SS-31 (Elamipretide): The Cardiolipin-Targeting Tetrapeptide in the Research Literature

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

SS-31, also published as elamipretide, MTP-131 and Bendavia, is a four-residue synthetic peptide that concentrates in the inner mitochondrial membrane and binds the phospholipid cardiolipin. This article explains its chemistry, traces the mechanism studies from 2004 to 2020, summarizes the clinical development record through the September 2025 FDA decision, and sets out why a lyophilized research vial is a different article from the approved drug product.

What SS-31 is: a D-Arg-Dmt-Lys-Phe-NH₂ tetrapeptide

SS-31 is the amidated tetrapeptide D-Arg-Dmt-Lys-Phe-NH₂, where Dmt is 2',6'-dimethyltyrosine, a non-proteinogenic tyrosine with two extra methyl groups on the phenol ring. The free base has the formula C₃₂H₄₉N₉O₅ and a molecular weight of 639.8 (PubChem CID 11764719; CAS 736992-21-5). The approved drug product uses the hydrochloride salt; research-grade lots are commonly supplied as acetate or trifluoroacetate salts, and the counter-ion must be read from the certificate rather than assumed.

Two features of the sequence explain most of its physical behavior. First, the chain alternates aromatic and basic residues (aromatic Dmt, basic Lys, aromatic Phe, with the D-Arg adding a third positive charge at physiological pH). That alternating motif was the design principle of the whole Szeto-Schiller series, and it gives the molecule an amphipathic character that favors partitioning into anionic membrane surfaces. Second, two of the four residues are non-canonical: a D-arginine and the dimethylated tyrosine. Both resist common proteases, and both are invisible to a mass spectrometer in the sense that matters for a certificate: MS confirms the elemental composition but cannot tell D-Arg from L-Arg. The identity line on a certificate therefore confirms the expected mass, and the chirality is a matter of synthesis records and, where a lab needs it, chiral amino acid analysis. The LC-MS identity article covers that limitation in detail.

The name history is worth decoding once. "SS" stands for Szeto-Schiller, the two chemists whose laboratories designed the series; SS-31 was the thirty-first analog. MTP-131 was the development code at Stealth BioTherapeutics, Bendavia the early trade name, elamipretide the International Nonproprietary Name, and Forzinity the approved US brand. All five names denote the same tetrapeptide, as discussed in why one peptide has many names.

Where the series came from: a 2004 cell and mitochondria study

The Szeto-Schiller peptides began as an offshoot of opioid chemistry, and the first mitochondrial paper appeared in the Journal of Biological Chemistry in 2004. Zhao and colleagues reported that a set of short aromatic-cationic peptides built around dimethyltyrosine were cell-permeable and accumulated roughly a thousand-fold in the inner mitochondrial membrane. In cultured mouse neuroblastoma N2A cells challenged with tert-butyl hydroperoxide, the peptides lowered intracellular reactive oxygen species and cell death with half-maximal concentrations in the nanomolar range. In isolated mitochondria, they reduced calcium-induced swelling, inhibited the permeability transition and prevented cytochrome c release. Analogs lacking the dimethyltyrosine did not reproduce these measurements, which placed the scavenging function on that residue.

A 2014 account in Clin Pharmacol Ther by Szeto and Birk describes how the series was repurposed: the peptides had been designed as receptor ligands, and their mitochondrial accumulation was an unplanned observation that redirected the program. That history matters when reading later papers, because the inventor is also a founder of the company that licensed the technology, and most mechanism papers through 2014 carry that disclosure. Independent laboratories entered the mechanism literature in 2020, as the next section describes.

The cardiolipin mechanism: what each study measured

The current mechanistic picture is that SS-31 binds cardiolipin, the dimeric anionic phospholipid concentrated in the inner mitochondrial membrane, and alters how cardiolipin interacts with cytochrome c and other membrane proteins. Four papers carry most of the evidence, and each used a different measurement.

