SS-31 (Elamipretide)

Overview

SS-31 (sequence: D-Arg-Dmt-Lys-Phe-NH2, where Dmt = 2',6'-dimethyltyrosine) is a tetrapeptide-a four-amino acid peptide-that represents a unique category of therapeutic peptide: a mitochondrial-targeting agent. SS-31 is also known by the trade name Elamipretide, the research designation MTP-131 (mitochondrial-targeting peptide 131), and the previous clinical name Bendavia.

SS-31 was originally developed by Stealth BioTherapeutics, a company founded specifically to develop mitochondrial-targeted peptides. In 2020, Stealth merged with Larimar Therapeutics, and the company now operates as Larimar Therapeutics (formerly Stealth BioTherapeutics), continuing the clinical development of SS-31. This represents one of the most significant clinical development programs for any investigational peptide.

As of April 2026, SS-31 is the most clinically advanced investigational peptide profiled on PeptideLibraryHub. Multiple Phase 3 clinical trials are underway or recently completed for Barth syndrome (a rare genetic mitochondrial disorder), and the FDA granted Fast Track designation to the Barth syndrome development program. This represents genuine clinical development with regulatory oversight, distinguishing SS-31 from the majority of investigational peptides discussed in research communities.

SS-31's unique property is its extraordinary capacity to target and concentrate in mitochondria-achieving concentrations 1,000-5,000 times higher in mitochondria than in blood. This targeting property allows SS-31 to deliver its pharmacological effects directly at the site of cellular energy production, theoretically avoiding systemic adverse effects associated with broad-spectrum mechanisms.

FDA and Clinical Development Status

As of April 2026:

• NOT FDA-approved for any indication
• Phase 3 Clinical Trials Active: TAZPOWER (Barth syndrome), MMPOWER (primary mitochondrial myopathy)
• FDA Fast Track Designation: Barth syndrome indication (expedited review pathway)
• IND Application: Active investigational new drug application with FDA
• Developed by Larimar Therapeutics (formerly Stealth BioTherapeutics)

Clinical Trial History

SS-31 has an extensive clinical trial history spanning multiple years and disease indications. Completed trials include Phase 2 studies in acute heart failure, Phase 2 studies in macular degeneration, and Phase 2 studies in primary mitochondrial myopathy. Currently active Phase 3 trials focus on Barth syndrome (TAZPOWER trial) and primary mitochondrial myopathy (MMPOWER trial). Data from the Phase 3 TAZPOWER trial (in Barth syndrome) have been presented at medical conferences, with reports indicating improvements in the six-minute walk test and other functional measures compared to placebo.

Regulatory Pathway

SS-31 is being developed through the standard FDA regulatory pathway for new drugs: preclinical testing, IND application (approval to begin human trials), Phase 1/2 trials, Phase 3 trials, and ultimately New Drug Application (NDA) submission for regulatory approval. The company (Larimar) has regular interactions with the FDA and follows established protocols for clinical development. This represents a fundamental difference from most investigational peptides, which lack formal development programs.

Mechanism of Action: Mitochondrial-Targeting and Cardiolipin Binding

SS-31's mechanism of action is distinct among peptide therapeutics and involves targeting subcellular organelles (mitochondria) with remarkable specificity. Unlike most drugs that circulate throughout the body and interact with cells broadly, SS-31 concentrates in mitochondria-the "power plants" of cells-and binds to a specific lipid within the inner mitochondrial membrane.

Mitochondrial Targeting and Concentration

SS-31 has the extraordinary property of concentrating in mitochondria at 1,000-5,000 fold higher concentrations than in blood plasma. This remarkable targeting occurs through the peptide's positive charge and specific structural features that allow it to cross cellular membranes and accumulate in mitochondria. The mechanism of this targeting is not fully understood but likely involves the peptide's cationic charge and its specific interaction with the mitochondrial membrane.

Cardiolipin Binding

Once in mitochondria, SS-31 binds to cardiolipin, a unique lipid found almost exclusively in the inner mitochondrial membrane. Cardiolipin is essential for the assembly and function of the electron transport chain (ETC),the protein complexes that generate the chemical energy (ATP) that powers all cellular functions. In diseased or dysfunctional mitochondria, cardiolipin undergoes oxidative damage, and its distribution in the inner membrane becomes abnormal, disrupting electron transport chain assembly and function.

