Elamipretide (SS-31): mitochondria-repair peptide for Barth syndrome
A tiny synthetic peptide that helps the cell's energy factories (mitochondria) work properly; in late-stage trials for Barth syndrome, a rare genetic disorder; not yet an approved drug.
- Class
- Mitochondrial tetrapeptide; cardiolipin-targeting cell-penetrating peptide
- Status
- FDA accelerated approval (September 19, 2025) as Forzinity for improving muscle strength in adult and pediatric Barth syndrome patients weighing ≥30 kg. Confirmatory post-marketing trial required and scheduled to begin first half 2026.
- Best-supported effect
- Improved muscle strength and 6-Minute Walk Test performance in Barth syndrome patients (TAZPOWER open-label extension; human clinical); strong cardiolipin-binding mechanism and mitochondrial bioenergetics support across in vitro, ex vivo, and animal studies.
- Main caveat
- Approved indication is ultra-narrow (Barth syndrome, <1,000 US patients). Pivotal trials in heart failure (PROGRESS-HF), acute MI (EMBRACE-STEMI), and dry AMD (ReCLAIM) missed primary endpoints. MMPOWER-3 in primary mitochondrial myopathy missed primary with genotype-specific post-hoc signals only. Accelerated approval is conditional on confirmatory trial. No controlled trials exist for healthy-aging or general longevity use.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
SS-31 (elamipretide, brand name Forzinity) is a synthetic four-amino-acid peptide that homes to the inner membrane of mitochondria — the compartments inside cells that produce ATP — where it binds a phospholipid called cardiolipin and helps mitochondria run more efficiently. On September 19, 2025, the FDA granted it accelerated approval as Forzinity for improving muscle strength in adults and children (≥30 kg) with Barth syndrome, an ultra-rare X-linked genetic disorder affecting roughly 150 people in the United States. This made elamipretide the first cardiolipin-directed mitochondrial therapeutic and the first mitochondria-targeted peptide ever approved by the FDA. The stored sequence shown here (RYKF) is the standard one-letter approximation; the actual drug is H-D-Arg-2′,6′-dimethyltyrosine-Lys-Phe-NH₂ — the first residue is the D-stereoisomer of arginine, the second is the non-natural amino acid 2′,6′-dimethyltyrosine (Dmt) rather than standard tyrosine, and the C-terminus is amidated, none of which the four-letter raw sequence reveals.
History
SS-31 emerged from the Szeto-Schiller peptide program at Cornell University Medical College in the late 1990s and early 2000s, where Hazel Szeto and Peter Schiller were systematically engineering small cationic peptides that could penetrate cell membranes and concentrate inside mitochondria. SS-31 — the fourth generation of that series — distinguished itself by binding cardiolipin on the inner mitochondrial membrane independent of mitochondrial membrane potential, solving a limitation of earlier triphenylphosphonium-based targeting strategies. Stealth BioTherapeutics licensed the molecule and advanced it under successive names — Bendavia for cardiovascular indications, MTP-131 in early trial documentation, and ultimately elamipretide as the International Nonproprietary Name.
The clinical-development arc spanned more than a decade. EMBRACE-STEMI in acute ST-elevation myocardial infarction missed its primary endpoint (Daubert and colleagues 2017; PMID 28534645). PROGRESS-HF in heart failure with reduced ejection fraction was negative (Butler and colleagues, J Card Fail 2020; PMID 32068002). ReCLAIM in dry age-related macular degeneration failed. MMPOWER-3 in primary mitochondrial myopathy missed its primary endpoint (Karaa and colleagues, Neurology 2023; PMID 37268435), though a post-hoc analysis suggested signals in genotype-defined subgroups (Karaa and colleagues 2024; PMID 39574155). The Barth syndrome program — the TAZPOWER Phase 2/3 trial and its 168-week open-label extension (Reid Thompson and colleagues 2021, PMID 33077895; Reid Thompson and colleagues 2024, PMID 38602181), paired with a natural-history comparison study (Reid Thompson and colleagues 2022; PMID 36056411) — produced the package that ultimately persuaded the FDA. Accelerated approval as Forzinity was granted on September 19, 2025, with a confirmatory post-marketing trial required under the accelerated-approval pathway (Hoy, Drugs 2025, "Elamipretide: First Approval"; PMID 41335372).
