Elamipretide (SS-31)

What it is

Elamipretide is a four-residue synthetic peptide with an unusual design brief: get inside a mitochondrion and stay there. It carries a D-form arginine, a non-standard residue called 2′,6′-dimethyltyrosine, a lysine and a phenylalanine. That alternating cationic-aromatic pattern lets it concentrate in the inner mitochondrial membrane without needing a transporter.

It has had several names across two decades of development — SS-31 in the academic literature, MTP-131 and bendavia in cardiology trials, elamipretide as the international non-proprietary name, and Forzinity as the approved brand. Papers describing all four are describing the same molecule, which matters when assessing how many times it has been tested.

What it does in the body

Its target is cardiolipin, a phospholipid found almost exclusively on the inner mitochondrial membrane. Cardiolipin shapes the folded cristae where the respiratory complexes sit, and it organises those complexes into supercomplexes for efficient oxidative phosphorylation. It also governs a decision point: the interaction between cardiolipin and cytochrome c determines whether cytochrome c behaves as an electron carrier or as a peroxidase.

Elamipretide binds cardiolipin through electrostatic and hydrophobic interactions. By doing so it prevents cardiolipin from converting cytochrome c into a peroxidase while preserving its electron-carrying role, protects cristae structure and supports oxidative phosphorylation. The designer’s own framing was a first-in-class cardiolipin-protective compound intended to restore mitochondrial bioenergetics.

This is the most specific and best-characterised mechanism of anything in this atlas. It is also a useful lesson: a precise mechanism and a clear molecular target did not produce a high trial success rate.

What the human evidence shows

Elamipretide has been through more controlled trials than every other compound in this atlas combined. Most of them failed.

Barth syndrome — the approval. Barth syndrome is caused by mutations in TAZ, which impair tafazzin, the enzyme that remodels cardiolipin. So the drug’s target is the exact molecule the disease disrupts. In TAZPOWER, a randomised double-blind placebo-controlled crossover trial, 12 patients took 40 mg/day or placebo for 12 weeks each with a washout between. Neither primary endpoint was met in the randomised phase. Ten patients continued into an open-label extension, and by week 36 there were significant improvements in the six-minute walk test (+95.9 m, p=0.024) and in a Barth symptom assessment (p=0.031). The 168-week extension results, published in 2024, reported a cumulative 96.1 m improvement in walk distance (p=0.003) in the eight patients who reached that point, improved fatigue scores, improved three-dimensional left-ventricular volumes, and improved monolysocardiolipin-to-cardiolipin ratios.

On 19 September 2025 the FDA granted accelerated approval to Forzinity to improve muscle strength in adult and paediatric patients with Barth syndrome weighing at least 30 kg, at 40 mg subcutaneously once daily. The approval rests on improved knee-extension strength — a surrogate the agency judged reasonably likely to predict benefit — and the FDA is requiring a post-approval randomised placebo-controlled confirmatory trial. The approval came after a complete response letter earlier in 2025. Barth syndrome affects approximately 150 people in the United States.

Primary mitochondrial myopathy — failed. MMPOWER-3 randomised 218 patients with genetically confirmed disease to 40 mg/day or placebo for 24 weeks. It missed both primary endpoints: the difference in six-minute walk distance was −3.2 m (95% CI −18.7 to 12.3, p=0.69) and the fatigue score difference was −0.07 (p=0.37). The paper is classified as providing Class I evidence that elamipretide does not improve these outcomes. It was well tolerated.

Heart attack — failed. EMBRACE STEMI, a randomised double-blind phase 2a trial, infused the drug during primary angioplasty for first anterior ST-elevation myocardial infarction. No significant reduction in infarct size by creatine kinase-MB area under the curve, and no improvement in MRI, angiographic, electrocardiographic or clinical outcomes. Safe and well tolerated.

