SS-31 (Elamipretide): What the 2025 FDA Approval Really Means

SS-31 is approved. That approval is far narrower than the way it is being used in marketing.

In September 2025, elamipretide received accelerated FDA approval under the brand name Forzinity, to improve muscle strength in adult and paediatric patients with Barth syndrome weighing at least 30 kg. Barth syndrome affects roughly 150 people in the United States.

In the three larger indications elamipretide was tested for, it did not succeed. Trials in primary mitochondrial myopathy, dry age-related macular degeneration and heart failure all missed their primary endpoints.

Both facts belong in any honest description of this compound, and you will rarely see them together.

What SS-31 is

SS-31 is an aromatic-cationic tetrapeptide, a chain of four amino acids. The name comes from Szeto and Schiller, the researchers who developed the series. It has also been known as MTP-131 and Bendavia, and is now marketed as Forzinity.

Its mechanism is unusually well characterised for a compound in this space. SS-31 concentrates in mitochondria and associates with cardiolipin, a phospholipid of the inner mitochondrial membrane. Stabilising cardiolipin is proposed to improve mitochondrial efficiency and reduce production of reactive oxygen species.

Cardiolipin is central to Barth syndrome, which is caused by variants in the TAFAZZIN gene that disrupt cardiolipin remodelling. The mechanism and the approved indication line up precisely, and that is not a coincidence.

What the approval says

The details matter here.

Accelerated approval, not standard approval. Accelerated approval permits authorisation on an intermediate endpoint reasonably likely to predict clinical benefit, with confirmatory evidence expected afterwards. Continued approval can depend on that confirmation.

The endpoint was knee extensor muscle strength, not survival, not cardiac outcomes, not quality of life.

The population is narrow: Barth syndrome patients weighing at least 30 kg. The sponsor stated it was continuing to work with the FDA on data to extend the indication to lighter children.

The supporting evidence came largely from the open-label portion of the TAZPOWER trial and a natural history control study. An FDA advisory committee had voted 10 to 6 in favour in October 2024, which is a notably split vote for an efficacy question.

This is a real approval and a genuine first, both as a treatment for Barth syndrome and as a mitochondria-targeted therapeutic. It is also an ultra-rare disease approval on an intermediate endpoint, and that is a different thing from a broad efficacy finding.

The trials that did not work

This is the part that gets omitted, and it is the most informative section for anyone considering SS-31 for general use.

Primary mitochondrial myopathy. Elamipretide went through Phase 3 and missed its primary endpoint.

Dry age-related macular degeneration. Missed its primary endpoint.

Heart failure. Missed its primary endpoint.

These were properly conducted trials under an investigational new drug application, which puts elamipretide in a different evidentiary class from most compounds sold as research peptides. It has been tested seriously. In the larger indications it was tested for, it did not demonstrate the effect.

Anyone using SS-31 for energy, athletic performance or anti-ageing is extrapolating well beyond an approval granted for a single ultra-rare genetic disease, into territory where the compound has already been tested and come up short.

Safety, as far as it is known

The trial safety picture is reasonably reassuring within the tested populations. The most commonly reported effects were injection-site reactions, including itching, redness, firmness, bruising and pain, generally mild to moderate and often resolving without intervention. The failed trials in myopathy and heart failure produced minimal safety signals, which is itself useful information.

Two caveats. The trial populations were patients with specific diseases, studied over defined periods, under supervision. And elamipretide is a once-daily subcutaneous injection, not an occasional or oral product, so the exposure pattern for anyone using it long-term is substantial.

Status in Ireland and the EU

Forzinity's approval is a US approval. It does not confer any status in Ireland or the EU.

Elamipretide holds orphan drug designation from the European Medicines Agency for Barth syndrome. Orphan designation is a development incentive granted early in a programme, providing protocol assistance and fee reductions. It is not a marketing authorisation and it does not permit sale. Anyone wanting the current EU position should check the EMA medicines register directly.

There is also a point that the marketing tends to invert. Now that an approved elamipretide product exists, material sold online as SS-31 is an unapproved copy of an approved medicine. That is a weaker regulatory position than before the approval, not a stronger one. An approved product has a manufacturing specification, a licence and a regulator standing behind it. Research-grade powder has none of those, whatever the molecule is called.

What would change the picture

The confirmatory work supporting the accelerated approval, and any expansion of the indication, are the things to watch in Barth syndrome.

For the broader claims, what is missing is a properly powered randomised trial in a general population measuring something meaningful, from a group independent of the original developers. The three trials that have come closest to testing broad efficacy did not deliver it.

About this article

This is a summary of published trial results and regulatory status. It is not medical advice and it is not legal advice.

Elamipretide is a prescription medicine in the jurisdiction where it is approved, and eligibility is determined by a clinician. If you have or suspect a mitochondrial condition, that is a conversation for a specialist rather than a supplier.

Regulatory positions change. Verify against the FDA and EMA registers before relying on anything above.