SS-31 (Elamipretide) Research Guide: Cardiolipin and FDA Status

SS-31 now occupies two evidence contexts that must be kept separate. One is the research literature: biophysical experiments, cellular systems, animal models, and trials conducted under the names SS-31, MTP-131, Bendavia, and elamipretide. The other is a current U.S. finished-drug context created by FDA’s 2025 accelerated approval of FORZINITY. The active peptide links those contexts, but formulation, manufacturing controls, labeling, indication, and regulatory evidence do not transfer between them.

Key takeaways

• SS-31 and elamipretide refer to the same four-residue active peptide; FORZINITY is a specific approved finished-drug formulation.
• Cardiolipin association anchors the mechanism literature, but reported downstream effects remain model- and assay-dependent.
• Preclinical findings span many disease and aging models; they do not establish approved uses outside the label.
• The large MMPOWER-3 primary mitochondrial myopathy trial did not meet its primary endpoints.
• FDA granted FORZINITY accelerated approval on September 19, 2025, to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg.
• The label states that continued approval may depend on verification and description of clinical benefit in a confirmatory trial.

What Is SS-31 (Elamipretide)?

SS-31 is a synthetic tetrapeptide in the Szeto-Schiller peptide family. Its sequence is commonly written D-Arg-Dmt-Lys-Phe-NH2, where Dmt denotes 2′,6′-dimethyltyrosine and NH2 marks the C-terminal amide. The alternating aromatic and cationic character is central to the way the family was designed and studied.

The names describe different stages or contexts. “SS-31” is the research-program name common in mechanistic and animal literature. “MTP-131” and “Bendavia” are historical development codes. “Elamipretide” is the nonproprietary drug name. “FORZINITY” is the brand name for the approved finished drug. A name change does not alter the active peptide, but it can signal a different formulation, development stage, or regulatory owner.

Reagent identity record

These fields let a laboratory check a received material against an independent record before any work begins.

How Was SS-31 Developed?

SS peptides emerged from work by Hazel Szeto and Peter Schiller on small aromatic-cationic peptides capable of crossing biological membranes. Unlike approaches that depend on a large lipophilic cation and membrane potential, the SS family was reported to concentrate at the inner mitochondrial membrane through a different physicochemical pattern. A historical review describes the discovery as a sequence of observations that moved from opioid-receptor research toward mitochondrial bioenergetics.^[1]

The lineage matters because “mitochondria-targeted” is sometimes treated as a complete mechanism. Localization, lipid association, protein interaction, redox effects, cristae organization, respiration, and organism-level outcomes are distinct experimental layers, and a paper supporting one cannot claim the rest.

This page owns the SS-31/elamipretide entity and its evidence boundaries. The MOTS-c research guide covers a different mitochondrial-derived peptide encoded within mitochondrial DNA. The two compounds differ in origin, sequence, proposed targets, and evidence record; “mitochondrial peptide” does not make them interchangeable.

How Does SS-31 Interact With Cardiolipin?

Cardiolipin is a distinctive phospholipid enriched in the inner mitochondrial membrane. It contributes to cristae architecture and the organization of respiratory-chain proteins. Mechanistic studies reported that SS-31 interacts with cardiolipin-containing systems and can modify the behavior of the cytochrome c/cardiolipin complex.^[2][3] In vitro, the peptide bound only to liposomes and bicelles containing cardiolipin, in roughly a 1:1 ratio, and increased state 3 respiration and ATP-synthesis efficiency in fresh isolated mitochondria.^[3] In rats, pretreatment before renal ischemia protected cristae membranes, prevented mitochondrial swelling, and sped ATP recovery on reperfusion.^[2]

Together these support a cardiolipin-centered working model in which inner-membrane association affects protein-lipid interactions, cristae or respiratory organization, and downstream bioenergetic measurements. Later proteomic work mapped a broader mitochondrial protein-interaction landscape, reinforcing that the cellular response cannot be reduced to one static lock-and-key target.^[4] That cross-linking mass-spectrometry survey reports no quantitative endpoint; its finding is categorical — every SS-31-interacting protein it identified was already a known cardiolipin binder.

Cautions follow in order: a membrane-system result does not fix the magnitude of effect in an intact cell, a cell response does not fix a whole-organism outcome, and a favorable preclinical result does not transfer to a different disease, formulation, or population. Cardiolipin provides mechanistic coherence; it does not erase the evidence ladder.

What Do Preclinical SS-31 Studies Show?

