SS-31

This article is for informational and educational purposes only and does not constitute medical advice. SS-31 (elamipretide) is supplied by Wholesale Peps as lyophilized research-grade material for in vitro laboratory use only and is not approved by the FDA for human or veterinary use.

Wholesale Peps is not affiliated with, endorsed by, or in any way connected to Stealth BioTherapeutics. This research review is compiled from publicly available peer-reviewed literature for educational purposes only.

1. Background

1.1 The Szeto-Schiller Peptide Class

The SS peptide class was developed at Weill Cornell Medical College through work by Hazel Saris (Szeto) and Peter Schiller, from whose names the “SS” designation derives. The original research program focused on the observation that peptides with an alternating aromatic-cationic pharmacophore pattern showed preferential accumulation in mitochondria, independent of the mitochondrial membrane potential. This was unexpected because most mitochondria-targeted compounds rely on the large negative membrane potential (−180 mV) of the inner mitochondrial membrane for electrophoretic driving of lipophilic cations, a mechanism first exploited by triphenylphosphonium-conjugated compounds [1].

SS-31 is the most clinically developed member of the class, numbered in the series of synthesized peptides. Its INN (International Nonproprietary Name) is elamipretide, and it was also designated MTP-131 in the clinical development program conducted by Stealth Biotherapeutics (later restructured as Relianta Therapeutics). The commercial development program covered multiple rare and common disease indications where mitochondrial dysfunction is proposed to contribute to pathology.

1.2 Cardiolipin as the Target

The eventual identification of cardiolipin as the molecular target of SS-31 provided a mechanistic framework for understanding its mitochondrial selectivity and biological effects. Cardiolipin is a unique dimeric phospholipid found almost exclusively in the inner mitochondrial membrane (IMM), where it constitutes approximately 15–20% of the IMM lipid content. Its unusual four-acyl chain structure and head group chemistry make it structurally distinct from other membrane phospholipids and enable it to fulfill specific structural functions unavailable to other lipids [2].

Cardiolipin serves three major structural roles in the IMM: it stabilizes the cristae membrane ultrastructure, facilitates the assembly and stability of ETC supercomplexes (particularly the I+III&sub2;+IV “respirasome”), and acts as a tethering site for cytochrome c in its electron carrier role between complex III and complex IV. Under conditions of mitochondrial stress or injury, cardiolipin can undergo oxidative peroxidation, which disrupts these functions: cristae become disorganized, ETC supercomplex assembly is impaired, and cytochrome c is released — initiating apoptotic signaling. Szeto and colleagues proposed that SS-31 binding to cardiolipin protects these structural and functional roles under stress conditions [1,2].

2. Molecular Structure

The pharmacophore of SS-31 is defined by the alternating arrangement of cationic and aromatic residues: D-Arg (cationic), 2′,6′-Dmt (aromatic), Lys (cationic), Phe (aromatic). This pattern — (+, Ar, +, Ar) — was identified as the structural requirement for mitochondrial membrane accumulation and cardiolipin binding. The dimethyl groups on Dmt’s ring prevent hydrogen bonding at the ortho positions, constraining the hydroxyl group and enhancing membrane interaction.

Two structural features confer enhanced proteolytic stability relative to a standard all-L-amino acid linear tetrapeptide. The D-arginine at position 1 creates a D-L scissile bond that is poorly recognized by most endopeptidases and aminopeptidases, which are stereoselective for L-amino acid substrates. The C-terminal amide eliminates the free carboxyl group recognized by carboxypeptidases. Together, these modifications extend the peptide’s half-life in biological fluids compared with unmodified analogues.

3. Proposed Mechanisms of Action

4. Key Research Findings

4.1 Cardiolipin Binding — Mechanistic Characterization

Birk et al. (2013) published a mechanistically detailed study demonstrating that SS-31 directly interacts with cardiolipin in mitochondrial membranes, using a combination of liposome binding assays, electron microscopy of mitochondrial morphology, and measurements of respiratory chain function in isolated ischemic mitochondria [2]. The study reported that SS-31 treatment of ischemic mitochondria restored cristae morphology, improved complex I and IV activity, increased ATP production, and reduced hydrogen peroxide generation. These findings provided the first direct mechanistic evidence linking cardiolipin binding to the full spectrum of SS-31’s reported effects on mitochondrial function.

Szeto (2014) reviewed the broader SS peptide pharmacology and proposed a unified mechanistic model in which cardiolipin binding acts as the primary event upstream of all downstream effects: by protecting cardiolipin from peroxidation, SS-31 preserves ETC supercomplex integrity, prevents cytochrome c peroxidase acquisition, and maintains the bioenergetic capacity of stressed mitochondria [1]. This model has been referenced by multiple independent groups as a framework for interpreting their own preclinical findings.

4.2 Preclinical Disease Models

SS-31 has been studied across a wider range of preclinical disease models than most research peptides, with findings reported by multiple independent research groups:

• Ischemia-reperfusion injury: Reported reductions in infarct size, improved cardiac function, and reduced markers of oxidative damage in rodent myocardial infarction models. These findings motivated the human EMBRACE STEMI reperfusion trial [3].
• Heart failure: In pressure overload and volume overload heart failure models, SS-31 treatment has been associated with preserved mitochondrial cristae architecture, improved ATP production, and attenuation of cardiac remodeling.
• Acute kidney injury: Multiple independent groups reported protective effects against ischemia-reperfusion and cisplatin-induced kidney injury in rodent models, with reduced tubular apoptosis and preserved renal function.
• Aging: Szeto and colleagues reported that SS-31 treatment in aged mice improved mitochondrial morphology, reduced ROS, and partially restored physical performance in aging skeletal muscle.
• Primary mitochondrial myopathy models: Preclinical models of ETC complex deficiency showed improved muscle fiber survival and exercise capacity with SS-31 treatment, providing the biological rationale for the clinical MMPOWER program.

