What Is SS-31 (Elamipretide)?

SS-31 (also known as Elamipretide, MTP-131, or Bendavia) is a synthetic aromatic-cationic tetrapeptide with the sequence D-Arg-2′,6′-Dmt-Lys-Phe-NH₂. It was designed to penetrate the inner mitochondrial membrane and bind selectively to cardiolipin — a phospholipid essential for electron transport chain (ETC) supercomplex assembly and optimal ATP production. By stabilizing cardiolipin, SS-31 restores mitochondrial bioenergetics in tissue under ischemic or oxidative stress. It has received FDA Fast Track designation for Barth syndrome (a pediatric mitochondrial cardiomyopathy) and has completed randomized phase 2 trials in heart failure.

Note on naming: "SS-31" refers to the Szeto-Schiller peptide nomenclature used in preclinical research. "Elamipretide" is the International Non-proprietary Name (INN); "MTP-131" and "Bendavia" are earlier developmental names used by Stealth Biotherapeutics. All refer to the same D-Arg-Dmt-Lys-Phe-NH₂ tetrapeptide.

The Short Answer

SS-31 is a cell-permeable, inner-mitochondrial-membrane–targeted tetrapeptide that binds cardiolipin to stabilize electron transport chain supercomplexes, reduce reactive oxygen species generation, and restore ATP production in energy-deprived or damaged tissue. In a randomized, placebo-controlled phase 2 trial (REPAIR-SHF), IV elamipretide significantly reduced left ventricular end-systolic and end-diastolic volume at end-infusion in patients with HFrEF, with a favorable safety profile.[2] A separate ex vivo study (Chatfield et al., 2019) in ventricular tissue from failing human hearts documented improved mitochondrial oxygen flux and ETC supercomplex-associated activity.[3]

Key Concepts

Molecular structure:

• Sequence: D-Arg-2′,6′-Dmt-Lys-Phe-NH₂ (alternating aromatic and cationic residues)
• Molecular weight: ~639 Da
• Design rationale: The alternating cationic (D-Arg, Lys) and aromatic (Dmt, Phe) residues enable spontaneous uptake across both the outer and inner mitochondrial membranes without relying on membrane potential — a key advantage over cationic compounds that accumulate via charge gradient and may lose efficacy when mitochondrial potential is compromised.[1]

Mechanism of action:

1. Cardiolipin binding: SS-31 preferentially associates with cardiolipin, the signature phospholipid of the inner mitochondrial membrane. Cardiolipin is required for the assembly and stability of respiratory supercomplexes (Complexes I, III, and IV). Loss of cardiolipin integrity — occurring under oxidative stress, ischemia, and aging — destabilizes supercomplexes and impairs electron flow.[1]

2. ETC supercomplex stabilization: By binding cardiolipin, SS-31 preserves supercomplex architecture, maintaining the substrate channeling efficiency that maximizes ATP yield per oxygen consumed.[1]

3. ROS reduction: Stabilized ETC supercomplexes produce fewer electrons that escape to form superoxide. SS-31 thereby reduces mitochondrial reactive oxygen species (ROS) without acting as a direct antioxidant scavenger.[1]

4. Cytochrome c retention: Cardiolipin anchors cytochrome c to the inner membrane for electron shuttling between Complexes III and IV. Loss of cardiolipin releases cytochrome c into the cytosol, triggering apoptosis. SS-31 protects this interaction.[1]

5. Membrane potential-independent uptake: Because SS-31 does not rely on ΔΨm for mitochondrial accumulation, it remains effective in severely depolarized mitochondria — precisely the conditions encountered in ischemic or failing tissue.[1]

Human Clinical Evidence

Current regulatory status: FDA Fast Track for Barth syndrome; phase 3 trials ongoing. Not FDA-approved for any indication as of the date of this publication. Research use only outside of approved clinical trials.

Dosage Reference

Educational reference only. The doses below derive from published clinical trials and preclinical data. SS-31 is not approved for unsupervised human use.

Reconstitution (10 mg vial): Add 1.0 mL bacteriostatic water → 10 mg/mL. On a U-100 insulin syringe, 1 unit = 0.01 mL = 0.1 mg.

Clinical trial reference doses:

• REPAIR-SHF (IV): 0.05–0.25 mg/kg; at 70 kg, that is 3.5–17.5 mg.
• Barth syndrome (SC): 40 mg/day administered as a continuous 2-hour SC infusion via pump in clinical trials; community SC injection protocols typically use 5–15 mg once daily.

For full reconstitution steps and syringe tables, see the SS-31 10 mg Dosage Protocol.

Safety Profile

Clinical trial safety data:

• Injection-site reactions: Redness, pain, and induration at the SC injection site are frequently reported across elamipretide SC trials; they are manageable and have not required trial discontinuation in published studies.
• IV infusion (REPAIR-SHF): Well tolerated at both doses; no serious adverse events attributed to SS-31; transient mild flushing reported in some participants.[2]
• Systemic safety: No significant changes in renal function, hepatic enzymes, haematology, or ECG parameters in published trials.

Contraindications and cautions:

• No clinical data in pregnancy or lactation; avoid
• Long-term safety data beyond the published trial durations are limited
• Not approved for human use outside of clinical trials

Comparison: SS-31 vs. Related Mitochondrial-Targeted Agents

Frequently Asked Questions

What does SS-31 treat?

In published human trials, SS-31 has been studied for ischemic heart failure (REPAIR-SHF) and Barth syndrome (a mitochondrial cardiomyopathy). It is not approved for any indication in the US. Preclinical studies report beneficial effects in models of acute kidney injury, Parkinson's disease, optic neuropathy, and skeletal muscle wasting.

How does SS-31 differ from antioxidant supplements like CoQ10?

CoQ10 and related antioxidants primarily act as electron carriers or scavengers. SS-31 does not function as a direct antioxidant scavenger; instead, it binds cardiolipin to preserve ETC supercomplex structure, which reduces ROS generation at the source. This mechanistic distinction means SS-31 remains effective in mitochondria that are too depolarized for potential-driven accumulation of cationic antioxidants.

Is SS-31 the same as MTP-131 or Bendavia?

Yes — all three names refer to the same tetrapeptide (D-Arg-2′,6′-Dmt-Lys-Phe-NH₂). MTP-131 and Bendavia were development-phase names used by Stealth Biotherapeutics; elamipretide is the INN; SS-31 is the original research designation from the Szeto-Schiller peptide series.

What are the most common side effects?

In the REPAIR-SHF IV trial, SS-31 was well tolerated with no serious adverse events attributed to the peptide.[2] Injection-site reactions (redness, pain, induration) are the primary adverse effect reported across elamipretide SC dosing in published clinical programs.

What is the evidence that SS-31 works in humans?

The REPAIR-SHF randomized trial (n=36, HFrEF EF ≤35%) showed significant reductions in LV end-systolic and end-diastolic volume at end-infusion in the 0.25 mg/kg/h cohort vs. placebo.[2] A separate ex vivo study (Chatfield et al., 2019) demonstrated improved mitochondrial oxygen flux in failing human ventricular tissue.[3] Phase 3 programs (Barth syndrome and cardiovascular outcomes) are ongoing; FDA approval is pending.