Preclinical studies have tested SS-31 across ischemia, aging, skeletal muscle, cardiac, renal, and other mitochondrial-stress models. In one aged-mouse study, a single intraperitoneal dose of 3 mg/kg restored resting and maximal mitochondrial ATP production, oxidative-phosphorylation coupling (P/O), and cell energy state (PCr/ATP) in 27-month-old mice to the levels measured in 5-month-old mice within one hour, with no observable effect in young muscle; eight days of dosing increased whole-animal endurance capacity.^[5] A second study gave 26-month-old female C57BL/6Nia mice 3 mg/kg/day for 8 weeks and reported reversal of the age-related decline in maximum ATP production and P/O coupling, greater gastrocnemius mass and fatigue resistance, and increased treadmill endurance — without any increase in mitochondrial content.^[6]

Mouse age, strain, disease model, tissue, exposure window, comparator, and assay all shape those results. They do not establish a universal “mitochondrial repair” effect, and they do not establish the approved human indication.

The preclinical record is strongest as a map of hypotheses: where cardiolipin-centered membrane effects might matter, which biomarkers may be informative, and which models respond. It cannot determine whether an untested condition will respond, or whether a different material has equivalent identity, exposure, or activity.

Additional reported findings

Two further reported results, each linked to its source record.

What Do Elamipretide Clinical Trials Show?

Primary mitochondrial myopathy

The early MMPOWER phase 1/2 study randomized 36 adults with genetically confirmed primary mitochondrial myopathy to intravenous elamipretide at 0.01, 0.1, or 0.25 mg/kg/h, or placebo, over 2 hours in a dose-escalating sequence. After 5 days, participants on the highest dose walked a mean of 64.5 m farther on the six-minute walk test versus 20.4 m in the placebo group (p = 0.053), with a dose-dependent increase across arms (p = 0.014).^[7] It generated human tolerability and signal-finding data, but its size and 5-day exposure limit efficacy inference; two errata were later published against the record.

The later MMPOWER-3 randomized trial enrolled 218 participants and tested subcutaneous elamipretide 40 mg/day against placebo over 24 weeks, from a baseline mean walk distance of 336.7 ± 81.2 m. It did not meet either primary endpoint: the least-squares mean difference was −3.2 m (95% CI −18.7 to 12.3; p = 0.69) for six-minute walk distance and −0.07 (95% CI −0.10 to 0.26; p = 0.37) for the prespecified fatigue score.^[8] Mechanistic plausibility and the earlier signal did not translate into success on that trial’s primary endpoints.

Barth syndrome

Barth syndrome is caused by pathogenic variants affecting tafazzin, which alters cardiolipin remodeling. The TAZPOWER crossover study randomized 12 participants to elamipretide 40 mg/day or placebo for 12 weeks, then switched arms after a 4-week washout; that randomized portion met neither primary endpoint. Ten participants continued into the open-label extension, where walk distance improved by 95.9 m (p = 0.024) and the symptom-assessment score by 2.1 points (p = 0.031) at week 36 among the 8 who reached that visit.^[9] A propensity-matched comparison of those 8 treated patients against 19 untreated natural-history controls reported least-squares mean differences of 79.7 m at week 64 (P = 0.0004) and 91.0 m at week 76 (P = 0.0005) in walk distance, with handheld-dynamometry differences of 40.8 N and 56.7 N at the same visits.^[10] By week 168 of the extension, cumulative walk-distance improvement was 96.1 m (P = 0.003), with injection-site reactions the most common adverse event.^[11] Those are open-label and non-randomized comparisons, and carry the limits of both.

What Is the Current FDA Status of FORZINITY?

On September 19, 2025, FDA granted accelerated approval to FORZINITY (elamipretide), NDA 215244, to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg. FDA described it as the first approved treatment for Barth syndrome. The current FDA approval announcement, approval letter, and prescribing information are the primary authorities for that status.

The label states that the indication was approved under accelerated approval based on improvement in knee-extensor muscle strength, an intermediate clinical endpoint. It also states that continued approval may depend on verification and description of clinical benefit in a confirmatory trial. Accelerated approval is real FDA approval, but its evidentiary and confirmatory boundary should remain visible.

The label does not approve elamipretide for general mitochondrial health, normal aging, primary mitochondrial myopathy, heart failure, or kidney disease, and it does not approve an Apex research reagent. FDA reviews a specific sponsor, manufacturing package, finished formulation, labeling, and evidence submission.

Outside that single US indication the position is unchanged: no EMA, NMPA, or other non-US marketing authorization for elamipretide is confirmed in the records reviewed here, and every other use remains investigational or unapproved. Apex research-grade SS-31 holds no regulatory approval in any jurisdiction.

How Should SS-31 Evidence Be Read?

Begin with the name and material. A mechanistic SS-31 paper, an elamipretide clinical trial, and a FORZINITY regulatory record may concern the same active peptide but not the same formulation or question. Then identify the evidence level: liposome, isolated mitochondrion, cultured cell, animal model, uncontrolled extension, randomized trial, and FDA review carry different inferential weight.

