Elamipretide (SS-31): The Mitochondrial Peptide's Real Trial Record

Elamipretide — also called SS-31 or MTP-131 — is a synthetic tetrapeptide developed by Stealth BioTherapeutics specifically to target cardiolipin, a phospholipid concentrated in the inner mitochondrial membrane. Unlike most compounds marketed in the wellness space with minimal trial data, elamipretide has accumulated a genuine and extensive clinical-trial record across four disease programs: primary mitochondrial myopathy, heart failure, Barth syndrome, and dry age-related macular degeneration (AMD). The honest conclusion from that record is mixed at best. The Phase 3 MMPOWER-3 trial in primary mitochondrial myopathy missed its primary endpoint^[1], and two Phase 2 heart failure trials produced neutral primary results^[4][5]. The one win — FDA accelerated approval for Barth syndrome in 2025^[8] — covers an ultra-rare orphan disease affecting perhaps a few hundred patients worldwide. Elamipretide is not approved for, and has not yet demonstrated efficacy in, mitochondrial myopathy, heart failure, or dry AMD — though a fully-enrolled Phase 3 dry-AMD trial (ReNEW) is active and has not yet reported. This article reviews the actual trial record. See the peptides hub for context on how peptides are studied, and our mitochondrial-energy evidence review for related NAD+ biology.

What is elamipretide (SS-31, MTP-131)?

Elamipretide is a synthetic, water-soluble tetrapeptide with the sequence D-Arg-2′,6′-Dmt-Lys-Phe-NH₂. Its key property is a high binding affinity for cardiolipin — a unique four-tailed phospholipid found almost exclusively in the inner mitochondrial membrane (IMM) where it plays a structural role in the electron transport chain (ETC) complexes, particularly Complexes I and III, and in the assembly of ATP synthase. In cells with damaged or dysfunctional mitochondria, cardiolipin undergoes oxidative modification, destabilizing ETC supercomplex architecture and reducing the efficiency of oxidative phosphorylation. Elamipretide is thought to bind and stabilize cardiolipin, protecting ETC complexes from oxidative disruption and helping restore proton gradient efficiency and ATP synthesis.

The compound was developed by Stealth BioTherapeutics (originally Stealth Peptides International) and has been studied primarily as a subcutaneous injection. It is not orally bioavailable in a clinically meaningful form and is not commercially available as a supplement or peptide vial from standard compounding sources. The rationale for its development was specific to diseases where mitochondrial cardiolipin pathology is a key driver — not as a general-purpose mitochondrial booster or anti-aging peptide. This distinction matters enormously when evaluating wellness-clinic or peptide-market claims about the compound.

The four clinical trial programs

Primary mitochondrial myopathy (MMPOWER program)

The MMPOWER program was the largest and longest clinical investment in elamipretide. Karaa and colleagues published a Phase 1/2 randomized dose-escalation trial in Neurology in 2018 (n=36), establishing safety, tolerability, and preliminary pharmacodynamic signals in adults with primary mitochondrial myopathy (PMM)^[2]. A Phase 2 randomized crossover trial followed in 2020 (J Cachexia Sarcopenia Muscle, Karaa et al.), designed to evaluate functional endpoints including the six-minute walk distance (6MWD) and patient-reported fatigue^[3].

The pivotal trial, MMPOWER-3, was a Phase 3, multicenter, randomized, double-blind, placebo-controlled trial in 218 adults with genetically confirmed primary mitochondrial myopathy. The primary endpoint was a composite of 6MWD and total fatigue score. The 2023 Neurology publication by Karaa and colleagues reported that elamipretide did not meet its primary composite endpoint — the defining miss in the compound's mitochondrial myopathy program^[1]. A 2024 post-hoc analysis explored genotype-specific effects, finding possible signals in subgroups with certain mitochondrial DNA mutations, but post-hoc subgroup analyses are hypothesis-generating, not confirmatory. The MMPOWER program did not succeed in its Phase 3 objective.

Heart failure (Daubert 2017 + PROGRESS-HF)

Two Phase 2 randomized controlled trials examined elamipretide in heart failure with reduced ejection fraction (HFrEF). Daubert and colleagues (2017) published a randomized, placebo-controlled trial in Circulation: Heart Failure that evaluated elamipretide in patients with stable systolic heart failure. The primary cardiovascular endpoints did not reach statistical significance, though the trial was early-stage and not powered as a definitive efficacy study^[4]. The larger PROGRESS-HF trial (Butler et al., 2020, Journal of Cardiac Failure) was a Phase 2 randomized trial evaluating elamipretide's effect on left ventricular function in HFrEF patients. The primary endpoint — change in left ventricular end-systolic volume index — was not significantly different from placebo, though some secondary and exploratory analyses showed signals in specific subgroups^[5]. Neither trial provided evidence supporting elamipretide for heart failure, and no Phase 3 heart failure program has been publicly announced.

Barth syndrome (TAZPOWER) — the one FDA approval

Barth syndrome is an X-linked genetic disorder caused by mutations in TAZ, the gene encoding tafazzin — an enzyme essential for cardiolipin remodeling. Because Barth syndrome is fundamentally a cardiolipin disease, and elamipretide binds cardiolipin directly, the mechanistic fit is tighter here than in any other indication. The TAZPOWER Phase 2/3 trial (Reid Thompson et al., 2021, Genetics in Medicine) was a randomized, double-blind, placebo-controlled crossover study in patients with Barth syndrome. It reported clinically meaningful improvements in several outcome measures including the 6MWD and patient-reported outcomes^[6].

