What SS-31 Mitochondrial Function Trials Measure

Category: Longevity & Cellular Aging

What SS-31 Mitochondrial Function Trials Measure

Posted on August 24, 2026

Introduction

When fatigue changes with age, it is tempting to ask whether the problem is “low energy” inside the muscle. That phrase feels personal, but researchers need something more concrete: can aging muscle’s mitochondrial energy output be measured, and does changing that measure actually translate into less fatigue?

The SS-31 mitochondrial function trial literature gives a useful, cautious answer. Researchers can measure mitochondrial ATP production directly in living human muscle. In one small older-adult trial, elamipretide, also called SS-31 or MTP-131, changed that biological measure briefly. The harder outcomes, including fatigue resistance, walking distance, and patient-reported fatigue, have been mixed or negative.

Research Confidence

★★★★☆ — Confidence is strong for one narrow conclusion: SS-31 mitochondrial function trial readouts and everyday function can diverge. That conclusion rests on a 39-person older-adult trial, two smaller primary mitochondrial myopathy trials, a 218-person phase 3 trial, and a rare-disease Barth syndrome trial. It does not support a healthy-aging efficacy claim.

Study Snapshot

• Study type: randomized, double-blind, placebo-controlled trial
• Participants: 39 healthy older adults, ages 60-85, 46% female
• Duration: single 2-hour infusion, with immediate and day 7 assessments
• Measured: ATPmax by phosphorus magnetic resonance spectroscopy in a hand muscle
• Found: ATPmax improved transiently, but the signal was absent at day 7 and fatigue resistance did not improve
• Funding: Stealth BioTherapeutics funded the study; some author consulting relationships were disclosed

Why This Matters

Fatigue is a slippery word. One person means heavy legs on stairs. Another means needing longer recovery after the same workout. A clinician may measure walking distance, grip strength, or a fatigue questionnaire. A biologist may measure ATP, the molecule cells use as an energy currency.

Those are not interchangeable.

This matters especially in midlife and older adulthood, when muscle preservation, exercise tolerance, and recovery become more practical than abstract. If a trial changes a mitochondrial measurement but not fatigue, the result is still scientifically interesting. It just answers a smaller question than most people are asking.

What ATPmax Means In An SS-31 Mitochondrial Function Trial

The older-adult SS-31 mitochondrial function trial used phosphorus magnetic resonance spectroscopy, a non-invasive scan that follows energy chemistry inside muscle. The main measure was ATPmax, an estimate of the muscle’s maximum capacity to make ATP after exercise.

In plain English, ATPmax is closer to testing the engine’s production capacity than asking whether the car feels easier to drive.

In the human randomized, double-blind, placebo-controlled trial, 39 adults aged 60 to 85 were selected for low mitochondrial function in a small hand muscle. “Double-blind” means neither participants nor investigators knew who received elamipretide or placebo during the trial. The elamipretide group received a single intravenous infusion, and outcomes were measured right after and again seven days later.

The biological signal was real but short-lived: ATPmax rose versus placebo by percent-change analysis immediately after infusion. By day 7, that difference was gone. The same trial also measured fatigue resistance using force over repeated effort, and that measure did not improve.

That is the central lesson: a mitochondrial readout moved, but the functional readout did not.

Walking Distance And Fatigue Did Not Always Follow

Primary mitochondrial myopathy, or PMM, is a group of genetic mitochondrial disorders that can affect muscle function. These trials are not healthy-aging studies, but they help show whether elamipretide signals translate into outcomes people can feel.

In a human phase 1/2 randomized trial of 36 adults with genetically confirmed PMM, five days of intravenous elamipretide produced a dose-related trend on the 6-minute walk test. The highest-dose group walked 64.5 metres farther versus 20.4 metres with placebo at day 5, narrowly missing the usual pairwise statistical cutoff.

A later human randomized crossover trial enrolled 30 PMM participants. A crossover trial lets each participant receive elamipretide and placebo in separate periods, with a washout break between them. After four weeks, the 6-minute walk difference favoured elamipretide by 19.8 metres but was not statistically significant. Some fatigue patient-reported scores improved, while other functional and activity measures did not.

Then came the larger test. In the 218-person human phase 3 MMPOWER-3 randomized clinical trial, elamipretide did not meet either co-primary endpoint. “Primary endpoint” means the main result the trial was built to judge. The between-group difference was -3.2 metres on 6-minute walk distance and -0.07 on the PMM fatigue score. The authors classified it as Class I evidence that elamipretide did not improve those PMM outcomes.

Where Barth Syndrome Fits, And Where It Does Not

Barth syndrome is a rare genetic disorder of mitochondrial cardiolipin metabolism that primarily affects males. It is biologically relevant to elamipretide, but it is not a model for ordinary aging fatigue.

In a human phase 2/3 randomized crossover trial, 12 Barth syndrome participants aged 12 to 35 received elamipretide and placebo in separate 12-week periods. The controlled phase did not meet its 6-minute walk or total fatigue endpoints. In the open-label extension, where participants knew they were receiving the drug and there was no placebo comparison, functional and symptom measures improved over time. The authors noted that placebo or learning effects could not be ruled out.

