SS-31 Peptide (Elamipretide): Sequence, Cardiolipin & FDA Status

SS-31 is one of the few research compounds whose regulatory status changed recently while its scientific description stayed exactly the same. In September 2025 the FDA granted accelerated approval to Forzinity, an elamipretide hydrochloride injection, for Barth syndrome. That single event made this tetrapeptide the first mitochondria-targeted therapeutic approved in the United States, and it is now cited across supplier pages in a form that implies considerably more than it establishes. This article separates the molecule from the pharmaceutical product, the biophysics from the shorthand, and the preclinical record from the human one.

What Is SS-31 Peptide?

SS-31 is a synthetic tetrapeptide with the sequence D-Arg-Dmt-Lys-Phe-NH₂, studied for its tendency to accumulate at the inner mitochondrial membrane and alter the physical properties of that membrane. It belongs to the Szeto-Schiller family of aromatic-cationic peptides, named after Hazel Szeto and Peter Schiller, whose numbering convention gives the compound its designation.

Two features distinguish it from most peptides in a research catalogue. It has no identified receptor. And it is short enough — four residues — that its behaviour is governed by charge distribution and hydrophobicity rather than by folding into a binding conformation. Researchers working with SS-31 as a research material are generally studying peptide-membrane biophysics, mitochondrial bioenergetics, or oxidative stress models rather than receptor pharmacology.

SS-31, Elamipretide, MTP-131, Bendavia, Forzinity

SS-31, elamipretide, MTP-131 and Bendavia all name the same molecule. Forzinity does not — it names a finished pharmaceutical product containing that molecule as a hydrochloride salt. Collapsing the two is the single most common error in secondary writing about this compound, and it matters because approval attaches to products, not to substances.

A useful test: a name that appears in a chemical registry alongside a CAS number describes a substance. A name that appears alongside an indication and a patient weight threshold describes a product.

SS-31 Molecular Profile: Sequence, Structure and Mass

What the Sequence Actually Contains

The four residues alternate between charge and aromaticity, which is the structural point of the molecule rather than an incidental feature.

• D-Arg carries a guanidinium group that stays protonated across physiological pH. Its D-configuration resists aminopeptidase cleavage at the N-terminus.
• Dmt is 2',6'-dimethyltyrosine, a non-proteinogenic residue. The two methyl groups flanking the phenol contribute bulk and hydrophobicity, and the phenol itself has been proposed as a radical-scavenging moiety.
• Lys contributes the second positive charge.
• Phe supplies a second aromatic ring, and the C-terminal amide removes the negative charge a free carboxylate would carry.

The net result is a peptide carrying roughly +3 charge in a molecule of only 640 daltons — an unusually high charge density for its size.

The 639.79 figure describes the free base. The approved pharmaceutical is a hydrochloride salt, and research material is frequently supplied as an acetate or trifluoroacetate. Those forms weigh more per mole, so a stated milligram quantity means different amounts of peptide depending on which basis a supplier used. The salt form belongs on a certificate of analysis, not in an assumption.

How Does SS-31 Target Mitochondria?

SS-31 concentrates at the inner mitochondrial membrane because that membrane carries a strong negative surface charge, and cardiolipin is the principal reason it does. The peptide partitions into the membrane interfacial region — the crowded zone of lipid headgroups, water and ions that separates bulk solvent from the hydrophobic core — rather than passing through into the matrix and binding a target protein.

What Cardiolipin Is, and Why the Inner Membrane Matters

Cardiolipin is an unusual phospholipid: four acyl chains on a dimeric glycerophosphate backbone, giving it two negative charges and a cone-like shape. In mammalian cells it is found almost exclusively in mitochondria and is concentrated in the inner membrane, where it makes up a substantial fraction of total lipid. It stabilises respiratory chain complexes, participates in cristae curvature, and anchors cytochrome c to the membrane.

That distribution is what makes it a plausible address. A polycationic peptide entering a cell encounters no other compartment with a comparable density of anionic lipid.

What the Binding Evidence Actually Shows

This is where the widely repeated phrasing overshoots. Mitchell and colleagues (J Biol Chem, 2020) examined SS-31 against model and mitochondrial membranes using biophysical and computational methods, and reported three findings that constrain the mechanism.

• Binding affinity and binding density scaled with membrane surface charge, not with cardiolipin's specific chemistry.
• SS-31 did not destabilise lamellar bilayers even at saturating concentrations, but did cause measurable changes in lipid packing.
• SS-20, an SS-family analogue lacking the Dmt residue, was indistinguishable from SS-31 in its effect on membrane surface potential.

