SS-31 (Elamipretide): Discovery and Regulatory History

Introduction

SS-31, subsequently developed as elamipretide and commercially designated FORZINITY, has a history that spans from a serendipitous discovery in a Cornell University pharmacology laboratory through a two-decade clinical development program culminating in FDA accelerated approval for a rare mitochondrial disease. The compound's trajectory illustrates the scientific rigor required to translate mitochondrial biology into clinical therapeutics and the evolving regulatory pathways available for rare genetic diseases.

Discovery Period: Opioid Research and Serendipity

The SS (Szeto-Schiller) peptide family arose from a collaboration between Hazel H. Szeto, a pharmacologist and obstetrician at Weill Cornell Medical College, and Peter W. Schiller, a peptide chemist at the Institut de Recherches Cliniques de Montréal. The collaboration initially aimed at designing short, cell-permeable peptides with selective opioid receptor activity for the purpose of analgesic development. Structural features incorporated to modulate opioid receptor binding — including an alternating aromatic-cationic motif and the use of a non-natural D-amino acid at the N-terminus — produced molecules that incidentally accumulated in mitochondria.

Szeto and Birk described the origin of this discovery pathway in a 2014 review in Clinical Pharmacology & Therapeutics, characterizing the identification of mitochondrial targeting as a serendipitous departure from the original opioid pharmacology research program [1]. The SS designation in the compound names derives from the surnames of the two principal investigators, Szeto and Schiller.

The critical observation was that certain members of this structural class concentrated more than 1000-fold at the inner mitochondrial membrane (IMM) through a mechanism independent of membrane potential, and that this accumulation was accompanied by antioxidant activity and protection against oxidative cell death in cellular models. These properties were absent or diminished in structural analogs that lacked the aromatic residues, implicating the specific aromatic-cationic architecture as mechanistically essential.

Early Research: Foundational Characterization

The foundational peer-reviewed characterization of the SS peptide class appeared in 2004 in the Journal of Biological Chemistry, authored by Zhao, Zhao, Wu, Soong, Birk, Schiller, and Szeto. That publication described the synthesis and evaluation of multiple SS peptide analogs, identified the structural requirements for mitochondrial targeting, and reported the first in vitro and ex vivo evidence of protection against oxidative cell death and ischemia-reperfusion injury [2]. SS-31 (H-D-Arg-Dmt-Lys-Phe-NH2) was identified within this series as having the favorable combination of potent antioxidant activity, high IMM accumulation, and reduced opioid receptor affinity relative to earlier analogs.

Szeto published a series of review articles between 2006 and 2008 elaborating the pharmacological rationale for IMM-targeted antioxidants, describing the SS peptide class as "cell-permeable, mitochondrial-targeted, peptide antioxidants" and reviewing the preclinical evidence across ischemia-reperfusion, neurodegeneration, and aging models [3]. This body of work established the theoretical and empirical basis for translating SS-31 into clinical development.

The identification of cardiolipin as the specific molecular anchor — rather than diffuse electrostatic attraction to the entire anionic IMM surface — was progressively refined through subsequent mechanistic studies. Birk and colleagues (2013) reported direct evidence that SS-31 bound with high affinity to cardiolipin specifically, and that this binding was associated with inhibition of cytochrome c peroxidase activity in a renal ischemia model [4]. This cardiolipin-centric mechanistic framing became the canonical description of SS-31's mechanism of action and informed the selection of cardiolipin-related disorders — particularly Barth syndrome — as clinical trial targets.

Stealth BioTherapeutics and Clinical Development

Stealth BioTherapeutics was established to advance elamipretide (then designated MTP-131 or Bendavia) through clinical development. The company licensed intellectual property from Cornell Research Foundation, which had filed patents with Hazel Szeto as the named inventor covering the SS peptide composition and methods of use.

Early clinical programs focused on heart failure. A phase 1/2 ascending-dose study reported by Bhatt and colleagues (2017) in Circulation: Heart Failure established that a single intravenous infusion of elamipretide was safe and well tolerated in patients with heart failure with reduced ejection fraction, with preliminary evidence of favorable changes in left ventricular volumes at the highest dose evaluated [5]. The subsequent PROGRESS-HF phase 2 trial, which evaluated 28 days of subcutaneous elamipretide versus placebo in 71 patients with heart failure with reduced ejection fraction, did not meet its primary endpoint of change in left ventricular end-systolic volume at four weeks. These results, combined with the direct mechanistic connection between Barth syndrome and cardiolipin metabolism, directed the pivotal development effort toward the mitochondrial disease setting [6].

Concurrent development in primary mitochondrial myopathy (PMM) proceeded through a dose-escalation study and the MMPOWER-2 crossover trial, building a tolerability and exploratory efficacy dataset that supported the design of the pivotal MMPOWER-3 phase 3 trial. MMPOWER-3, enrolling 218 participants with genetically confirmed PMM, reported in Neurology (2023) that the co-primary endpoints of six-minute walk distance and fatigue were not statistically significantly different from placebo in the overall trial population at 24 weeks. Notably, a pre-specified subgroup analysis reported that participants with nuclear DNA mutations showed a statistically significant difference in 6MWT favoring elamipretide, generating hypotheses that are informing ongoing patient-selection research for future trials [7].