In 2025, the FDA granted accelerated approval to elamipretide for the treatment of Barth syndrome — making it the first FDA-approved therapy for that condition and the first cardiolipin-directed mitochondrial therapeutic to receive any U.S. regulatory approval^[8]. Accelerated approval is a regulatory pathway for serious conditions with unmet medical need, where approval is based on a surrogate or intermediate clinical endpoint reasonably likely to predict clinical benefit. Confirmatory evidence of clinical benefit is required post-approval. The approval is specifically for Barth syndrome, a disease estimated to affect roughly 150–300 individuals in the United States at any given time.

Dry AMD (ReCLAIM program)

Age-related macular degeneration has been proposed as another setting where mitochondrial dysfunction in retinal pigment epithelium contributes to disease progression, making the cardiolipin-targeting mechanism potentially relevant. The ReCLAIM program has advanced further than the other elamipretide indications outside Barth syndrome. Allingham and colleagues (2022) published Phase 1 ReCLAIM results in Ophthalmology Science, establishing that elamipretide was tolerable as a subcutaneous injection in the AMD population and reporting exploratory imaging outcomes^[7]. ReCLAIM-2, a completed Phase 2 randomized, double-masked, placebo-controlled trial (n=176; Ehlers et al. 2024, Ophthalmology Science^[9]), tested daily subcutaneous elamipretide 40 mg for 48 weeks in non-central geographic atrophy and missed both co-primary endpoints (change in low-luminance visual acuity and change in geographic-atrophy area). It did, however, show a significant secondary/exploratory signal: a 43–47% reduction in the progression of ellipsoid-zone (photoreceptor-layer) attenuation versus placebo, plus significantly more patients gaining ≥10 letters of low-luminance visual acuity (14.6% vs 2.1%). The trial investigators reported that this ellipsoid-zone endpoint would carry forward as the regulator-agreed primary endpoint for the Phase 3 program. That Phase 3 trial, ReNEW (NCT06373731; n=313, fully enrolled), is active and no longer recruiting as of August 2026, with an estimated primary completion date of August 2027^[10] — no Phase 3 AMD trial has completed or reported results yet, but one is well under way, using a different primary endpoint (ellipsoid-zone preservation) than the visual-acuity and GA-area endpoints ReCLAIM-2 missed.

Clinical trial record at a glance

Why did Barth syndrome succeed where mitochondrial myopathy did not?

The mechanistic specificity of elamipretide's cardiolipin-binding action likely explains the divergence. Barth syndrome is caused directly by defective cardiolipin remodeling — the TAZ gene product tafazzin is responsible for mature cardiolipin composition, and loss-of-function mutations produce structurally abnormal cardiolipin that impairs mitochondrial ETC function across multiple organ systems. In this context, a drug that stabilizes cardiolipin is targeting the root pathophysiological mechanism of the disease. Primary mitochondrial myopathy (PMM) is a heterogeneous category — the clinical diagnosis encompasses mutations across dozens of different mitochondrial and nuclear genes, not all of which involve cardiolipin as a primary mediator. A molecule that works by improving cardiolipin stability may not address the specific mitochondrial defects present in much of the PMM population.

This pattern — promising mechanism, mixed results in heterogeneous disease populations, better signal in mechanistically well-matched orphan disease — is common in mitochondrial pharmacology. It underscores why trial evidence, not mechanism plausibility, is the relevant standard. The fact that elamipretide works in Barth syndrome does not extend its efficacy to broader mitochondrial conditions any more than a disease-specific orphan-drug approval generalizes to off-label populations.

What about the wellness and peptide market?

Elamipretide (marketed in some wellness contexts as "SS-31") appears in peptide-optimization and biohacking discussions as a mitochondrial anti-aging compound. The wellness-market narrative typically runs: SS-31 improves mitochondrial function → more ATP → more energy → better performance, recovery, and aging outcomes. This chain is plausible at the level of mechanism but has never been demonstrated in a controlled human trial in healthy individuals. Every published human RCT of elamipretide was conducted in patients with significant disease (mitochondrial myopathy, heart failure, Barth syndrome, dry AMD) — and even in those diseased populations, primary endpoints were mostly missed. The compound is not available as an FDA-approved drug for any general indication, and compounding-pharmacy versions of elamipretide operate in a regulatory gray zone.

If you encounter elamipretide or SS-31 marketed for energy enhancement, mitochondrial biogenesis, or longevity, the honest characterization is that the compound has failed its primary endpoint in the largest controlled trial ever conducted in its core indication (mitochondrial disease, MMPOWER-3), and has produced neutral results in heart failure — the two trials most likely to be informative about broader mitochondrial benefits. Wellness-market claims outrun the evidence by a wide margin. See our NAD+ peptide guide for a broader comparison of mitochondrial-support approaches. For mitochondrial energy and aging evidence specifically, see our NAD vs. NMN vs. NR evidence review.