SS-31 (Elamipretide): Mitochondria-Targeted Tetrapeptide

Abstract

SS-31 (H-D-Arg-Dmt-Lys-Phe-NH₂, also known as elamipretide, Bendavia, MTP-131) is a water-soluble, cell-permeable tetrapeptide that selectively concentrates >1,000-fold in the inner mitochondrial membrane due to its alternating aromatic-cationic motif. It interacts electrostatically with cardiolipin, stabilizes cristae architecture, inhibits cytochrome c peroxidase activity, reduces mitochondrial ROS production, and optimizes electron transport chain function.

Introduction

Mitochondrial dysfunction is a convergent feature of ischemic injury, chronic heart failure, neurodegenerative diseases, and primary mitochondrial disorders. Strategies to pharmacologically protect mitochondria have historically been limited by poor inner-membrane penetration. SS-31 was rationally designed to exploit the negative membrane potential (Δψₘ ≈ -180 mV) of energized mitochondria.

Molecular Background

SS-31 is a tetrapeptide with D-amino acids at positions 1-2 for protease resistance. Key structural features include:

• 2,6-dimethyltyrosine (Dmt): Lipophilic aromatic component
• Arginine and lysine: Delocalized positive charges
• C-terminal amide: Enhanced stability

At physiological pH, SS-31 carries a net +3 charge and concentrates >1,000-fold in mitochondria within minutes. It binds cardiolipin via electrostatic and hydrophobic interactions.

Mechanisms of Action

AMPK Activation

SS-31 activates AMPK within 15-30 minutes in ischemic myocardium and skeletal muscle via restoration of ATP/AMP ratio and reduced ROS-mediated inhibition of LKB1.

SIRT1 Pathway

Indirect activation occurs through increased NAD⁺/NADH ratio and reduced oxidative inactivation. In Barth syndrome models, SS-31 restores SIRT1-mediated deacetylation of PGC-1α.

mTOR Regulation

In heart failure models, SS-31 suppresses pathological mTORC1 activation while preserving physiological anabolic signaling in skeletal muscle during regeneration.

NRF2 Induction

Strong induction of NRF2 nuclear translocation and HO-1 expression occurs in multiple models, mediated by reduced KEAP1 oxidation and enhanced AKT-GSK3β signaling.

Peptide-Specific Pathways

• Cardiolipin stabilization and cristae preservation
• Selective inhibition of cytochrome c peroxidase activity
• Optimization of ETC supercomplex assembly (I/III/IV)
• Prevention of mPTP opening
• Reduction of mitochondrial ROS at complex I and III

Preclinical Research

In Vitro Studies

• Isolated mitochondria: 1-10 nM SS-31 restores respiration, reduces H₂O₂ emission
• Cardiomyocytes under hypoxia-reoxygenation: ATP preserved, mPTP inhibited

Animal Models

• Myocardial infarction (mouse/rat): Infarct size ↓ 40-60%, EF ↑ 20-30%
• Aged mice (27 mo): Treadmill endurance ↑ 50%, mitochondrial complex IV activity restored
• SOD1-G93A ALS mice: Delayed disease onset, extended survival 15-20%
• mdx Duchenne mice: Diaphragm force ↑ 100%, fibrosis ↓ 50%

Human Research

Completed and ongoing trials as of December 2025:

• Phase II HFpEF (n=84): 4 mg IV daily × 7 days → improved diastolic function
• Phase II Barth syndrome: 6-minute walk test ↑ 95 m, cardiac stroke volume ↑
• Phase II/III mitochondrial myopathy: 40 mg SC daily → 6MWT ↑ 35 m (trend)
• Phase II dry AMD: Geographic atrophy progression slowed

Safety profile: >1,000 subjects exposed with mild injection-site reactions as most common adverse event.

Summary Table

Conclusion

SS-31/elamipretide is unique among mitochondrial therapeutics as the only molecule clinically advanced that selectively targets the inner mitochondrial membrane and stabilizes cardiolipin without inhibiting respiration. The consistency of cardioprotective and neuromuscular effects across species, models, and independent laboratories is notable.

Ongoing Phase III studies will determine its translational potential. Regardless of ultimate commercial outcome, SS-31 has established proof-of-principle that small, rationally designed peptides can safely accumulate in mitochondria and exert meaningful biological effects.