SS-31 Peptide Elamipretide · MTP-131 · Cardiolipin-targeting tetrapeptide

SS-31 (also known as elamipretide) is a four–amino-acid synthetic peptide studied for its affinity for cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane. It is one of the most extensively characterised mitochondria-targeted compounds in the published literature, with a research record stretching from isolated mitochondria through to human clinical trials.

What is SS-31?

SS-31 belongs to a family of small, cell-permeable peptides first described by Hazel Szeto and Peter Schiller. The “SS” prefix is simply their initials, and the number is the compound's position in the series that came out of that collaboration. The series was originally investigated for entirely different reasons, and the mitochondrial targeting that the compound is now known for was an unexpected observation rather than a design goal — a detail worth knowing, because it shapes how the early literature is written.

The compound was later advanced through clinical development by Stealth BioTherapeutics under the international nonproprietary name elamipretide. Across two decades of publications the same molecule appears under at least four labels: SS-31 in the foundational pharmacology work, MTP-131 in earlier company literature, Bendavia in cardiovascular trial reporting, and elamipretide in current regulatory and clinical contexts. Anyone searching the literature should run all four, because results split across them and a search for one name alone will miss a substantial fraction of the record.

Structurally it is unusual for a peptide of its size. It carries an alternating aromatic–cationic motif — a repeating pattern of positively charged and ring-containing residues — that allows it to cross cell membranes without a dedicated transporter. Once inside a cell it does not distribute evenly. It concentrates in mitochondria at levels far above those in the surrounding cytosol, and within the mitochondrion it associates with the inner membrane rather than the matrix. That selective partitioning, rather than any single downstream pathway, is what the bulk of the research literature is concerned with.

One further structural note matters for laboratory work: the molecule includes D-arginine and 2′,6′-dimethyltyrosine, both non-standard residues. Non-standard amino acids resist the proteases that would rapidly degrade a conventional four-residue peptide, which is a large part of why the compound survives long enough to be studied at all.

Why the inner mitochondrial membrane matters

To follow the SS-31 literature you need a working picture of where it acts. A mitochondrion has two membranes. The outer one is relatively smooth and permeable. The inner one is folded into deep invaginations called cristae, and it is functionally the important surface in the organelle: the protein complexes of the electron transport chain are embedded in it, and the proton gradient that drives ATP synthesis is maintained across it.

That folding is not decorative. Cristae dramatically increase the available membrane area, and their curvature concentrates the respiratory complexes into dense, organised arrays rather than letting them drift apart. The geometry affects the chemistry. When cristae flatten or fragment, the complexes disperse, electron transfer becomes less efficient, and a greater proportion of electrons leak prematurely to oxygen and generate reactive species instead of contributing to the gradient. Loss of structure and loss of efficiency are the same event viewed from two angles.

Cardiolipin: the molecular target

Cardiolipin is the phospholipid that holds that architecture together. It is structurally unusual — most phospholipids carry two fatty-acid tails, while cardiolipin carries four, joined through a double-glycerol backbone. That shape is conical rather than cylindrical, which is precisely what a membrane needs in order to curve tightly. Cardiolipin accumulates at the points of highest curvature in the cristae.

It is also effectively exclusive to this one membrane. Cardiolipin is synthesised in the mitochondrion and stays there, which is what makes it useful as a target: a compound with genuine cardiolipin affinity has, in practical terms, only one address in the entire cell.

Beyond structure, cardiolipin participates directly in function. Several respiratory-chain complexes require bound cardiolipin for stability and activity, and cytochrome c — which shuttles electrons between complexes — is held loosely at the membrane through an interaction with it. Cardiolipin is also chemically vulnerable: its four tails are rich in unsaturated bonds, and it sits adjacent to the main site of reactive oxygen species production in the cell. Oxidative damage to cardiolipin is therefore both an early consequence of mitochondrial stress and a cause of further dysfunction, which is the self-reinforcing loop that much of this literature describes.

How SS-31 is reported to act

SS-31 associates with cardiolipin through a combination of electrostatic attraction to the phosphate head groups and hydrophobic insertion of its aromatic residues into the membrane. It is described in the literature as a cardiolipin binder rather than an enzyme inhibitor or receptor agonist, and that distinction carries real experimental consequences.

