SS 31

So What Does This Actually Mean?

Plain English summary — no PhD required

SS-31 (Elamipretide) is a 4-amino-acid peptide that targets cardiolipin in the inner mitochondrial membrane. Phase II trials showed encouraging signals, but the pivotal Phase 3 MMPOWER-HF trial (n=360, 2023) did not meet its primary endpoint. Efficacy is not demonstrated in humans. Anti-aging and longevity effects are preclinical only.

What It Does

SS-31 concentrates 1,000-fold in the inner mitochondrial membrane, stabilizing cardiolipin and the electron transport chain complexes that produce ATP. This reduces ROS production, improves energy output, and protects against mitochondrial dysfunction.

Why It Matters

Mitochondrial dysfunction is a hallmark of aging and a driver of heart failure and neurodegeneration. SS-31 concentrates at the exact site of ROS generation — a precision that conventional antioxidants lack.

The Bottom Line

SS-31 is a mitochondria-targeted tetrapeptide with a well-characterized mechanism and the most advanced clinical program of any mitochondria-targeted peptide. However, the Phase 3 MMPOWER-HF trial failed its primary endpoint. Human efficacy is not demonstrated. Anti-aging effects are preclinical only.

Overview

What is SS-31?

SS-31 (also known as Elamipretide or MTP-131) is a tetrapeptide that selectively concentrates in the inner mitochondrial membrane, where it binds cardiolipin — a phospholipid essential for electron transport chain function. The proposed mechanism involves stabilizing cardiolipin and restoring cristae architecture. Phase II trials in heart failure showed encouraging signals, but the pivotal Phase 3 MMPOWER-HF trial (n=360, 2023) did not meet its primary endpoint. Efficacy is not demonstrated in humans. Anti-aging and longevity effects are preclinical only.

Key Takeaways

• SS-31 (Szeto-Schiller peptide 31, also known as Elamipretide) is a tetrapeptide (D-Arg-Dmt-Lys-Phe-NH2) developed by Hazel Szeto at Weill Cornell Medicine, designed to selectively target the inner mitochondrial membrane.

• Mechanism: the alternating aromatic-cationic structure enables SS-31 to selectively concentrate in the inner mitochondrial membrane (IMM) where it binds cardiolipin — a phospholipid essential for electron transport chain (ETC) organization and cristae structure.

• Cardiolipin binding stabilizes cytochrome c in the ETC, reduces electron leak (and thus reactive oxygen species production), and preserves mitochondrial cristae architecture during ischemia-reperfusion injury.

• Clinical translation: Elamipretide has been evaluated in Phase 2 and Phase 3 trials. The pivotal Phase 3 MMPOWER-HF trial (n=360, 2023) did not meet its primary endpoint. The Barth syndrome Phase 3 program (MMPOWER-3) also did not meet its primary endpoint. Efficacy is not demonstrated; SS-31 is not FDA-approved.

• Unlike MOTS-c (which acts via AMPK signaling), SS-31 acts directly at the mitochondrial membrane level — making it a complementary tool for studying mitochondrial biology from a structural vs. signaling perspective.

Composition

Molecular Composition

Amino Acid Sequence

D-Arg-2',6'-Dmt-Lys-Phe-NH2

SS-31 is a 4-amino-acid tetrapeptide with an alternating aromatic-cationic motif (D-Arg-2',6'-Dmt-Lys-Phe-NH2). The dimethyltyrosine (Dmt) residue provides antioxidant activity, while the cationic residues (D-Arg, Lys) drive electrostatic concentration in the inner mitochondrial membrane, achieving 1,000-fold enrichment relative to cytoplasm.

Mechanism of Action

How Does It Work?

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SS-31 restores mitochondrial function by stabilizing cardiolipin in the inner mitochondrial membrane, improving ATP production efficiency and reducing damaging free radical leakage.

SS-31 concentrates in the inner mitochondrial membrane via electrostatic interactions, where it binds cardiolipin — a phospholipid that anchors electron transport chain complexes. By stabilizing cardiolipin, SS-31 maintains optimal cristae architecture and ETC complex conformation, improving ATP synthesis efficiency and reducing electron leak (and consequent ROS generation).

SS-31 Mechanism of Action — Simplified signaling pathway diagram. For research reference only.

"SS-31 targets the inner mitochondrial membrane and restores mitochondrial function by stabilizing cardiolipin, reducing ROS, and improving ATP production." — Szeto, Antioxidants & Redox Signaling, 2014

Signaling Pathways

Key Research Pathways

Cardiolipin Stabilization / Cristae Architecture

Binds cardiolipin in the inner mitochondrial membrane, stabilizing cristae structure and maintaining optimal conformation of electron transport chain complexes for efficient ATP synthesis.

ROS Scavenging / Mitochondrial Antioxidant Defense

Reduces mitochondrial superoxide and hydrogen peroxide production by preventing electron leak from the electron transport chain.

Ischemia-Reperfusion Protection

Protects against ischemia-reperfusion injury by maintaining mitochondrial membrane potential and preventing mPTP opening during reperfusion.

Research Highlights

Key Findings from the Literature

• Phase 2 trials showed encouraging signals; pivotal Phase 3 MMPOWER-HF trial (n=360, 2023) did not meet its primary endpoint

• Phase 3 Barth syndrome program (MMPOWER-3): primary endpoint not met; some secondary endpoints improved

• Reversed age-related decline in skeletal muscle function in old mice (Science, 2020)

• 1,000-fold concentration in inner mitochondrial membrane vs. cytoplasm

• Reduces mitochondrial ROS by stabilizing cardiolipin and preventing electron leak

Outcome Matrix

Evidence by Claimed Outcome

Each outcome rated by the highest level of evidence available. Tiers follow our 5-tier methodology.

StrongModeratePreliminaryPreclinicalTheoretical

Claimed Outcome

Evidence Tier

Studies

Key Evidence Note

Mitochondrial membrane protection

Moderate

4

Phase II trial in heart failure (PROGRESS-HF); cardiolipin targeting confirmed

Cardiac function improvement

Moderate

3

Phase II data; PROGRESS-HF showed improved 6MWT

Renal protection

Preliminary

2

Phase I/II data in CKD; mechanistically plausible

Exercise tolerance

Preliminary

1

Secondary endpoint in PROGRESS-HF; positive trend

Study counts reflect peer-reviewed publications in the evidence database below. "Theoretical" outcomes have mechanistic rationale only. Learn about our evidence tiers →

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Grounded in peer-reviewed research data

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Researcher Notes

Important Research Context

SS-31 (Elamipretide) has the most advanced clinical data of any mitochondria-targeted peptide, but the pivotal Phase 3 MMPOWER-HF trial (n=360, 2023) did not meet its primary endpoint in heart failure. The Barth syndrome Phase 3 program (MMPOWER-3) also did not meet its primary endpoint. Anti-aging and longevity effects are preclinical only. A 2020 study in Science demonstrated SS-31 reversed age-related decline in skeletal muscle function in old mice — this is animal data, not human evidence.