The scientific story of how two researchers created a class of molecules with intrinsic mitochondrial selectivity, and why that discovery took two decades to be taken seriously by the pharmaceutical world.
Every important scientific discovery has a moment when it seems improbable. For SS-31, that moment was when two researchers proposed that it was possible to build a molecule that would concentrate selectively in the mitochondrion, without needing any vehicle, without needing any special conjugation. The structure of the peptide would do the work on its own.
Today, two decades later, that hypothesis is no longer a bet. It is the validated mechanism that took SS-31 to Phase III clinical trials. But the journey between an idea in the laboratory and a pivotal trial in thousands of patients is rarely told, and rarely simple.
The question that changed everything
In the early 2000s, Hazel Szeto was a researcher at Weill Cornell Medicine in New York. Her area was physiology and pharmacology, with a focus on compounds that affected the mitochondrion. The problem that drew her was specific: how to make a molecule reach the inner mitochondrial membrane selectively?
The inner mitochondrial membrane is one of the most protected structures of the cell. To get there, a compound must cross the plasma membrane, escape the lysosomes, pass through the outer mitochondrial membrane, and only then access the space where the respiratory chain operates. Most available antioxidants do not come close to that, they distribute throughout the whole body, in concentrations that, at the site where they matter, are insufficient.
The solution Szeto and Peter Schiller proposed was structural. Instead of using a vehicle, they designed peptides with an intrinsic chemical property: the alternation of aromatic and positively charged residues. That sequence created selective affinity for cardiolipin, the phospholipid exclusive to the inner mitochondrial membrane. The peptide did not need to be guided. It simply went to where the cardiolipin was.
It was not a vehicle that carried the molecule to the mitochondrion. It was the very structure of the peptide that created this affinity, a selectivity inscribed in the sequence of amino acids.
SS-31 was the most studied member of that new class, named the Szeto-Schiller peptides, or SS peptides. The number of publications that followed Szeto’s seminal study, published in the Annals of the New York Academy of Sciences in 2014, is indicative of the interest the hypothesis generated: in a decade, SS-31 accumulated one of the largest volumes of peer-reviewed literature among mitochondrial peptides.
The timeline of a discovery
| 2004-07 | Initial preclinical evidence. First studies in animal models document neuroprotection via reduction of mitochondrial ROS. The intrinsic selectivity begins to be characterized. |
| 2013 | Renal mechanism confirmed. Birk et al. publish in the Journal of the American Society of Nephrology: SS-31 re-energizes ischemic mitochondria via interaction with cardiolipin in renal models. |
| 2014 | Mechanistic framework consolidated. Szeto publishes the seminal study describing the Szeto-Schiller class, the cardiolipin-binding mechanism, and the intrinsic mitochondrial selectivity. Central reference of the field. |
| 2017 | Phase II clinical trial, heart. Daubert et al. publish in Circulation: Heart Failure the data from the Elamipretide trial in HFpEF patients. Improvement in functional capacity documented in humans. |
| 2021 | Phase II/III, Barth syndrome. EMPOWER trial published in JACC Heart Fail. A genetic disease caused by a specific defect in cardiolipin, maximum mechanistic specificity among all the compound’s trials. |
| 2026 | Phase III underway. Pivotal trial for HFpEF in progress. Data not yet published. SS-31 / Elamipretide as the mitochondrial compound with the most advanced clinical stage available in the literature. |
Why it took so long
This is the part of the story that rarely appears in scientific articles. SS-31 had a clear mechanism and robust preclinical data since the mid-2000s. But the path to clinical trials took nearly a decade.
Part of that is inherent to the process. Clinical trials are expensive, logistically complex, and regulatorily demanding. A peptide molecule without clinical precedent must prove safety before proving efficacy, and each phase takes years. But part is also historical: the field of mitochondrial medicine was, until recently, considered peripheral by the pharmaceutical industry. The mitochondrion was seen as an important organelle, but not as a primary therapeutic target.
That changed. The combination of longevity data, studies on cellular aging, and growing evidence that cardiovascular, neurological, and metabolic diseases have a significant mitochondrial component placed the mitochondrion at the center of translational research. SS-31 benefited from that shift in perspective, but also fed it, providing evidence that it was possible to reach the inner mitochondrial membrane precisely and study the effects of that intervention in humans.
The pharmaceutical industry took years to recognize the mitochondrion as a primary target. SS-31 was one of the compounds that helped change that perception.
What comes next
The Phase III data have not yet been published. This means the story of SS-31 does not yet have a definitive ending, at least not the ending cardiology is waiting for. But what already exists in the literature is unusual enough to justify attention.
For researchers, SS-31 represents a convergence point: a compound with a precise mechanism, demonstrated intrinsic selectivity, and a chain of evidence that runs from the laboratory to the clinic coherently. It is not something one sees often in peptide research.
For Axion, SS-31 is the anchor compound of the Mitochondrial Energy System, the one with the most advanced clinical stage, the largest volume of literature, and the best-characterized mechanism. It represents the standard against which other compounds in the system are compared. And it represents, too, a window into where mitochondrial research is going, and why it is worth following closely.
