SS-31

01 · What it is

SS-31 is the only compound in this family that holds a drug approval, and it earned that approval in a disease whose randomised trial enrolled twelve people — after failing the largest trial it ever ran.

SS-31 is a four-residue aromatic-cationic peptide, D-arginyl-2,6-dimethyltyrosyl-lysyl- phenylalaninamide, developed under several names: SS-31, MTP-131, Bendavia, and — as an approved drug — elamipretide ¹⁶. The alternating aromatic and cationic residues give it a property that almost no other peptide in this project has: it concentrates in the inner mitochondrial membrane without a transporter and without a receptor, driven by charge and by its affinity for a specific phospholipid.

That phospholipid is cardiolipin, and it is the whole story. Cardiolipin is found almost exclusively in the inner mitochondrial membrane, where it organises the cristae folds and holds the electron transport chain complexes together in supercomplexes. SS-31 binds it, and the downstream effects — restored cristae architecture, improved electron transport efficiency, reduced peroxidase activity at the cytochrome c–cardiolipin complex — follow from that single interaction ¹⁷.

The clinical record is unusually complete and unusually mixed. Elamipretide has been through randomised placebo-controlled trials in heart failure with reduced ejection fraction ¹, primary mitochondrial myopathy at phase 1/2 ², phase 2 crossover ⁵ and phase 3 ¹¹, Barth syndrome ¹³, dry age-related macular degeneration at phase 1 ^6,7 and phase 2 ¹⁴, and Leber hereditary optic neuropathy ¹². This is a real drug development programme with real comparators, and it is the reason this guide reads differently from the rest of the family.

The phase 3 in primary mitochondrial myopathy did not work. The post hoc analysis published by the same investigators states it plainly in its own background section: the MMPOWER-3 clinical trial did not demonstrate a significant benefit of elamipretide treatment in a genotypically diverse population of adults with primary mitochondrial myopathy ¹⁵. That analysis went on to look for signal within genotype subgroups, and describes its findings as a foundation for further work rather than as a result ¹⁵.

The approval came from a different disease. On 19 September 2025 the US Food and Drug Administration granted accelerated approval to elamipretide, as Forzinity, for Barth syndrome in patients weighing at least 30 kg ²¹. Barth syndrome is a rare X-linked disorder of cardiolipin remodelling — which is to say, the one disease where the drug’s single molecular target is the thing that is broken. The randomised portion of the trial supporting it, TAZPOWER, enrolled twelve patients in a 28-week double-blind placebo-controlled crossover, followed by a 168-week open-label extension ^10,13.

That combination — a mechanism confirmed at the molecular level, an approval in the one disease that matches the mechanism exactly, and a failure in the broader population — is the shape of this compound’s evidence, and it is the shape the rest of this guide follows.

02 · Evidence at a glance

Evidence grade

Approved Pharma — accelerated approval for Barth syndrome, 19 September 2025 ²¹

Structure

D-Arg-2,6-dimethylTyr-Lys-Phe-NH2; aromatic-cationic tetrapeptide

Other names

SS-31, MTP-131, Bendavia, elamipretide, Forzinity

Molecular target

Cardiolipin, in the inner mitochondrial membrane ¹⁷

Receptor

None — uptake is charge- and affinity-driven, not receptor-mediated

Approved indication

Barth syndrome, patients ≥30 kg ²¹

Basis of approval

Accelerated; improvement in knee extensor muscle strength, a measure considered reasonably likely to predict benefit ²¹

Randomised trial supporting it

TAZPOWER, 28-week double-blind placebo-controlled crossover, 12 patients, plus 168-week open-label extension ¹³

Largest trial

MMPOWER-3, phase 3 in primary mitochondrial myopathy — did not demonstrate significant benefit ^11,15

Other randomised trials

HFrEF ascending dose ¹; PMM phase 1/2 ² and phase 2 crossover ⁵; dry AMD phase 2 ¹⁴; LHON phase 2 topical ¹²

Routes studied in humans

Subcutaneous, intravenous, topical ophthalmic

Safety in early-phase trials

No severe adverse events reported ⁸

Approval scope

One indication, one jurisdiction, one weight band

03 · Mechanism of action

Cardiolipin, and why a single lipid explains the whole compound

Cardiolipin is a four-tailed phospholipid confined almost entirely to the inner mitochondrial membrane. It curves that membrane into cristae, it anchors cytochrome c, and it holds respiratory chain complexes together in the supercomplex arrangements that make electron transfer efficient. When cardiolipin content or acyl composition is abnormal, cristae flatten, supercomplexes dissociate, electron transfer becomes leaky, and reactive oxygen production rises.

