Mitochondria-targeting tetrapeptide · FDA-approved (Barth syndrome)

SS-31 (Elamipretide / FORZINITY)

Used for: Barth syndrome (as FORZINITY) · studied for: primary mitochondrial disease, mitochondrial dysfunction

SS-31 is the original designation for elamipretide, a mitochondria-targeting tetrapeptide developed by Stealth BioTherapeutics. It selectively binds cardiolipin on the inner mitochondrial membrane, stabilizes cristae, and reduces oxidative damage. The FDA granted accelerated approval to elamipretide as FORZINITY on 19 September 2025 for Barth syndrome — the first FDA-approved therapy for that rare genetic disorder.

Research designation
SS-31 · Bendavia · MTP-131
INN / brand
Elamipretide · FORZINITY
Structure
D-Arg–2',6'-dimethylTyr–Lys–Phe-NH₂ (tetrapeptide)[1]
Mechanism target
Cardiolipin, inner mitochondrial membrane[1]
Developer
Stealth BioTherapeutics
FDA approval
Accelerated approval, 19 Sep 2025 (Barth syndrome)[2][3]
Other programs
NuPower Phase 3 — primary mitochondrial disease[4]
FDA-approved as FORZINITY only for Barth syndrome. This page summarizes the published mechanism and clinical record. It is not a substitute for the FORZINITY prescribing information. Material sold as "SS-31" by research-chemical vendors is not the FORZINITY drug product. This page does not recommend off-label use, acquisition, or dosing.

Last reviewed April 22, 2026 by the Enhanced Health Editorial Team. Reviewed for source alignment, FDA-status language, and medical-claim boundaries. See the editorial policy.

What SS-31 is

SS-31 is a synthetic cell-permeable tetrapeptide consisting of four residues (D-Arg–2',6'-dimethylTyr–Lys–Phe) with an amidated C-terminus. The "SS" prefix reflects its original development in the laboratory of Hazel Szeto and colleagues. It has been designated in various developmental stages as SS-31, Bendavia, and MTP-131, and carries the generic name elamipretide.[1]

On 19 September 2025, the FDA granted accelerated approval to elamipretide under the brand name FORZINITY (Stealth BioTherapeutics) for the treatment of Barth syndrome — a rare X-linked genetic disorder affecting mitochondrial cardiolipin metabolism. This was the first FDA-approved therapy for Barth syndrome.[2][3]

Mechanism

Elamipretide's mechanism is distinctive among mitochondrial therapeutics because it does not act on a classical enzyme or receptor. Instead, it selectively binds a structural lipid:[1]

  • Cardiolipin targeting. Cardiolipin is a negatively charged phospholipid concentrated in the inner mitochondrial membrane and essential for cristae architecture and electron-transport-chain function. Elamipretide's positive charges produce electrostatic attraction to cardiolipin, driving mitochondrial accumulation.
  • Cristae stabilization. Binding stabilizes cardiolipin, preserving the curved cristae structure that maximizes inner membrane surface area for oxidative phosphorylation.
  • Reduced peroxidation. Elamipretide reduces cardiolipin peroxidation, a driver of mitochondrial dysfunction.
  • ATP preservation and ROS reduction. Net effect: improved membrane potential, reduced reactive oxygen species, preserved ATP production.

Clinical trial and regulatory history

  • TAZPOWER (Barth syndrome). Phase 2/3 randomized trial and open-label extension. Knee-extensor muscle strength improved more than 45% versus baseline on elamipretide, with significant correlation to 6-minute walk test improvement. In the open-label extension, eight patients improved 6-minute walk distance by an average of 96.1 m over baseline; cardiac stroke volume also improved.[3][4]
  • FDA regulatory path. After an initial Complete Response Letter, the NDA resubmission sought accelerated approval based on improvement in knee-extensor muscle strength as an intermediate clinical endpoint. An FDA advisory committee in October 2024 concluded elamipretide was effective for Barth syndrome. Accelerated approval was granted on 19 September 2025.[2][3]
  • NuPower Phase 3. Ongoing evaluation in nuclear-DNA-related primary mitochondrial disease at the time of this page's review.[4]
  • Other indications explored. Cardiac ischemia-reperfusion (MITOCARE, EMBRACE), dry macular degeneration, Leber hereditary optic neuropathy — with mixed outcomes across programs.[1]

