cardiolipin dysfunction is the direct genetic cause. In broader cardiac and mitochondrial-disease populations, results are inconsistent; this fits a compound whose benefit may depend heavily on how central cardiolipin damage is to the specific condition being treated.
Typical research parameters
| Parameter | Detail |
|---|---|
| Common vial sizes | 5 mg, 10 mg |
| Supplied as | Lyophilized (freeze-dried) powder |
| Reconstitution | Sterile saline (0.9% NaCl) or sterile water |
| Storage (lyophilized) | Refrigerated (2–8°C); stable at -20°C long-term; avoid repeated freeze-thaw |
| Administration studied | Subcutaneous injection and intravenous infusion in clinical trials; subcutaneous in most animal studies [3] |
| Mitochondrial targeting | Concentrates in inner mitochondrial membrane driven by membrane potential; no external targeting sequence required [1] |
References
- [1] Szeto HH, Birk AV. Serendipity and the discovery of novel compounds that restore mitochondrial plasticity. Clinical Pharmacology and Therapeutics. 2014;96(6). PubMed ↗
- [2] Zhao W, Xu Z, Cao J, et al. Elamipretide (SS-31) improves mitochondrial dysfunction, synaptic and memory impairment induced by lipopolysaccharide in mice. Journal of Neuroinflammation. 2019;16. PubMed ↗
- [3] Dai DF, Chen T, Szeto H, et al. Mitochondrial targeted antioxidant Peptide ameliorates hypertensive cardiomyopathy. Journal of the American College of Cardiology. 2011;58. PubMed ↗
- [4] Whitson JA, Martín-Pérez M, Zhang T, et al. Elamipretide (SS-31) treatment attenuates age-associated post-translational modifications of heart proteins. GeroScience. 2021;43. PubMed ↗
- [5] Pharaoh G, Kamat V, Kannan S, et al. The mitochondrially targeted peptide elamipretide (SS-31) improves ADP sensitivity in aged mitochondria by increasing uptake through the adenine nucleotide translocator (ANT). GeroScience. 2023;45. PubMed ↗
- [6] U.S. Food and Drug Administration. FDA Grants Accelerated Approval to First Treatment for Barth Syndrome. Press Announcement, September 19, 2025. FDA.gov ↗
- [7] Karaa A, Haas R, Goldstein A, Vockley J, Cohen BH. A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy (MMPOWER-2). Journal of Cachexia, Sarcopenia and Muscle. 2020;11(4):909-918. PMC ↗
- [8] 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. PMC ↗
- [9] 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. PubMed ↗
- [10] Gibson CM, et al. EMBRACE-STEMI study: safety and tolerability of intravenous MTP-131 during primary percutaneous coronary intervention. European Heart Journal. 2016;37:1296–1303. doi:10.1093/eurheartj/ehv597
- [11] Phase 2a Clinical Trial of Mitochondrial Protection (Elamipretide) During Stent Revascularization in Patients With Atherosclerotic Renal Artery Stenosis. Circulation: Cardiovascular Interventions. 2017. PubMed ↗