SS-31

What is SS-31?#

SS-31 (also known as Elamipretide, Bendavia, and MTP-131) is a synthetic mitochondria-targeted tetrapeptide consisting of four amino acids arranged in an alternating aromatic-cationic motif: D-Arg-Dmt-Lys-Phe-NH2, where Dmt represents 2',6'-dimethyltyrosine. It is a member of the Szeto-Schiller (SS) peptide family, named after its co-discoverers Hazel H. Szeto and Peter W. Schiller, who developed a series of cell-permeable peptide antioxidants in the early 2000s.

The defining characteristic of SS-31 is its ability to selectively concentrate more than 1,000-fold within the inner mitochondrial membrane (IMM), driven by electrostatic interaction between its positively charged residues and cardiolipin, a signature phospholipid of the IMM. This selective targeting distinguishes SS-31 from other antioxidants and mitochondrial agents, providing direct access to the site of oxidative phosphorylation and electron transport.

SS-31 is being developed by Stealth BioTherapeutics (now Larimar Therapeutics following acquisition of the elamipretide program) and has progressed through multiple clinical trials. It has received Orphan Drug Designation and Fast Track Designation from the FDA for the treatment of Barth syndrome, a rare X-linked genetic disorder characterized by cardiomyopathy, skeletal myopathy, and neutropenia caused by mutations in the tafazzin gene that lead to defective cardiolipin remodeling.

Mechanism of Action#

Szeto-Schiller Peptide Design#

The SS peptide family was designed based on a structural motif that enables both cell penetration and mitochondrial targeting without requiring the mitochondrial membrane potential that is necessary for other mitochondria-targeted compounds such as triphenylphosphonium (TPP) conjugates. The alternating aromatic-cationic motif (aromatic-cationic-aromatic-cationic) allows SS-31 to freely cross cell membranes and concentrate in mitochondria even in cells with depolarized mitochondria, a critical advantage in pathological conditions where mitochondrial membrane potential is compromised.

The incorporation of D-arginine at the N-terminus provides resistance to aminopeptidase degradation, while the 2',6'-dimethyltyrosine residue contributes both to the aromatic character necessary for membrane interaction and to the intrinsic radical scavenging activity of the molecule. The C-terminal amidation eliminates the negative charge that would otherwise impede membrane penetration.

Cardiolipin Binding and Stabilization#

The primary molecular target of SS-31 is cardiolipin, a unique diphosphatidylglycerol lipid found almost exclusively in the inner mitochondrial membrane. Cardiolipin plays essential structural and functional roles in mitochondrial bioenergetics, including proper assembly and function of electron transport chain (ETC) complexes, formation of ETC supercomplexes, and activity of ATP synthase. Cardiolipin also plays a role in mitochondrial dynamics, cristae formation, and the regulation of apoptosis through its interaction with cytochrome c.

SS-31 binds to cardiolipin through electrostatic interactions between its cationic residues and the phosphate headgroups of cardiolipin, along with hydrophobic interactions between its aromatic residues and the acyl chains. This binding interaction stabilizes cardiolipin within the IMM, protecting it from oxidative damage by reactive oxygen species (ROS) generated during electron transport.

By preserving cardiolipin integrity, SS-31 maintains the structural organization of ETC complexes and supercomplexes, thereby optimizing electron transfer efficiency and reducing electron leak that would otherwise generate additional ROS. This creates a virtuous cycle where reduced ROS production further protects cardiolipin from oxidative damage.

Electron Transport Chain Optimization#

Through its stabilization of cardiolipin, SS-31 has been demonstrated to improve the efficiency of mitochondrial oxidative phosphorylation. In preclinical studies, SS-31 treatment has been shown to restore the formation of ETC supercomplexes (sometimes called respirasomes), which are higher-order assemblies of Complexes I, III, and IV that facilitate more efficient electron channeling.

Disruption of these supercomplexes is a common feature of mitochondrial dysfunction in aging, heart failure, and genetic mitochondrial diseases. By restoring supercomplex formation, SS-31 reduces the generation of ROS at Complexes I and III, improves the coupling of electron transport to ATP synthesis, and increases the maximum capacity for oxidative phosphorylation.

This improvement in mitochondrial bioenergetics is considered the primary mechanism underlying the therapeutic benefits observed in preclinical and clinical studies, rather than direct antioxidant scavenging activity that was originally hypothesized for the SS peptide series.