That last point is the awkward one for any account built on Dmt's antioxidant chemistry: if a peptide without Dmt tunes surface electrostatics identically, then surface-charge modulation is a property of the cationic scaffold rather than of the distinctive residue. The honest description is electrostatically driven interfacial partitioning at anionic membranes, in which cardiolipin is the dominant contributor because of where it sits and how much charge it carries.

What Research Has Investigated Beyond the Membrane

The lipid picture and the protein picture are not competing accounts. Chavez and colleagues (PNAS, 2020) used chemical cross-linking with mass spectrometry to identify direct protein interactors of SS-31 in mitochondria, and found that every interactor identified was itself a known cardiolipin binder. They clustered into oxidative phosphorylation components and 2-oxoglutarate metabolic enzymes, with cross-linked residues frequently sitting near the regions where those proteins contact cardiolipin.

Each row belongs to a category, and the categories do not transfer. A cristae observation in a rat heart is not a human outcome, and an interactome map is a hypothesis generator rather than a demonstrated mechanism of benefit.

What Human Trials Have Found

The human record is larger than most write-ups suggest, and considerably more mixed.

Two details in that table repay attention. TAZPOWER is the trial the approval rests on; it enrolled twelve participants, its randomised phase did not separate elamipretide from placebo on either primary endpoint, and the muscle-strength improvement emerged in the open-label extension, where every participant knew they were receiving the compound and there was no concurrent control. And in the ophthalmology programme, the endpoint now serving as the Phase 3 primary — ellipsoid zone attenuation — was a predefined secondary in a trial that missed both of its primaries on nominal p-values.

Is SS-31 FDA Approved?

Forzinity, an elamipretide hydrochloride injection, holds FDA accelerated approval granted 19 September 2025 to improve muscle strength in adult and paediatric patients with Barth syndrome weighing at least 30 kg. Nothing broader than that sentence is approved.

Three qualifications sit inside it.

1. Accelerated approval is conditional. It was granted on knee-extensor strength as an intermediate endpoint considered reasonably likely to predict benefit. The FDA is requiring a post-approval randomised, double-blind, placebo-controlled confirmatory trial.
1. One indication only. Mitochondrial myopathy, heart failure and dry AMD remain investigational, and the myopathy programme's Phase 3 trial did not meet its endpoints.
1. Approval attaches to a product. It covers a specific manufacturer's hydrochloride formulation made under pharmaceutical manufacturing controls, with a label, a defined route and a defined population. A research-use-only SS-31 preparation is a different article entirely: not approved, not manufactured to those standards, and not intended for human or veterinary use.

Barth syndrome closes a neat loop with the mechanism. It is caused by variants in the TAFAZZIN gene, whose product remodels cardiolipin acyl chains. The disease is, at the molecular level, a cardiolipin disorder — which is why a cardiolipin-interacting peptide was tested there first.

Verifying SS-31 Identity in a Research Lot

Documentation for this compound has to answer more questions than it does for a conventional peptide.

• Identity by mass spectrometry establishes the molecular mass. Given the Dmt residue and the D-arginine, it does not establish that the correct residue was incorporated in the correct configuration.
• Purity by HPLC is a chromatographic area percentage under one stated method. It is a different measurement from peptide content, which requires a quantitative method.
• Chemical form — free base, acetate, trifluoroacetate, hydrochloride — determines what a stated milligram figure means.
• Batch traceability ties every one of those numbers to the vial in hand rather than to a representative document.

Where the Evidence Stops

Four boundaries are worth stating plainly. Selectivity for cardiolipin over other anionic lipids is inferred from mitochondrial cardiolipin abundance rather than demonstrated as chemical specificity. The contribution of Dmt's proposed radical scavenging is not resolved, given that an analogue without it modulates surface potential identically. The downstream chain from altered membrane electrostatics to improved bioenergetics is proposed rather than closed. And the human efficacy record is one narrow accelerated approval against several trials that missed their primary endpoints.

None of that makes SS-31 uninteresting. It makes it a compound whose mechanistic literature is genuinely rich and whose translational literature is genuinely thin — an unusual and useful combination for a research tool.

SS-31 Among Other Mitochondrial Peptides

SS-31 is often grouped with the mitochondrial-derived peptides, which is a category error worth correcting. MOTS-c and Humanin are encoded within the mitochondrial genome and act as signalling molecules through defined cellular pathways; our MOTS-c research overview covers the AMPK evidence in detail. SS-31 is fully synthetic, has no genomic origin, and acts on the physical properties of a membrane rather than through a signalling cascade. The overlap is the organelle, not the mechanism. The same distinction separates it from NAD⁺ precursor approaches, which work on cofactor availability rather than membrane biophysics.