Heart failure — signal only. In an ascending-dose trial, 36 patients with reduced ejection fraction received a single four-hour infusion or placebo. No serious adverse events. At the highest dose, left-ventricular end-diastolic volume fell 18 mL (p=0.009) and end-systolic volume 14 mL (p=0.005) versus placebo, correlating with peak plasma concentration. A single-infusion imaging change, not an outcome.

Macular degeneration — missed primaries, interesting secondaries. ReCLAIM-2 randomised 176 patients with dry AMD and non-central geographic atrophy to 40 mg/day for 48 weeks. Both primary endpoints — low-luminance visual acuity and square-root-converted atrophy area — were missed. But progression of total ellipsoid zone attenuation was reduced by 43% and partial attenuation by 47% (nominal p=0.003 and p=0.004), and more patients gained at least ten letters of low-luminance acuity (14.6% versus 2.1%, nominal p=0.040). Adverse events occurred in 86% on drug versus 71% on placebo, mostly injection-site reactions. The ellipsoid zone endpoint was accepted as the primary endpoint for phase 3.

Claims and what backs them

Risks and unknowns

Tolerability is genuinely good. Across trials the dominant adverse events were mild-to-moderate injection-site reactions — pruritus, pain, bruising, erythema. In ReCLAIM-2 any adverse event occurred in 86% of treated patients versus 71% on placebo, which is a modest excess driven largely by those local reactions. No serious safety signal emerged in the heart-failure infusion study.

The surrogate-endpoint caveat is the FDA’s own. Accelerated approval means the agency accepted a measure reasonably likely to predict clinical benefit, not a demonstration of it. Commentary on the approval has noted that it rests on a surrogate, not on confirmed long-term outcomes such as survival or quality of life. If the confirmatory trial fails, accelerated approvals can be withdrawn.

Weight and age limits. The approval covers patients weighing at least 30 kg; the manufacturer has said it will work with the FDA on a possible label expansion for smaller patients. Nothing is established below that threshold.

Everything outside Barth syndrome is off-label and mostly tested-and-failed. That is an unusual and important distinction. For most peptides here the problem is absence of evidence. For elamipretide in myopathy and in myocardial infarction, there is evidence of absence.

Where regulators stand

The bottom line

Elamipretide is the most rigorously tested peptide in this atlas and the only approved one, and it belongs here mainly as a reference point. It shows what twenty years of development, a precise molecular target and a dozen trials actually buy: one accelerated approval in an ultra-rare genetic disease whose defect is in the exact pathway the drug addresses, with a confirmatory trial still outstanding.

It also shows how often a convincing mechanism fails in people. The same molecule, in the same formulation, at the same dose, did not reduce heart-attack damage, did not help mitochondrial myopathy, and did not hit its primary endpoints in macular degeneration. Anyone selling a peptide on the strength of mitochondrial mechanism alone should be asked to account for this record.

References

1. US Food and Drug Administration. FDA grants accelerated approval to first treatment for Barth syndrome. Press announcement, 19 September 2025 Source
2. Szeto HH. First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics. Br J Pharmacol. 2014;171(8):2029–2050 PMID 24117165
3. 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. Genet Med. 2021;23(3):471–478 PMID 33077895
4. Thompson WR, Manuel R, Abbruscato A, 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 38602181
5. 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(3):e238–e252 PMID 37268435
6. Gibson CM, Giugliano RP, Kloner RA, et al. EMBRACE STEMI study: a phase 2a trial to evaluate the safety, tolerability, and efficacy of intravenous MTP-131 on reperfusion injury. Eur Heart J. 2016;37(16):1296–1303 PMID 26586786
7. Daubert MA, Yow E, Dunn G, et al. Novel mitochondria-targeting peptide in heart failure treatment: a randomized, placebo-controlled trial of elamipretide. Circ Heart Fail. 2017;10(12):e004389 PMID 29217757
8. Ehlers JP, Hu A, Boyer D, et al. ReCLAIM-2: a randomized phase II clinical trial evaluating elamipretide in age-related macular degeneration. Ophthalmol Sci. 2025;5(1):100628 PMID 39605874