What Is SS-31?

SS-31 belongs to a class called Szeto-Schiller peptides, named after the researchers Hazel Szeto and Peter Schiller who developed them in the early 2000s. The compound is a tetrapeptide, meaning it is built from four amino acids arranged in the sequence D-Arg-Dmt-Lys-Phe-NH2. That sequence gives SS-31 an alternating aromatic-cationic structure that is central to how researchers believe it works.

The peptide carries a net positive charge at physiological pH, which is thought to drive it toward the strongly negative electrical potential of the inner mitochondrial membrane. Once there, it is believed to bind to cardiolipin, a phospholipid found almost exclusively in that membrane. Cardiolipin plays a structural role in organizing the electron transport chain complexes and is easily oxidized under conditions of cellular stress. Its oxidation is associated with mitochondrial dysfunction across a wide range of disease models.

SS-31 is also known by the development name elamipretide and was studied under the brand name Bendavia by Stealth BioTherapeutics. It is a research compound. No formulation of SS-31 or elamipretide has received FDA approval for any condition as of mid-2025, and it should not be treated as equivalent to any approved prescription drug.

How Do Researchers Think SS-31 Works?

The leading mechanistic hypothesis centers on cardiolipin stabilization. Cardiolipin normally keeps cytochrome c anchored to the inner mitochondrial membrane, where it functions as an electron carrier. When cardiolipin is oxidized, cytochrome c can detach and trigger apoptotic signaling. In cell culture and animal studies, SS-31 has been shown to reduce cardiolipin oxidation, which researchers interpret as a way the peptide may preserve electron transport chain efficiency and reduce the leak of reactive oxygen species.

A 2014 paper in the Journal of Biological Chemistry by Birk and colleagues used isolated mitochondria to show that SS-31 binding to cardiolipin altered the lipid's interaction with cytochrome c in a way that reduced peroxidase activity without blocking electron transfer. That in-vitro finding has been cited widely as a mechanistic anchor for the broader SS-31 research program, though in-vitro results do not confirm the same effects occur in living organisms at comparable concentrations.

Some researchers have also proposed that SS-31 improves mitochondrial cristae architecture, the folded inner-membrane structures where ATP synthesis occurs. A 2020 study in Nature Communications by Sabbah and colleagues, conducted in dogs with heart failure, reported improvements in cristae density alongside functional cardiac improvements. That study involved a small number of animals and its findings cannot be generalized to humans.

What Does the Animal Research Show?

The bulk of SS-31 research is preclinical. Studies in rodents have examined the peptide across models of ischemia-reperfusion injury, acute kidney injury, heart failure, and age-related skeletal muscle decline. In a 2013 study published in Aging Cell, Siegel and colleagues reported that old mice treated with SS-31 showed improvements in skeletal muscle mitochondrial function and exercise tolerance compared to untreated old mice. The sample sizes in that study were small, and the findings have not been replicated in large controlled animal trials.

Kidney research has been another active area. A 2013 study in the Journal of the American Society of Nephrology by Szeto and colleagues used a rat model of chronic kidney disease and reported that SS-31 reduced tubular cell mitochondrial fragmentation and slowed disease progression markers. Rodent models of kidney disease do not map cleanly onto human chronic kidney disease, and the translational relevance remains uncertain.

Animal studies are useful for generating hypotheses and understanding mechanisms, but they are not evidence that a compound works in humans. Peptides that show strong effects in rodents frequently fail to replicate those effects in human trials, and the SS-31 research record includes examples of that gap, particularly in the cardiovascular space.

Has SS-31 Been Studied in Humans?

Yes, SS-31 has reached human clinical trials, which places it ahead of most research peptides on the evidence ladder. The most extensively studied indication was heart failure with preserved ejection fraction (HFpEF). Stealth BioTherapeutics ran a Phase 2 trial called PROGRESS-HFpEF, which enrolled 71 patients and tested subcutaneous elamipretide against placebo. Results published in 2020 in JACC: Heart Failure showed no statistically significant improvement in the primary endpoint of six-minute walk distance, though some secondary measures trended in a favorable direction. The company subsequently discontinued its HFpEF program.