What it does
In healthy mitochondria, the inner membrane is densely packed with cardiolipin, an unusual four-chain phospholipid found almost exclusively in this one location. Cardiolipin organizes the electron transport chain into supercomplexes and holds the cristae — the deep folds where ATP synthesis happens — in their tight pleated shape. When cardiolipin is damaged, oxidized, or mis-remodeled (as in Barth syndrome, aging, or ischemic stress), those supercomplexes destabilize, ATP output drops, and the chain leaks electrons that become reactive oxygen species.
SS-31 binds directly to cardiolipin. Once bound, it stabilizes the lipid, preserves cristae architecture, restores electron transport chain supercomplex integrity, and improves ATP synthesis efficiency. Unlike classical antioxidants that scavenge free radicals after they form, SS-31 acts upstream — by stabilizing electron flow through the respiratory chain, it limits the upstream generation of ROS in the first place. Its alternating aromatic-cationic structure also allows passive membrane penetration without dependence on mitochondrial membrane potential, which means it accumulates preferentially in dysfunctional mitochondria rather than being driven only by healthy ones. The mechanism has been characterized in detail across review and primary literature (Mitchelson and colleagues 2024, PMID 40294492; Allen and colleagues 2025, PMID 39940712; Szeto-program review, PMID 35037146).
In Barth syndrome, loss-of-function mutations in the TAFAZZIN gene disable a phospholipid-remodeling enzyme; without it, cardiolipin accumulates in an immature monolysocardiolipin-enriched form, cristae become disorganized, and oxidative phosphorylation falters — producing cardiomyopathy, skeletal myopathy, growth delay, and cyclic neutropenia. Because SS-31 acts directly on cardiolipin architecture, the mechanistic match to Barth biology is unusually direct, which is the rationale the FDA accepted for accelerated approval. Through this same cardiolipin-stabilization pathway, SS-31 sits in the broader mitochondrial-derived peptide neighborhood alongside peptides such as MOTS-c (/card/pep-00003), which acts through a different mechanism (AMPK activation) on overlapping mitochondrial-health endpoints.
Evidence
- Human (Barth syndrome — approved indication): The TAZPOWER Phase 2/3 crossover trial in adolescent and adult Barth syndrome patients showed mixed results in its initial 12-week blinded phase, while the 168-week open-label extension reported an average 96-meter improvement on the 6-Minute Walk Test in 8 of 10 patients along with cardiac stroke volume gains (Reid Thompson and colleagues 2024; PMID 38602181). A natural-history comparison study supported the magnitude of the OLE effect against untreated Barth patients (PMID 36056411). The FDA accepted this package for accelerated approval on September 19, 2025 (Hoy 2025; PMID 41335372).
- Human (other indications — mixed to negative): PROGRESS-HF in HFrEF missed its primary endpoint over 28 days (Butler and colleagues 2020; PMID 32068002). MMPOWER-3 in primary mitochondrial myopathy missed its primary 6MWT endpoint at 24 weeks (Karaa and colleagues 2023; PMID 37268435), with genotype-specific signals only in post-hoc subgroup analysis (PMID 39574155). A randomized dose-escalation crossover trial in primary mitochondrial myopathy produced mixed signals (Karaa and colleagues 2018; PMID 29500292). EMBRACE-STEMI in acute ST-elevation MI missed its primary endpoint (Daubert and colleagues 2017; PMID 28534645). A Phase 2a trial in atherosclerotic renal artery stenosis reported published results (Saad and colleagues 2017; PMID 28916603). A randomized topical-ophthalmic trial in Leber hereditary optic neuropathy reported published results (PMID 37923251). A small randomized single-dose trial in older adults showed improved in vivo skeletal-muscle ATP production by phosphorus magnetic resonance spectroscopy (Roshanravan and colleagues 2021; PMID 34264994) — an acute physiology signal, not a chronic-benefit demonstration.
