SS-31

Overview

SS-31 (better known by its drug-development name elamipretide, and historically as MTP-131 and Bendavia) is a small, synthetic mitochondria-targeting tetrapeptide. It belongs to the Szeto-Schiller (SS) peptide family, named after researchers Hazel Szeto and Peter Schiller, who described this class of cell-permeable, aromatic-cationic peptides in the early 2000s.

Unlike most peptides discussed in a longevity context, SS-31 does not act on a hormone receptor. Instead, it accumulates inside cells on the inner mitochondrial membrane, where it binds the signature phospholipid cardiolipin. The stated goal is to help preserve the folded membrane structures called cristae and to support efficient operation of the electron transport chain, the machinery that produces most cellular ATP. This places it alongside other mitochondrial research compounds such as MOTS-c, though the two work by entirely different mechanisms.

SS-31 has been studied in an unusually large number of human clinical trials, run by its developer Stealth BioTherapeutics, across mitochondrial myopathy, Barth syndrome, heart failure, and retinal disease. The results have been genuinely mixed: several large trials missed their primary endpoints. At the same time, in September 2025 the US FDA granted accelerated approval to the hydrochloride form (elamipretide HCl, brand name Forzinity) for the ultra-rare Barth syndrome, making it one of the few peptides in this category to reach any regulatory approval.

Chemistry and structure

SS-31 is a tetrapeptide (four amino-acid residues) with an amidated C-terminus and a distinctive alternating aromatic / cationic side-chain pattern that is characteristic of the SS-peptide family.

Two structural features are notable. The peptide includes a D-arginine residue and a modified tyrosine, 2,6-dimethyltyrosine (Dmt), which contribute to its stability and its chemical behavior. The mix of positively charged residues (arginine, lysine) and aromatic residues (Dmt, phenylalanine) gives the molecule the amphipathic character that lets it penetrate cells and associate with mitochondrial membranes.

Mechanism of action

SS-31's proposed mechanism centers on cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane. Cardiolipin is essential for organizing the respiratory complexes of the electron transport chain and for shaping the tightly folded cristae where oxidative phosphorylation occurs.

The peptide is thought to work roughly as follows:

1. Cell and mitochondrial uptake. Because it is cell-permeable and aromatic-cationic, SS-31 enters cells and concentrates at the inner mitochondrial membrane. This accumulation is described as largely independent of membrane potential, unlike many other mitochondria-targeted molecules.
2. Cardiolipin binding. SS-31 associates with cardiolipin through electrostatic and hydrophobic interactions. Biophysical work (Mitchell et al., 2020) indicates it binds lipid bilayers and modulates membrane surface electrostatics, which the authors argue is a key part of how it acts.
3. Cristae and complex stabilization. By interacting with cardiolipin, SS-31 is proposed to help stabilize cristae architecture and the organization of respiratory supercomplexes, supporting more efficient electron transport and ATP production.
4. Reduced oxidative stress. The interaction is also associated with less cardiolipin peroxidation and lower production of reactive oxygen species (ROS) in preclinical models.

The net preclinical picture is improved mitochondrial bioenergetics and reduced oxidative damage. It is important to distinguish this mechanistic, largely cell- and animal-based rationale from clinical outcomes in patients, which (as below) have been inconsistent.

Clinical trials and evidence

SS-31 / elamipretide has one of the larger human trial programs among mitochondrial-targeted agents. The honest summary is that results are mixed, with multiple primary-endpoint misses alongside one accelerated approval and several encouraging secondary signals.

Primary mitochondrial myopathy (MMPOWER program)

The MMPOWER trials studied elamipretide in primary mitochondrial myopathy (PMM).

• MMPOWER-3 (Karaa et al., Neurology, 2023) was a Phase 3, randomized, placebo-controlled trial in 218 adults. Over 24 weeks of daily subcutaneous elamipretide (40 mg) versus placebo, the drug did not meet its co-primary endpoints: the distance on the 6-minute walk test and the Total Fatigue Score on a symptom assessment did not significantly improve relative to placebo.
• A post hoc analysis (Karaa et al., Orphanet Journal of Rare Diseases, 2024) reported that a genetic subgroup appeared to improve on the 6-minute walk test, while the mtDNA-variant subgroup did not. The responders were patients carrying nuclear DNA (nDNA) pathogenic variants, particularly mtDNA-maintenance / replisome genes (e.g., those presenting as chronic progressive external ophthalmoplegia). The authors framed this as a hypothesis-generating result motivating a follow-up trial in that subgroup.

The overall MMPOWER-3 result is best described as a negative primary outcome with a genotype-specific signal that has not yet been confirmed.