Next, examine endpoint ownership. A change in oxygen-consumption rate does not establish improved walking performance, and an intermediate muscle-strength endpoint does not establish every patient-centered outcome. A negative result in one indication does not prove the mechanism false, but it does bar that mechanism from being presented as confirmed benefit in that population.

Finally, keep analytical identity separate from biological and regulatory evidence. Mass spectrometry tests whether observed ions support the expected analyte mass, and HPLC reports chromatographic purity under a defined method. Neither establishes clinical formulation, sterility, cardiolipin binding, efficacy, or equivalence to FORZINITY. The Apex guides to mass-spectrometry identity testing and HPLC purity interpretation explain those boundaries.

Frequently Asked Questions

What is SS-31?

SS-31 is an aromatic-cationic tetrapeptide with the sequence D-Arg-Dmt-Lys-Phe-NH2. It is also known as elamipretide and has been studied for its interaction with cardiolipin-rich inner mitochondrial membranes.

Are SS-31 and elamipretide the same peptide?

Yes. SS-31 is the research-program designation and elamipretide is the nonproprietary drug name for the same active tetrapeptide. Formulation and regulatory context still have to be identified separately.

What is FORZINITY?

FORZINITY is the FDA-approved elamipretide finished drug under NDA 215244. It is distinct from an Apex SS-31 research reagent even though both refer to the same active-peptide identity.

When did FDA approve FORZINITY?

FDA granted accelerated approval on September 19, 2025, to improve muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kg.

What does accelerated approval mean here?

The indication was approved based on improvement in an intermediate clinical endpoint. The label states that continued approval may depend on verification and description of clinical benefit in a confirmatory trial.

Did the MMPOWER-3 trial meet its primary endpoints?

No. The 24-week randomized primary mitochondrial myopathy trial did not meet either primary endpoint, which is important evidence against extending the Barth syndrome approval to that indication.

What can an SS-31 certificate of analysis establish?

A lot-specific certificate can report the material identity, stated methods, chromatographic result, and mass result for that lot. It cannot establish sterility, cardiolipin binding, clinical efficacy, regulatory approval, or equivalence to FORZINITY.

References

1. Szeto HH, et al. Serendipity and the discovery of novel compounds that restore mitochondrial plasticity. Clin Pharmacol Ther. 2014;96(6):672-83. PMID: PMID 25188726.
2. Birk AV, et al. The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin. J Am Soc Nephrol. 2013;24(8):1250-61. PMID: PMID 23813215.
3. Birk AV, et al. Targeting mitochondrial cardiolipin and the cytochrome c/cardiolipin complex to promote electron transport and optimize mitochondrial ATP synthesis. Br J Pharmacol. 2014;171(8):2017-28. PMID: PMID 24134698.
4. Chavez JD, et al. Mitochondrial protein interaction landscape of SS-31. Proc Natl Acad Sci U S A. 2020;117(26):15363-15373. PMID: PMID 32554501.
5. Siegel MP, et al. Mitochondrial-targeted peptide rapidly improves mitochondrial energetics and skeletal muscle performance in aged mice. Aging Cell. 2013;12(5):763-71. PMID: PMID 23692570.
6. Campbell MD, et al. Improving mitochondrial function with SS-31 reverses age-related redox stress and improves exercise tolerance in aged mice. Free Radic Biol Med. 2019;134:268-281. PMID: PMID 30597195.
7. Karaa A, et al. Randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy. Neurology. 2018;90(14):e1212-e1221. PMID: PMID 29500292.
8. Karaa A, et al. Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial. Neurology. 2023;101(3):e238-e252. PMID: PMID 37268435.
9. Reid Thompson W, 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(3):471-478. PMID: PMID 33077895.
10. Hornby B, et al. Natural history comparison study to assess the efficacy of elamipretide in patients with Barth syndrome. Orphanet J Rare Dis. 2022;17(1):336. PMID: PMID 36056411.
11. Thompson WR, et al. Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER. Genet Med. 2024;26(7):101138. PMID: PMID 38602181.
12. Zhao C, et al. Elamipretide: The first cardiolipin-directed mitochondrial therapeutic for Barth syndrome approved under accelerated approval. Drug Discov Ther. 2026;19(6):435-436. PMID: PMID 41260682.

Written by Nicholas Tremelling

Reviewed by the Apex Laboratory Editorial Team

Reviewed July 25, 2026 for SS-31 identity, cardiolipin mechanism, preclinical-to-clinical evidence boundaries, MMPOWER-3 results, current FORZINITY accelerated-approval status, FDA primary sources, PMID verification, and research-use framing. See the Apex Laboratory editorial standards.