Regulatory status is specific. In September 2025, the FDA granted accelerated approval to Forzinity, the elamipretide injection product, for Barth syndrome patients weighing at least 30 kg, based on knee extensor muscle strength. Accelerated approval means continued approval may depend on later confirmation of clinical benefit. It does not imply approval for aging, fatigue, PMM, or general mitochondrial support.

What This Means For You

If you are reading mitochondrial research because your stamina feels different than it used to, the key is to separate lab signal from lived outcome. ATPmax can tell researchers something important about muscle bioenergetics. It cannot, by itself, tell you whether someone will walk farther, feel less fatigued, or preserve muscle with age.

A reasonable question to bring to a clinician or researcher is not “does this raise ATP?” but “which outcome matters here?” For daily life, that might be walking distance, strength, repeated-effort fatigue, recovery time, or a validated fatigue scale. A validated scale is a questionnaire tested to measure the same symptom reliably across people.

What This Tells Us About Women Specifically

The most relevant older-adult trial included women, but only 46% of participants were female, and menopausal status was not reported. Results were not meaningfully broken out by sex.

The PMM trials included many women, with roughly 64% to 83% female participation depending on the trial, but they also did not report menopause-specific analyses. The Barth syndrome trial enrolled only males, which fits the disease biology but limits extrapolation.

That gap matters. Midlife fatigue, muscle preservation, hormone therapy use, activity level, and menopause stage can overlap in real life. These trials do not tell us whether SS-31 mitochondrial function trial signals differ across those contexts.

Questions This Study Couldn't Answer

• The older-adult trial tested a single infusion and a small hand muscle, not long-term daily function.
• The ATPmax signal disappeared by day 7, so durability is unknown.
• Fatigue resistance did not improve in the older-adult trial.
• The larger PMM phase 3 trial did not improve walking distance or fatigue.
• Menopause status, hormone therapy use, and sex-stratified outcomes were not adequately reported.
• Barth syndrome findings come from a rare, mostly male genetic disease and should not be generalized to healthy aging.
• Several trials involved the drug developer through funding, trial support, or disclosed author relationships.

Future Research

The next step is not another ATP-only result. Confidence would rise with a preregistered, adequately powered randomized trial in midlife-to-older adults that tracks functional endpoints such as gait speed, endurance, strength, fatigability, and daily activity. Sex-stratified analysis, menopause-status reporting, longer follow-up, and transparent adverse-event and dropout reporting would make the evidence far more useful.

Sources

1. Roshanravan B, Liu SZ, Ali AS, et al. In vivo mitochondrial ATP production is improved in older adult skeletal muscle after a single dose of elamipretide in a randomized trial. PLoS One. 2021;16(7):e0253849. Human randomized, double-blind, placebo-controlled trial, n=39 analyzed. DOI: 10.1371/journal.pone.0253849. PubMed

2. Karaa A, Haas R, Goldstein A, Vockley J, Weaver WD, Cohen BH. Randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy. Neurology. 2018;90(14):e1212-e1221. Human phase 1/2 randomized, double-blind, dose-escalation trial, n=36. DOI: 10.1212/WNL.0000000000005255. PubMed

3. Karaa A, Haas R, Goldstein A, Vockley J, Cohen BH. A randomized crossover trial of elamipretide in adults with primary mitochondrial myopathy. J Cachexia Sarcopenia Muscle. 2020;11(4):909-918. Human randomized, double-blind, placebo-controlled crossover trial, n=30. DOI: 10.1002/jcsm.12559. PubMed

4. Karaa A, Bertini E, Carelli V, et al. Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial. Neurology. 2023;100(23):e2387-e2398. Human phase 3 randomized, double-blind, placebo-controlled trial, n=218 randomized. DOI: 10.1212/WNL.0000000000207402. PubMed

5. Thompson WR, Hornby B, Manuel R, et al. A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome, a genetic disorder of mitochondrial cardiolipin metabolism. Genet Med. 2021;23(3):471-478. Human phase 2/3 randomized, double-blind, placebo-controlled crossover trial, n=12 randomized. DOI: 10.1038/s41436-020-01006-8. Nature

6. FDA. Forzinity (elamipretide) prescribing information. Revised September 2025. NDA 215244. Regulatory label. FDA label

Research Use And Availability

Nuforme supplies SS-31 for laboratory research use in Canada. It is listed for research contexts only, with documentation and third-party testing standards relevant to laboratory materials. The Research peptide glossary and SS-31 product category can help readers distinguish research materials from approved clinical products. The evidence discussed here includes human trials, but Nuforme’s SS-31 is not supplied for administration or personal use.

Final Thoughts

The honest answer is measured. Human trials show that mitochondrial energy production in muscle can be measured, and SS-31 has produced a short-term ATPmax signal in older adults. They do not show that this translates into less fatigue, better walking performance, or healthy-aging benefit. For now, the measurement is ahead of the everyday meaning.