What is SS-31? Elamipretide, what it does and what it is for

What is SS-31?

SS-31 is the research name for elamipretide, a peptide only four amino acids long that targets mitochondria, the energy-producing parts of cells. It binds cardiolipin, a fat molecule in the inner mitochondrial membrane. In the United States it is an approved medicine, FORZINITY, for one rare genetic disease, Barth syndrome.

SS-31 is a laboratory code name. The drug's official name is elamipretide, and older papers also call it MTP-131 [4]. Its US label calls it a mitochondrial cardiolipin binder [1], which is a compact description of everything it is meant to do.

Chemically it is tiny. The label gives its sequence as D-Arg-2,6-dimethyl-Tyr-Lys-Phe-NH2: four amino acids, arginine, a modified tyrosine, lysine and phenylalanine [1]. All of them are in the normal L form except arginine, which is in the mirror-image D form [1]. Researchers describe it as a very polar tetrapeptide, a four-amino-acid peptide that is strongly charged [2].

So "what is SS-31 for?" has a precise answer and a looser one. The precise answer: in the United States it is approved to improve muscle strength in people with Barth syndrome who weigh at least 30 kg [1]. The looser answer is that it has also been tested in heart attacks, heart failure, mitochondrial muscle disease and eye disease. The sections below go through both.

How SS-31 works: cardiolipin and the inner membrane

Mitochondria make energy on a folded inner membrane called the cristae. Cardiolipin, a fat found on that membrane, holds the folds together. SS-31 binds cardiolipin with high affinity, and in rats it protected the cristae during loss of blood flow and helped cells recover their energy supply faster.

Mitochondria make most of a cell's ATP, its energy currency, on a heavily folded inner membrane. The folds are called cristae, and losing them is how mitochondria fail when blood flow is cut: ischemia destroys the cristae membranes required for ATP synthesis [2].

The molecule that holds those folds together is cardiolipin. In a 2013 study, researchers used a glowing version of SS-31 to show that it binds with high affinity to cardiolipin, a phospholipid on the inner mitochondrial membrane that is required for cristae formation [2]. Bound together, the SS-31 and cardiolipin pair also blocked an enzyme activity of cytochrome c that damages cardiolipin during ischemia [2]. In rats given SS-31 before the blood supply to the kidney was cut off, it protected the cristae and prevented mitochondrial swelling [2], and the authors report that it speeds the recovery of ATP after ischemia [2].

The approved label puts the same idea in one sentence: elamipretide localizes to the inner mitochondrial membrane and improves mitochondrial morphology and function [1]. A review of its heart research describes the target as the cardiolipin-cytochrome c supercomplex, and notes it raised the heart's pumping fraction in dog models of heart failure and prevented harmful remodelling of the heart in rats [3]. Those are animal results. The human trials, below, are where the idea was actually tested.

Is SS-31 FDA approved?

Yes, for one use. Elamipretide, sold as FORZINITY, is approved in the United States to improve muscle strength in Barth syndrome, under accelerated approval. That approval rests on knee strength gains seen in an open-label extension, after the randomized part of the trial missed both of its primary endpoints.

Barth syndrome is a rare, X-linked genetic disorder [1] caused by defects in a gene called TAZ, which leaves people with abnormal cardiolipin on the inner mitochondrial membrane [4]. Because SS-31 binds cardiolipin [2], it was a logical disease to test it in, and it is the one disease the approved label covers [1].

The approval is an accelerated approval, which rests on an intermediate measure rather than a final clinical outcome. The label says it was granted based on an improvement in knee extensor muscle strength, an intermediate clinical endpoint, and that continued approval may depend on a confirmatory trial [1]. The label is dated September 2025 [1].

The trial behind it, TAZPOWER, is worth reading closely because it is small and its results are mixed:

• The randomized part did not work on its main goals. 12 people took elamipretide or placebo for 12 weeks each, in random order [4]. Neither primary endpoint, walking distance in six minutes and a fatigue score, was met [4], and the label says the drug was not superior to placebo on them [1].
• Strength did not rise during that part either. According to the label, increases in knee strength were not seen in the randomized trial but were seen during the open-label extension [1].
• The extension looked better, without a control group. 10 people continued on the drug, and 8 reached week 168 [5]. By then, walking distance had improved by a cumulative 96.1 m from the start of the extension [5].

