SS-31

SS-31, known by the international nonproprietary name elamipretide and the code MTP-131, is the tetrapeptide D-Arg-Dmt-Lys-Phe-NH2, where Dmt denotes 2,6-dimethyltyrosine. It carries a C-terminal amide and alternates basic and aromatic residues, giving a net charge of plus three alongside two aromatic side chains. Two of its four positions are non-standard: a D-arginine and a doubly methylated tyrosine.

The targeting mechanism is what distinguishes this compound from most mitochondrially directed molecules. Rather than accumulating in response to the membrane potential, as lipophilic cations such as triphenylphosphonium conjugates do, it is reported to associate directly with cardiolipin, the four-tailed phospholipid found almost exclusively in the inner mitochondrial membrane. The alternating charge and aromatic pattern is what enables that interaction.

The practical consequence of the distinction is that potential-driven accumulation collapses precisely when a mitochondrion is depolarised, whereas cardiolipin remains present. A compound targeted by lipid binding therefore behaves differently in a depolarised system than one targeted by charge, which is central to how experiments with this molecule are designed.

Direct binding work uses isolated cardiolipin-containing liposomes with isothermal titration calorimetry and nuclear magnetic resonance to characterise the interaction, and this biophysical layer is the foundation of the mechanistic case. Cardiolipin also organises the supercomplexes of the electron transport chain, which is the proposed link between a lipid-binding peptide and bioenergetic endpoints.

Downstream work in isolated mitochondria and cultured cells covers cristae architecture by electron microscopy, respirometry, and reactive oxygen species production. Rodent models of tissue injury and ageing form much of the in vivo literature. The experiment that carries the most weight is the one connecting a measured change in membrane organisation to a measured change in respiration, since the reported effects on both are otherwise correlations.

The non-standard residues are what a lot analysis has to establish. 2,6-dimethyltyrosine adds twenty-eight daltons relative to tyrosine, so a mass spectrum immediately distinguishes correct incorporation from substitution with ordinary tyrosine, which would be a far cheaper building block and a materially different molecule. C-terminal amidation gives a mass one dalton below the free acid and is confirmed in the same measurement.

The D-arginine at position one presents the impurity class that mass cannot detect, since the L-epimer has an identical formula and mass while the stereochemistry is part of the design. Chromatographic resolution is the only route to detecting it, so the HPLC method needs to be developed against that separation rather than simply producing one narrow peak.

With a net charge of plus three and only four residues, the peptide is poorly retained on reversed-phase columns without an ion-pairing reagent, and the counterion contribution to weighed mass is proportionally large for a molecule this small.