Interaction with Cytochrome c#

SS-31 has also been shown to interact directly with cytochrome c, modifying its function as an electron carrier. Specifically, SS-31 alters the interaction between cytochrome c and cardiolipin in a manner that preserves the electron carrier function of cytochrome c while inhibiting its peroxidase activity.

The peroxidase activity of the cytochrome c-cardiolipin complex is responsible for cardiolipin oxidation during mitochondrial stress, which in turn triggers the release of cytochrome c from the IMM and the initiation of the intrinsic apoptosis pathway. By inhibiting this peroxidase activity, SS-31 may provide cytoprotection against apoptotic cell death in addition to its bioenergetic benefits.

Clinical Evidence#

TAZPOWER Trial for Barth Syndrome#

The TAZPOWER trial was a pivotal Phase 3, randomized, double-blind, placebo-controlled crossover study evaluating elamipretide in patients with Barth syndrome. Barth syndrome is caused by mutations in the tafazzin gene, which encodes the enzyme responsible for cardiolipin remodeling. As a result, patients with Barth syndrome have defective cardiolipin composition, making this condition a particularly rational target for a cardiolipin-stabilizing agent.

The trial enrolled 12 patients with genetically confirmed Barth syndrome and evaluated subcutaneous elamipretide (40 mg/day) in two 12-week treatment periods separated by a washout period. The primary endpoint was the six-minute walk test (6MWT) distance.

While the trial demonstrated trends toward improvement in 6MWT distance and reported statistically significant improvements in certain secondary endpoints including the five-times sit-to-stand test, the overall results were mixed due to the small sample size inherent to ultra-rare disease trials.

Stealth BioTherapeutics submitted a New Drug Application (NDA) to the FDA based on data from the TAZPOWER trial and open-label extension studies, but the FDA issued a Complete Response Letter requesting additional data. Subsequent analyses and longer-term follow-up from open-label extension studies have provided additional evidence of clinical benefit, and the development program has continued under Larimar Therapeutics.

Heart Failure Studies#

SS-31 has been evaluated in multiple clinical trials for heart failure. The EMBRACE STEMI trial was a Phase 2a study in patients with ST-elevation myocardial infarction (STEMI) undergoing percutaneous coronary intervention. The trial evaluated intravenous elamipretide administered as a single dose at the time of reperfusion.

While the primary endpoint of reduction in creatine kinase-MB (CK-MB) release was not met, the trial demonstrated trends toward reduced infarct size as measured by cardiac MRI. These imaging results suggested a potential cardioprotective effect that warranted further investigation.

The Phase 2 trial in heart failure with reduced ejection fraction (HFrEF) evaluated 4 weeks of subcutaneous elamipretide and demonstrated improvements in left ventricular end-systolic volume and trends toward improved cardiac output. These findings supported the concept that mitochondrial dysfunction contributes to heart failure pathophysiology and that targeting cardiolipin can improve cardiac function.

Primary Mitochondrial Myopathy#

A Phase 2 trial evaluated elamipretide in patients with primary mitochondrial myopathy due to nuclear or mitochondrial DNA mutations. The trial assessed changes in the 6MWT distance over 5 days of intravenous elamipretide. While numerical improvements in walking distance were observed, the short treatment duration and small sample size limited the conclusiveness of the results.

Therapeutic Applications#

The most advanced clinical application of SS-31 is in Barth syndrome, where the direct mechanistic rationale (correcting the consequences of defective cardiolipin remodeling) provides a strong biological basis for therapeutic intervention. Subcutaneous injection is the primary route of administration for chronic treatment, with intravenous administration used in acute care settings such as myocardial infarction.

Beyond Barth syndrome, SS-31 is being investigated for a range of conditions characterized by mitochondrial dysfunction, including heart failure, primary mitochondrial myopathies, age-related macular degeneration (in collaboration with Allergan/AbbVie), and age-related decline in skeletal muscle function. The breadth of potential applications reflects the central role of mitochondrial dysfunction in numerous disease states.

In preclinical models of aging, SS-31 has demonstrated the ability to reverse age-related mitochondrial dysfunction in the heart, skeletal muscle, kidneys, and brain, restoring function to levels more consistent with younger tissues. These findings have generated interest in SS-31 as a potential geroprotective agent, though clinical development in aging indications remains at an early stage.

Evidence Gaps and Limitations#

Despite a robust preclinical profile and a substantial clinical development program, several significant evidence gaps remain for SS-31. The pivotal TAZPOWER trial in Barth syndrome, while demonstrating trends toward benefit and significant improvements on some endpoints, did not meet its primary endpoint with conventional statistical significance, in part due to the very small sample size imposed by the rarity of the disease. The FDA's Complete Response Letter underscored the regulatory challenges of demonstrating efficacy in ultra-rare diseases.

