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SS-31, known in clinical development as elamipretide (MTP-131), is a synthetic tetrapeptide with an alternating aromatic-cationic structure: D-Arg-Dmt-Lys-Phe-NH2. That structure allows it to cross cell membranes without a transporter and to concentrate selectively in mitochondria, reaching intramitochondrial levels far above those in the surrounding cytosol.

Its molecular target is unusual and well characterised. SS-31 binds cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane, where it is essential to cristae curvature and to holding the electron transport chain complexes in their functional supercomplex arrangement. By associating with cardiolipin, SS-31 is described in the literature as stabilising cristae architecture and improving the efficiency of electron transport, rather than acting as a conventional antioxidant scavenger.

Elamipretide has been studied in primary mitochondrial myopathy and related conditions but has not received marketing authorisation. Supplied in a 3 mL pre-filled pen at 10 mg/mL (30 mg total peptide).