Electron Transport Chain Stabilization

SS-31 is proposed to stabilize cardiolipin and preserve the organization and function of electron transport chain complexes. By binding to cardiolipin and potentially reducing its oxidation, SS-31 theoretically preserves the assembly and function of the ETC. This would maintain ATP synthesis, reduce reactive oxygen species (ROS) production from dysfunctional electron transport, and preserve mitochondrial function in diseased cells.

Reduction of Mitochondrial ROS Production

Dysfunctional mitochondria produce excessive reactive oxygen species (ROS), free radicals that damage cellular components including proteins, lipids, and DNA. By stabilizing the electron transport chain, SS-31 reduces ROS production from damaged mitochondria. This reduction in oxidative stress is proposed to preserve mitochondrial and cellular function. Preclinical studies report that SS-31 treatment reduces mitochondrial ROS production in cells and tissues from patients with mitochondrial disease.

ATP Synthesis Enhancement

The ultimate goal of electron transport chain function is the synthesis of ATP, the universal energy currency of cells. By preserving electron transport chain organization and function, SS-31 is proposed to enhance ATP synthesis in cells with dysfunctional mitochondria. Preclinical studies and early clinical research report improvements in cellular ATP levels in tissues from patients treated with SS-31.

Clinical Trial Data and Study Results

TAZPOWER Trial (Barth Syndrome, Phase 3)

The TAZPOWER trial is a Phase 3 randomized controlled trial examining SS-31 in adolescent and adult patients with Barth syndrome, a rare genetic mitochondrial disorder caused by mutations in the TAZ gene (which encodes proteins involved in cardiolipin remodeling). Barth syndrome presents with progressive cardiac dysfunction, skeletal muscle weakness, and growth delay. Data from the TAZPOWER trial presented at medical conferences (as of April 2026) reported that patients treated with SS-31 40 mg subcutaneously daily showed improvements in the six-minute walk test (a measure of exercise capacity) compared to placebo, with statistical significance reported in some patient cohorts. Secondary endpoints examining cardiac function and biomarkers have been examined. This represents genuine clinical efficacy evidence in a rare disease population.

MMPOWER Trial (Primary Mitochondrial Myopathy, Phase 3)

The MMPOWER trial is a Phase 3 trial examining SS-31 in patients with genetically confirmed primary mitochondrial myopathy (conditions involving genetic mutations in mitochondrial genes or nuclear genes encoding mitochondrial proteins). These patients present with progressive muscle weakness, exercise intolerance, and systemic symptoms related to mitochondrial energy failure. The trial employs similar dosing and clinical endpoints as TAZPOWER, examining whether SS-31 improves exercise capacity and muscle function in these severe genetic diseases.

ReCLAIM Trial (Age-Related Macular Degeneration, Phase 2)

Earlier Phase 2 trials examined SS-31 in age-related macular degeneration (AMD), a progressive eye disease involving retinal degeneration. The retina is a highly metabolically active tissue with high mitochondrial density, making mitochondrial dysfunction a potential contributor to AMD pathology. Phase 2 ReCLAIM trial data examined whether SS-31 could slow retinal degeneration in AMD patients.

Heart Failure Trials (Phase 2)

Earlier Phase 2 trials examined SS-31 in patients with acute decompensated heart failure, hypothesizing that improving cardiac mitochondrial function could improve heart function and reduce the severity of heart failure. These trials generated preliminary data supporting continued clinical development of SS-31 in cardiac indications.

Research Applications Beyond Current Clinical Trials

While SS-31's clinical development focuses on Barth syndrome and primary mitochondrial myopathy, research has examined potential therapeutic applications in other conditions involving mitochondrial dysfunction:

Neurodegenerative Diseases

Mitochondrial dysfunction is implicated in Parkinson's disease, Alzheimer's disease, and other neurodegenerative conditions. Preclinical research has examined SS-31's effects in cell culture and animal models of neurodegeneration, with reports of neuroprotection and reduced neuronal cell death when exposed to toxins or conditions that induce neurodegeneration.

Ischemia-Reperfusion Injury

When tissues experience interrupted blood flow (ischemia) followed by restoration of blood flow (reperfusion), paradoxical damage occurs from ROS production and calcium overload. SS-31 has been examined preclinically in models of stroke, myocardial infarction, and kidney ischemia-reperfusion injury, with reports of reduced tissue damage when administered during reperfusion.

Diabetic Complications

Mitochondrial dysfunction is proposed to contribute to diabetic complications including diabetic neuropathy and diabetic kidney disease. Preclinical studies have examined SS-31 in models of diabetes-induced tissue damage, with reports of improved tissue preservation.

