SS-31: What Researchers Should Know About Elamipretide

SS-31 (also known as Elamipretide, MTP-131, or Bendavia) is a mitochondria-targeted tetrapeptide that has gained increasing attention in biomedical research over recent years. On 19 September 2025 the FDA granted the pharmaceutical formulation (FORZINITY) accelerated approval in the US, narrowly indicated to improve muscle strength in adult and paediatric patients with Barth syndrome weighing at least 30 kg. That approval is US-only and specific to that indication; every other use, and the research-grade peptide itself, remains investigational. For researchers, the question is: what distinguishes research-grade SS-31 from the approved drug, and what matters when sourcing it?

SS-31longevity

Mitochondria-targeted tetrapeptide (Elamipretide) that stabilizes cardiolipin and prevents ROS formation at the source.

Structure and Biochemical Properties

SS-31 is a synthetic tetrapeptide with the sequence:

D-Arg-2',6'-dimethylTyr-Lys-Phe-NH2

The structure was specifically designed for mitochondrial uptake. Two properties are central:

Positive charge: SS-31's cationic residues favour electrostatic interactions with anionic, cardiolipin-containing membranes. Unlike lipophilic cations such as MitoQ, uptake of SS-31 has been reported as independent of mitochondrial membrane potential.

Amphipathic character: The aromatic residues (phenylalanine and 2',6'-dimethyltyrosine) partially shield the positive charge, enabling passage through cell membranes. The amphipathic structure - hydrophobic and hydrophilic regions - allows both membrane interaction and solubility in aqueous environments.

Mechanism of Action: Cardiolipin Binding

The central mechanism of action of SS-31 is based on selective binding to cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane. Cardiolipin plays a structural and functional role in organising respiratory chain complexes and maintaining cristae structure.

What SS-31 does at cardiolipin:

• Stabilisation of cristae architecture: The binding protects cardiolipin from oxidative damage, thereby maintaining the spatial organisation of the respiratory chain.
• Reduction of ROS production: By stabilising the electron transport chain, electron leakage is reduced, which decreases the formation of reactive oxygen species.
• Preservation of ATP synthesis: The intact cristae structure and stabilised membrane potential support oxidative phosphorylation.
• Modulation of membrane electrostatics: A 2020 biophysical study found that SS-31 altered surface electrostatics in model and mitochondrial membranes. The authors proposed several possible downstream mechanisms; enhanced assembly of cardiolipin-dependent protein complexes was not directly demonstrated.

FDA Approval: FORZINITY for Barth Syndrome

On 19 September 2025, the FDA granted accelerated approval to FORZINITY (elamipretide injection) to improve muscle strength in adult and paediatric patients with Barth syndrome weighing at least 30 kg.

What is Barth syndrome?

Barth syndrome (BTHS) is a rare, X-linked mitochondrial disorder affecting approximately 150 individuals in the United States. It is caused by mutations in the TAFAZZIN gene, which is responsible for cardiolipin remodelling. The result: abnormal cardiolipin, impaired mitochondrial function, cardiomyopathy, skeletal muscle weakness, and neutropenia.

Why is the approval significant?

• First FDA-approved therapy for Barth syndrome
• First FDA-approved mitochondrial cardiolipin binder, with continued approval subject to confirmatory verification of clinical benefit

Important limitation: FDA states that accelerated approval was based on improvement in knee extensor muscle strength, an intermediate clinical endpoint. Continued approval may be contingent on verification and description of clinical benefit. The FDA approval letter dated 19 September 2025 scheduled confirmatory-trial initiation for March 2026 and study completion for September 2029.

TAZPOWER Study: The Basis for Approval

The approval evidence included TAZPOWER, a randomised, double-blind, placebo-controlled crossover trial in 12 patients with genetically confirmed Barth syndrome, followed by a single-arm open-label extension. The randomised period did not meet its primary six-minute walk and fatigue endpoints. Knee extensor strength was a secondary endpoint, showed no increase during the randomised period, and increased only during the uncontrolled extension. FDA used the extension-period strength finding as an intermediate clinical endpoint for accelerated approval; continued approval may depend on confirmatory verification of clinical benefit.

Study design:

• Phase 2 study with cross-over design
• 40 mg elamipretide administered subcutaneously daily in the trial
• 192-week open-label extension; eight participants reached Week 168 and three reached Week 192

Results reported from the uncontrolled open-label extension:

• 6-minute walk test: Cumulative improvement of 96.1 metres from OLE baseline (p = 0.003)
• Muscle strength: Knee extensor strength increased during the uncontrolled extension. It had been a secondary endpoint in the randomised period, where no increase was observed.
• Cardiac parameters: Clinically meaningful improvements in left ventricular end-diastolic and end-systolic volumes as well as stroke volume
• Biomarker: Significant decrease in the MLCL:CL ratio (an elevated ratio is pathognomonic for BTHS)
• Fatigue: Mean BTHS-SA Total Fatigue scores were lower than baseline, but the Week 168 change was not statistically significant (P = 0.21).

Safety in TAZPOWER: Ten patients entered the OLE; eight reached the 168-week visit. Elamipretide was well tolerated. The most common adverse events were injection site reactions.

Further Clinical Trials

SS-31/Elamipretide is being investigated in several indications beyond Barth syndrome.