A separate and more encouraging human data set comes from Barth syndrome, a rare X-linked genetic disorder of cardiolipin metabolism. A 2020 open-label trial published in JCI Insight by Thompson and colleagues enrolled 12 patients and reported improvements in skeletal muscle strength and exercise capacity after 12 weeks of elamipretide. Open-label trials without a placebo arm are a lower tier of evidence than randomized controlled trials, and the small sample size limits what conclusions can be drawn, but the Barth syndrome findings are considered among the more compelling human signals in the SS-31 literature.

A Phase 2 trial in primary mitochondrial myopathy (MMPOWER-3) enrolled 218 patients and was randomized and placebo-controlled, making it the largest and most rigorous SS-31 human trial to date. Results published in 2021 in Neurology showed no significant improvement on the primary endpoint. The trial did report some patient-reported outcome improvements, but the overall result was not considered sufficient to support regulatory filing. These outcomes illustrate why preclinical promise does not reliably translate to human benefit.

Honest Limits of the Evidence

SS-31 has a more developed research record than most peptides discussed in the research community, including multiple randomized human trials. That is a meaningful distinction. Even so, the compound's two largest placebo-controlled trials, in HFpEF and primary mitochondrial myopathy, did not meet their primary endpoints. Researchers continue to investigate whether specific subpopulations, delivery methods, or disease stages might yield different results, but those questions remain open.

Most of the positive findings in the SS-31 literature come from animal studies or small open-label human studies, both of which sit lower on the evidence hierarchy than large randomized controlled trials. In-vitro mechanistic work is valuable for understanding how a compound might act, but it does not establish clinical benefit. Readers should weight the evidence accordingly.

SS-31 is not available as an approved drug. Research-grade versions sold by peptide suppliers are not manufactured under the same quality controls as pharmaceutical-grade compounds used in clinical trials, and their purity, potency, and safety profiles are not independently verified. The research record described here reflects studies using pharmaceutical-grade material under controlled conditions, which is a different context from unregulated research chemical supply chains.

Frequently asked questions

Is SS-31 the same thing as elamipretide?

Yes. SS-31 is the original laboratory designation for the compound. Elamipretide is the international nonproprietary name assigned during clinical development, and Bendavia was the brand name used by Stealth BioTherapeutics during its drug development program. All three names refer to the same tetrapeptide with the sequence D-Arg-Dmt-Lys-Phe-NH2.

Why did Stealth BioTherapeutics stop developing SS-31 for heart failure?

The PROGRESS-HFpEF Phase 2 trial, which enrolled 71 patients with heart failure with preserved ejection fraction, did not meet its primary endpoint of improving six-minute walk distance. After that result and the subsequent failure of the MMPOWER-3 trial in primary mitochondrial myopathy to meet its primary endpoint, Stealth BioTherapeutics ceased operations in 2022. The compound is still studied by academic researchers, particularly in Barth syndrome and aging-related contexts.

What makes SS-31 different from antioxidant supplements like CoQ10?

SS-31 is designed to concentrate specifically inside the inner mitochondrial membrane rather than distributing broadly through the body. Its proposed mechanism involves direct interaction with cardiolipin at the site where mitochondrial oxidative stress originates. CoQ10 supplementation increases circulating coenzyme Q10 levels but does not selectively target the inner mitochondrial membrane in the same way. Whether that targeting difference produces meaningful clinical advantages has not been established in head-to-head human trials.

Sources

  1. Birk et al., 2014, Journal of Biological Chemistry In-vitro mechanistic study on SS-31 and cardiolipin
  2. Szeto et al., 2013, Journal of the American Society of Nephrology Rat model study of SS-31 in chronic kidney disease
  3. Thompson et al., 2021, JCI Insight, Barth syndrome open-label trial 12-patient open-label human trial in Barth syndrome
  4. Chatfield et al., 2019, JACC Heart Failure, PROGRESS-HFpEF Phase 2 RCT of elamipretide in HFpEF
  5. Karaa et al., 2021, Neurology, MMPOWER-3 trial Largest RCT of elamipretide in mitochondrial myopathy

Educational and informational content only. This is not medical advice, diagnosis, or treatment. The compounds discussed are research compounds that are not approved for human use outside specific prescribed contexts. Always consult a qualified, licensed clinician before making any health decision.