Longevity & Cellular Aging
CAS
736992-21-5
Molecular Weight
640
Da
A mitochondria-targeting tetrapeptide developed by Hazel Szeto at Weill Cornell that selectively concentrates in the inner mitochondrial membrane and stabilizes cardiolipin, a phospholipid essential for electron transport chain function. In September 2025 it became the first mitochondria-targeted therapy the FDA has approved, cleared under accelerated approval as FORZINITY for the ultra-rare Barth syndrome. That approval is narrow: the pivotal myopathy trial (MMPOWER-3) failed its primary endpoints, the Barth data came from a 12-patient crossover trial, and every other use remains investigational. Strong foundational science, mixed human translation.
Injectable
Intranasal Suitable
No
Prescription
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SS-31 discussion in longevity and biohacking communities is growing but remains niche, partly because subcutaneous injection is a practical barrier, partly because the MMPOWER-3 failure tempered enthusiasm among more evidence-following community members. The most engaged discussion clusters in mitochondrial disease patient communities (Barth syndrome, MELAS) where TAZPOWER results are treated with genuine cautious optimism. In general longevity circles, SS-31 is frequently discussed alongside MOTS-c as an example of a "mitochondria-targeted" approach distinct from NAD+ supplementation, intellectually compelling, but with a higher translation-to-clinic risk profile than it initially appeared. Injection site reactions are frequently mentioned in self-experiment accounts as the primary tolerability concern.
SS-31 is a synthetic tetrapeptide belonging to the Szeto-Schiller peptide family, designed with specific structural features that allow it to penetrate cell membranes and selectively accumulate in mitochondria at concentrations 1,000 to 5,000 times higher than in the cytoplasm. This selectivity is achieved through an alternating aromatic-cationic amino acid pattern that facilitates interaction with the inner mitochondrial membrane. Once there, SS-31 binds cardiolipin, a phospholipid unique to the inner mitochondrial membrane , stabilizing mitochondrial cristae architecture and optimizing the organization and function of the electron transport chain. The compound represents a distinct mechanistic approach from NAD+ precursors (NMN) or mitophagy enhancers: rather than increasing mitochondrial production, it improves the efficiency and structural integrity of existing mitochondria.
SS-31 binds to cardiolipin in the inner mitochondrial membrane, where cardiolipin serves as an essential structural scaffold for the respiratory complex supercomplexes (Complexes I, III, and IV) that perform oxidative phosphorylation. Mitochondrial dysfunction, whether from aging, disease, or ischemic injury, involves cardiolipin peroxidation and subsequent disruption of cristae structure, which impairs electron transport efficiency and increases reactive oxygen species (ROS) leakage. By binding peroxidized cardiolipin, SS-31 stabilizes cristae morphology, reduces ROS production, restores electron transport efficiency, and enhances ATP synthesis. It also inhibits cytochrome c peroxidase activity, preventing further cardiolipin oxidation. In preclinical models, this translates to reduced ischemia-reperfusion injury, improved cardiac function, protection against muscle atrophy, and reversal of age-related mitochondrial dysfunction. In human trials, the translation has been more variable.
Clinical trial indications: Primary mitochondrial myopathy (MMPOWER-2/3), Barth syndrome (TAZPOWER), heart failure with reduced ejection fraction (PROGRESS-HF), age-related macular degeneration (ReCLAIM)
Community/longevity use: Mitochondrial function optimization, age-related energy decline, exercise recovery, neuroprotection
Active research: Friedreich's ataxia, ischemic kidney disease, dry AMD
In clinical trials, the most commonly reported adverse event was injection site reaction (redness or swelling at the subcutaneous injection site), generally mild but occurring in a meaningful proportion of participants. The FORZINITY prescribing information also notes hypersensitivity reactions. No serious adverse events were attributed to the compound in MMPOWER-3, TAZPOWER, or PROGRESS-HF, and the Barth open-label extension has now followed a small number of patients past week 168, though long-term data outside that ultra-rare population remains limited. Oral bioavailability is negligible because the peptide is degraded in the GI tract, so subcutaneous or intravenous administration is required for any systemic effect. This is a significant practical consideration for community use, as the compound cannot be effectively taken orally.
SS-31 now has one FDA-approved commercial formulation: FORZINITY (elamipretide HCl), an injectable approved in September 2025 for muscle strength in Barth syndrome patients weighing at least 30 kilograms. It is dispensed through a single specialty pharmacy to eligible patients, not sold over the counter or broadly prescribed. For every other use, including the longevity and mitochondrial-support applications that drive most community interest, there is no approved formulation. In clinical trials the compound was administered subcutaneously (MMPOWER-3, TAZPOWER) or intravenously (PROGRESS-HF). Oral administration is ineffective because the peptide is degraded in the GI tract, this is a firm pharmacokinetic constraint, not a dosing preference, so any systemic use requires injection. Outside the approved Barth indication, SS-31 is sold as a research chemical by peptide synthesis suppliers, typically as a lyophilized powder requiring reconstitution, and it remains investigational. Stealth BioTherapeutics, the company that developed it, was taken private in 2022 through a merger led by Morningside Venture Investments rather than shut down, continued development as a private company, and brought FORZINITY through to accelerated approval.
SS-31 (elamipretide) has one FDA-approved indication. On September 19, 2025, the FDA granted accelerated approval to FORZINITY (elamipretide HCl) to improve muscle strength in adult and pediatric Barth syndrome patients weighing at least 30 kilograms, making it the first mitochondria-targeted therapeutic the agency has approved. Because it is an accelerated approval, continued approval can be made contingent on confirmatory trials that verify the benefit, and the approval could be revisited if those trials fall short. The approval is also narrow. It does not cover primary mitochondrial myopathy (where the pivotal MMPOWER-3 trial failed its primary endpoints), heart failure, macular degeneration, or the general longevity and mitochondrial-support uses that drive most community interest, all of which remain investigational. Elamipretide was studied for years under multiple Investigational New Drug applications by Stealth BioTherapeutics, which was taken private in a 2022 merger led by Morningside Venture Investments, not shut down, and continued development through to this approval after an earlier rejection in 2025. Now that an approved elamipretide product exists, compounding it faces the same restriction that applies to other approved drugs, since a compounded version would be essentially a copy of a commercially available product. Outside the approved Barth indication, it continues to occupy a research-compound gray zone. No other major regulatory agency has approved it to date.
https://pubmed.ncbi.nlm.nih.gov/29500292/
https://pubmed.ncbi.nlm.nih.gov/33077895/
https://pubmed.ncbi.nlm.nih.gov/24117165/
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