MMPOWER-3: Primary Mitochondrial Myopathy

The pivotal phase 3 study MMPOWER-3 evaluated elamipretide in 218 patients with genetically confirmed primary mitochondrial myopathy (PMM). Result: the primary endpoints (6-minute walk test and fatigue score) were not met. The difference between elamipretide and placebo was -3.2 metres on the 6MWT (p = 0.69).

A prespecified exploratory genotype subgroup analysis, incorporating a post hoc correction of three misclassified participants, found a nominal 6MWT signal in participants with nuclear-DNA (nDNA) pathogenic variants, while the mitochondrial-DNA (mtDNA) subgroup showed no difference versus placebo. Because this was an exploratory subgroup result and was not multiplicity-adjusted, it is hypothesis-generating rather than confirmatory; the later analyses of specific nDNA genotypes were post hoc.

ReCLAIM and ReNEW: Dry Age-Related Macular Degeneration (AMD)

The phase 1 ReCLAIM study, an open-label safety study, evaluated subcutaneous elamipretide injections in intermediate AMD and geographic atrophy. The subsequent randomised, placebo-controlled phase 2 ReCLAIM-2 study did not meet either primary endpoint, change in low-luminance best-corrected visual acuity or square-root-transformed geographic atrophy area. Nominal exploratory ellipsoid-zone findings informed further study but were not confirmatory evidence of efficacy. The phase 3 ReNEW study (NCT06373731) reached the 50% mark of the planned 360 participants in March 2025. The primary endpoint is the rate of change in macular photoreceptor loss, measured by SD-OCT and ellipsoid zone mapping. It is a 96-week randomised study; the registry lists it as active and no longer recruiting, with estimated primary completion in 2027, not 2026 (ClinicalTrials.gov NCT06373731, checked July 2026).

ReGAIN has been described as a second study in the phase 3 dry AMD programme. No registry identifier is cited here, so no current recruitment or completion timeline is claimed.

PROGRESS-HF: Heart Failure

The phase 2 PROGRESS-HF study evaluated elamipretide in patients with heart failure with reduced ejection fraction (HFrEF). Elamipretide was well tolerated but did not significantly improve left ventricular end-systolic volume at four weeks compared to placebo.

Safety Profile

The safety profile of SS-31/Elamipretide has been consistently documented across multiple clinical trials.

MMPOWER-2 safety (n = 30): In this crossover trial, injection-site reactions during elamipretide treatment were most commonly erythema (57%), pruritus (47%), pain (20%), urticaria (20%), and irritation (10%). Most were mild, although some moderate reactions occurred. These are MMPOWER-2-specific rates, not pooled rates across the clinical programme.

Injection site reactions were generally mild and self-limiting, and in the TAZPOWER study they could be managed with oral antihistamines or topical corticosteroids.

Intravenous administration: Early trials used different historical regimens. MMPOWER administered 0.01, 0.1, or 0.25 mg/kg/h for two hours on five consecutive days. A separate HFrEF dose-ranging study administered single four-hour infusions of 0.005, 0.05, or 0.25 mg/kg/h. These were trial regimens, not handling instructions for research-grade material.

Serious adverse events did occur in the programme: in the phase 3 MMPOWER-3 trial they were reported in 5 of 109 elamipretide participants versus 3 of 109 on placebo, though none were considered treatment-related, and the FORZINITY label warns of serious hypersensitivity reactions. It is therefore accurate to say no treatment-related deaths were reported, not that no serious adverse events occurred at all.

Research-Grade SS-31 vs. FORZINITY: Key Differences

For researchers, the distinction between the approved drug and research-grade peptide is essential.

Quality Criteria When Purchasing

When sourcing SS-31 for research, analytical transparency is key.

Storage: Follow the supplier's documented, lot-specific storage instructions for the unopened lyophilised material. After reconstitution, use only a validated assay-specific stability and handling procedure; research-grade SS-31 has no universal post-reconstitution temperature or shelf-life.

EU shipping: For researchers inside the EU customs territory, sourcing within the EU avoids import clearance and tends to shorten transit. PeptidesDirect ships from the EU with tracking; see the shipping page for current destination-specific estimates.

Mechanistic Comparison with Other Research Compounds

SS-31 has proposed research mechanisms distinct from those discussed for MOTS-c, Epitalon and NAD+. This article identifies no controlled evidence establishing the efficacy, safety or synergy of combining SS-31 with any of them. The comparisons below are mechanistic only and are not recommendations for combined use.

SS-31 and MOTS-c: MOTS-c is a mitochondrially encoded peptide linked in research to AMPK activation and metabolic adaptation. Its proposed metabolic-signalling role differs from SS-31's cardiolipin-related membrane effects; no combined-use benefit is established here.

SS-31 and Epitalon: Epitalon is discussed in ageing research in connection with telomere-related hypotheses. Those hypotheses differ from SS-31's proposed mitochondrial-membrane mechanism; no combined-use benefit is established here.

SS-31longevity

Mitochondria-targeted tetrapeptide (Elamipretide) that stabilizes cardiolipin and prevents ROS formation at the source.

Frequently Asked Questions

Order SS-31

When sourcing SS-31 for research, look for batch-matched third-party laboratory reports that state the tests performed and disclose the commissioning party and sample source, EU shipping with tracking, and transparent quality documentation. The clear distinction between research-grade peptide and the approved drug FORZINITY is a sign of a serious supplier.

SS-31longevity

Mitochondria-targeted tetrapeptide (Elamipretide) that stabilizes cardiolipin and prevents ROS formation at the source.