- Animal: Preclinical literature spans cardiac ischemia-reperfusion (Brown and colleagues 2014; PMID 24288396), myocardial infarction in rats with the SS-peptide series (Cho and colleagues 2007; PMID 17429296), kidney disease (review, PMID 35707274), neurodegeneration (review, PMID 35111001), neurovascular coupling and cognition in aged mice (Tarantini and colleagues 2018; PMID 29405550), and LPS-induced cognitive impairment models (PMID 31747905).
- In vitro / ex vivo: Cardiolipin-binding characterization and electron-transport-chain supercomplex preservation are documented in biochemical studies of the inner mitochondrial membrane. Ex vivo studies in human failing-heart tissue support the cardiolipin-stabilization mechanism distinct from clinical efficacy trials.
Of the published RCT-class clinical reports in the elamipretide development program, the Barth syndrome program is the only one whose data formed the basis for regulatory approval; trials in heart failure, acute MI, primary mitochondrial myopathy (broadly), and dry AMD have either missed primary endpoints or remain ongoing. ReNEW (Phase 3, dry AMD) is fully enrolled with readout expected late 2027, and SPIMD-301 (Phase 3, polymerase-gamma / nDNA-mutation mitochondrial disease) is announced as the next regulatory target.
Myths and misconceptions
- "Elamipretide is FDA-approved as a general anti-aging or longevity drug." No. Forzinity is approved only for improving muscle strength in Barth syndrome patients ≥30 kg. There are no controlled trials of elamipretide in healthy adults for general mitochondrial support or anti-aging endpoints. Single-dose pharmacodynamic measurements of muscle ATP synthesis in older adults (PMID 34264994) are acute physiology, not durable healthy-aging benefit.
- "Research-grade SS-31 is the same product as Forzinity." The molecule is chemically the same. The products are not. Forzinity is GMP-manufactured with controlled identity, purity, sterility, and stability, supplied through a single specialty pharmacy (AnovoRx). Research-chemical SS-31 is typically labeled "not for human use," carries no identity or sterility assurances, and is not a substitute for the approved drug.
- "Accelerated approval means the drug is validated for all mitochondrial diseases." Accelerated approval is specific to Barth syndrome and rests on a surrogate-endpoint package in a small population, with a confirmatory post-marketing trial required. Separate trials in heart failure (PROGRESS-HF), broader primary mitochondrial myopathy (MMPOWER-3), and dry AMD (ReCLAIM) failed or produced mixed results. The Barth approval does not imply benefit across mitochondrial disease generally.
- "Strong mechanism guarantees clinical efficacy." Cardiolipin-binding pharmacology is genuinely well-characterized, and the Barth mechanistic match is unusually direct. But elamipretide's broader clinical record is mixed: positive enough in Barth to support accelerated approval, negative in major heart failure and AMD trials, and mixed in primary mitochondrial myopathy. Elegant mechanism is necessary but not sufficient.
Known effects
- Improving muscle strength in Barth syndrome — FDA-approved (Forzinity, September 19, 2025); accelerated approval based on TAZPOWER OLE and natural-history comparison.
- Cardiac stroke-volume improvement in Barth syndrome — Open-label extension finding (PMID 38602181).
- In vivo skeletal-muscle ATP synthesis (older adults) — Single-dose Phase 2 RCT positive on a phosphorus-MRS endpoint (PMID 34264994); does not establish chronic benefit in healthy aging.
- Stabilization of cardiolipin and ETC supercomplexes — Mechanistic, characterized across in vitro and ex vivo studies.
- Heart failure (HFrEF) outcomes — Phase 2 RCT negative on primary endpoint (PROGRESS-HF; PMID 32068002).
- Primary mitochondrial myopathy outcomes (general population) — Phase 3 RCT missed primary (MMPOWER-3; PMID 37268435); genotype-defined subgroup signals in post-hoc only (PMID 39574155).