The 2013 paper is the origin of the "cardiolipin binder" description. Its central finding is biochemical: cytochrome c, when bound to cardiolipin, can act as a peroxidase that oxidizes cardiolipin, and the SS-31/cardiolipin complex suppressed that peroxidase activity while protecting the heme iron. The 2014 follow-up asked whether the electron-carrier function of cytochrome c survived the same interaction, and reported that it did. The two 2020 papers come from University of Connecticut and University of Washington groups with no financial interest declared in the compound, and they shifted the description from "a specific cardiolipin ligand" toward "a polybasic peptide that tunes membrane surface charge and sits where cardiolipin-binding proteins sit." Those descriptions are compatible rather than competing, but they are different claims, and a research program should cite the one it is actually testing.

Clinical development and the 2024-2025 FDA decisions

Elamipretide has an unusually long and public regulatory record. Stealth BioTherapeutics ran randomized trials in primary mitochondrial myopathy and in Barth syndrome, an X-linked disorder of cardiolipin remodeling caused by variants in the TAZ gene. A 2020 crossover trial in adults with primary mitochondrial myopathy, published in the Journal of Cachexia, Sarcopenia and Muscle, measured six-minute walk distance and symptom scores. The larger MMPOWER-3 trial, published in Neurology in 2023, did not meet its primary endpoints on those measures. In Barth syndrome, the TAZPOWER program published in Genetics in Medicine in 2021 reported a 12-week randomized crossover phase that did not reach significance on its primary endpoints, followed by an open-label extension in which the authors reported changes in walk distance and strength measures over time.

The regulatory history that followed is documented in Drugs@FDA and summarized in a 2026 Drugs "First Approval" review. The sponsor submitted NDA 215244 for Barth syndrome; an FDA advisory committee met on the application in October 2024; the agency issued a complete response letter in 2025; and on September 19, 2025, the FDA granted accelerated approval. The product is elamipretide hydrochloride, approved by the FDA as Forzinity, for improving muscle strength in adults and children with Barth syndrome weighing 30 kg or more. Accelerated approval is conditioned on confirmatory evidence, so the label and its basis can change. Elamipretide remains investigational for every other indication the sponsor has studied, including dry age-related macular degeneration, and the FDA status reference table lists it accordingly.

Why a research vial is not Forzinity

A lyophilized SS-31 vial sold for laboratory research is not Forzinity and should never be described as such. Forzinity is a finished, sterile drug product manufactured under an approved application, with a specified salt, concentration, container closure and label. A research vial is a chemical reagent: synthesized and lyophilized in the USA, characterized by an independent US laboratory, and documented by a lot-matched certificate of analysis. The two articles may share a peptide sequence, but nothing else about them is interchangeable, and the research product carries no approval of any kind.

For the research product, the documentation that matters is the certificate. For SS-31 10 mg and SS-31 30 mg the posted certificate reports identity by mass spectrometry against the expected mass of 639.8, purity by HPLC, net peptide content, endotoxin, sterility and heavy metals, with the label mass referring to the lyophilized fill. Because this sequence carries two positive charges from Lys and D-Arg plus a charged N-terminus, counter-ion mass is a meaningful fraction of the fill, so the net peptide content line should be read alongside purity rather than in place of it. The peptide is hydrophilic and dissolves readily in water, and the dimethyltyrosine phenol is the residue most sensitive to oxidation in solution; the aliquots and oxidation article covers how labs log that.

Reading the SS-31 literature critically

Three distinctions keep a literature survey honest. First, model system: the mechanism papers used liposomes, bicelles, isolated mitochondria and cultured cells, with rodent ischemia models in the 2013 work; the clinical papers measured functional endpoints in people enrolled in trials of the drug product. A finding from one tier does not transfer to another. Second, sponsorship: through 2014 nearly every mechanistic paper came from the inventor's laboratory, and the 2020 cross-linking and biophysics papers are the first substantial independent confirmations of the cardiolipin-proximal binding picture. Third, endpoint: the randomized trials that failed their primary endpoints and the accelerated approval that followed are both part of the record, and a balanced summary reports both. The compound sits with humanin and MOTS-c in the mitochondria-focused group of the catalog, and the three-way comparison explains why SS-31 is the only one of the three that is synthetic rather than genome-encoded.

Frequently Asked Questions

Is SS-31 the same thing as elamipretide?

