Article

Elamipretide (SS-31): the 2025 accelerated approval on the record

Published 21 October 2026 · KlowSignal Research Desk

Abstract illustration of folded inner membrane layers in soft blue and amber tones, with a small four-part chain resting against the folds

Prepared by: Research Desk. Sources are linked for verification.

Published 21 October 2026

At a glance

Compound: SS-31, a synthetic tetrapeptide (D-Arg-Dmt-Lys-Phe-NH2)
Other names: elamipretide, MTP-131
Research focus: binding to cardiolipin, a phospholipid of the inner mitochondrial membrane
Regulatory event: on 19 September 2025, FDA granted accelerated approval to Forzinity (elamipretide) for Barth syndrome in people weighing at least 30 kg
Scope: the approval covers that prescription product only, not SS-31 sold as a laboratory reagent

What SS-31 is

SS-31 is a synthetic peptide of four amino acids: D-arginine, 2',6'-dimethyltyrosine (Dmt), lysine and phenylalanine, with an amidated C-terminus. It belongs to the Szeto-Schiller (SS) peptides, named after Hazel Szeto and Peter Schiller, who developed the series.

The structure alternates aromatic and positively charged residues. According to the research literature, this pattern lets the peptide cross cell membranes and concentrate in the inner mitochondrial membrane.

The SS peptides were first developed in opioid receptor research. Researchers later noticed that some members of the series accumulated in mitochondria, and SS-31 became the lead compound for that work. A 2014 review by Szeto in the British Journal of Pharmacology describes the discovery and development of SS-31 as the first compound designed to interact with cardiolipin.

Cardiolipin: the laboratory findings

Cardiolipin is a phospholipid with four fatty-acid chains. It is found almost only in the inner mitochondrial membrane, where it shapes the folds known as cristae and helps organise the respiratory chain. The preclinical record on SS-31 centres on this lipid:

Binding. Using a fluorescent analogue, Birk et al. (2013) reported that SS-31 binds cardiolipin with high affinity.
Cytochrome c. The same work reported that the SS-31-cardiolipin complex inhibited the peroxidase activity of cytochrome c, a reaction that damages cardiolipin under stress.

These are laboratory findings about mitochondrial structure and chemistry. Results in cell-free systems, cultured cells or animal models do not predict outcomes in other conditions or in people.

The clinical record before approval

SS-31 is unusual among the compounds covered on this site because it has been through formal clinical development, and that record includes a negative result.

Primary mitochondrial myopathy. A randomised crossover trial in adults by Karaa et al. (2020) was followed by MMPOWER-3, a phase 3 randomised placebo-controlled trial reported by Karaa et al. (2023) with 218 participants. MMPOWER-3 did not meet its primary endpoints, the six-minute walk test and a fatigue score.
Barth syndrome. Barth syndrome is a rare genetic disease of cardiolipin metabolism. Smaller trials in this condition formed the basis of the 2025 FDA decision.

A trial that misses its primary endpoint has not shown the effect it set out to measure. That result sits alongside the later approval in a different, much narrower setting.

The 19 September 2025 decision

On 19 September 2025, FDA granted accelerated approval to Forzinity (elamipretide) for Barth syndrome, limited to people weighing at least 30 kg.

Two conditions shape how the decision should be read:

An intermediate endpoint. The approval was based on knee extensor muscle strength, an intermediate measure rather than a direct measure of long-term clinical outcome.
A confirmatory trial. FDA required a confirmatory randomised, placebo-controlled trial.

The primary records are FDA's approval document, the Drug Trials Snapshot for Forzinity and the prescribing information. Readers who want the regulator's own wording should go to these documents rather than to secondary summaries.

What accelerated approval means here

Accelerated approval is an FDA pathway that allows approval on the basis of an intermediate or surrogate endpoint, on the condition that a confirmatory trial follows. Three points follow from the record:

The approval is conditional on the confirmatory trial FDA required.
The approval is narrow: one disease and a body-weight limit.
The approval does not settle questions in other conditions. The MMPOWER-3 result remains part of the record.

Our guide on how to read a research signal explains why intermediate endpoints and conditional approvals need careful reading.

Why the approval does not extend to research-grade material

A prescription approval applies to a specific product: one manufacturer's material, made to a defined specification, with a reviewed label, for a stated indication. It does not attach to the molecule in general.

SS-31 sold as a laboratory reagent is not Forzinity. It has not been evaluated by FDA, it is not covered by the approval, and the approval says nothing about its identity or purity. For laboratory work, batch-level certificates of analysis are the minimum documentation a laboratory should expect, because product identity and purity affect whether results can be reproduced.

Elamipretide does not appear on the agenda of FDA's July 2026 Pharmacy Compounding Advisory Committee meeting, which covers other peptides nominated for the 503A bulks list. We found no EU marketing authorisation for elamipretide as of 19 October 2026.

Open research questions

How cardiolipin binding translates into measurable changes in tissue function across conditions.
Whether findings in Barth syndrome, a disease of cardiolipin itself, apply to other mitochondrial disorders.
How long-term exposure affects mitochondrial membranes and cardiolipin content in animal models.
What the required confirmatory trial will report.

Limitations of the evidence

The phase 3 trial in primary mitochondrial myopathy did not meet its primary endpoints.
The Barth syndrome approval rests on an intermediate endpoint and is conditional on a confirmatory trial.
Preclinical findings in laboratory and animal models do not predict outcomes in other conditions.

Key terms

Cardiolipin. A phospholipid with four fatty-acid chains, found almost only in the inner mitochondrial membrane.
Cristae. The folds of the inner mitochondrial membrane, where much of the respiratory chain sits.
Intermediate endpoint. A measure taken as an early indication of an outcome, rather than the outcome itself.
Primary endpoint. The main outcome a trial is designed to test.

Where researchers source it

Purita is a store that lists SS-31 for laboratory research use only. Its product page labels the item for research use only and not for human or animal administration.

Purita product page

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