In 2023, a small molecule developed at Saint Louis University did something that had eluded pharmacology for decades: it produced the metabolic and transcriptomic fingerprint of endurance training in sedentary mice. SLU-PP-332, a synthetic pan-agonist of the estrogen-related receptors (ERRα, ERRβ, ERRγ), increased treadmill running capacity by roughly 50% and reprogrammed skeletal muscle toward an oxidative, fatigue-resistant phenotype — all without a single bout of voluntary exercise. The compound has since become the lead reference molecule in the emerging field of ERR-targeted exercise mimetics.
What Is SLU-PP-332?
SLU-PP-332 is a synthetic small molecule developed in the laboratory of Dr. Thomas Burris, then at Saint Louis University, designed as a direct agonist of the estrogen-related receptor family — a trio of orphan nuclear receptors (ERRα/NR3B1, ERRβ/NR3B2, ERRγ/NR3B3) that have no known endogenous ligand but function as master transcriptional regulators of oxidative metabolism. Unlike earlier ERR ligands, which were primarily inverse agonists or selective for a single isoform, SLU-PP-332 binds all three ERRs and activates their transcriptional output, making it the first true pan-ERR agonist suitable for in vivo pharmacology.[1]
The molecule is orally bioavailable in rodents, with plasma exposure sufficient to drive measurable changes in skeletal muscle gene expression within days of dosing. In published preclinical work it has been administered intraperitoneally at 10–20 mg/kg and produces robust metabolic effects without overt toxicity over the studied dosing windows.[1][2]
How SLU-PP-332 Works
ERRα Activation: ERRα is the most abundant ERR isoform in skeletal muscle, heart, and brown adipose tissue, and it functions as the obligate transcriptional partner of PGC-1α — the coactivator widely regarded as the master regulator of mitochondrial biogenesis. By directly activating ERRα, SLU-PP-332 amplifies the PGC-1α/ERRα transcriptional program even in the absence of the upstream signals (calcium flux, AMPK activation, β-adrenergic tone) that normally trigger it during exercise.[1][3]
Mitochondrial Biogenesis: Downstream of ERRα activation, SLU-PP-332 upregulates nuclear-encoded genes of oxidative phosphorylation, the TCA cycle, fatty acid β-oxidation, and mitochondrial dynamics. In treated mouse muscle, mitochondrial DNA copy number and oxidative enzyme content rise, mirroring the adaptations seen after weeks of endurance training.[1]
Fiber Type Shifting: ERR signaling drives skeletal muscle toward type I (slow oxidative) and type IIa (fast oxidative) fibers at the expense of glycolytic type IIb fibers. SLU-PP-332 treatment in mice reproduces this shift, increasing myoglobin expression and capillary density markers and contributing to the observed increase in fatigue resistance.[1]
Pan-Isoform Engagement: Activation of ERRβ and ERRγ — which are enriched in heart and oxidative muscle — extends the metabolic program beyond what selective ERRα agonism alone can produce. ERRγ in particular is critical for postnatal cardiac oxidative metabolism, and its activation appears to contribute to the cardiac and exercise-capacity phenotypes seen with SLU-PP-332.[3]
Research Findings
Endurance Capacity: In the landmark 2023 study published in The Journal of Pharmacology and Experimental Therapeutics, Billon and colleagues showed that sedentary mice treated with SLU-PP-332 for two weeks increased their treadmill running time by approximately 45% and total running distance by approximately 70% compared with vehicle-treated controls. Crucially, the treated mice were not exercise-trained — the gains were purely pharmacological.[1]
Transcriptomic Overlap With Training: RNA-sequencing of skeletal muscle from SLU-PP-332–treated mice revealed substantial overlap with the gene expression signature induced by chronic endurance exercise, with shared upregulation of OXPHOS subunits, fatty acid oxidation enzymes, and slow-twitch contractile genes. This positions SLU-PP-332 as one of the more transcriptionally faithful exercise mimetics described to date.[1]
Metabolic and Obesity Effects: A follow-up study in diet-induced obese mice demonstrated that SLU-PP-332 reduced fat mass, improved glucose tolerance, and increased energy expenditure without changing food intake. The compound did not produce significant weight loss in lean mice, suggesting that its effects are most pronounced when oxidative metabolism is the relevant constraint.[2]