SS-31’s alternating aromatic and basic residues let it partition into that membrane and associate with cardiolipin. The consequences reported across the mechanistic literature are structural before they are biochemical: cristae architecture is restored, supercomplex assembly improves, electron transport efficiency rises, and peroxidase activity at the cytochrome c–cardiolipin complex falls ¹⁷. A 2025 review by investigators across the programme’s academic and industrial groups sets out the contemporary account and the newer biophysical work behind it ¹⁷.

This is the cleanest mechanism-to-target mapping of any compound in this project. There is one binding partner, it is a lipid rather than a protein, and the effects downstream are consistent with what that lipid does.

Why Barth syndrome is the test case the mechanism predicts

Barth syndrome is caused by mutations in TAZ, the gene encoding the transacylase that remodels immature cardiolipin into its mature form. Patients accumulate monolysocardiolipin and lack mature cardiolipin; cristae are abnormal; cardiac and skeletal muscle fail. A compound whose sole target is cardiolipin, given in the one disease defined by defective cardiolipin, is as close to a mechanistic prediction as drug development offers.

The approval followed that logic and the regulator’s summary describes the drug as binding the inner part of the mitochondria and improving mitochondrial structure and function ²¹.

No receptor, and what follows from that

SS-31 has no receptor. It is not an agonist, and dose-response does not depend on receptor occupancy or desensitisation. Distribution is governed by mitochondrial membrane potential and cardiolipin abundance, which means uptake is highest in the tissues with the most mitochondria — heart, skeletal muscle, retina, kidney — and this is reflected in the indications the programme has pursued ^{1,2,6,7,12,14}.

The absence of a receptor is usually a weakness in this field. Here it is the opposite: the target is a defined molecule, the distribution follows from physical chemistry, and neither requires an unidentified binding site to explain.

04 · Key research findings

Primary mitochondrial myopathy — three trials, ending in a negative phase 3. MMPOWER was a phase 1/2 multicentre randomised double-blind placebo-controlled dose-escalation trial in 36 participants with genetically confirmed mitochondrial disease ². MMPOWER-2 was a randomised double-blind placebo-controlled crossover trial in 30 participants; the protocol used 40 mg/day subcutaneously ⁵. Early reports from these described safety and suggested improvement in six-minute walk test performance and fatigue scales ⁴. MMPOWER-3 was the pivotal phase 3 ¹¹. Its own investigators’ subsequent post hoc analysis opens by stating that MMPOWER-3 did not demonstrate a significant benefit of elamipretide in a genotypically diverse population of adults with primary mitochondrial myopathy, and presents genotype-specific findings as a foundation for further work ¹⁵.

A promising phase 2 signal in a heterogeneous rare disease population did not survive a phase 3. This is the most common outcome in mitochondrial medicine and it happened here in full public view.

Barth syndrome — the approval. TAZPOWER was a 28-week randomised, double-blind, placebo-controlled crossover trial followed by an open-label extension; the 168-week extension results reported long-term efficacy and safety ¹³. Because the randomised population was twelve patients, the programme supplemented it: a phase 3 observational, retrospective, non-interventional study established a natural history control cohort against which the open-label extension could be compared ⁹, and a separate analysis used hierarchical clustering on wearable-device time-series data — heart rate, respiratory rate, activity, posture — alongside functional measures to identify responders within the randomised crossover data ¹⁰.

Twelve randomised patients is not a limitation the programme concealed; it is a limitation the programme built three additional analyses to work around. Whether those analyses substitute for a larger trial is a judgement the accelerated approval pathway was designed to make, and it is why the approval is accelerated rather than full ²¹.

