SS-31: The Most-Tested Mitochondrial Peptide Still Waiting for Approval

Most compounds in this series have the same underlying problem: too little human testing to say much with confidence. SS-31 — also known as elamipretide — is the rare exception, and it flips the usual story entirely. This is one of the most extensively human-trialed peptides in existence, tested across three separate real disease programs for over a decade. And it still hasn't been approved for anything.

A Different Kind of Mitochondrial Peptide

SS-31 works differently from MOTS-c, the mitochondrial peptide covered earlier in this series. Instead of acting through a cell-signaling pathway like AMPK, SS-31 is a small tetrapeptide engineered to physically concentrate inside mitochondria and bind directly to cardiolipin — a lipid in the inner mitochondrial membrane essential for organizing the electron transport chain. Its structure, an alternating pattern of aromatic and positively charged amino acids, lets it cross into mitochondria without needing an intact membrane potential — an advantage over other mitochondria-targeting compounds, which often can't reach damaged mitochondria at all, precisely the ones most in need of help.

A Real, Extensive Trial Program

Developed by Stealth BioTherapeutics (the program has since been acquired by Larimar Therapeutics), SS-31 earned FDA Orphan Drug, Fast Track, and Rare Pediatric Disease designations for Barth syndrome — a genetic disorder that damages cardiolipin metabolism directly, giving SS-31 about as strong a mechanistic rationale as any peptide gets. Those designations reflect genuine regulatory recognition of unmet medical need. They are not approval.

What the Actual Results Show

This is where the story gets genuinely instructive:

• An early trial in primary mitochondrial myopathy tested 5 days of intravenous dosing and found a dose-dependent trend toward improved walking distance on a 6-minute walk test — suggestive, but the overall effect narrowly missed statistical significance (p = 0.053), in a short, small trial.
• A Phase 2/3 crossover trial in Barth syndrome (Reid Thompson et al., Genetics in Medicine, 2021) tested 12 weeks of daily dosing in 12 subjects. Neither primary endpoint — walking distance or a symptom scale — was met in that initial phase.
• But an open-label extension of the same trial, running out to 36–48 weeks, told a different story: significant improvements in walking distance, symptom scores, and even cardiac stroke volume emerged — suggesting a real effect that simply takes far longer to become measurable than the original trial window allowed for.
• Then MMPOWER-3, a large, well-powered, 218-participant randomized trial in primary mitochondrial myopathy, tested the compound again — and did not meet its primary endpoints.

The Lesson Here Isn't "It Doesn't Work"

It's more specific than that, and more useful: a compound can have a genuinely strong mechanistic rationale, years of real Phase 1 through Phase 3 testing, regulatory fast-track status, and multiple different disease programs — and still not produce a single clean, unambiguous pivotal win. That's a meaningfully different situation than most of the compounds in this series, where the honest problem is simply that almost no human testing exists yet. Here, the testing is extensive. The results are just genuinely mixed, indication by indication, trial by trial.

Where Things Stand

As of 2026, SS-31 remains unapproved for any indication. It's available primarily through clinical trials and compassionate-use programs rather than general prescription, and development continues under Larimar Therapeutics, with the strongest remaining signal centered on Barth syndrome's longer-duration outcomes.

The Honest Summary

SS-31 is arguably the best-tested mitochondrial peptide anywhere, and that's exactly what makes it worth understanding: extensive clinical testing and a strong mechanism don't automatically add up to a clear result. Some trials came close. One large trial missed entirely. The clearest positive signal took nearly a year of treatment to show up. That's a more honest, more complicated picture than "in clinical trials" usually implies — and a good reminder that the amount of testing a compound has been through and the strength of its actual results are two different questions.  SS-31 (elamipretide) has gone through more human clinical trials than almost any other research peptide, spanning Barth syndrome, primary mitochondrial myopathy, and heart failure programs over more than a decade, backed by FDA Orphan Drug and Fast Track designations — yet it still hasn't produced a single clearly positive pivotal trial, including the large 218-participant MMPOWER-3 study. Here's what that mixed, extensively-documented record actually reveals about the difference between being tested and being proven.

For more compounds documented at this level of trial-stage detail, Peptide AtlasX catalogs research peptides with citations back to the underlying published studies.