SS-31

SS-31 is a four-amino-acid peptide, also known as elamipretide, developed from work by Hazel Szeto and Peter Schiller — the SS in the name is theirs. Its target is unusually specific: not a receptor, not an enzyme, but a particular lipid found in one membrane.

That lipid is cardiolipin, and it is peculiar to the inner mitochondrial membrane. Where an ordinary membrane lipid has two fatty acid tails, cardiolipin has four, which gives it a cone shape rather than a cylinder. Packed together, cone-shaped lipids force a membrane to curve — and the inner mitochondrial membrane is folded into deep pleats called cristae, which is where the machinery generating ATP is arranged.

The compound alternates charged and aromatic residues in a pattern that draws it to cardiolipin and holds it there. It accumulates in the inner mitochondrial membrane not because it is transported but because that is where its binding partner is. Targeting a lipid rather than a protein is a genuinely unusual design, and it is what gives the molecule its specificity.

• Cardiolipin and membrane shape: This lipid's shape is what allows the inner membrane to fold, and the folding is what packs in the energy-generating machinery. A molecule that binds it is acting on mitochondrial architecture.

• Efficiency of electron transfer: The proteins of the electron transport chain sit within that membrane and depend on its organisation. Disorganisation makes electron transfer leak, which is one of the main sources of reactive oxygen.

• Reducing damage at source: Most antioxidants neutralise reactive molecules after they form. Improving the efficiency of the process that produces them addresses the problem earlier, which is a different strategy entirely.

• Completed clinical trials: It has been through controlled human studies in mitochondrial disease, with results that did not meet their primary endpoints. That makes it one of the few compounds here with real clinical evidence, including its limits.

Mitochondria generate energy by passing electrons along a chain of protein complexes embedded in their inner membrane, using the energy released to pump protons across it and then letting those protons back through a turbine that makes ATP. The efficiency of this depends on the complexes being arranged correctly and close together.

That arrangement depends on the membrane's shape. The inner membrane is folded into cristae, greatly increasing its area and packing the machinery into an ordered layout, and cardiolipin is what makes the folding possible. It also binds several of the complexes directly, holding them in position. When cardiolipin is damaged — which reactive oxygen readily does — the structure loosens and the chain becomes leaky.

A leaky chain is the problem, because electrons escaping the sequence react with oxygen to form the very molecules that caused the damage. Ageing and disease produce a feedback loop of exactly this kind. SS-31 binds cardiolipin and is proposed to stabilise the arrangement, improving efficiency and reducing escape. It is not neutralising anything; it is reducing how much gets produced, which is why it is described as targeting the source rather than the symptom.

The underlying science is strong. Cardiolipin's role in cristae structure and in organising the respiratory complexes is established, and the binding of this peptide to it has been demonstrated. Preclinical work across several models of mitochondrial dysfunction is substantial and comes from multiple groups.

Elamipretide has been through controlled trials in primary mitochondrial myopathy and Barth syndrome; the principal ones did not meet their primary endpoints, with some secondary measures reported as favourable. It is supplied for laboratory research only. Few compounds in this field have clinical data at all, and its mechanism is among the best defined here — a tetrapeptide that binds one specific lipid in one specific membrane.

MOTS-c is the complementary mitochondrial compound — a signal encoded by the organelle's own DNA, where this one acts on the physical structure of its membranes.

NAD+ is the coenzyme carrying the electrons this machinery transfers, and Glutathione neutralises the reactive molecules that escape it. The three together cover supply, structure and clean-up in the same system.