Elamipretide
SS-31, MTP-131, RX-31, Szeto-Schiller peptide 31, D-Arg-Dmt-Lys-Phe-NH2
Elamipretide is a four-residue mitochondria-targeting peptide that concentrates in the inner mitochondrial membrane and binds cardiolipin, the signature phospholipid of that membrane. It is the only compound in this class with regulator-approved labelling: in September 2025 the FDA granted it accelerated approval for improving muscle strength in Barth syndrome. Its trial record is otherwise mostly negative, with large randomised studies in mitochondrial myopathy, heart failure and macular degeneration failing their primary endpoints.
Mechanism
Elamipretide carries an alternating aromatic-cationic motif with a D-arginine at the N-terminus and a C-terminal amide, a design that resists proteolysis and permits cell entry without a transporter. Once inside the cell it partitions into the inner mitochondrial membrane, driven not by the membrane potential but by direct affinity for cardiolipin. Cardiolipin is a four-tailed phospholipid found almost exclusively in the inner mitochondrial membrane, where it organises the respiratory chain supercomplexes and anchors cytochrome c to the membrane surface.
The binding is electrostatic and hydrophobic rather than a classical receptor interaction. Biophysical work has shown that elamipretide binds lipid bilayers and modulates their surface electrostatics, and that it interacts selectively with cardiolipin-containing membranes. The functional consequence proposed is that it stabilises cardiolipin-protein interactions and preserves cristae architecture, which in turn improves supercomplex assembly and electron-transport efficiency and reduces electron leak to superoxide. It also blocks the peroxidase activity that cytochrome c acquires when it dissociates from oxidised cardiolipin, a step upstream of apoptosis. This is the basis for calling it a 'cardiolipin binder' rather than an antioxidant: it is not scavenging radicals stoichiometrically but restoring the structural conditions under which the respiratory chain leaks fewer of them.
Barth syndrome is the mechanistic proof of concept. It is caused by loss-of-function mutations in TAFAZZIN, the enzyme that remodels immature cardiolipin into its mature tetralinoleoyl form. Patients accumulate abnormal monolysocardiolipin, and their mitochondria show disorganised cristae. A drug that binds and stabilises cardiolipin has a coherent rationale in exactly that disease and a much weaker one in generic ageing.
What the research shows
The clinical programme is unusually large for a mitochondrial drug and mostly disappointing. In primary mitochondrial myopathy, the phase 1/2 MMPOWER trial randomised 36 participants to intravenous elamipretide at 0.01, 0.1 or 0.25 mg/kg/h infused over two hours, or placebo, in a dose-escalating sequence, with six-minute walk distance measured after five days of treatment. Participants on the highest dose walked a mean of 64.5 metres further, against 20.4 metres in the placebo group, which did not reach significance on its own (p=0.053); the dose-response trend did (p=0.014), as did a covariate-adjusted analysis (51.2 versus 3.0 metres, p=0.0297). Note that this was a short intravenous study, not the subcutaneous regimen later taken forward. The definitive study, MMPOWER-3 (NCT03323749, n=218 randomised 1:1, 24 weeks, 40 mg daily subcutaneously), then failed: the between-group difference in six-minute walk distance was -3.2 metres (95% CI -18.7 to 12.3, p=0.69) and the fatigue score difference was likewise null (p=0.37). The publication carries a Class I evidence rating for that negative result, and the registry record for the trial is marked terminated. A post hoc genotype-stratified reanalysis suggested benefit in a nuclear-DNA mutation subgroup, but post hoc subgroup findings in a failed trial are hypothesis-generating only.
In Barth syndrome, TAZPOWER (NCT03098797) randomised 12 patients to 40 mg daily or placebo for 12 weeks, followed by a four-week washout and crossover to the opposite arm. Neither primary endpoint was met in that randomised phase. Ten patients continued into an open-label extension and eight reached 36 weeks, at which point six-minute walk distance had improved by 95.9 metres (p=0.024) and the Barth syndrome symptom score by 2.1 points (p=0.031), alongside gains in knee extensor strength and some cardiac parameters. It was this open-label signal, not the randomised comparison, that supported FDA accelerated approval in September 2025 on knee extensor muscle strength as an intermediate clinical endpoint reasonably likely to predict benefit. A confirmatory postmarketing trial is required. The honest reading is that the approval rests on uncontrolled data in an ultra-rare disease where a controlled trial of adequate size is close to impossible.
Elsewhere the results are negative. In heart failure with reduced ejection fraction, the four-week PROGRESS-HF phase 2 trial found no improvement in left ventricular end-systolic or end-diastolic volume. In dry age-related macular degeneration, ReCLAIM-2 (NCT03891875, n=176), a randomised phase 2 study, did not meet its primary endpoint of change in low-luminance visual acuity, though some anatomical measures moved. Trials in Leber hereditary optic neuropathy, Fuchs corneal dystrophy and reperfusion injury were completed or terminated without establishing efficacy.