Storage and handling

  • FORZINITY (commercial). Handled according to the FDA prescribing information — consult the official label.
  • Research-vendor lyophilized powder. Stored sealed per the supplier's certificate of analysis — typically refrigerated at 2–8 °C or frozen at −20 °C.
  • After reconstitution. Research vials are typically reconstituted in bacteriostatic water (0.9% benzyl alcohol) and stored refrigerated at 2–8 °C. Avoid shaking and freeze–thaw cycles; discard cloudy solutions.

Reconstitution math (research vials)

This math is for research-vendor lyophilized SS-31 vials. FORZINITY is a regulated pharmaceutical; use the FDA prescribing information for clinical dosing and administration.

Example 1 · 20 mg vial + 2 mL bac water concentration = 20 mg / 2 mL = 10 mg/mL = 10,000 mcg/mL
1 unit = 0.01 mL = 100 mcg = 0.1 mg
→ 1 mg = 10 units
→ 5 mg = 50 units
→ 10 mg = 100 units (full syringe)
Example 2 · 20 mg vial + 1 mL bac water concentration = 20 mg / 1 mL = 20 mg/mL = 20,000 mcg/mL
1 unit = 0.01 mL = 200 mcg = 0.2 mg
→ 1 mg = 5 units
→ 5 mg = 25 units
→ 10 mg = 50 units
→ 20 mg = 100 units (full syringe)
Example 3 · 10 mg vial + 2 mL bac water concentration = 10 mg / 2 mL = 5 mg/mL = 5000 mcg/mL
1 unit = 0.01 mL = 50 mcg
→ 1 mg = 20 units
→ 5 mg = 100 units (full syringe)

Frequently asked questions

What is SS-31?

Original designation for elamipretide, now FDA-approved as FORZINITY for Barth syndrome (Sep 2025). Mitochondria-targeting tetrapeptide.[1][2]

How does it work?

Binds cardiolipin on the inner mitochondrial membrane, stabilizes cristae, reduces cardiolipin peroxidation, preserves ATP production.[1]

Is SS-31 FDA-approved?

As FORZINITY for Barth syndrome — yes (accelerated approval 19 Sep 2025). Not approved for other indications.[2][3]

What did the trials show?

TAZPOWER: > 45% improvement in knee-extensor strength vs baseline, correlated with 6-minute walk test improvement; ~96 m avg 6MWT gain in open-label extension; cardiac stroke-volume improvement.[3][4]

How do I reconstitute a 20 mg research vial?

Typical: 20 mg + 2 mL bac water = 10 mg/mL → 1 unit = 0.1 mg; 5 mg = 50 units. Using 1 mL gives 20 mg/mL (0.2 mg per unit). Research math — not for FORZINITY clinical use.

Who developed it?

Stealth BioTherapeutics, building on foundational work by Hazel Szeto's laboratory (hence "SS" prefix).

Primary references

  1. Stefanovic-Racic M, et al. Elamipretide: a review of its structure, mechanism of action, and therapeutic potential. Int J Mol Sci, 2025;26(3):944 (PMC11816484). MDPI / Int J Mol Sci 2025 · PMC11816484
  2. United Mitochondrial Disease Foundation. FDA approves first mitochondrial disease therapy: Stealth BioTherapeutics' elamipretide for Barth syndrome. 19 September 2025 announcement. umdf.org
  3. Johns Hopkins University. FDA approves drug for treatment of rare mitochondrial disorder. Hub news, 25 September 2025. hub.jhu.edu
  4. Reid Thompson W, et al. A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome. Genet Med, 2021 (TAZPOWER). Genetics in Medicine
  5. Stealth BioTherapeutics. Science & pipeline. stealthbt.com

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