SS-31 is elamipretide: the structure, and what FDA approved

SS-31 is elamipretide: a synthetic four-residue peptide, D-Arg-Dmt-Lys-Phe-NH2, in which Dmt is 2,6-dimethyl-L-tyrosine. As a free base it is C32H49N9O5, molecular weight 639.8. On September 19, 2025, FDA approved one drug containing it, Forzinity (elamipretide hydrochloride, NDA 215244, Stealth BioTherapeutics), under accelerated approval. That approval belongs to Forzinity. Research-grade SS-31 sold as a lyophilized powder is not Forzinity and is not an approved drug; ours is sold for laboratory research use only.

One molecule, several names

PubChem files the compound as CID 11764719 under its international nonproprietary name, elamipretide, and lists the names it has carried through development: SS-31, "Szeto-Schiller peptide", MTP-131, Bendavia and Ocuvia. The CAS registry number is 736992-21-5. All of them point at the same structure, so a certificate or a paper that says SS-31 and one that says elamipretide are describing the same peptide. What differs between products is the salt form, which changes the weight of the powder but not the peptide.

The structure, residue by residue

Read in order, the residues alternate: basic, aromatic, basic, aromatic. FDA's label places Forzinity in the established pharmacologic class "mitochondrial cardiolipin binder"; cardiolipin is a phospholipid of the inner mitochondrial membrane.

The molecule has three basic groups: the free N-terminal amine, the guanidino group of arginine and the side-chain amine of lysine. That is why the approved drug's salt is a trihydrochloride. The label gives it as hydrochloride (1:3), C32H49N9O5·3HCl, molecular weight 749.2, against 639.8 for the free base. A research powder may be an acetate or another salt; the peptide inside is the same, and only the mass per milligram of powder changes. See net peptide content for how that is measured.

Because one residue is D and one is not coded at all, no gene encodes SS-31. It can only be made by chemical synthesis.

What FDA approved, as of November 7, 2026

Drugs@FDA lists one application with elamipretide as its active ingredient:

The approval was granted under FDA's accelerated approval pathway, which allows FDA to keep an approval conditional on a confirmatory trial; the label says so in its first section. Forzinity is a ready-to-use sterile solution of the hydrochloride salt made under that application.

An approval is specific to the approved product: its applicant, its manufacturing, its formulation and its labeling. It does not make elamipretide approved in any other form. A vial of research SS-31 is not Forzinity, was not made under NDA 215244, and is not an approved drug. It is sold for laboratory research use only and is not for human or veterinary use. Our table of FDA-approved peptides lists elamipretide alongside the other approved peptide drugs.

SS-31 and MOTS-c are different molecules

The two are often named together, so the difference is worth stating in structural terms. MOTS-c is a 16-residue peptide, MRWQEMGYIFYPRKLR, entirely made of coded L-amino acids, with a molecular weight of 2174.6. UniProt (A0A0C5B5G6) records it as a mitochondrial-derived peptide encoded within the mitochondrial MT-RNR1 gene. SS-31 has four residues, one D-amino acid, one non-coded residue and an amidated C-terminus, and no gene encodes it. They share a research subject, mitochondria, and nothing in their structure.

What our certificates show

Every SS-31 size we list has a current certificate of analysis from Bioviridian Inc., the testing laboratory. All three were issued August 28, 2026, with purity by reversed-phase HPLC at 214 nm and identity by LC-MS/MS:

An earlier 10 mg lot, SS31-10-0318, is kept as an archived certificate: 99.7% purity, identity by MALDI-MS, issued April 3, 2026.

For identity, the number to look for is the peptide's own mass, whatever the salt. The monoisotopic mass of C32H49N9O5 is 639.39. By electrospray, that appears as ions at m/z 640.39 (one proton), 320.70 (two) and 214.14 (three), since each of the three basic groups can carry a proton. A mass spectrum does not show the salt: a certificate that gives 749 as the observed mass is quoting the trihydrochloride's formula weight, not a measurement. Our guide to mass spectrometry and peptide identity explains charge states, and how to verify a lab report covers checking a certificate number with the laboratory.

Common questions

Is SS-31 the same as elamipretide?

Yes. SS-31 is a development name for elamipretide, its international nonproprietary name. PubChem lists both, with MTP-131 and Bendavia, for the same compound, CID 11764719, CAS 736992-21-5.

Is SS-31 FDA approved?

Elamipretide is approved in one product only: Forzinity, elamipretide hydrochloride, NDA 215244, approved September 19, 2025 under accelerated approval. Research SS-31 is not that product and is not an approved drug. Checked against Drugs@FDA on November 7, 2026.

What is the structure of SS-31?

D-Arg-Dmt-Lys-Phe-NH2: D-arginine, 2,6-dimethyl-L-tyrosine, L-lysine and L-phenylalanine amide. The free base is C32H49N9O5 with a molecular weight of 639.8; the trihydrochloride salt in the approved drug is C32H49N9O5·3HCl, 749.2.

What mass should an SS-31 certificate of analysis show?

The peptide's monoisotopic mass, 639.39, seen by electrospray as m/z 640.39, 320.70 or 214.14 depending on charge. The salt does not change the mass the instrument sees for the peptide.

How should SS-31 be stored?

Our SS-31 is supplied as a lyophilized powder and listed for storage at -20 °C, desiccated. General handling is covered in storage and stability.

Sources

• Elamipretide, CID 11764719, PubChem
• Drugs@FDA: NDA 215244, Forzinity, FDA
• Forzinity prescribing information (label), FDA, 2025
• NDA 215244 approval letter, FDA, September 19, 2025
• openFDA drugsfda endpoint, searched by active ingredient, FDA, November 7, 2026
• UniProt A0A0C5B5G6, Mitochondrial-derived peptide MOTS-c (human), UniProt Consortium