SS-31

SS-31 is a synthetic tetrapeptide of the Szeto-Schiller series. It is also written SS31, and its international nonproprietary name is elamipretide. Its sequence is D-Arg-Dmt-Lys-Phe-NH2, where Dmt is 2,6-dimethyltyrosine (PubChem CID 11764719). Its design motif alternates aromatic and basic residues. This gives the molecule a net positive charge. The motif also lets SS-31 cross cell membranes and concentrate in the inner mitochondrial membrane.

There, reported levels are more than 1000-fold higher than outside the cell (Zhao et al., 2004; Szeto, 2006). Since then it has been studied in isolated mitochondria and cell culture. It has also been studied in rodent models of ischemia-reperfusion and aging, and in human clinical trials.

Elamipretide is a drug.

The SS-31 sold here is research-grade lyophilized peptide for laboratory use. It is not that drug product. It is not intended for that or any clinical indication, and it is not for use in people.

Sequence and aliases come from PubChem (CID 11764719). The figures above describe the free peptide. Salt forms add mass.

Published work on SS-31 falls into six areas. Each summary below states the model used. Human studies used the drug product in clinical settings. They describe that product, not the research material sold here.

Design and mitochondrial targeting

The founding study screened peptide antioxidants that can enter cells. They were built on alternating aromatic and basic residues. In N2A nerve cells, they cut intracellular reactive oxygen species and cell death caused by t-butylhydroperoxide. Half-maximal effects were in the nanomolar range. In isolated mitochondria, they lowered reactive oxygen species production. They inhibited the permeability transition and swelling. They also stopped calcium-induced cytochrome c release. Analogs lacking dimethyltyrosine did not inhibit mitochondrial reactive oxygen species or swelling (Zhao et al., 2004). A review of the series set out the targeting rationale (Szeto, 2006).

Cardiolipin binding and cristae structure

One team used a polarity-sensitive fluorescent analog. They reported that SS-31 binds tightly to cardiolipin. Cardiolipin is the anionic phospholipid of the inner mitochondrial membrane. Cristae need it to form. The team found that the SS-31 and cardiolipin complex inhibited cytochrome c peroxidase activity. It did so by protecting the heme iron of cytochrome c. In rat kidneys put under ischemia, SS-31 given beforehand preserved cristae membranes. It also prevented mitochondrial swelling (Birk et al., 2013).

A review by the same group framed cardiolipin itself as the drug target. It also summed up how SS-31 acts (Szeto, 2014). Another study looked at rat hearts after ischemia-reperfusion. It used serial block face scanning electron microscopy with 3D reconstruction. This showed that cristae networks broke apart less. The drop in complex I, II and IV activity in permeabilized ventricular fibers was smaller. Mass spectrometry showed that cardiolipin levels were not restored (Allen et al., 2020).

Biophysics and protein interactions

Biophysical and computational work used model and mitochondrial membranes. It found that the peptide partitions into the interfacial region. Its affinity there is linked to surface charge. It does not destabilize lamellar bilayers. It alters lipid packing and surface electrostatics. Divalent cation distribution changed, and calcium stress in mitochondria fell (Mitchell et al., 2020). Cross-linking mass spectrometry found the protein partners of SS-31 in mitochondria. They act in oxidative phosphorylation and 2-oxoglutarate metabolism. All are known cardiolipin binders. They were often cross-linked near cardiolipin-interacting regions (Chavez et al., 2020).

Rodent ischemia-reperfusion models

In a rat model of kidney ischemia-reperfusion, SS-31 protected mitochondrial structure and respiration during early reperfusion. It sped up ATP recovery and cut tubular apoptosis and necrosis. It also lowered markers of oxidative stress and inflammation (Szeto et al., 2011). The cardiac work above used the same model in rat hearts (Allen et al., 2020).

Aged rodent energetics

One study used young (5 month) and old (27 month) mice. SS-31 was given once at 3 milligrams per kilogram. In old mice, resting and maximal mitochondrial ATP production returned to young levels within one hour. So did coupling of oxidative phosphorylation and muscle cell energy state. These were measured by combined optical and phosphorus-31 magnetic resonance spectroscopy. There was no observable effect in young muscle (Siegel et al., 2013).

An 8-week study in 26-month-old female mice reported a reversal of the decline in maximal mitochondrial ATP production and coupling. It also found a shift in glutathione redox status. Cysteine S-glutathionylation was reversed across the muscle proteome. This came without more mitochondrial protein (Campbell et al., 2019). In old mice, 8 weeks of SS-31 reversed much of the diastolic dysfunction. It also normalized cardiomyocyte proton leak and lowered mitochondrial reactive oxygen species. It added nothing further in mice already expressing mitochondrial-targeted catalase (Chiao et al., 2020).