Published interaction mapping describes SS-31 associating with a broad set of inner-membrane proteins rather than one target with high selectivity. The interpretation generally offered is that the compound acts on the membrane environment those proteins occupy — stabilising cardiolipin, preserving curvature and keeping complexes in proximity — rather than switching any individual protein on or off. In preclinical models this association is reported alongside restored cristae architecture, improved coupling between electron transport and ATP synthesis, and reduced electron leak.

The preclinical research record

SS-31 has an unusually deep literature for a peptide of this size. Published preclinical work spans isolated mitochondria, cultured cells, and animal models, and the breadth is largely explained by the target: because cardiolipin integrity is relevant wherever mitochondrial density is high, the same compound shows up across otherwise unrelated fields.

Recurring areas in the literature include ischemia-reperfusion models, where mitochondrial damage follows the restoration of blood flow rather than its interruption; cardiac and skeletal muscle models, both tissues with very high mitochondrial content; renal models, for the same reason; neurodegeneration models, where mitochondrial dysfunction is a common feature across several otherwise distinct conditions; and models of age-related mitochondrial decline, where cardiolipin content and cristae organisation both change measurably with age.

Researchers reviewing this body of work should note two things. First, outcomes vary considerably by model, timing and administration route, and the compound's reported effects are consistently larger in systems with demonstrable mitochondrial dysfunction than in healthy controls — which is what a mechanism based on restoring impaired structure would predict. Second, a substantial portion of the record was produced with pharmaceutical-grade material under controlled administration, which is not the same thing as research-grade powder in a laboratory assay.

Clinical development and regulatory status

Elamipretide has been evaluated in registered human trials across several indications, including primary mitochondrial myopathy, Barth syndrome, heart failure and dry age-related macular degeneration, under trial programmes with names such as MMPOWER, TAZPOWER and ReCLAIM. The programme is notable for having produced both positive and negative readouts: several trials did not meet their primary endpoints, and the development path narrowed over time toward the rare-disease indications where the mitochondrial defect is genetically defined and unambiguous.

In September 2025 the United States Food and Drug Administration granted accelerated approval to elamipretide under the brand name Forzinity, for Barth syndrome — an ultra-rare genetic disorder involving a defect in cardiolipin remodelling. It is generally described as the first approved therapy that directly targets mitochondrial function.

Handling and storage in the laboratory

Like most lyophilised research peptides, SS-31 is comparatively stable as a dry powder under cold storage and considerably less stable once reconstituted. General laboratory practice for this class of compound is to keep sealed vials frozen, protected from light and moisture, and to allow a vial to reach room temperature before opening so that atmospheric moisture does not condense inside it. Opening a cold vial is one of the most common avoidable sources of degradation in peptide work.

Reconstitution solvent, working concentration and aliquoting strategy depend entirely on the assay being run and should follow the protocol of the study in question rather than any generic figure. The one piece of guidance that generalises is to avoid repeated freeze-thaw cycling: cycling is the most frequent cause of unexplained variability between runs, which is why most laboratories aliquot into single-use volumes immediately after reconstitution rather than returning a stock vial to the freezer.

It is also worth recording lot numbers against experimental runs. Peptide purity varies between suppliers and between batches from the same supplier, and a batch change midway through a series is a confound that is very difficult to diagnose after the fact if it has not been logged.

Verification and certificates of analysis

Identity and purity claims are only as good as the documentation behind them, and in this market that documentation is frequently absent, generic, or supplied only on request after purchase. Every SS-31 lot NexPept lists is tested by an independent third-party laboratory using high-performance liquid chromatography and mass spectrometry, and the report is published openly rather than offered on request.

The current SS-31 batch is NP-SS31-062426-01, independently tested at 99.60% purity. That report, along with every other batch we have published, is available in the NexPept Certificate of Analysis library. From late September 2026 onward we have also been adding heavy-metal and endotoxin testing to our published reports.

When reading any certificate of analysis, check that the batch number on the document matches the batch you were sent, that the test date is recent enough to be meaningful, that both an identity method and a purity method are reported rather than purity alone, and that the issuing laboratory is named. Our quality standards page sets out how our batches are tested and documented, and the broader guide to research peptides in Canada covers how to read a COA in more detail and what the common gaps look like.

Related cellular research compounds

SS-31 is catalogued under cellular research. Researchers working on mitochondrial and cellular-maintenance questions frequently hold these alongside it, either as comparators or because they bear on adjacent parts of the same system — NAD+ on the redox chemistry that feeds the electron transport chain, Epitalon on the cellular-ageing literature, and the tissue-model peptides on downstream structural questions.

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