Heart failure. A double-blind, placebo-controlled, ascending-dose trial gave patients with heart failure and reduced ejection fraction (≤35%) a single four-hour infusion, with eight patients in the first cohort ¹. Reviews of the cardiac programme note that early-phase administration produced no severe adverse events and showed improvements in cardiac haemodynamics at the highest doses, while stating that long-term data were lacking ⁸. Interest in mitochondria-targeted compounds for cardiovascular disease more broadly has been reviewed alongside this work ³.

Haemodynamic improvement after a single infusion is a pharmacodynamic result, not an outcome. No cardiovascular outcome trial has been reported.

Retina. Two single-centre, open-label, 24-week phase 1 trials examined elamipretide in dry age-related macular degeneration — one in patients with intermediate AMD and high-risk drusen ⁷, one in patients with non-central geographic atrophy ⁶. Both reported exploratory analyses suggesting a positive effect on visual function, particularly under low luminance. ReCLAIM-2 followed as a prospective, phase 2, randomised, placebo-controlled, double-masked, multicentre trial in patients aged 55 and over with geographic atrophy, evaluating geographic atrophy growth, visual function and ellipsoid zone preservation, with a primary safety endpoint ¹⁴.

Open-label exploratory analyses of visual function under low luminance are the weakest kind of positive finding, and the programme correctly treated them as hypothesis-generating by running a masked randomised trial afterwards.

Leber hereditary optic neuropathy. A phase 2 prospective, randomised, vehicle-controlled, single-centre trial assessed a topical ophthalmic solution for safety, tolerability and potential efficacy ¹².

A topical formulation of a mitochondria-targeting peptide, tested in a mitochondrial optic neuropathy, is another instance of the programme going where the mechanism points.

Contemporary appraisal. Reviews published since the phase 3 result place elamipretide as the leading mitochondria-targeted therapeutic while noting that phase 3 evidence remains incomplete ^16,17,18. Two 2026 reviews of peptides marketed directly to patients list elamipretide among compounds where approved and unapproved use overlap ^19,20.

05 · Evidence overview

06 · Safety profile

Animal data. The preclinical package is that of a compound that completed a full development programme, spanning cardiac, renal, retinal and skeletal muscle models, and is summarised in the mechanistic and clinical reviews ^3,8,17.

Human data. This is the most substantial human safety dataset in Family E by a wide margin. Reviews of the early-phase programme report that administration did not result in any severe adverse events ⁸. The Barth syndrome open-label extension provides 168 weeks of continuous exposure data ¹³. Trials have been conducted by subcutaneous, intravenous and topical ophthalmic routes ^1,2,5,12. The US label carries the approved safety information for the indication and weight band covered by the approval ²¹.

What is genuinely unknown. The safety of long-term administration outside Barth syndrome, since the 168-week extension data comes from a disease population of twelve randomised patients ¹³. Whether the accelerated approval’s surrogate — knee extensor strength — predicts clinical benefit; that is precisely the question the accelerated approval pathway defers to a confirmatory trial ²¹. Consequences of chronic cardiolipin binding in people with normal cardiolipin, which no trial has studied, because every trial population had mitochondrial pathology. Effects in patients weighing under 30 kg, explicitly outside the approval ²¹. Whether the negative phase 3 in mitochondrial myopathy reflects an inactive drug, an insensitive endpoint, or a population too genotypically heterogeneous to show an effect — the post hoc analysis raises the third possibility and does not settle it ¹⁵. And, for material obtained outside the approved supply chain, everything: no published analysis has examined the identity or purity of any SS-31 preparation sold as a research compound.

07 · US regulatory status

Current as of 6 September 2026. Elamipretide holds accelerated approval from the US Food and Drug Administration, granted 19 September 2025 under NDA 215244 and marketed as Forzinity, for the treatment of Barth syndrome in patients weighing at least 30 kg ²¹. It is not a controlled substance.

Three features of that approval belong in any honest account of it. It is accelerated, which means it rests on an endpoint considered reasonably likely to predict clinical benefit rather than on a demonstrated outcome, and it carries a confirmatory obligation. The endpoint was muscle strength at the knee ²¹. And the indication is one ultra-rare disease, not mitochondrial dysfunction generally — the trial in a broader mitochondrial population did not succeed ^11,15.

Nothing about this approval extends to other uses. A drug approved for Barth syndrome in patients above 30 kg is approved for that, and the evidence supporting it was generated in that population.

