Abstract
A comprehensive review of SS-31 (elamipretide), the mitochondria-targeted tetrapeptide that binds cardiolipin to optimize electron transport chain function, reduce oxidative stress, and enhance ATP production, with clinical trials in heart failure, Barth syndrome, and mitochondrial myopathy.
SS-31, also known by its clinical names elamipretide, Bendavia, and MTP-131, is a synthetic mitochondria-targeted tetrapeptide with the sequence D-Arg-dimethylTyr-Lys-Phe-NH2 that selectively concentrates in the inner mitochondrial membrane where it binds to cardiolipin to optimize electron transport chain function, reduce reactive oxygen species generation, and enhance ATP production. Developed from the Szeto-Schiller peptide family, it was discovered through a serendipitous finding by Hazel H. Szeto and Peter W. Schiller who observed that certain cell-penetrating peptides showed unexpected mitochondrial protective properties independent of their original design purpose. SS-31 has since advanced through extensive preclinical research and multiple clinical trials conducted by Stealth BioTherapeutics for conditions including heart failure, Barth syndrome, primary mitochondrial myopathy, and age-related mitochondrial decline.
The structural design of SS-31 features an alternating aromatic-cationic motif that enables its unique pharmacological behavior. The D-arginine provides a positive charge, the 2,6-dimethyltyrosine (Dmt) contributes both aromatic character and potent radical scavenging activity, the lysine adds another positive charge, and the phenylalanine completes the aromatic-cationic pattern. The C-terminal amidation enhances stability against carboxypeptidase degradation. This specific arrangement enables SS-31 to penetrate cell membranes and concentrate within the inner mitochondrial membrane at levels 1000 to 5000-fold higher than cytoplasmic concentrations, driven by the electrochemical potential across the membrane.
The mechanism of action of SS-31 centers on its reversible binding to cardiolipin, a unique diphosphatidylglycerol lipid found almost exclusively in the inner mitochondrial membrane. Cardiolipin plays an essential structural role in organizing the electron transport chain complexes (I, III, and IV) into supercomplexes or respirasomes that enable efficient substrate channeling of electrons. Cardiolipin also anchors cytochrome c to the outer surface of the inner membrane, positioning it for optimal electron shuttling between Complex III and Complex IV. With aging and disease, cardiolipin becomes oxidized and depleted, disrupting supercomplex organization, increasing electron leak that generates superoxide radicals, reducing ATP production efficiency, and destabilizing cytochrome c binding.
By binding cardiolipin, SS-31 stabilizes the cardiolipin-protein interactions that maintain supercomplex integrity, modulates the cardiolipin-cytochrome c interaction to favor electron transfer over peroxidase activity, inhibits opening of the mitochondrial permeability transition pore, prevents cytochrome c release into the cytoplasm (which would trigger apoptosis), maintains mitochondrial membrane potential, and enhances ATP production while suppressing reactive oxygen species generation. Importantly, these effects are most pronounced in dysfunctional mitochondria with disrupted cardiolipin organization, meaning SS-31 preferentially improves function in diseased or aged cells while having minimal effect on healthy mitochondria that already have intact supercomplex organization.
Preclinical research has demonstrated the efficacy of SS-31 across a wide range of mitochondrial dysfunction models. In aged mice treated with 5 to 10 mg per kg per day via intraperitoneal injection for 8 to 12 weeks, SS-31 restores expression of mitochondrial biogenesis genes including PGC-1alpha, Nrf1, Nrf2, and TFAM, normalizes mitochondrial fusion proteins Mfn1 and Mfn2, restores synaptic proteins PSD-95 and synaptophysin, and reduces inflammatory markers NF-kB, IL-6, and TNF-alpha. These findings indicate that SS-31 not only restores mitochondrial function but also reverses broader age-related changes in gene expression and tissue inflammation.
In cardiac research, SS-31 has shown particular promise. It protects against ischemia-reperfusion injury by reducing reactive oxygen species, preserving ATP levels, and reducing infarct size. In models of heart failure, SS-31 improves diastolic function, reduces cardiac remodeling, and normalizes phosphocreatine-to-ATP ratios that indicate myocardial energetic status. A notable finding from combination studies with NMN demonstrated that while SS-31 restores diastolic function and NMN restores systolic function during high workload in aged hearts, the combination normalizes both parameters synergistically.
Barth syndrome, a rare X-linked genetic disorder caused by mutations in the tafazzin gene that result in abnormal cardiolipin remodeling, represents the most direct application for SS-31. In Barth syndrome, defective cardiolipin composition disrupts mitochondrial supercomplex assembly, causing cardiomyopathy, skeletal myopathy, neutropenia, and growth failure. SS-31's ability to stabilize cardiolipin-protein interactions addresses the fundamental molecular defect, and Stealth BioTherapeutics has developed the compound under the brand name FORZINITY for this indication.
Neurological applications of SS-31 have been explored in models of Alzheimer's disease, traumatic brain injury, and optic neuropathy. In Alzheimer's models, SS-31 reduces amyloid-beta pathology and promotes mitochondrial biogenesis. In traumatic brain injury, it upregulates SIRT1 and PGC-1alpha to support mitochondrial recovery. For optic neuropathies including Leber's hereditary optic neuropathy, SS-31 promotes BDNF signaling and mitochondrial motility in optic nerve cells, supporting axonal transport and neuronal survival.
Additional preclinical findings span metabolic and renal disease. In diabetic nephropathy models, SS-31 lowers proteinuria and reduces oxidative stress markers. In models of lipopolysaccharide-induced neuroinflammation, the peptide restores superoxide dismutase activity, BDNF levels, and synaptic protein expression. SS-31 also upregulates frataxin in models relevant to Friedreich's ataxia, suggesting potential application in this mitochondrial disease.
Clinical trials have advanced SS-31 through Phase 2 and Phase 3 studies. The EMBRACE trial in heart failure patients demonstrated reduced NT-proBNP levels (a biomarker of heart failure severity) and improved six-minute walk distance. The MMPOWER trials in primary mitochondrial myopathy evaluated subcutaneous administration and showed mixed results, with some improvements in exercise tolerance but failure to meet all primary endpoints. Trials in dry age-related macular degeneration and kidney disease have also been conducted with variable outcomes.
The safety profile of SS-31 has been favorable across clinical trials. The peptide is well tolerated with subcutaneous administration at dosages up to 0.25 mg per kg per day or 40 mg per day for periods up to 24 weeks. The most commonly reported adverse events are mild injection site reactions. No mitochondrial toxicity has been observed in healthy tissues, consistent with the compound's preferential action on dysfunctional mitochondria. The absence of effects on normal mitochondria provides an important safety margin.
Regarding regulatory status, SS-31 has received FDA Orphan Drug designation, Fast Track designation, and Rare Pediatric Disease designation for Barth syndrome and primary mitochondrial myopathy. Full regulatory approval has not yet been granted for any indication. Research continues for aging-related conditions including cognitive decline, muscle function, and cardiac energetics, though these broader indications are at earlier stages of clinical development.