Yes. SS-31, MTP-131, Bendavia, elamipretide and Forzinity all name the tetrapeptide D-Arg-Dmt-Lys-Phe-NH₂. SS-31 is the laboratory code, elamipretide the nonproprietary name, and Forzinity the US brand of the hydrochloride drug product approved in September 2025.

Is SS-31 approved by the FDA?

Elamipretide hydrochloride was approved by the FDA as Forzinity on September 19, 2025, under the accelerated approval pathway for Barth syndrome (NDA 215244). That approval applies to the finished drug product only. A research-grade SS-31 vial is not Forzinity and is not an approved drug.

What does SS-31 bind to in mitochondria?

Published studies report that SS-31 binds cardiolipin, the anionic phospholipid of the inner mitochondrial membrane. A 2013 study used a fluorescent analog to show high-affinity cardiolipin binding, 2014 NMR work on bicelles showed the aromatic residues penetrating cardiolipin-containing bilayers, and a 2020 cross-linking study found its protein neighbors were all known cardiolipin binders.

What is Dmt in the SS-31 sequence?

Dmt is 2',6'-dimethyltyrosine, a tyrosine with two methyl groups added to the aromatic ring. In the 2004 founding paper, analogs without Dmt did not reduce mitochondrial reactive oxygen species or swelling, so the residue is regarded as essential to the molecule's activity in those assays.

Citations

1. Zhao K, Zhao GM, Wu D, Soong Y, Birk AV, Schiller PW, Szeto HH. Cell-permeable peptide antioxidants targeted to inner mitochondrial membrane inhibit mitochondrial swelling, oxidative cell death, and reperfusion injury. J Biol Chem. 2004;279:34682-34690. PMID 15178689
2. Szeto HH, Birk AV. Serendipity and the discovery of novel compounds that restore mitochondrial plasticity. Clin Pharmacol Ther. 2014;96:672-683. PMID 25188726
3. Birk AV, Liu S, Soong Y, et al. The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin. J Am Soc Nephrol. 2013;24:1250-1261. PMID 23813215
4. Birk AV, Chao WM, Bracken C, Warren JD, Szeto HH. Targeting mitochondrial cardiolipin and the cytochrome c/cardiolipin complex to promote electron transport and optimize mitochondrial ATP synthesis. Br J Pharmacol. 2014;171:2017-2028. PMID 24134698
5. Mitchell W, Ng EA, Tamucci JD, et al. The mitochondria-targeted peptide SS-31 binds lipid bilayers and modulates surface electrostatics as a key component of its mechanism of action. J Biol Chem. 2020;295:7452-7469. PMID 32273339
6. Chavez JD, Tang X, Campbell MD, et al. Mitochondrial protein interaction landscape of SS-31. Proc Natl Acad Sci USA. 2020;117:15363-15373. PMID 32554501
7. Karaa A, Haas R, Goldstein A, et al. A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy. J Cachexia Sarcopenia Muscle. 2020;11:909-918. PMID 32096613
8. Karaa A, Bertini E, Carelli V, et al. Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial. Neurology. 2023;101:e238-e252. PMID 37268435
9. Reid Thompson W, Hornby B, Manuel R, et al. A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome, a genetic disorder of mitochondrial cardiolipin metabolism. Genet Med. 2021;23:471-478. PMID 33077895
10. Shirley M. Elamipretide: First Approval. Drugs. 2026;86:377-383. PMID 41335372
11. US FDA. Drugs@FDA: NDA 215244, Forzinity (elamipretide hydrochloride), approved September 19, 2025. Drugs@FDA
12. PubChem. Elamipretide, CID 11764719. PubChem

This article is for laboratory research reference only. US Prime Peptides products are supplied strictly for in vitro research. Not for human consumption.

Compliance Notice: US Prime Peptides products are supplied strictly for laboratory and academic research purposes only. They are not intended for human or veterinary consumption, diagnosis, treatment, or prevention of any disease. All content on this page is educational in nature and does not constitute medical advice or product claims. Researchers are responsible for handling these compounds in accordance with their institution’s safety protocols and applicable laws.