SS-31 / Elamipretide

SS-31 peptide, in one line: SS-31 (SS31, elamipretide, brand Forzinity) is a 4-amino-acid peptide that binds cardiolipin in the inner mitochondrial membrane, and the FDA granted it accelerated approval for Barth syndrome on September 19, 2025, at 40 mg subcutaneously once daily (FDA).

SS-31 is a mitochondria-targeted peptide, a compound engineered to act inside the cell's energy-producing organelles rather than at the cell surface. It belongs to a family of aromatic-cationic peptides, sometimes called Szeto-Schiller peptides after the researchers who developed them, that share the ability to accumulate within mitochondria. SS-31 is a tetrapeptide, meaning it is built from just four amino acids, and its compact sequence D-Arg-Dmt-Lys-Phe gives it both stability and its unique targeting behavior.

SS-31's targeting is not achieved by attaching it to a bulky delivery molecule, which is how many mitochondria-directed compounds work. Instead, its intrinsic chemistry, an alternating pattern of aromatic and positively charged residues, allows it to be drawn into and concentrated within the inner mitochondrial membrane. Once there, it associates with cardiolipin, a specialized phospholipid found almost exclusively in that membrane and critical to the operation of the energy-generating machinery.

SS-31, developed under the name elamipretide, has been tested in people across a range of conditions united by mitochondrial dysfunction, and is now sold by prescription as Forzinity for Barth syndrome. The underlying concept is a shift in strategy: rather than trying to neutralize oxidative damage after it has spread through the cell, SS-31 acts at the source, protecting the mitochondrial membrane where much of that damage originates. This upstream approach is what makes the peptide scientifically notable within the growing field of mitochondrial medicine.

SS-31 works by binding to cardiolipin in the inner mitochondrial membrane and stabilizing the structure and function of the energy-producing machinery. This single point of action, at the membrane where cellular energy is generated, produces a cascade of protective effects on mitochondrial performance and cell survival.

Stabilizing Cardiolipin

SS-31 concentrates in the inner mitochondrial membrane and associates with cardiolipin, the phospholipid that helps organize this membrane and its embedded electron transport chain. In healthy mitochondria, cardiolipin supports the tight, folded architecture and the proper arrangement of the protein complexes that produce energy. Under stress, cardiolipin becomes oxidized and disordered, and the machinery it supports begins to fail. By binding and stabilizing cardiolipin, SS-31 helps maintain this critical structure.

Improving Efficiency and Cutting Damage

SS-31 improves the efficiency of the electron transport chain, the series of protein complexes that pass electrons along to generate the gradient used to make ATP. When cardiolipin is disorganized, electrons leak from this chain and react with oxygen to form damaging reactive oxygen species. By keeping the chain properly organized, SS-31 both raises energy output and reduces electron leak, which lowers reactive oxygen species production at their point of origin rather than mopping them up afterward.

Protecting Against Cell Death

SS-31 also helps guard against the mitochondrial events that drive programmed cell death. When the inner membrane is damaged, mitochondria can release factors that trigger apoptosis, contributing to tissue loss in conditions like heart attack and neurodegeneration. By preserving cardiolipin and membrane integrity, SS-31 supports the mitochondria's stability and reduces these death signals, an effect that underlies its study in acute injury and chronic disease alike.

SS-31, as elamipretide, has been tested in human trials and in animal work targeting conditions marked by mitochondrial dysfunction. One program, Barth syndrome, ended in FDA accelerated approval in September 2025; most others missed their main goals.

Barth Syndrome (the approved use)

In TAZPOWER, 12 patients with Barth syndrome took 40 mg/day or placebo for 12 weeks each in a crossover. Neither primary endpoint was met (six-minute walk -0.8 m vs placebo, p = 0.97). In the open-label extension, the 8 patients who reached week 36 walked 95.9 m further than at baseline and gained 56 N of knee extensor strength, without a placebo group for comparison (Thompson et al., Genet Med 2021). The FDA based its accelerated approval on the knee-strength improvement (FDA, 2025).