Sepsis and Critical Illness

Mitochondrial dysfunction occurs in sepsis and critical illness, contributing to multiorgan failure. Preclinical research has examined SS-31's effects in endotoxemia and sepsis models, with reports of improved organ function and survival in animal studies.

Clinical Trial Dosing and Administration

Phase 3 Trial Dosing (Barth Syndrome and Mitochondrial Myopathy)

In the currently active Phase 3 trials (TAZPOWER and MMPOWER), SS-31 is administered via subcutaneous injection at a dose of 40 mg daily. This represents the dose selected from earlier Phase 1/2 dose-ranging studies as having optimal efficacy-to-safety profile. The injection is subcutaneous (self-administered or administered by healthcare providers) once daily.

Administration and Infusion Considerations

SS-31 is supplied as a solution for injection. In clinical trials, administration is straightforward subcutaneous injection. The compound is generally well-tolerated via this route, with the primary adverse effects related to the injection itself (injection site reactions) rather than systemic toxicity.

Treatment Duration

Current Phase 3 trials examine SS-31 administration for 12-24 months, with ongoing efficacy and safety assessment. This represents the longest clinical trial duration examined for SS-31 to date, though ultimate long-term safety would only be established post-approval through ongoing surveillance and Phase 4 studies.

Safety Profile and Adverse Effects

Reported Adverse Effects in Clinical Trials

Across Phase 2 and Phase 3 trials, SS-31 has been reported to be generally well-tolerated. The adverse effects most commonly reported include:

• Injection site reactions: Local erythema, swelling, pain, and reactions at the injection site-common with any daily subcutaneous injection
• Headache: Reported in some trial participants, generally mild to moderate
• Nausea: Mild nausea reported in some patients, generally transient
• Fatigue: Reported in some patients, though this symptom may also be related to the underlying disease

No serious adverse events causally related to SS-31 have been reported across clinical trials to date. Serious adverse events that have occurred during trials (hospitalizations, disease progression) have been deemed unrelated to the study drug or related to the underlying disease process.

Long-Term Safety Considerations

SS-31 has been administered in clinical trials for up to 24 months in some studies. Long-term safety beyond this period has not been formally studied, as the drug remains investigational. Long-term concerns (development of tolerance, effects on organ function with chronic use, immunological effects) would need to be addressed post-approval through surveillance and Phase 4 studies.

Special Populations

Clinical trials in Barth syndrome have enrolled pediatric and adolescent patients (ages 6-17 in TAZPOWER) as well as adults, providing some safety data in pediatric populations. This is important for rare genetic diseases where the affected population includes children and adolescents.

Stacking Considerations

In research community discussions, SS-31 is sometimes described in theoretical "mitochondrial health" or "bioenergetic optimization" stacking protocols, combined with other mitochondrial-function-supporting peptides and compounds. The proposed rationale is that SS-31's mitochondrial membrane-protective mechanism could be complemented by agents supporting mitochondrial biogenesis, NAD+-dependent metabolism, or other aspects of cellular energy production. However, no published human studies have examined the safety or efficacy of SS-31 combined with other peptides or compounds.

Commonly Discussed Research Combinations

Reported protocols in research contexts sometimes describe SS-31 stacked with agents such as NAD+ precursors (NMN, NR), MOTS-C (mitochondrial-derived peptide), sirtuins activators, or other mitochondrial-support compounds. The theoretical basis is that simultaneous support of mitochondrial membrane integrity (SS-31), mitochondrial biogenesis (MOTS-C), and NAD+-dependent longevity pathways might produce additive benefits for cellular energy and aging. Other discussions involve combining SS-31 with neuroprotective peptides in the context of neurodegenerative research. These combinations remain entirely speculative, based on mechanistic reasoning about mitochondrial biology without human validation.

Safety Considerations in Combination

Combining multiple agents targeting mitochondrial function introduces theoretical risks: excessive mitochondrial stress responses, dysregulation of cellular energy metabolism, unknown interactions affecting ATP production, and potential off-target effects. Without human safety data for SS-31 in combination with other agents, responsible research practice emphasizes single-agent clinical studies before advancing to combination protocols.

Evidence Status: No published human studies have examined SS-31 combined with other peptides, metabolic agents, or mitochondrial-support compounds. All reported stacking discussions represent theoretical constructs without clinical evidence of safety or efficacy.

Frequently Asked Questions

Side Effects & Safety Profile

SS-31 (Elamipretide) has been evaluated in multiple clinical trials across Barth syndrome, heart failure, and mitochondrial myopathy. The side effect data below reflects findings from published clinical trial data, primarily from the TAZPOWER and ReCOVER-HF studies.