In September 2025 the US FDA gave elamipretide — the mitochondria-targeted tetrapeptide better known in research circles as SS-31 — accelerated approval as Forzinity for Barth syndrome in patients weighing at least 30 kg. That made it the first FDA-approved drug aimed directly at mitochondria. What most SS-31 research summaries leave out is that the approval came with homework attached, and the homework only started being handed in this year: in July 2026 the sponsor announced the first patient dosed in 4TAZPower, the Phase 3b/4 confirmatory trial whose primary completion date is 30 September 2029. If you want to know what is SS-31's actual evidentiary standing, that date matters more than the approval headline.

What is SS-31, and what was actually approved?

SS-31 is a four-amino-acid, cell-permeable peptide that accumulates in the inner mitochondrial membrane and binds cardiolipin — the phospholipid that organises cristae folds and helps anchor electron transport chain supercomplexes. The proposed mechanism is membrane stabilisation rather than antioxidant scavenging. Our SS-31 profile covers that biophysics in more detail.

Barth syndrome is an X-linked disorder of cardiolipin remodelling caused by TAFAZZIN variants, which makes it the cleanest possible test case for a cardiolipin-binding drug. It is also ultra-rare: the pivotal randomised trial, TAZPOWER, enrolled 12 genetically confirmed patients.

The approval rests on an intermediate endpoint from an open-label extension

This is the part worth stating plainly. The 12-week randomised, placebo-controlled crossover portion of TAZPOWER did not meet either of its co-primary endpoints — the six-minute walk test and the Barth Syndrome Symptom Assessment. Improvements in the 6MWT, symptom scores, knee extensor strength and some cardiac parameters emerged later, in the open-label extension, where every participant knew they were on drug and there was no concurrent placebo group. The 168-week open-label extension data were published in Genetics in Medicine in 2024; eight patients reached the week 168 visit.

Accelerated approval was granted on knee extensor muscle strength as an intermediate clinical endpoint — a measure the FDA accepted as reasonably likely to predict clinical benefit, not as proof of it. Accelerated approval is explicitly conditional: continued marketing depends on verifying clinical benefit in a confirmatory trial. An earlier complete response letter, before the 2025 approval, is a reminder that this file was not a comfortable one for reviewers.

4TAZPower: the trial that has to do the verifying

The confirmatory study (ClinicalTrials.gov NCT07531251, sponsor Stealth BioTherapeutics — the company now trading as Mighty Therapeutics after a June 2026 rename) is a randomised, double-blind, parallel-group, placebo-controlled trial of once-daily subcutaneous elamipretide over 72 weeks, in patients aged 5 and over with genetically confirmed Barth syndrome. Estimated primary completion is 30 September 2029.

Two details are worth flagging for anyone tracking SS-31 research:

It re-tests the same endpoint that triggered approval

The stated objective is to confirm efficacy on the basis of knee extensor muscle strength. That is a reasonable regulatory choice — you confirm what you were approved on — but it means the 2029 read-out will still be a strength measure, not mortality, cardiac events or transplant-free survival. Longer-horizon cardiac outcomes in Barth syndrome will remain a natural-history and registry question for some time.

Parallel-group, not crossover

Moving from a 12-patient crossover to a 72-week parallel-group design with a real placebo arm is a meaningful methodological upgrade. It is also the reason the answer is three years away: recruiting a placebo-controlled trial in an ultra-rare disease where a drug is already approved in one market is genuinely hard.

SS-31 Australia: is SS-31 legal in Australia?

Elamipretide is not on the Australian Register of Therapeutic Goods. There is no TGA-registered SS-31 product, and no provisionally registered one either. The practical consequences:

  • No Australian approval means no Australian label. A US accelerated approval does not create TGA registration. The closest local analogue is the TGA's provisional approval pathway, which grants registration on preliminary clinical data for a maximum of six years, with a binding obligation to supply confirmatory efficacy and safety data before that window closes. Nothing about the FDA decision obliges a sponsor to lodge that application here.
  • Access for a diagnosed patient would run through unapproved-goods pathways — the Special Access Scheme, an Authorised Prescriber, or a clinical trial — always clinician-initiated, always for a specific patient, never something a consumer arranges. Expanded and emergency access programs have reportedly supplied elamipretide to a few dozen Barth syndrome patients worldwide, including infants.
  • Research-only SS-31 sold online is not the approved product. Forzinity is a prescription injection with a defined weight threshold and a monitoring context. Material labelled "not for human consumption" is a different thing entirely, with no assurance of identity, purity or sterility. We don't cover suppliers, and nothing here is a recommendation to obtain it.

What this does and doesn't say about SS-31 beyond Barth syndrome

A rare-disease approval in a cardiolipin-remodelling defect is the narrowest possible beachhead, and it should not be read across to general use. Elamipretide's Phase 3 in primary mitochondrial myopathy missed both of its endpoints. Programs in polymerase gamma-related mitochondrial disease and dry age-related macular degeneration continue. For healthy adults seeking energy, performance or anti-ageing effects, there is still no published randomised human trial supporting those uses — the supporting work is cell-culture and animal-model data across ischaemia-reperfusion, doxorubicin cardiotoxicity, ageing muscle and acute kidney injury. Preclinical, and in many cases unreplicated in humans.

One housekeeping note for readers who check registries themselves: trial databases now carry demonstration and low-provenance records for popular peptides. Before you treat a registry entry as news, confirm the sponsor is a real, traceable organisation. In this case it is — Stealth BioTherapeutics/Mighty Therapeutics has a published trial history and an approved product.

What to watch next

Enrolment updates for 4TAZPower, any TGA provisional determination or registration application for elamipretide in Australia, and whether the polymerase gamma and dry AMD programs generate randomised data. We track those across the peptides section, and you can join the free updates list if you'd rather have them arrive than hunt for them.

FAQ

What is SS-31 (elamipretide)?

SS-31 is a mitochondria-targeted tetrapeptide that binds cardiolipin in the inner mitochondrial membrane. As elamipretide, it received FDA accelerated approval in September 2025 as Forzinity for Barth syndrome in patients weighing at least 30 kg.

Is SS-31 legal in Australia?

There is no SS-31 or elamipretide product on the ARTG, so it cannot be supplied as a registered medicine in Australia. Access for a diagnosed patient would depend on clinician-initiated unapproved-goods pathways such as the Special Access Scheme, an Authorised Prescriber, or a clinical trial.

Does FDA approval mean SS-31 is proven to work?

Not in the usual sense. Accelerated approval was based on knee extensor muscle strength — an intermediate endpoint — observed in an open-label extension after the randomised portion of TAZPOWER missed both co-primary endpoints. A confirmatory Phase 3b/4 trial has a primary completion date of 30 September 2029.

Is there human evidence for SS-31 for energy or anti-ageing?

No published randomised human trial supports SS-31 for energy, athletic performance or anti-ageing in healthy adults. That literature is preclinical — cell and animal models — and its Phase 3 in primary mitochondrial myopathy missed both endpoints.

Sources

Not medical advice. SS-31 (elamipretide) holds FDA accelerated approval only for Barth syndrome in patients ≥30 kg and is not on the ARTG; retatrutide.net.au is independent, does not sell or source peptides, and publishes for information only.