Barth syndrome (TAZPOWER) and FDA accelerated approval

Barth syndrome is an ultra-rare, X-linked mitochondrial disorder (caused by TAFAZZIN mutations affecting cardiolipin remodeling), so it is mechanistically relevant to a cardiolipin-binding peptide.

• The pivotal TAZPOWER trial included a 28-week randomized, double-blind, placebo-controlled phase followed by a long open-label extension (168-week results reported by Thompson et al., 2024).
• In the randomized portion, elamipretide did not statistically separate from placebo on the pre-specified primary endpoints (a 6-minute walk distance and a Barth-specific fatigue assessment).
• However, during the open-label extension, investigators reported improvements in knee extensor muscle strength and in some functional and cardiac measures.

On September 19, 2025, the FDA granted accelerated approval to elamipretide HCl (Forzinity) to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg. The approval rested on the knee extensor strength improvement as an intermediate clinical endpoint, and, as is standard for accelerated approval, continued approval may depend on confirming clinical benefit in further (confirmatory) study.

Heart failure and cardiac ischemia (Bendavia era)

Under the name Bendavia / MTP-131, the peptide was studied in cardiology:

• Preclinical: chronic elamipretide improved left-ventricular and mitochondrial function in dogs with advanced heart failure (Sabbah et al., Circulation: Heart Failure, 2016).
• EMBRACE-STEMI (Phase 2a) tested intravenous elamipretide during primary PCI for acute anterior ST-elevation myocardial infarction. It did not meet its primary endpoint of reduced infarct size, though some secondary observations were reported.
• Phase 2 work in heart failure (e.g., PROGRESS-HF and related studies) explored left-ventricular function with mixed/limited findings.

Dry AMD / geographic atrophy (ReCLAIM program)

Because retinal cells are highly energy-dependent, elamipretide was tested in dry age-related macular degeneration (AMD) with geographic atrophy (GA):

• ReCLAIM-2 (Phase 2) administered daily subcutaneous elamipretide for 48 weeks. It did not meet its primary endpoints (low-luminance visual acuity and GA growth).
• The investigators highlighted a secondary signal, slowed progression of ellipsoid zone (EZ) attenuation on imaging, which Stealth described as informing a planned Phase 3 program using an EZ-based endpoint.

Summary table

Safety and reported effects

In published trials, elamipretide given by daily subcutaneous injection was generally described as well tolerated, with most adverse events mild to moderate.

• The most frequently reported issue was injection-site reactions (redness, irritation, or discomfort at the injection site).
• Across the controlled trials, serious adverse events were not reported as clearly attributable to the drug at the doses studied, and discontinuations were relatively limited.
• Because much of the clinical data comes from defined, time-limited trials (commonly weeks to a couple of years in extension studies), very long-term safety across broad populations is less fully characterized, and reporting reflects supervised use of pharmaceutical-grade material.

This section summarizes published research observations and the approved product's reported profile. It is not a safety endorsement, and trial safety findings do not transfer to unregulated material used outside medical supervision.

Regulatory status

The regulatory picture is specific and limited, and easy to overstate.

• Approved (US, 2025): elamipretide HCl (Forzinity) received FDA accelerated approval on September 19, 2025, only to improve muscle strength in Barth syndrome patients weighing at least 30 kg. It carried Orphan Drug, Fast Track, Priority Review, and Rare Pediatric Disease designations. Accelerated approval is based on an intermediate endpoint, and continued approval may be contingent on confirmatory trials.
• Investigational (everything else): for primary mitochondrial myopathy, heart failure, dry AMD/geographic atrophy, and any general "longevity," "anti-aging," or performance use, SS-31 / elamipretide is not approved and remains investigational.
• Not a supplement: SS-31 is not a dietary supplement and is not an approved general-wellness product. Material sold to consumers as "SS-31," frequently labeled "for research use only," falls outside the manufacturing, purity, and oversight standards of the clinical program described here and is not the same as an approved medicine.

Regulatory status is jurisdiction-specific and can change. This page is not legal advice.

How it compares

SS-31 is often grouped with other peptides studied in longevity and mitochondrial contexts, especially MOTS-c and Epitalon. The most important distinction is concrete: SS-31 is the only one of the three with a substantial human clinical-trial program and any regulatory approval — but that approval is narrow and rests on contested data (a single accelerated approval atop several failed randomized primary endpoints).

