SS-31

What is SS-31?

SS-31 (elamipretide) is a synthetic tetrapeptide that selectively binds cardiolipin on the inner mitochondrial membrane. It received FDA accelerated approval in September 2025 under the brand name Forzinity for improving muscle strength in Barth syndrome patients weighing ≥30 kg — making it the first cardiolipin-directed mitochondrial therapeutic and the first mitochondria-targeted peptide ever approved. Trials continue in dry age-related macular degeneration (ReNEW, readout expected late 2027), primary mitochondrial myopathy from polymerase gamma / nDNA mutations (SPIMD-301), and other indications.

What SS-31 Is Investigated For

SS-31 (elamipretide) is investigated across Barth syndrome, primary mitochondrial myopathy, heart failure with reduced ejection fraction, dry age-related macular degeneration, ischemia-reperfusion injury, and broader mitochondrial-dysfunction indications. The strongest evidence — and the only approved indication — is Barth syndrome, a rare X-linked cardiolipin-metabolism disorder where the mechanistic case is unusually direct and long-term open-label extension data supports durable benefit. The honest caveats are substantial: pivotal trials in heart failure (PROGRESS-HF), acute MI (EMBRACE-STEMI), and dry AMD (ReCLAIM) missed their primary endpoints, and MMPOWER-3 in primary mitochondrial myopathy missed its primary with only genotype-specific post-hoc signals. No controlled trials exist for healthy-aging or general longevity use, research-chemical SS-31 is not equivalent to clinical-grade elamipretide, and strong mechanism — cardiolipin stabilization on the inner mitochondrial membrane — has not translated into broad clinical efficacy outside the one rare disease where the biology aligned most directly.

History & Discovery

SS-31 emerged from work by Hazel Szeto and Peter Schiller in the late 1990s and early 2000s on small peptides designed to penetrate cell membranes and concentrate inside mitochondria. Originally developed at Cornell as one of the 'Szeto-Schiller' (SS) series of mitochondrially targeted peptides, SS-31 (chemically D-Arg-2',6'-Dmt-Lys-Phe-NH2) was distinguished by its ability to selectively bind cardiolipin on the inner mitochondrial membrane without depending on a membrane potential — solving a problem that had limited earlier mitochondria-targeting strategies. The drug was licensed and developed by Stealth BioTherapeutics under the names Bendavia, MTP-131, and ultimately elamipretide. Stealth advanced the molecule through trials across multiple indications: ischemia-reperfusion injury after acute myocardial infarction (EMBRACE-STEMI, which missed its primary endpoint), heart failure with reduced ejection fraction (PROGRESS-HF, also negative), primary mitochondrial myopathy (MMPOWER-3, missed primary but suggested signals in subgroups), Barth syndrome (TAZPOWER and natural-history comparisons, ultimately supporting approval), and dry age-related macular degeneration (ReCLAIM trials). The regulatory journey included FDA Complete Response Letters, accelerated-approval discussions, and persistent debate about the right endpoints for ultra-rare mitochondrial diseases. Approval for Barth syndrome — a rare X-linked cardiolipin-metabolism disorder where the mechanistic case for elamipretide is unusually direct — represents the most clinically mature landing point for a mitochondria-targeted peptide to date. On September 19, 2025, the FDA granted accelerated approval to elamipretide hydrochloride under the brand name Forzinity for improving muscle strength in adult and pediatric Barth syndrome patients weighing ≥30 kg, making it the first approved cardiolipin-directed mitochondrial therapeutic. Commercial supply began November 2025 and AnovoRx became the exclusive US specialty pharmacy in December 2025; a confirmatory post-marketing trial is scheduled to begin in the first half of 2026 as a condition of the accelerated-approval pathway. Stealth BioTherapeutics has announced plans to pursue primary mitochondrial disease from polymerase gamma / nDNA mutations (SPIMD-301, Phase 3) as the next indication.

How It Works

SS-31 concentrates in the powerhouses of your cells (mitochondria) and helps stabilize the structures that produce cellular energy. By improving mitochondrial efficiency, it may support heart function, exercise capacity, and cellular health.

SS-31 selectively targets cardiolipin on the inner mitochondrial membrane. Cardiolipin is essential for the organization and function of electron transport chain complexes. By stabilizing cardiolipin, SS-31 optimizes electron transfer, reduces reactive oxygen species (ROS) production, and improves ATP synthesis efficiency. It also prevents cardiolipin peroxidation, which is a key event in mitochondrial dysfunction.

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about SS-31:

• 01Heart failure and AMD efficacy — trials in heart failure with reduced ejection fraction (PROGRESS-HF) and dry AMD (ReCLAIM) failed to meet primary endpoints; the Phase 3 ReNEW trial in dry AMD is fully enrolled with readout expected in late 2027, which is the next major data point. Whether subgroups benefit and whether different dosing strategies might succeed in heart failure remains unresolved.
• 02Primary mitochondrial myopathy — MMPOWER-3 missed its primary endpoint, with genotype-specific post-hoc signals. Stealth has subsequently focused on the polymerase gamma / nDNA-mutation subpopulation through the SPIMD-301 Phase 3 trial, which is the announced next regulatory target. Whether this more homogeneous population will produce a positive pivotal remains to be seen.
• 03Long-term safety in chronic use — Barth syndrome open-label extension data is the longest continuous human exposure dataset, but this is a small population, and long-term safety in larger populations is not characterized.
• 04Healthy-aging or longevity use — there are no controlled trials of elamipretide in healthy adults for general mitochondrial support or anti-aging endpoints; extrapolation from rare-disease data is not justified.
• 05Optimal route and formulation — daily subcutaneous injection has been the standard; an oral or longer-acting formulation has not reached late-stage development.
• 06Cardiolipin-targeting selectivity in vivo — while in vitro and animal mechanism is well characterized, tissue-level selectivity and downstream consequences in long-term human use are still being mapped.

Forms & Administration

SS-31 is administered via subcutaneous injection. It is primarily available through clinical trials. All injectable peptides should only be administered under the guidance of a qualified healthcare provider. Never self-administer without clinician oversight.

Dosing & Protocols

The ranges below reflect protocols commonly discussed in the literature and by clinicians — not a prescription. Actual dosing for any individual should be determined by a qualified healthcare provider who knows the patient.

Dosing for SS-31/elamipretide outside its approved indication is not supported by clinical trial data. The numbers above describe what has been studied, not a recommendation for off-label use.

Timeline of Effects

Common Questions

Who SS-31 Is NOT For

Drug & Supplement Interactions

Elamipretide has a relatively limited published drug-interaction profile, in part because it is given subcutaneously and largely cleared by peptidase-mediated degradation rather than CYP-mediated metabolism. No major CYP-based interactions have been highlighted in regulatory documents. Clinical-trial protocols typically allowed concurrent standard-of-care therapies for the underlying disease (heart-failure medications in PROGRESS-HF, mitochondrial-disease supportive care in MMPOWER-3 and TAZPOWER), without signals of clinically meaningful interaction. Patients with Barth syndrome are commonly on cardiac medications (beta-blockers, ACE inhibitors), and these have been used alongside elamipretide in trials. For patients considering SS-31 for unapproved indications, the lack of comprehensive interaction data outside the rare-disease populations studied is a significant gap. Disclosure to all prescribing clinicians is appropriate given the absence of broader pharmacology data.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions