SS-31

SS-31 (elamipretide) is a cell-permeable tetrapeptide that selectively targets and concentrates in the inner mitochondrial membrane, achieving concentrations 1000-5000 fold higher within mitochondria than in the cytoplasm. This remarkable selectivity is driven by the peptide's alternating aromatic-cationic motif (D-Arg-Dmt-Lys-Phe), which exploits the steep mitochondrial membrane potential to accumulate at the inner membrane independent of conventional mitochondrial targeting sequences.

Once localised at the inner mitochondrial membrane, research suggests SS-31 binds selectively to cardiolipin, a unique phospholipid found exclusively in the inner mitochondrial membrane that is essential for the structural integrity and function of electron transport chain (ETC) complexes. By stabilising cardiolipin-protein interactions, SS-31 optimises the organisation of ETC supercomplexes (respirasomes), enhancing electron transfer efficiency between Complex III and Complex IV. This improved electron flow reduces electron leak and the generation of reactive oxygen species (ROS) at their primary mitochondrial source.

Studies indicate that SS-31's mechanism is fundamentally different from conventional antioxidants. Rather than scavenging ROS after they are formed, SS-31 prevents their overproduction by restoring normal mitochondrial electron transport. This upstream approach preserves physiological ROS signalling while preventing pathological oxidative stress. Additionally, SS-31 protects against cardiolipin peroxidation, prevents cytochrome c release (a key trigger of apoptosis), and maintains mitochondrial membrane potential — collectively preserving mitochondrial bioenergetics in conditions of cellular stress, ischaemia, and ageing.

SS-31 was discovered by Hazel Szeto and Peter Bhatt Schiller at Weill Cornell Medical College in the early 2000s as part of a programme to develop cell-permeable peptide antioxidants. The Szeto-Schiller (SS) peptide series was designed based on the principle that alternating aromatic and cationic amino acids could enable mitochondrial targeting without conventional mitochondrial targeting sequences. SS-31 emerged as the lead compound due to its exceptional mitochondrial concentration and its unique interaction with cardiolipin (CAS 736992-21-5).

The peptide entered clinical development under the names Bendavia and later elamipretide (MTP-131), with Stealth BioTherapeutics leading multiple Phase I, II, and III trials. Clinical programmes have investigated SS-31 in primary mitochondrial myopathy (the MMPOWER trials), heart failure (the EMBRACE and PROGRESS-HF trials), age-related macular degeneration (the ReCLAIM trials), and renal ischaemia-reperfusion injury. While the MMPOWER-3 Phase III trial for primary mitochondrial myopathy did not meet its primary endpoint in 2020, subsequent analyses and ongoing trials in Barth syndrome and other mitochondrial disorders continue. Research into SS-31's effects on ageing has been particularly compelling, with preclinical studies demonstrating reversal of age-related mitochondrial dysfunction in cardiac, skeletal muscle, and renal tissues, positioning it as one of the most advanced mitochondria-targeted therapeutics in development.