SS-31 (Elamipretide): The Cardiolipin Peptide That Became Forzinity

Most mitochondrial peptides discussed in longevity circles are still research compounds. SS-31 is the exception. The same tetrapeptide sold in catalogs as SS-31 or MTP-131 is the active moiety in Forzinity (elamipretide), which the FDA granted accelerated approval in September 2025 as the first treatment for Barth syndrome.

That dual identity is the story. The mechanism is elegant, the rare-disease data are real, and the broader mitochondrial-myopathy program failed its primary endpoints. Educational only — not medical advice, not a protocol, and not a recommendation to use research-grade SS-31 as a stand-in for the approved drug.

What SS-31 actually is

SS-31 is a cell-permeable tetrapeptide: D-Arg-2′,6′-dimethylTyr-Lys-Phe-NH2. It belongs to the Szeto-Schiller family of aromatic-cationic peptides designed to concentrate on the inner mitochondrial membrane rather than act as generic antioxidants.

Unlike MOTS-c, which is encoded inside mitochondrial DNA and behaves like an exercise-responsive hormone, SS-31 is a synthetic ligand for cardiolipin. Cardiolipin holds respiratory-chain supercomplexes together, shapes cristae, and anchors cytochrome c. When it is lost, remodeled, or peroxidized, ATP production falls and reactive oxygen species rise at the source.

Szeto’s 2014 review in the British Journal of Pharmacology described the binding: cationic residues interact with cardiolipin’s anionic headgroups, while the aromatic residues insert into the acyl-chain region. The peptide does not primarily scavenge radicals after they form. It stabilizes cardiolipin so the electron-transport chain leaks fewer electrons in the first place (Szeto, 2014).

Barth syndrome is the cleanest clinical test of that idea. Pathogenic variants in TAZ disable tafazzin, the enzyme that remodels cardiolipin. Patients accumulate monolysocardiolipin, lose mature cardiolipin, and develop cardiomyopathy, skeletal myopathy, neutropenia, and early mortality.

TAZPOWER: the 12-week crossover did not win

TAZPOWER (NCT03098797) was a Johns Hopkins phase 2/3 crossover with a long open-label extension. Twelve ambulatory males aged 12–35 with genetically confirmed Barth syndrome and body weight above 30 kg received daily subcutaneous elamipretide 40 mg or placebo for 12 weeks, then crossed over after a washout (Thompson et al., Genetics in Medicine, 2021).

The blinded primary endpoints did not separate from placebo:

• 6-minute walk test: −0.8 meters versus placebo (p = 0.97)
• Barth Syndrome Symptom Assessment total fatigue: +0.06 (p = 0.89)
• Knee extensor strength by handheld dynamometry (secondary): +6.7 newtons (p = 0.65)

A 12-week, n = 12 crossover in an ultra-rare disease is underpowered for walk distance. A negative primary analysis is still a negative primary analysis.

The open-label extension is why the drug exists

Ten participants entered the extension. Eight completed week 36 in the 2021 report. Against their own baseline, the same endpoints moved:

• 6-minute walk: +60.5 m at week 12 of the extension (16%, p = 0.02) and +95.9 m at week 36 (25%, p = 0.02)
• Knee extensor strength: +37.9 N at week 12 (30%, p = 0.003) and +56.0 N at week 36 (42%, p = 0.001)
• Total fatigue score: −1.6 points at week 12 and −2.1 at week 36
• Indexed left-ventricular stroke volume: about 16% higher by week 36

The 168-week extension kept most of that signal in the eight people who stayed on drug. Cumulative 6-minute-walk improvement was 96.1 meters at week 168 (p = 0.003). Fatigue scores stayed below baseline. Cardiac volumes and the monolysocardiolipin/cardiolipin ratio moved in the expected direction (Thompson et al., 2024).

Injection-site reactions — erythema, pain, and induration — were the dominant adverse events and the reason two participants left the extension. The FDA later noted that serious reactions have also been reported.

The honest shape of the Barth data: short blinded comparison negative, longer open-label follow-up better than each patient’s baseline, tiny sample, no concurrent control in the extension. For an ultra-rare lethal disease with no other approved therapy, regulators treated the strength change as an intermediate endpoint reasonably likely to predict benefit.

What the FDA actually approved

On 19 September 2025 the FDA granted accelerated approval to Forzinity (elamipretide) injection to improve muscle strength in adult and pediatric Barth syndrome patients weighing at least 30 kg. It is the first approved treatment for the disease and the first mitochondria-targeted therapeutic to clear that bar.

Accelerated approval is not full approval. The agency said knee-extensor strength is reasonably likely to predict being able to stand more easily or walk farther, and it required a confirmatory randomized, double-blind, placebo-controlled trial. The labeled product is a once-daily subcutaneous injection. Research vials labeled SS-31 do not inherit that indication, manufacturing package, or risk-management framework.

The other mitochondrial program failed

If SS-31 were a general mitochondrial tonic, the larger primary mitochondrial myopathy program should have shown it. MMPOWER-3 randomized 218 adults with genetically confirmed PMM to daily subcutaneous elamipretide 40 mg or placebo for 24 weeks. It missed both co-primary endpoints: 6-minute walk and PMMSA total fatigue. The walk-test difference was −3.2 meters (95% CI −18.7 to 12.3; p = 0.69). That is Class I evidence of no benefit on those endpoints in a heterogeneous PMM population (Karaa et al., Neurology, 2023).

Earlier, smaller MMPOWER studies had suggested a walk-distance signal. Phase 3 did not confirm it. Later post-hoc genotype splits are hypothesis-generating, not a substitute for the primary result. Dry AMD and other programs remain investigational.

How to read the research-peptide conversation

SS-31 shows up next to MOTS-c and humanin because all three touch mitochondria. They are not interchangeable. MOTS-c is an endogenous mitochondrial-encoded peptide tied to AMPK and exercise. Humanin is another mitochondrial-derived peptide with mostly preclinical cytoprotection data. SS-31 / elamipretide is a synthetic cardiolipin stabilizer with one ultra-rare approved use and a failed broader myopathy phase 3.

Pro-peptide does not mean indiscriminate. A compound that can change knee strength and stroke volume in Barth syndrome is scientifically interesting. That is not evidence that research-grade SS-31 restores energy, slows aging, or substitutes for Forzinity. Sourcing still matters; a certificate of analysis is the minimum filter, not a clinical endorsement. Compare listed compounds on the Price Index, and if you are mapping licensed clinics rather than research vendors, start with the provider directory. For the class-level primer, see Peptides 101.

This article is for educational purposes only and does not constitute medical advice. Forzinity (elamipretide) is FDA-approved only for Barth syndrome in patients meeting the labeled weight criterion. Research-grade SS-31 is not a substitute for the approved product. Consult a qualified clinician before considering any peptide.