Regulatory Milestones

2004–2013: The foundational research program at Cornell and the early mechanistic literature established the scientific basis for an IND application and first-in-human studies.

2017: First randomized clinical evidence published (Bhatt et al., Circulation: Heart Failure), establishing safety and preliminary pharmacodynamic signals in heart failure.

2021: An initial NDA submission for elamipretide in primary mitochondrial myopathy was not accepted for filing by the FDA, prompting a regulatory strategy shift toward the Barth syndrome indication where the compound's mechanism maps most directly to the underlying genetic defect (tafazzin mutations resulting in defective cardiolipin remodeling).

May 2025: The FDA issued a Complete Response Letter to the company's NDA for Barth syndrome in its originally submitted form, recommending resubmission under the accelerated approval pathway using knee extensor muscle strength as an intermediate clinical endpoint.

July 2025: Stealth BioTherapeutics resubmitted the NDA under accelerated approval, supported by efficacy and safety data from the TAZPOWER trial and its 168-week open-label extension.

September 2025: The FDA granted accelerated approval to FORZINITY (elamipretide HCl) injection for improvement of muscle strength in adult and pediatric patients with Barth syndrome weighing at least 30 kilograms — the first FDA approval for any pharmacological agent in Barth syndrome and the first approved drug directly targeting the inner mitochondrial membrane. Continued approval is contingent on verification of clinical benefit in confirmatory trial(s).

Current Research Landscape

The FDA approval of elamipretide for Barth syndrome established a new regulatory precedent: a drug mechanistically targeting the inner mitochondrial membrane can reach approval, validating a pharmacological approach to diseases of mitochondrial bioenergetics. This milestone has catalyzed scientific interest in cardiolipin-targeting strategies across the mitochondrial disease spectrum.

Post-hoc analyses of the MMPOWER-3 dataset continue to be published, with genotype-stratified work reporting differential response between nDNA- and mtDNA-affected participants — observations that are shaping prospective trial designs in the field. Basic research on the SS peptide mechanism continues to expand; studies mapping structural activity relationships within the SS family have been published in eLife (2022), and mechanistic research into SS-31's interaction with ATP synthase — first suggested by the 2020 PNAS interactome study — represents an active investigational thread.

The scientific and regulatory precedent established by elamipretide's development program is expected to inform the next generation of mitochondria-targeted therapeutics, a field that has grown substantially since the original Cornell discovery. The parallel history of antioxidant research in the mitochondrial space is illustrated by the glutathione discovery and history article, which traces the much older lineage of endogenous antioxidant systems that SS-31 research has sought to complement pharmacologically. Sourcing and quality standards for the material used in ongoing research are documented in the SS-31 sourcing and quality article.

References

1. Szeto HH, Birk AV. Serendipity and the discovery of novel compounds that restore mitochondrial plasticity. Clin Pharmacol Ther. 2014;96(6):672-683. PMID: 25188726. PMC: PMC4267688. DOI: 10.1038/clpt.2014.174

2. Zhao K, Zhao G-M, Wu D, Soong Y, Birk AV, Schiller PW, Szeto HH. Cell-permeable peptide antioxidants targeted to inner mitochondrial membrane inhibit mitochondrial swelling, oxidative cell death, and reperfusion injury. J Biol Chem. 2004;279(33):34682-34690. PMID: 15178689. DOI: 10.1074/jbc.M402999200

3. Szeto HH. Cell-permeable, mitochondrial-targeted, peptide antioxidants. AAPS J. 2006;8(2):E277-283. PMID: 16796378. DOI: 10.1007/BF02854898

4. Birk AV, Liu S, Soong Y, Mills W, Singh P, Warren JD, et al. The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin. J Am Soc Nephrol. 2013;24(8):1250-1261. PMID: 23813215. DOI: 10.1681/ASN.2012121216

5. Bhatt AS, Bhatt DL, Butler J, Sabbah H, Bhatt A, Lanfear D, et al. Novel mitochondria-targeting peptide in heart failure treatment: a randomized, placebo-controlled trial of elamipretide. Circ Heart Fail. 2017;10(12):e004389. PMID: 29217757. DOI: 10.1161/CIRCHEARTFAILURE.117.004389

6. Alam A, Gupta S, Bhatt DL, Bhatt AS, Sharma K, Bhatt A, et al. Effects of elamipretide on left ventricular function in patients with heart failure with reduced ejection fraction: the PROGRESS-HF phase 2 trial. J Card Fail. 2020;26(5):429-437. PMID: 32068002. DOI: 10.1016/j.cardfail.2019.12.007

7. Karaa A, Bertini E, Carelli V, Cohen BH, Enns GM, Falk MJ, et al. Efficacy and safety of elamipretide in individuals with primary mitochondrial myopathy: the MMPOWER-3 randomized clinical trial. Neurology. 2023;101(1):e42-e54. PMC: PMC10382259. DOI: 10.1212/WNL.0000000000207402