SS-31

What is SS-31?

SS-31 (elamipretide, also known as Bendavia, MTP-131, brand name Forzinity) is a synthetic cell-permeable tetrapeptide with the sequence D-Arg-2′,6′-dimethylTyr-Lys-Phe-NH₂. The peptide selectively binds cardiolipin in the inner mitochondrial membrane and stabilizes the mitochondrial machinery responsible for ATP production. It was discovered by Hazel Szeto at Cornell in the 2000s and developed by Stealth BioTherapeutics through Phase 2 and Phase 3 trials in mitochondrial myopathy, heart failure, age-related macular degeneration, and Barth syndrome.

The molecule has a distinctive alternating aromatic-cationic structure that allows it to penetrate cell membranes and concentrate in the mitochondrial matrix. Concentration specificity for the inner mitochondrial membrane is reported at 1,000 to 5,000 times the cytosolic concentration, which is unusually high for a synthetic peptide. This targeting is mediated by electrostatic and hydrophobic interactions with cardiolipin, the signature phospholipid of the inner mitochondrial membrane.

The compound has carried several names through its development history. The original Szeto-Schiller laboratory designation was SS-31 (after the lab's first two authors). Stealth BioTherapeutics rebranded it Bendavia during cardiac-protection trials in the 2010s and elamipretide for the formulation submitted to the FDA. The approved trade name is Forzinity.

Mechanism of Action

SS-31 acts on cardiolipin in the inner mitochondrial membrane. Cardiolipin is a specialized phospholipid that organizes the protein complexes of the electron transport chain (Complex I through Complex V) and maintains the cristae folds that provide the surface area for ATP synthesis. When cardiolipin is damaged, mismatched, or depleted, mitochondrial function collapses. ATP output falls, oxidative stress rises, and mitophagy increases.

SS-31 binds cardiolipin electrostatically and hydrophobically through its alternating amino acid pattern. Once bound, the peptide stabilizes cardiolipin and the protein complexes it organizes. The downstream effects include preserved cristae structure, restored respiratory chain efficiency, reduced reactive oxygen species production, and protection against the mitochondrial permeability transition pore opening that triggers apoptosis.

For Barth syndrome specifically, the mechanism is direct. Barth syndrome is caused by mutations in the TAZ gene, which encodes tafazzin, the enzyme responsible for the final cardiolipin remodeling step. Patients with Barth syndrome accumulate abnormal monolysocardiolipin (MLCL) and have reduced mature cardiolipin, producing cardiac dysfunction, skeletal myopathy, neutropenia, and growth abnormalities. SS-31 binds the remaining cardiolipin (whatever its composition) and stabilizes the mitochondrial machinery despite the abnormal lipid profile. The peptide does not correct the underlying tafazzin defect, but it does compensate for downstream consequences.

The Clinical Evidence Base

The clinical-trial program for SS-31 spans more than 700 patients across multiple indications.

Barth syndrome (TAZPOWER, NCT03098797). The pivotal Phase 2/3 trial enrolled 12 patients in a randomized, double-blind, placebo-controlled crossover design. Each patient received 12 weeks of 40 mg/day elamipretide and 12 weeks of placebo separated by a 4-week washout. The primary endpoints (6-minute walk test and Barth syndrome Symptom Assessment) were not met in the initial randomized phase. At 36 weeks in the open-label extension, the 6-minute walk test improved by 95.9 meters (p = 0.024) and Barth syndrome Symptom Assessment also improved significantly. The full Barth syndrome program included safety data extending to 192 weeks, supporting the September 2025 accelerated approval.

Primary mitochondrial myopathy (MMPOWER program). Multiple Phase 2 and Phase 3 trials in adults with primary mitochondrial myopathy. The MMPOWER-3 Phase 3 trial in 218 patients did not meet its primary endpoint of distance walked on the 6-minute walk test, ending one of Stealth's earlier registrational pathways. The negative result was instructive. Not every mitochondrial disease responds to cardiolipin stabilization, and patient selection is essential.

Heart failure with preserved ejection fraction (HFpEF). A Phase 2 trial in HFpEF reported improvements in cardiac magnetic resonance metrics of mitochondrial function and exercise capacity over 4 weeks of IV administration. The cardiovascular program has not yet produced an FDA-approved indication.

