SS-31 is an approved drug now. The trial behind that approval had 12 people.

What FDA approved, and what it did not

Barth syndrome is a serious, life-limiting mitochondrial disease that primarily affects males and typically begins with severe heart failure in infancy.[1] The company that developed elamipretide describes it as affecting approximately 150 individuals in the United States.[13] This is an ultra-rare-disease approval, and the accelerated pathway exists precisely for that situation: it allows earlier approval on a measure considered reasonably likely to predict benefit, rather than one that directly measures benefit.[12]

The label carries the accelerated-approval caveat in its own text: approval was granted on improved knee extensor muscle strength, and continued approval may be contingent on confirming clinical benefit in a confirmatory trial.[3]

Evidence ledger

The human record for elamipretide

Elamipretide has been through a substantial clinical programme, which is unusual for a compound also sold as a research chemical. That programme is worth reading in full rather than by its single positive headline: the largest randomized trials were negative, and the aging data are thin.

Counts are separate trials, not one pooled population. The two phase 3 programmes studied genetic mitochondrial disease, not aging.[4][7][8][9]

The blinded phase did not separate from placebo

TAZPOWER randomized 12 patients, six per sequence, to 12 weeks of elamipretide 40 mg daily or placebo, then crossed them over after a four-week washout. Its two pre-specified primary endpoints were six-minute walk distance and a three-question fatigue score. At the end of treatment, mean walk distance was 443.1 metres on elamipretide and 443.9 metres on placebo. Mean total fatigue was 6.305 and 6.240.[4]

The peer-reviewed report of that phase states plainly that it did not achieve its primary objectives.[5] Muscle strength by handheld dynamometry was a secondary measure in the blinded phase, and it did not separate either: mean strength moved from 131.2 to 135.9 newtons on elamipretide and from 123.1 to 129.3 newtons on placebo.[4]

The approval rests on the open-label extension

After the crossover, 10 of the 12 patients continued on elamipretide with no placebo group and no blinding; 8 reached the week-168 visit.[4][6] Knee extensor strength rose during that period. Registry results give mean changes from extension baseline of 57.8 and 61.9 newtons at week 168 for the two original sequences.[4] FDA judged that improvement reasonably likely to predict benefits such as standing more easily or walking farther.[1]

That is a defensible regulatory judgement about a devastating rare disease with no other treatment. It is also, methodologically, the weakest kind of comparison: an uncontrolled extension cannot separate a drug effect from expectation, practice on the test, added attention, or selective retention of the patients who were doing well. Two of the ten did not complete it.[4]

FDA required Stealth to run a randomized, double-blind, placebo-controlled trial in patients five years and older to verify that the strength change translates into benefit, with a final report due March 2030.[2] That trial, 4TAZPower, began recruiting on July 2, 2026, plans 48 participants over 72 weeks, and estimates primary completion in September 2029. Its own registry entry describes the approval as based on knee extensor strength, an intermediate clinical endpoint.[10] Until it reads out, the clinical benefit is predicted, not demonstrated.

The largest randomized trial of this peptide was negative

MMPOWER-3 randomized 218 people with genetically confirmed primary mitochondrial myopathy, 109 to elamipretide and 109 to placebo, at the same 40 mg daily subcutaneous dose later approved for Barth syndrome. Over 24 weeks the difference in six-minute walk distance was −3.2 metres (95% CI −18.7 to 12.3; p = 0.69) and the difference in total fatigue score was −0.07 (95% CI −0.10 to 0.26; p = 0.37).[7]

The authors classified the result as Class I evidence that elamipretide does not improve six-minute walk distance or fatigue at 24 weeks in primary mitochondrial myopathy. They also found it well tolerated.[7] A negative trial in one disease does not disprove an effect in another, and Barth syndrome has a specific cardiolipin defect that elamipretide targets directly. But it does mean the well-powered, blinded, placebo-controlled test of this peptide in muscle function has been run, and it did not show a benefit.

A second phase 3 trial, NuPOWER, enrolled 102 people with nuclear-DNA mitochondrial disease and completed in September 2024. No results were posted to its registry entry as of this writing, so we are not characterizing its outcome.

The longevity claim is where the evidence thins out

SS-31 is marketed in the research-chemical market largely on mitochondrial and anti-aging framing. The strongest human data point for that framing is a randomized, double-blind, placebo-controlled trial in 39 older adults aged 60 to 85 selected for impaired mitochondrial function. A single two-hour infusion raised in-vivo mitochondrial ATP production capacity relative to placebo immediately afterwards (%ΔATPmax, p = 0.045). By day 7 there was no difference between groups, and exercise tolerance did not significantly improve on the infusion day.[8]

That is a genuine mechanistic finding in humans, and it is not the same thing as a health benefit. A single dose moved a biochemical measure for less than a week without moving the function that measure is supposed to underwrite.

The multi-month benefits people cite for aging come from animal work. One representative study treated aged female mice for eight months and reported preserved exercise tolerance and left ventricular mass, with more modest effects on diastolic function and skeletal muscle force.[11] Aged mice are not aging humans, and that paper also notes elamipretide is not orally bioavailable.

The first registered study of daily dosing in healthy older adults is currently recruiting: an open-label, single-arm pilot in 30 adults aged 65 to 80, four weeks long, whose primary outcome is safety and tolerability rather than any performance measure.[9] That is the appropriate next step, and it is also a good indication of how early this line of research still is.

What the approved product is, in detail

Forzinity is 40 mg injected subcutaneously once daily for patients weighing at least 30 kg, reduced to 20 mg daily in adults with severe renal impairment.[3] The label warns that the formulation contains benzyl alcohol and must not be used in neonates, and instructs prescribers to monitor for hypersensitivity reactions.[3]

Injection-site reactions dominated the adverse-event profile: in the blinded phase, erythema was reported in every patient on elamipretide against 25% on placebo, with pain, induration and itching also more frequent on drug.[3][4] FDA additionally required two post-approval carcinogenicity studies, a 26-week mouse study and a two-year rat study, because spontaneous adverse-event reporting would not be sufficient to detect an unexpected serious carcinogenicity risk.[2] Long-term carcinogenic potential was not characterized at the time of approval.

Everything above describes one manufactured product with a known formulation, a stability-tested 48-month expiry, and a specific dose.[2] A vial labelled SS-31 in the research market shares a peptide name with it. It does not thereby share its identity, purity, formulation, or the evidence attached to it.

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