In heart failure, while early-phase trials have shown promising signals, no Phase 3 trial has yet demonstrated a definitive clinical benefit. The EMBRACE STEMI trial's failure to meet its primary CK-MB endpoint, despite trends on imaging endpoints, illustrates the challenge of translating mechanistic improvements in mitochondrial function into measurable clinical outcomes in acute settings.

The long-term safety profile of chronic SS-31 administration is still being characterized. While the open-label extension studies in Barth syndrome have provided encouraging safety data over months to years of treatment, the consequences of sustained cardiolipin stabilization and modified cytochrome c function over decades of use are unknown.

An important conceptual limitation is that SS-31 addresses the consequences of mitochondrial dysfunction (cardiolipin damage, ROS generation, ETC inefficiency) rather than the underlying causes. In genetic mitochondrial diseases, the causative mutations remain present, and SS-31 would likely need to be administered chronically. Whether the benefits of SS-31 are sustained with long-term use or whether compensatory mechanisms could attenuate its effects over time is an open question.

The transition of the elamipretide program from Stealth BioTherapeutics to Larimar Therapeutics has introduced uncertainties regarding the future development timeline and strategy. The regulatory path forward for the Barth syndrome indication, following the FDA's Complete Response Letter, will likely require additional clinical data or analyses, potentially delaying patient access.

Finally, while preclinical data supporting applications in aging and age-related disease are compelling, clinical translation in these areas is inherently challenging due to the long timelines required to demonstrate benefits on aging-related endpoints and the current lack of regulatory frameworks specifically designed for geroprotective interventions.

Key Research Findings#

TAZPOWER: A Randomized, Double-Blind, Placebo-Controlled, Crossover Trial of Elamipretide in Barth Syndrome, published in Cardiology in the Young (Thompson WR et al., 2021):

Pivotal Phase 3 crossover trial evaluating 40 mg daily SC elamipretide in 12 patients with genetically confirmed Barth syndrome. Demonstrated trends toward improved 6MWT distance and significant improvements in the five-times sit-to-stand test, though the primary endpoint did not achieve conventional statistical significance due to the very small sample size inherent to ultra-rare disease trials.

• Trends toward improved six-minute walk test distance
• Statistically significant improvements in five-times sit-to-stand test
• Generally well tolerated with injection site reactions as most common AE

EMBRACE STEMI: Evaluating Myocardial Effects of Bendavia for Reducing Reperfusion Injury in Patients with Acute Coronary Events, published in American Heart Journal (Gibson CM et al., 2016):

Phase 2a randomized, double-blind, placebo-controlled trial evaluating single IV dose of elamipretide (0.25 mg/kg) in first-time STEMI patients undergoing PCI. Primary endpoint of CK-MB AUC reduction was not met, but cardiac MRI showed trends toward reduced infarct size.

• Primary CK-MB endpoint not met
• Trends toward reduced infarct size on cardiac MRI
• Well tolerated in acute cardiac care setting

Phase 2 Trial of Elamipretide in Heart Failure with Reduced Ejection Fraction, published in Circulation Heart Failure (Daubert MA et al., 2017):

Phase 2 randomized trial evaluating 4 weeks of daily SC elamipretide (40 mg) in patients with HFrEF. Demonstrated improvements in left ventricular end-systolic volume and trends toward improved cardiac output, supporting the role of mitochondrial dysfunction in HF pathophysiology.

• Improvement in left ventricular end-systolic volume
• Trends toward improved cardiac output
• Supported mitochondrial targeting as a therapeutic strategy in heart failure

Phase 2 Trial of Elamipretide in Primary Mitochondrial Myopathy, published in Neurology (Karaa A et al., 2018):

Phase 2 trial evaluating 5 days of IV elamipretide (0.25 mg/kg daily) in patients with genetically confirmed primary mitochondrial myopathy. Numerical improvements in 6MWT distance were observed but were not conclusive due to short treatment duration and small sample size.

• Numerical improvements in six-minute walk test distance
• Demonstrated feasibility of mitochondrial targeting in genetically heterogeneous PMM population
• Well tolerated over 5-day IV treatment

Related Reading#

• SS-31 research studies and evidence
• SS-31 dosing protocols
• SS-31 side effects profile
• Cerebrolysin research guide