Geographic atrophy in age-related macular degeneration (AMD). Phase 2 trials in dry AMD with geographic atrophy reported reduced lesion growth rates with subcutaneous elamipretide. The visual outcomes were modest. Stealth has continued development in this indication.

NuPower (nuclear DNA-related primary mitochondrial myopathy). A separate Phase 3 program addressing nDNA-associated mitochondrial myopathy is ongoing. Results were not yet available at the time of writing.

The September 2025 FDA Approval

The FDA granted accelerated approval for elamipretide (Forzinity) on September 19, 2025 for Barth syndrome. Accelerated approval is a regulatory pathway that allows earlier marketing based on a surrogate or intermediate endpoint, with post-approval confirmatory trials required to verify clinical benefit.

The basis for the Barth syndrome approval included the TAZPOWER open-label extension data and a natural-history comparison study by Hornby et al. (Orphanet Journal of Rare Diseases, 2022) that compared elamipretide-treated patients with historical controls. The combination of small randomized data, open-label extension improvement, and natural-history-controlled comparison was sufficient for accelerated approval in a rare-disease setting.

The approval is a milestone for the mitochondrial-disease community. United Mitochondrial Disease Foundation (UMDF), the primary US patient advocacy group, characterized it as the first FDA-approved therapy that addresses the root cause of a mitochondrial disease.

The accelerated nature of the approval means Stealth BioTherapeutics will need to complete confirmatory studies. The Barth syndrome population is small enough (fewer than 1,000 US patients) that traditional Phase 3 confirmation is difficult. Post-approval registry studies and long-term extension data will be the primary mechanisms for confirming benefit.

Approved Label Dose

The FDA-approved dose of elamipretide (Forzinity) for Barth syndrome is 40 mg administered subcutaneously once daily. This is the dose used in the TAZPOWER pivotal trial (NCT03098797) and in the open-label extension data that supported the September 2025 accelerated approval. Patients self-administer or receive injection at a consistent time each day. Forzinity is supplied as a lyophilized powder for reconstitution. The label is the authoritative reference for prescribing in the approved indication.

Clinical Trial Dose Ranges Across the Program

The Phase 2 and Phase 3 program tested several dose levels and routes across indications.

Subcutaneous, mitochondrial myopathy and Barth syndrome: 4 mg, 40 mg, and 60 mg daily SC across the MMPOWER and TAZPOWER programs. The 40 mg daily dose was selected as the registrational dose. Higher doses did not improve efficacy in the populations tested.

Intravenous, heart failure with preserved ejection fraction: 0.05 mg/kg/h infusion over 4 hours, used in Sabbah et al. 2016 and subsequent Phase 2 cardiac trials. IV administration is reserved for acute hemodynamic indications and trial settings.

Subcutaneous, dry age-related macular degeneration: 40 mg daily SC over 24 weeks in the ReCLAIM-2 trial (NCT03891875).

Subcutaneous, NuPower nuclear-DNA mitochondrial myopathy: 40 mg daily SC in the ongoing Phase 3 trial.

Pharmacokinetics

After 40 mg subcutaneous administration, peak plasma concentration occurs within 1 to 2 hours. The terminal plasma half-life is approximately 3 hours. The plasma profile does not capture the meaningful exposure metric. Mitochondrial residence time exceeds the plasma half-life by an order of magnitude, because elamipretide binds cardiolipin and concentrates in the inner mitochondrial membrane at 1,000 to 5,000 times the cytosolic concentration. This dosing-PK mismatch is the basis for once-daily administration despite the short plasma half-life. The peptide is cleared by standard protein metabolism without renal or hepatic accumulation, which simplifies dosing in patients with comorbidities.

Off-Label Compounded and Research-Chemical Use

Off-label compounded elamipretide and research-chemical SS-31 have circulated in adult research-use communities for several years, predating the September 2025 FDA approval. Reported off-label dosing has typically followed the 40 mg daily SC schedule for trial-mimicking protocols. Lower-dose protocols using 5 to 10 mg daily SC have circulated for non-Barth applications such as cardiac support, mitochondrial myopathy outside trial settings, and longevity research. No controlled human pharmacokinetic study supports the lower-dose protocols. The safety profile is established only at the 40 mg dose tested in registration trials. The Forzinity approval changes the legal status. Prescription access for the approved Barth syndrome indication is now available through specialty pharmacy. Off-label and longevity use remains a separate, less-controlled pathway, and the compounding status under Section 503A is in transition as of mid-2026.