Primary Mitochondrial Disease

SS-31 has been studied in people with primary mitochondrial myopathies, inherited disorders in which defective mitochondria cause muscle weakness and exercise intolerance. Clinical trials examined whether the peptide could improve measures such as exercise capacity. The largest, MMPOWER-3, randomized 218 adults to 40 mg/day or placebo for 24 weeks and missed both primary endpoints: six-minute walk -3.2 m versus placebo (p = 0.69) and no fatigue benefit (Karaa et al., Neurology 2023). That result illustrates the difficulty of treating deeply rooted genetic mitochondrial defects.

Cardiovascular and Ischemia Research

SS-31 has been investigated in heart failure and in ischemia-reperfusion injury, the damage that occurs when blood flow returns to tissue after a period of deprivation. Because the heart is exceptionally dependent on mitochondrial energy, and because reperfusion generates a burst of reactive oxygen species, a mitochondria-protecting peptide is mechanistically attractive here. Human results have been negative so far. In EMBRACE STEMI, a one-hour infusion during stenting for a first anterior heart attack did not reduce infarct size (Gibson et al., Eur Heart J 2016). In PROGRESS-HF, 71 people with reduced-ejection-fraction heart failure took placebo, 4 mg or 40 mg daily for 28 days, with no change in left ventricular end-systolic volume (Butler et al., J Card Fail 2020).

Aging and Other Applications

SS-31 has also been explored in the context of aging and in conditions such as certain retinal diseases and kidney injury, all of which involve mitochondrial stress. Because mitochondrial decline is a recurring theme in aging tissues, research has asked whether protecting the inner membrane can preserve function in aged cells. This work has no approved use, and the overall body of SS-31 evidence is best summarized as compelling in mechanism, with one narrow approval and several negative larger trials.

The only FDA-approved SS-31 dose is 40 mg injected under the skin once daily, for Barth syndrome patients weighing at least 30 kg (Forzinity label). The table below lists that dose and the doses used in named human trials; it is context, not guidance for personal use.

No published human trial has used the 5 to 10 mg daily doses common on vendor sites. For syringe units and vial math, see the SS-31 dosage chart.

Formulation and Route

The prescription product, Forzinity, comes as a ready-mixed solution (280 mg in 3.5 mL, 80 mg/mL) that is discarded 8 days after the vial is first opened, per the label. Vendor SS-31 comes as a lyophilized (freeze-dried) powder that must be reconstituted before use. In clinical trials elamipretide was given by subcutaneous injection. Although its measured plasma half-life is relatively short, at roughly one to two hours, the peptide concentrates strongly within mitochondria, so its effect at the site of action can outlast its presence in the bloodstream.

Reconstitution

Administration in Research

SS-31, as elamipretide, has an FDA label, which gives it far more human safety data than most peptides sold online. Its most common reported effects relate to the injection itself: in the placebo-controlled Barth crossover on the Forzinity label, all 12 patients on drug had injection-site redness (vs 25% on placebo), 75% had pain (vs 42%) and 67% had hardening or itching (vs 17%) (Forzinity label).

Common Side Effects

SS-31 in clinical trials has most frequently been associated with reactions at the subcutaneous injection site.

Considerations

SS-31 safety considerations reflect its narrow approval and its mechanism:

• Injection site tolerability: Local reactions are the most frequent issue and are managed with site rotation and technique in trials
• Approval status: Elamipretide is approved only for Barth syndrome (patients 30 kg and over); long-term safety for any other use has not been studied
• Mixed efficacy context: Because some trials did not meet primary endpoints, benefit should not be assumed, and use remains within research
• Product quality: Research-grade material used outside trials lacks the quality assurance of a clinical program, adding risk
• Individual variation: Responses vary, and clinical monitoring is part of any legitimate study protocol

Overall Safety Picture

SS-31 has a comparatively informative human safety record for a research peptide, owing to its formal clinical development, with injection site reactions being the predominant finding. Outside Barth syndrome its considerations are those of any unapproved use: unestablished long-term safety, unproven benefit, and the importance of product quality and medical supervision.