Evidence assessment
High-quality evidence
Multiple adequately powered randomised placebo-controlled trials have been conducted and the compound now carries FDA-approved labelling, which meets the definition of strong evidence even though most of those trials were negative and the approval rests on an uncontrolled surrogate endpoint.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial High-quality evidence
Both primary endpoints missed: six-minute walk distance difference -3.2 m (95% CI -18.7 to 12.3, p=0.69) and fatigue score difference p=0.37. Reported as Class I evidence of no benefit; treatment was well tolerated.
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 Mixed evidence
Neither primary endpoint was met in the randomised phase; at 36 weeks of open-label treatment six-minute walk distance improved 95.9 m (p=0.024) and the symptom score 2.1 points (p=0.031), with gains in knee extensor strength and some cardiac parameters.
Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER Limited evidence
Sustained improvement from baseline in knee extensor muscle strength and cardiac measures; elamipretide was tolerated over more than three years.
Randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy Mixed evidence
Highest dose walked a mean 64.5 m further at day 5 versus 20.4 m on placebo (p=0.053); dose-dependent increase across doses p=0.014; covariate-adjusted change 51.2 versus 3.0 m (p=0.0297). No differences in other efficacy or safety endpoints.
Effects of Elamipretide on Left Ventricular Function in Patients With Heart Failure With Reduced Ejection Fraction: The PROGRESS-HF Phase 2 Trial High-quality evidence
No significant improvement in left ventricular end-systolic or end-diastolic volume versus placebo.
ReCLAIM-2: A Randomized Phase II Clinical Trial Evaluating Elamipretide in Age-related Macular Degeneration, Geographic Atrophy Growth, Visual Function, and Ellipsoid Zone Preservation High-quality evidence
Did not meet the primary endpoint of change in low-luminance best-corrected visual acuity, though some anatomical measures favoured treatment.
The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin Preclinical only
SS-31 binds cardiolipin, preserves cristae architecture, protects cytochrome c from becoming a peroxidase and restores ATP synthesis after ischaemia.
Safety
The safety database is genuinely substantial by the standards of this class, spanning several hundred trial participants with exposures up to 168 weeks. In MMPOWER-3 most adverse events were mild to moderate and the drug was described as well tolerated despite the null efficacy result. The dominant adverse effect is injection-site reaction, which is common, dose-related and was a frequent reason for discontinuation. Otherwise the tolerability profile across the trial programme was unremarkable, with no consistent organ toxicity signal. Two limits should be stated. First, all of this experience is in patients with mitochondrial disease, heart failure or retinal disease, not in healthy people using it for ageing, and there is no long-term safety data in that population. Second, material sold outside the approved supply chain as 'SS-31' is not the product that generated this safety record and its identity and purity are unverified.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | No MHRA marketing authorisation. Access in the UK would be through unlicensed-medicine or named-patient routes only. Material sold as 'SS-31' by research-chemical suppliers is unlicensed and outside any regulated supply chain. |
| United States | Granted FDA accelerated approval in September 2025 to improve muscle strength in adult and paediatric patients with Barth syndrome meeting a minimum weight threshold, based on knee extensor muscle strength as an intermediate clinical endpoint. A confirmatory postmarketing trial is required. It is not approved for mitochondrial myopathy, heart failure, macular degeneration or any ageing-related indication. |
| WADA (sport) | Not individually named on the Prohibited List. Before September 2025 it fell under section S0 as a non-approved substance; FDA approval for a rare disease complicates that analysis, and athletes should seek a determination from their own anti-doping organisation rather than rely on secondary summaries. |
Questions
Yes, but narrowly. Elamipretide received accelerated approval in September 2025 to improve muscle strength in Barth syndrome, an ultra-rare genetic disorder, subject to a minimum patient weight. It is not approved for mitochondrial myopathy, heart failure, eye disease or anything ageing-related, and a confirmatory trial is still required.
No. MMPOWER-3, the largest trial, randomised 218 patients to 40 mg daily for 24 weeks and found a six-minute walk difference of -3.2 metres (p=0.69) and no fatigue benefit. The publication carries a Class I rating for that negative result. The earlier positive signal came from a short intravenous dose-escalation study in 36 people, which the definitive trial did not confirm.
It concentrates in the inner mitochondrial membrane and binds cardiolipin, the phospholipid that organises respiratory chain supercomplexes and anchors cytochrome c. Stabilising cardiolipin is thought to preserve cristae structure, improve electron transport efficiency and reduce electron leak, rather than scavenging free radicals directly.
There is no evidence for that. Every completed trial was in a specific disease population, and the ones in less selected populations, heart failure and macular degeneration, were negative. Nothing in the trial record speaks to healthy ageing.