MOTS-c vs SS-31: Mitochondrial Peptide Comparison

MOTS-c and SS-31 are both discussed as mitochondrial peptides. Their targets differ.

MOTS-c is encoded by a short open reading frame in mitochondrial 12S rRNA. Lee et al. identified the 16-amino-acid peptide, showed AMPK-linked metabolic actions via folate-cycle inhibition, and found that treatment in mice prevented age-dependent and high-fat-diet insulin resistance and diet-induced obesity (PMID: 25738459). Kim et al. showed that MOTS-c translocates to the nucleus under metabolic stress in an AMPK-dependent manner and regulates nuclear genes, including antioxidant-response-element programs, interacting with NRF2 (PMID: 29983246). The MOTS-c mitochondrial signaling article covers this retrograde axis.

SS-31 (elamipretide) is a synthetic tetrapeptide that concentrates at the inner mitochondrial membrane. Zhao et al. described cell-permeable SS peptides that inhibit mitochondrial swelling, oxidative cell death, and reperfusion injury (PMID: 15178689). Birk et al. reported that SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin (PMID: 23813215). Later clinical development used the generic name elamipretide.

MOTS-c is an endogenous mtDNA-encoded signal. SS-31 is a synthetic cardiolipin-directed tetrapeptide.

MOTS-c

Interventional efficacy in the discovery paper is murine (PMID: 25738459). Reynolds et al. reported that MOTS-c enhanced physical performance in young, middle-aged, and old mice, and that late-life intermittent treatment (three times weekly from 23.5 months) increased physical capacity and healthspan measures in mice. In humans, the same paper reports that exercise induces endogenous MOTS-c expression in skeletal muscle and circulation. That is exercise physiology, not a randomized trial of injected MOTS-c (PMID: 33473109). No PubMed-indexed interventional RCT of exogenous MOTS-c in humans is cited above.

SS-31 / elamipretide

TAZPOWER (Barth syndrome). Reid Thompson et al. ran a randomized, double-blind, placebo-controlled crossover of elamipretide 40 mg/day versus placebo (12 weeks each, 4-week washout) in 12 subjects, then an open-label extension. In part 1, neither co-primary endpoint (6-minute walk test; Barth Syndrome Symptom Assessment) was met. Ten continued open-label; eight reached 36 weeks, with 6MWT +95.9 m (P=0.024) and BTHS-SA −2.1 points (P=0.031) versus baseline. Those later changes had no concurrent randomized control (PMID: 33077895).

MMPOWER-3 (primary mitochondrial myopathy). Karaa et al. ran a Phase 3 randomized trial of daily subcutaneous elamipretide 40 mg versus placebo for 24 weeks. The trial did not meet its co-primary endpoints of 6-minute walk distance and the Primary Mitochondrial Myopathy Symptom Assessment Total Fatigue Score (PMID: 37268435).

Heart-failure and geographic-atrophy programs also missed primary endpoints, and those uses are not approved. The record is extensive human testing, mostly missed primary endpoints, and a narrow accelerated approval.

MOTS-c. There is no interventional human safety RCT among the citations above. Mouse doses in the discovery work are not a human safety database (PMID: 25738459). Endogenous MOTS-c after exercise (PMID: 33473109) does not establish the safety of the exogenous peptide.

Elamipretide. TAZPOWER and MMPOWER-3 used daily subcutaneous 40 mg. Injection-site reactions are the main adverse-event class in the trial and label record. TAZPOWER’s randomized portion missed both co-primaries; the open-label extension is uncontrolled (PMID: 33077895). MMPOWER-3 also missed its co-primaries (PMID: 37268435).

FDA. On 19 September 2025 the FDA granted accelerated approval to elamipretide hydrochloride (Forzinity) to improve muscle strength in Barth syndrome patients weighing at least 30 kg (NDA 215244), using knee-extensor strength as an intermediate endpoint. Accelerated approval may require confirmatory benefit. MOTS-c is not approved. The Barth syndrome label does not cover metabolic aging, exercise performance, or other uses.

SS-31/elamipretide has a human trial program and a narrow FDA accelerated approval for Barth syndrome. That same program includes missed primary endpoints in TAZPOWER’s randomized crossover, MMPOWER-3, and other indications. The Barth syndrome label does not change those missed myopathy and heart-failure endpoints.

MOTS-c has a clearer mouse mechanism and an exercise-responsive endogenous signal in humans, with no published interventional human trial of the exogenous peptide. The human data do not support calling it an exercise-mimetic drug.

Exogenous MOTS-c in humans, elamipretide outside Barth syndrome, and any head-to-head trial are still unpublished. The MOTS-c clinical literature is thin. Mouse findings are not treated as human efficacy.