SS-31

SS-31, marketed as elamipretide, is a four-amino-acid peptide engineered by Hazel Szeto and Peter Schiller at Cornell to selectively concentrate in the inner mitochondrial membrane and bind cardiolipin — the unique phospholipid that organizes the electron transport chain and is exclusively localized to mitochondrial inner membranes. The molecule's structure (D-Arg-2',6'-dimethylTyr-Lys-Phe-NH2) combines alternating aromatic and basic residues with a D-stereochemistry N-terminal arginine that produces 1,000- to 5,000-fold concentration in the inner mitochondrial membrane relative to extracellular space. By stabilizing cardiolipin and supporting electron-transport-chain organization, SS-31 has been reported to improve mitochondrial respiration, reduce reactive oxygen species production under stress, and protect mitochondria in models of ischemia-reperfusion injury, heart failure, and primary mitochondrial disease. The mechanism is structural and pharmacological — direct organelle targeting — rather than receptor agonism, which distinguishes it sharply from every other peptide on this site.

SS-31 is the premium-tier mitochondrial peptide in the corpus and the molecule with the most ambitious clinical-development program. Its Phase III trial in primary mitochondrial myopathy (MMPOWER-3, Karaa et al., *Neurology* 2023) is also the most prominent negative trial in the corpus: across 27 sites in seven countries, 24 weeks of subcutaneous elamipretide 40 mg/day did not improve six-minute walk test distance or the primary fatigue endpoint compared to placebo. The trial provided Class I evidence that elamipretide does not improve those endpoints in unselected primary mitochondrial myopathy patients, ending Stealth BioTherapeutics' lead-indication program for the molecule. A prespecified subgroup of nuclear-DNA-mitochondrial-disease patients showed apparent response signals that were not the prespecified primary outcome but informed the subsequent NuPOWER program. The story has a partial recovery in a different rare disease. In September 2025, the FDA approved elamipretide as Forzinity for Barth syndrome — a rare X-linked cardiolipin-remodeling disorder where the cardiolipin-stabilizing mechanism of SS-31 maps directly onto the disease pathophysiology. The approval is narrow, the patient population small, and access is structured through specialist channels. SS-31 is now an FDA-approved peptide in a rare-disease indication, with a failed Phase III in the broader myopathy indication that originally drove the development program. The biohacker and longevity-research interest in SS-31 rests on different evidence: mostly preclinical work in heart failure, age-related mitochondrial decline, and ischemia-reperfusion injury, plus the broader appeal of "mitochondrial health" as a category. The honest framing here is that SS-31 has more rigorous clinical evidence behind it than nearly any peptide on this list — but the rigorous evidence is mostly negative for the broad indication, positive for a narrow one, and absent for the longevity-focused use cases that drive most current interest.