- Dry age-related macular degeneration outcomes — Phase 2 (ReCLAIM) missed primary endpoints; Phase 3 ReNEW fully enrolled, readout expected late 2027.
- Cognitive function in humans — Not established; rodent neurovascular-coupling and memory data only (PMID 29405550, PMID 31747905).
- General anti-aging or longevity use in healthy adults — No controlled trials. Preclinical signal in aging rodents; no human longevity endpoints studied.
Safety signals
Injection-site reactions (erythema, pain, pruritus) are the most commonly reported adverse events across the elamipretide clinical program and the Forzinity label. Serious hypersensitivity including anaphylaxis has been observed in clinical use; onset has been reported from minutes to months after initiation, so vigilance is required throughout treatment rather than only at first dose (Forzinity FDA label, 2025). Headache is reported across multiple trial populations; nausea and other gastrointestinal symptoms are infrequent and generally mild.
Elamipretide is cleared primarily by peptidase-mediated degradation rather than CYP-mediated metabolism, and no major CYP-based interactions were flagged in regulatory documents. In Barth syndrome trial populations, cardiac medications including beta-blockers and ACE inhibitors were used concurrently without reported interaction signals; granulocyte colony-stimulating factor and growth hormone, both used in selected Barth patients, were also concomitant without flagged interactions in the development program.
Long-term safety in non-Barth populations is not characterized. The 168-week TAZPOWER open-label extension provides the longest continuous human exposure dataset (PMID 38602181), but in a small cohort of a single rare-disease population. Long-term safety in heart failure, broader primary mitochondrial myopathy, dry AMD, or healthy aging is not established. Human pregnancy safety data are limited, and transfer into breast milk is not well characterized.
Research-chemical SS-31 sold outside the regulatory pathway carries no identity, purity, or sterility verification; safety data from non-GMP supply are not captured in the clinical program and cannot be extrapolated from Forzinity's regulatory record.
Regulatory status
- US (FDA): Accelerated approval granted September 19, 2025 for elamipretide hydrochloride as Forzinity, indicated for improving muscle strength in adult and pediatric Barth syndrome patients weighing ≥30 kg. Distribution is through AnovoRx as the exclusive specialty pharmacy. A confirmatory post-marketing trial is required under the accelerated-approval pathway and is scheduled to begin in the first half of 2026. No other indications are FDA-approved. Off-label SS-31 use through compounding or research-chemical channels is not equivalent to Forzinity and is not supported by FDA review (Hoy 2025; PMID 41335372).
- EU (EMA): Not broadly approved as of early 2026; evaluation pathways for rare-disease use are in progress. Availability outside the US depends on local approval and reimbursement decisions; patients and families should coordinate with national rare-disease specialty centers and the Barth Syndrome Foundation.
- WADA: Elamipretide is not specifically named on the WADA Prohibited List. Athletes using the approved Barth syndrome formulation would navigate the Therapeutic Use Exemption process. Research-chemical SS-31 from non-approved supply remains in a regulatory grey zone, S0-adjacent for substances not approved by any governmental health authority for the relevant use context.
Open questions
- Confirmatory post-marketing trial. Accelerated approval is conditional on a confirmatory trial scheduled to begin in the first half of 2026. Whether confirmatory results support full approval, on what timeline, and at what effect size remains unresolved.
- Heart failure and dry AMD efficacy. PROGRESS-HF and ReCLAIM missed primary endpoints. The Phase 3 ReNEW trial in dry AMD is fully enrolled with readout expected in late 2027; this is the next major data point. Whether dose optimization, patient stratification by mitochondrial-dysfunction severity, or different endpoints could produce positive results in these more prevalent conditions is the central question for the compound's broader relevance.
- Genotype-defined mitochondrial myopathy. MMPOWER-3 missed its primary but generated post-hoc signals in polymerase gamma / nDNA-mutation patients. SPIMD-301 (Phase 3) tests this more homogeneous population — whether this stratification produces a positive pivotal remains to be seen.