An open-label extension has no placebo group, so it cannot separate a drug effect from practice, expectation or the natural course of a disease. That is why the approval is conditional: the label says continued approval may depend on verifying the clinical benefit in a confirmatory trial [1].

What else has SS-31 been tested for?

Several conditions, and the controlled trials did not meet their main goals. A 218-person trial in mitochondrial muscle disease, a heart attack trial, a heart failure trial and a 176-person trial in dry macular degeneration all missed their primary endpoints, though the eye trial reported a secondary signal that a phase 3 programme will test.

Most people asking what SS-31 is for want to know what it does in people. Outside Barth syndrome, the honest answer from the controlled trials so far is that it has not done what it was tested for.

• Primary mitochondrial myopathy. These are genetic disorders that impair the mitochondria's energy production [6]. MMPOWER-3, a pivotal phase 3 trial [6], randomized 218 people to elamipretide or placebo [6] for 24 weeks [6]. It did not meet its primary endpoints of walking distance and fatigue [6].
• Heart attack. EMBRACE STEMI gave the drug, under its older name MTP-131, to people having a first major heart attack who were treated with an emergency procedure to reopen the blocked artery (PCI) [7]. It was safe and well tolerated [7], but it did not significantly reduce infarct size, the amount of heart muscle that died [7].
• Heart failure. PROGRESS-HF randomized 71 patients with heart failure with reduced ejection fraction [8]. After 4 weeks the drug was well tolerated but did not improve the size of the heart's main pumping chamber compared with placebo [8].
• Dry age-related macular degeneration. A small phase 1, single-centre, open-label study ran for 24 weeks [9] and its exploratory analyses suggested a possible positive effect on vision in dim light [9]. The controlled follow-up, ReCLAIM-2, randomized 176 patients [10] and did not meet its primary endpoints, vision in low light and the growth of the damaged area [10]. It did report slower loss of the retina's light-sensing layer on scans, a secondary measure [10], which its authors say will be the primary endpoint of a phase 3 programme [10].

None of these trials tested SS-31 as an anti-aging or general energy supplement, and no source here measures that in people. The longevity claims made for it online are not what these trials measured.

SS-31 side effects and safety

In trials, the most common side effects were injection-site reactions such as redness, pain, itching and bruising. The label also warns of allergic reactions, some serious enough to need emergency care, and notes frequent rises in eosinophils, a type of white blood cell, with use of 30 days or longer.

Elamipretide is given as an injection under the skin, and the injection site is where most of its side effects show up. The label lists the reactions seen more often on the drug than on placebo: redness, pain, hardening, itching, bruising and hives at the injection site [1]. Its most common adverse reactions overall are injection site reactions [1], and the long Barth syndrome extension reported the same [5].

The label's warnings also cover allergy. Hypersensitivity reactions, including serious allergic reactions requiring emergency medical intervention, have been reported, among them skin rashes and respiratory symptoms such as cough [1]. They may occur within minutes to months after treatment starts [1], and serious hypersensitivity to any ingredient is the label's one contraindication [1].

The label also flags a laboratory finding. Eosinophil counts, a type of white blood cell involved in allergy, rose frequently in studies where the drug was given for 30 days or more [1], though the rise was not linked to symptoms or to changes in other lab tests [1]. And the label says not to use it in newborns, because it contains benzyl alcohol [1].

Across the larger trials the drug was generally well tolerated: in MMPOWER-3 most adverse events were mild to moderate [6], and in the eye study all adverse events were mild or moderate [9]; the two serious ones, a urinary tract infection and sepsis in the same participant, were judged unrelated to the drug [9]. That describes supervised use of the approved product in trials. It says nothing about products sold online as "SS-31" for research, which are not the approved product.