Under the World Anti-Doping Code, elamipretide does not appear as a named substance in the classes this guide has reviewed. Competitors should consult the current Prohibited List directly rather than rely on secondary summaries, including this one.

08 · Limitations of the evidence

1. The phase 3 trial in the larger indication was negative. MMPOWER-3 did not demonstrate a significant benefit in a genotypically diverse population of adults with primary mitochondrial myopathy, as the investigators’ own subsequent paper states ^11,15. Any account of this compound that leads with the approval and omits this is incomplete.

1. The approval rests on twelve randomised patients and a surrogate endpoint. TAZPOWER’s randomised phase enrolled twelve people with Barth syndrome ¹³; the accelerated approval was based on improvement in knee extensor strength, a measure the agency judged reasonably likely to predict benefit rather than one demonstrating it ²¹. Both facts are ordinary for an ultra-rare disease and both limit what can be concluded.

1. Post hoc genotype analysis is hypothesis-generating. The MMPOWER-3 subgroup work was performed after an overall negative result, and its authors present it as a foundation for further study ¹⁵. Subgroup findings following a failed primary endpoint carry a high prior probability of not replicating.

1. The Barth efficacy case leans on non-randomised comparators. A retrospective natural history control cohort ⁹ and a responder-clustering analysis of wearable-device data ¹⁰ were constructed to strengthen an open-label extension. Both are defensible in a disease this rare and neither is a randomised comparison.

1. The retinal evidence is early and partly open-label. The two phase 1 trials were single-centre and open-label, with visual-function benefits reported as exploratory analyses ^6,7. ReCLAIM-2 was masked and randomised ¹⁴, and the programme should be judged on it rather than on the phase 1 work.

1. No outcome trial exists in any common disease. The heart failure work is a single-infusion ascending-dose study with haemodynamic endpoints ¹. Nothing has tested whether the compound changes mortality, hospitalisation or progression in a prevalent condition.

1. Every trial population had mitochondrial pathology. Barth syndrome, primary mitochondrial myopathy, LHON, heart failure, age-related macular degeneration. Nothing in the record describes what this compound does in a person whose mitochondria are functioning normally, and the longevity framing under which SS-31 is often discussed assumes exactly that population.

1. Research concentration is real, though less severe than elsewhere in this project. The mitochondrial myopathy programme shares a core investigator group across MMPOWER, MMPOWER-2 and MMPOWER-3 ^2,5,11,15, and the Barth work shares another ^9,10,13. The retinal and LHON trials were run by different groups ^6,7,12,14, which provides partial independence.

1. Publication of negative results here has been good, which is itself worth noting. The failed phase 3 was published in a major neurology journal ¹¹ and its shortfall restated in the post hoc paper’s own opening ¹⁵. That is not typical of this field and it makes the file more trustworthy than most in this project, not less.

1. Nothing is known about non-pharmaceutical material. The evidence above concerns a drug made to pharmaceutical specification. No published analysis has examined the identity, purity or stereochemistry of any preparation sold as SS-31 outside that supply chain — and this molecule contains a D-amino acid and a modified tyrosine, both of which are synthesis steps where a shortcut changes the molecule.

09 · References

1. 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 Dec;10(12):e004389.

   PMID 29217757 ↗

2. Karaa A, Haas R, Goldstein A, Vockley J, Weaver WD, Cohen BH. Randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy. Neurology. 2018 Apr 3;90(14):e1212–e1221.

   PMID 29500292 ↗

3. Eirin A, Lerman A, Lerman LO. Enhancing mitochondrial health to treat hypertension. Curr Hypertens Rep. 2018 Aug 17;20(10):89.

   PMID 30120623 ↗

4. de Barcelos IP, Emmanuele V, Hirano M. Advances in primary mitochondrial myopathies. Curr Opin Neurol. 2019 Oct;32(5):715–721.

   PMID 31408013 ↗

5. Karaa A, Haas R, Goldstein A, Vockley J, Cohen BH. A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy. J Cachexia Sarcopenia Muscle. 2020 Aug;11(4):909–918.

   PMID 32096613 ↗

6. Mettu PS, Allingham MJ, Cousins SW. Phase 1 clinical trial of elamipretide in dry age-related macular degeneration and noncentral geographic atrophy: ReCLAIM NCGA study. Ophthalmol Sci. 2021 Nov 27;2(1):100086.