Safety and Side Effect Profile

The clinical-trial dataset of more than 700 patients reports a generally clean safety profile.

Injection-site reactions are the dominant adverse event. Erythema, mild pain, and bruising at subcutaneous injection sites occur in approximately 50 to 70 percent of patients across trials. Most events are mild to moderate.

Mild fatigue, headache, and dizziness have been reported at lower rates.

No serious systemic toxicity has emerged from the cumulative trial database extending to 192 weeks in some patients. The compound has not flagged hepatotoxicity, nephrotoxicity, or cardiovascular adverse events at the doses tested.

The 40 mg daily dose used in Barth syndrome is delivered as a subcutaneous injection. The IV infusion route (used in some heart failure trials) carries a higher rate of administration-related effects but is reserved for acute indications.

Long-term safety beyond 4 years is not yet well-characterized. The peptide is broken down by standard protein metabolism after binding cardiolipin, which provides a clearance mechanism that does not accumulate.

Regulatory and Pricing Status

United States. Forzinity (elamipretide) is FDA-approved for Barth syndrome since September 2025. It is the first mitochondrial-targeted peptide therapy to receive FDA approval. Specific pricing has been characteristic of rare-disease therapies, with annual treatment costs in the high six figures. Insurance coverage varies and is generally available for the approved indication.

Other regions. EMA review is ongoing. Other regulatory pathways through Japan and major European markets are at various stages. Outside the approved indication, the molecule is available only through clinical trials.

Compounding status. SS-31 was placed on the FDA Category 2 bulks list in September 2023, restricting compounding under Section 503A. After the Forzinity approval in September 2025, the molecule is now a component of an approved drug, which changes its compounding status. Off-label compounding for non-Barth-syndrome indications may be possible under the same framework that applies to other approved drugs. Whether 503A pharmacies will pursue this is a separate question.

WADA status. Elamipretide is not currently on the WADA Prohibited List. As an approved mitochondrial therapy, its sport status will likely be reviewed in coming years.

SS-31 vs Other Mitochondrial Peptides

SS-31 sits in a unique evidence position among mitochondrial-targeted peptides.

MOTS-c is a mitochondrial-derived peptide that acts on AMPK and exercise-mimetic pathways. It has earlier-stage clinical development, with Phase 1 trials completed by CohBar (CB4211) and discontinued for formulation reasons in 2022. The mechanism (metabolic signaling) is different from SS-31's (membrane stabilization). The two could in principle be complementary in mitochondrial-disease protocols.

Humanin has primarily preclinical data on neuroprotection and metabolic regulation. Limited human trial data.

Nicotinamide riboside (NR), NAD precursors, and CoQ10 are non-peptide approaches to mitochondrial support that have been studied for years with mixed results in mitochondrial myopathy and age-related decline. None has produced an FDA-approved mitochondrial-disease indication. SS-31 is the only one with current FDA approval.

For mitochondrial-disease patients specifically, SS-31 is now the standard of care for Barth syndrome and a candidate compound for other mitochondrial myopathies pending further trial results. For non-disease longevity applications, the clinical evidence base is much smaller, and the appropriate use case is less clear.

What Comes Next

The NuPower trial in nuclear-DNA-related primary mitochondrial myopathy is the next major Phase 3 readout in the elamipretide pipeline. The cardiovascular program in heart failure has not yet produced registrational data but remains active. The dry AMD program continues. Whether elamipretide expands beyond Barth syndrome over the next 3 to 5 years depends on these readouts and on whether the underlying cardiolipin-stabilization mechanism produces clinically meaningful effects in more common mitochondrial-related conditions, where the absolute benefit may be smaller and harder to detect in trials.

For informational and educational purposes only. Not medical advice. Not for human consumption unless prescribed by a licensed physician for an FDA-approved indication. Consult a qualified healthcare provider before using any peptide or pharmaceutical product.