- Long-term safety in non-Barth populations. The longest continuous exposure data come from a small Barth open-label extension. Long-term safety in heart failure, mitochondrial myopathy, healthy aging, or other populations is not characterized.
- Healthy-aging and longevity use. No controlled trials of elamipretide in healthy adults for general mitochondrial support or anti-aging endpoints. Preclinical rationale is strong; clinical translation is not established. Extrapolation from rare-disease approval data to healthy aging is not supported by the evidence base.
- Oral or longer-acting formulation. Daily subcutaneous injection has been the route across the clinical program; no oral or longer-acting formulation has reached late-stage development, which limits chronic-use practicality.
Related peptides
- MOTS-c (/card/pep-00003) — mitochondrial-derived peptide encoded within the 12S rRNA region; acts via AMPK activation rather than direct cardiolipin binding, but occupies the same mitochondrial-health therapeutic neighborhood.
- Epithalon (/card/pep-00008) — synthetic pineal tetrapeptide proposed as a longevity candidate; same four-residue size class, very different mechanism (proposed telomerase / epigenetic effects, primarily Russian/Ukrainian research) and a much weaker clinical evidence base than elamipretide.
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Does a small structural change to one building block of this drug allow it to grip the fats inside mitochondria tightly enough to keep energy production organized?
Cells generate energy through tightly organized protein clusters inside mitochondria. If this structural detail turns out to matter, it could guide the design of better drugs for heart failure, aging, and rare mitochondrial diseases, by pointing chemists toward exactly which part of the molecule does the real work.
Would removing a tiny chemical modification on the peptide's tail stop it from reaching its target inside the cell?
If the capped tail turns out to be load-bearing for how this drug finds its target, it would establish a clear rule for building future versions of the drug, shortening the trial-and-error phase of drug development for people with mitochondrial diseases.
Could the very defect that causes Barth syndrome actually attract this drug, making it work harder where it is needed most?
Barth syndrome is a rare, life-threatening heart condition in boys caused by faulty fat processing inside mitochondria. If this peptide naturally seeks out the damaged lipids that pile up in those cells, it could be a more targeted therapy than previously understood, and future trials could focus on the patients most likely to benefit.
If Duchenne muscular dystrophy damages the same fats inside mitochondria that this drug protects, could it help boys with DMD too?
Duchenne muscular dystrophy affects roughly 1 in 3,500 boys and causes progressive muscle loss with limited treatment options. If elamipretide could reduce the downstream mitochondrial damage triggered by the primary defect, it might offer a new add-on strategy, and because the drug is already in clinical trials, the path to testing would be shorter.
If you swapped just one part of this peptide back to its natural form, would it fall apart in the blood before reaching the target?
Many promising peptide drugs never make it to market because the body digests them before they reach their target. If this one structural quirk is what prevents that, it gives drug designers a clear, simple rule for keeping similar future drugs stable long enough to work, potentially after a simple injection under the skin.
If nerve cell death in POLG disease begins with damaged fats inside mitochondria, could a drug that protects those fats slow the disease?
POLG mutations cause a progressive neurological disease involving seizures, coordination loss, and cognitive decline, with no disease-modifying treatment available. If elamipretide can slow the early mitochondrial damage in affected neurons, it could become the first pharmacological option for thousands of patients worldwide, and would strengthen the case for this drug class in neurodegeneration more broadly.
▸full evidence table1 metrics
| metric | value | tool |
|---|---|---|
| ranking score | 0.7965373396873474 | boltz-2 |
▸3-letter notation
▸recipeboltz-2 2.2.1
| parameter | value |
|---|---|
| model | boltz-2 2.2.1 |
| weights | — |
| hardware | vast_v100_32gb |
| mlx version | — |
| python | — |
| random seed | 1 |
| msa strategy | none_monomer |
| runtime | — |
| predicted by | — |
| predicted at | 2026-05-23 |
▸citationbibtex
@peptide{pep10802,
sequence = {RYKF},
target = {cardiolipin},
author = {peptidemodel},
year = {2026},
status = {bioassayed}
}