   PMID 36246181 ↗

7. Allingham MJ, Mettu PS, Cousins SW. Phase 1 clinical trial of elamipretide in intermediate age-related macular degeneration and high-risk drusen: ReCLAIM high-risk drusen study. Ophthalmol Sci. 2021 Dec 22;2(1):100095.

   PMID 36246187 ↗

8. Obi C, Smith AT, Hughes GJ, Adeboye AA. Targeting mitochondrial dysfunction with elamipretide. Heart Fail Rev. 2022 Sep;27(5):1925–1932.

   PMID 35037146 ↗

9. Hornby B, Thompson WR, Almuqbil M, Manuel R, Abbruscato A, Carr J, Vernon HJ. Natural history comparison study to assess the efficacy of elamipretide in patients with Barth syndrome. Orphanet J Rare Dis. 2022 Sep 2;17(1):336.

   PMID 36056411 ↗

10. Van den Eynde J, Chinni B, Vernon H, Thompson WR, Hornby B, Kutty S, Manlhiot C. Identifying responders to elamipretide in Barth syndrome: hierarchical clustering for time series data. Orphanet J Rare Dis. 2023 Apr 11;18(1):76.

    PMID 37041653 ↗

11. Karaa A, Bertini E, Carelli V, et al; MMPOWER-3 Trial Investigators. Efficacy and safety of elamipretide in individuals with primary mitochondrial myopathy: the MMPOWER-3 randomized clinical trial. Neurology. 2023 Jul 18;101(3):e238–e252.

    PMID 37268435 ↗

12. Karanjia R, Sadun AA. Elamipretide topical ophthalmic solution for the treatment of subjects with Leber hereditary optic neuropathy: a randomized trial. Ophthalmology. 2024 Apr;131(4):422–433.

    PMID 37923251 ↗

13. Thompson WR, Manuel R, Abbruscato A, Carr J, Campbell J, Hornby B, Vaz FM, Vernon HJ. Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER. Genet Med. 2024 Jul;26(7):101138.

    PMID 38602181 ↗

14. Ehlers JP, Hu A, Boyer D, et al; ReCLAIM-2 (SPIAM-202) Study Investigators. ReCLAIM-2: a randomized phase II clinical trial evaluating elamipretide in age-related macular degeneration, geographic atrophy growth, visual function, and ellipsoid zone preservation. Ophthalmol Sci. 2024 Oct 9;5(1):100628.

    PMID 39605874 ↗

15. Karaa A, Bertini E, Carelli V, et al; MMPOWER-3 Trial Investigators. Genotype-specific effects of elamipretide in patients with primary mitochondrial myopathy: a post hoc analysis of the MMPOWER-3 trial. Orphanet J Rare Dis. 2024 Nov 21;19(1):431.

    PMID 39574155 ↗

16. Tung C, Varzideh F, Farroni E, Mone P, Kansakar U, Jankauskas SS, Santulli G. Elamipretide: a review of its structure, mechanism of action, and therapeutic potential. Int J Mol Sci. 2025 Jan 23;26(3):944.

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17. Sabbah HN, Alder NN, Sparagna GC, et al. Contemporary insights into elamipretide’s mitochondrial mechanism of action and therapeutic effects. Biomed Pharmacother. 2025 Jun;187:118056.

    PMID 40294492 ↗

18. Bangeas A, Poulidou V, Liampas I, et al. Advances in management of mitochondrial myopathies. Int J Mol Sci. 2025 Jun 5;26(11):5411.

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19. Renke G, Chinellato L. Therapeutic peptides in aesthetic, metabolic and endocrine conditions: effects, safety, clinical applications, and future perspectives. Int J Mol Sci. 2026 Apr 27;27(9):3890.

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20. Mendias CL, Awan TM. Safety and efficacy of approved and unapproved peptide therapies for musculoskeletal injuries and athletic performance. Sports Med. 2026 Aug;56(8):1921–1935.

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21. US Food and Drug Administration. FDA grants accelerated approval to first treatment for Barth syndrome, 19 September 2025; approval letter and prescribing information, NDA 215244. Regulatory documents; no PMID.

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