4.3 Phase 2 Clinical Trials

Multiple phase 1 and phase 2 clinical trials were conducted by Stealth Biotherapeutics across different indications. Gibson et al. (2016) published results of the EMBRACE STEMI study, a phase 2a trial evaluating intravenous elamipretide administered at the time of primary percutaneous coronary intervention for ST-elevation myocardial infarction [3]. The trial did not demonstrate a statistically significant reduction in infarct size on the primary endpoint (area under the creatinine kinase-MB curve), though some pre-specified subgroup analyses suggested signals in patients with anterior STEMI. Safety and tolerability were acceptable.

Karaa et al. (2018) published a phase 2 randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy (PMM), demonstrating acceptable safety and tolerability at multiple dose levels and reporting improvements in some patient-reported outcome measures and mitochondrial function biomarkers [4]. These phase 2 findings motivated the pivotal phase 3 MMPOWER-3 trial.

4.4 MMPOWER-3 Phase 3 Trial — A Null Result in Primary Mitochondrial Myopathy

The MMPOWER-3 trial was a randomized, double-blind, placebo-controlled phase 3 study of subcutaneous elamipretide in adults with genetically confirmed primary mitochondrial myopathy. The trial enrolled participants across multiple international sites and used change in 6-minute walk distance (6MWD) as its primary efficacy endpoint — a validated measure of functional exercise capacity used in neuromuscular disease trials.

Stealth Biotherapeutics announced in 2020 that the MMPOWER-3 trial did not achieve statistical significance on the primary endpoint. The company noted that some pre-specified secondary endpoints — including patient-reported outcomes measuring fatigue and physical function — showed nominally significant improvements, though these secondary findings should be interpreted with caution given the primary endpoint failure. Following the MMPOWER-3 result, Stealth Biotherapeutics entered restructuring proceedings in 2022, and the clinical development program was substantially curtailed.

5. Evidence Status

What We Still Don’t Know

• Whether the failed MMPOWER-3 primary endpoint reflects true inefficacy or trial design: The 6-minute walk distance endpoint may not have been sufficiently sensitive to detect meaningful benefits in a heterogeneous PMM population. Some secondary patient-reported outcomes were nominally positive. Whether a different endpoint selection, patient enrichment strategy, or dose/duration would yield a different result has not been tested.
• Whether the cardiolipin mechanism translates to human mitochondrial disease: The mechanistic evidence from isolated mitochondria and animal models is compelling, but the relationship between cardiolipin binding in isolated systems and functional outcomes in patients with genetically heterogeneous mitochondrial disease is not established.
• Optimal disease indication: SS-31 has been studied across ischemia-reperfusion, heart failure, mitochondrial myopathy, Barth syndrome, and Friedreich’s ataxia. Which indication, if any, is most amenable to clinical benefit from this mechanism is unknown after MMPOWER-3.
• Pharmacodynamic target engagement biomarker: No validated blood or tissue biomarker of SS-31 target engagement (cardiolipin protection, ETC efficiency improvement) has been established for use in clinical trials. The absence of a pharmacodynamic marker makes dose selection and patient selection in trials difficult.
• HFpEF and other ongoing indications: The heart failure with preserved ejection fraction program (IMPROVE-HF) and Barth syndrome program generated some positive signals. The current status of these programs following the Stealth Biotherapeutics restructuring is uncertain, and the translational question remains open.
• Long-term safety with chronic subcutaneous dosing: Published phase 2 safety data covers treatment periods of weeks to months. Long-term safety data with chronic dosing schedules required for diseases like heart failure or mitochondrial myopathy has not been published.

6. Limitations of Current Research

References

1. Szeto HH. “First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics.” British Journal of Pharmacology. 2014;171(8):2029–2050. doi:10.1111/bph.12461
2. Birk AV, Liu S, Soong Y, Mills W, Singh P, Warren JD, Bhardwaj S, Bhardwaj G, Bhardwaj A, Szeto HH. “The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin.” Journal of the American Society of Nephrology. 2013;24(8):1250–1261. doi:10.1681/ASN.2012121216
3. Gibson CM, Giugliano RP, Kloner RA, et al. “EMBRACE STEMI Study: a Phase 2a Trial to Evaluate the Safety, Tolerability, and Efficacy of Intravenous MTP-131 on Reperfusion Injury in Patients Undergoing Primary Percutaneous Coronary Intervention.” European Heart Journal. 2016;37(16):1296–1303. doi:10.1093/eurheartj/ehv597
4. Karaa A, Haas R, Goldstein A, Vockley J, Weaver WD, Cohen BH. “Randomized dose-escalation trial of elamipretide in adults with primary mitochondrial myopathy.” Neurology. 2018;90(14):e1212–e1221. doi:10.1212/WNL.0000000000005255
5. Szeto HH, Birk AV. “Serendipitous discovery of Szeto-Schiller (SS) peptides: mitochondria-targeted peptides for metabolic diseases.” Pharmaceutical Research. 2014;31(10):2669–2679. doi:10.1007/s11095-014-1352-x