Key takeaways:

• Different mechanisms, very different evidence levels. SS-31 stabilizes mitochondrial membranes via cardiolipin; MOTS-c is a mitochondrial-derived metabolic regulator; Epitalon targets telomere/pineal signaling. Only SS-31 has reached regulatory approval.
• SS-31 has by far the most human data, but those trials targeted specific diseases (not general anti-aging), several missed their primary endpoints, and the lone approval is a narrow, conditional accelerated approval — not proof of broad benefit.
• MOTS-c and Epitalon remain predominantly preclinical for any longevity claim, with no approvals and no robust human longevity outcomes.

Common misconceptions

• "SS-31 is an approved anti-aging or longevity drug." No. Its only approval is a narrow US accelerated approval for Barth syndrome (2025). There is no approved longevity, anti-aging, or performance indication.
• "All its trials succeeded." The opposite is closer to true: several large trials (MMPOWER-3 in myopathy, ReCLAIM-2 in AMD, EMBRACE-STEMI in heart attack, and the randomized phase of TAZPOWER) missed their primary endpoints.
• "SS-31 and elamipretide are different things." They are the same molecule, along with MTP-131 and Bendavia (and the approved salt, Forzinity / elamipretide HCl).
• "It boosts mitochondria like a stimulant or hormone." It is not a stimulant or hormone. Its proposed action is structural / biophysical, binding cardiolipin to help stabilize the inner mitochondrial membrane, not direct receptor signaling.
• "A strong mechanism means proven benefit." SS-31 has a well-studied mechanism, yet clinical outcomes have been inconsistent; mechanism and confirmed clinical benefit are not the same thing.
• "Research-grade SS-31 equals the approved medicine." Approved elamipretide is a characterized, manufactured pharmaceutical used under medical supervision; unregulated "research" material is not equivalent and was not part of the trials summarized here.

Community claims & recent evidence

These points address claims circulating in the peptide community — including popular video "masterclasses" that pair SS-31 with MOTS-c and stage it as a cancer or "all-disease" protocol — checked against primary sources. A knowledgeable creator is not peer review: every statement below was traced to a real study or dropped.

Verified additions

• The foundational cardiolipin mechanism has a specific, often-miscited source. SS-31 binds with high affinity to cardiolipin on the inner mitochondrial membrane and inhibits the cytochrome-c peroxidase activity that drives cardiolipin peroxidation during ischemia, "re-energizing" damaged mitochondria in isolated-organelle and animal kidney-injury models. [Animal]/[In vitro] (Birk et al., Journal of the American Society of Nephrology 2013). Community talks often cite this as a "2013 Journal of Clinical Investigation" paper — right year, wrong journal.
• Beyond cardiolipin, a 2023 study identified a second molecular target: the ADP/ATP carrier (ANT). In aged mouse muscle, elamipretide improved mitochondrial ADP sensitivity by increasing ADP uptake through the adenine nucleotide translocator and lowering ANT s-glutathionylation, and rescued muscle force and cardiac systolic function. [Animal] (Pharaoh et al., GeroScience 2023).
• SS-31 crosses the blood-brain barrier and acts on neuronal mitochondria in animal models. In an Alzheimer's (APP) mouse model it reduced amyloid-β, oxidative stress and synaptic/mitochondrial deterioration; in an LPS-inflammation mouse model it improved mitochondrial function, synaptic markers and memory. [Animal] (Reddy et al., Human Molecular Genetics 2017; Zhao et al., Journal of Neuroinflammation 2019). This is preclinical only — there is no human cognition or dementia trial.

Claims that don't hold up

• "SS-31 helps cure cancer / resensitizes cancer cells to apoptosis — proven in 2020 Cancer Research." No verifiable primary study supports SS-31 as a cancer treatment, and the cited paper does not resolve. Mechanistically the claim runs backwards: SS-31's documented role is cytoprotective — it stabilizes cristae and reduces ROS and cytochrome-c release to help prevent apoptosis (Birk et al. 2013; Tung et al., Int. J. Mol. Sci. 2025), the opposite of a pro-apoptotic "cancer killer." There is no oncology trial of SS-31. [Hypothesis]
• "It boosts ATP output by well over 30% and stabilizes Complex I — proven in a 2016 Nature Communications paper." The specific paper and the ">30%" figure do not resolve to a real primary source. SS-31 does improve electron-transport-chain and supercomplex organization in preclinical models, but there is no confirmed 30% ATP figure — and in humans the large trials repeatedly missed their primary endpoints (see Clinical trials above). [Hypothesis]
• "You must run MOTS-c first, then SS-31 — it's a non-negotiable law of physiological sequencing." No human or animal study has tested this MOTS-c→SS-31 combination or sequence; it is an untested hypothesis presented as an established rule. MOTS-c itself has essentially no controlled human trials, so a mandatory pairing order cannot be evidence-based. [Hypothesis]