Heart Failure Models: Because ERRs are central to cardiac fuel metabolism, SLU-PP-332 has been tested in murine heart failure models. In a 2024 study, the compound improved cardiac function and reduced pathological remodeling in mice subjected to pressure overload, consistent with restoration of oxidative metabolic capacity in the failing heart.[4]
Safety Profile and Pharmacokinetic Considerations
SLU-PP-332 is a tool compound, not an approved drug. There are no human safety data, no clinical trials, and no pharmacokinetic studies in humans as of this writing. Published rodent work has not reported acute toxicity at the doses studied, but durations have generally been limited to weeks rather than months, and long-term oncologic, cardiac structural, and reproductive safety have not been characterized in any species.[1][2]
Several theoretical risks warrant explicit attention. ERRα is overexpressed in a subset of breast, colon, and prostate cancers, where it has been associated with more aggressive disease and worse prognosis. Whether sustained pharmacological ERR agonism would accelerate growth of latent ERR-driven malignancies is unknown but biologically plausible and is a significant translational concern.[5]
Cardiac hypertrophy is another consideration. While ERRγ activation appears beneficial in models of established heart failure, chronic supraphysiological activation of the oxidative transcriptional program could in principle produce maladaptive remodeling, particularly in healthy hearts already at peak oxidative capacity. The therapeutic window in humans — if one exists — has not been defined.
The pharmacokinetic profile in rodents (oral bioavailability, sufficient muscle exposure with intermittent dosing) is favorable for further development, but species differences in hepatic metabolism mean human PK cannot be predicted from murine data. SLU-PP-332 should be regarded as a research chemical and not used outside of formal preclinical research settings.
SLU-PP-332 vs Other Exercise Mimetics
Versus AMPK Activators (AICAR, Metformin): AMPK agonists work upstream of PGC-1α and produce broader metabolic effects, including hepatic gluconeogenesis suppression. However, their effects on skeletal muscle oxidative capacity are more modest than those of SLU-PP-332, and the transcriptomic signature is less specifically “endurance-like.”
Versus PPARδ Agonists (GW501516): GW501516 was the prototypical exercise mimetic of the 2000s, acting on PPARδ to drive fatty acid oxidation. It was abandoned for human use after long-term rodent carcinogenicity studies showed accelerated tumor formation across multiple organ systems. SLU-PP-332 acts on a parallel but distinct nuclear receptor axis; whether it shares the carcinogenicity liability is not yet known and will be the central translational question for any future development.
Versus MOTS-c: MOTS-c, a mitochondrial-derived peptide, also activates AMPK and produces exercise-like metabolic effects but requires injection and acts primarily through a metabolic-stress-response pathway. SLU-PP-332 acts directly at the transcription factor level and is orally bioavailable, giving it a cleaner pharmacological profile but a narrower mechanism.
Versus Actual Exercise: No pharmacological agent reproduces the full systemic adaptation to training — exercise produces cardiovascular, neurocognitive, musculoskeletal, and psychological benefits that no small molecule has been shown to replicate. SLU-PP-332 is best understood as a tool to interrogate which adaptations are downstream of ERR signaling specifically, and as a potential therapeutic for patients who cannot exercise due to disease, disability, or aging-related frailty — not as a substitute for training in healthy individuals.
References
- Billon C, et al. “Synthetic ERR agonists augment functional mitochondrial capacity and improve aerobic exercise tolerance in mice.” The Journal of Pharmacology and Experimental Therapeutics. 2023;387(2):232-240.
- Billon C, et al. “A synthetic ERR agonist alleviates metabolic syndrome.” The Journal of Pharmacology and Experimental Therapeutics. 2024;388(2):232-240.
- Giguère V. “Transcriptional control of energy homeostasis by the estrogen-related receptors.” Endocrine Reviews. 2008;29(6):677-696.
- Xu W, et al. “Estrogen-related receptor agonism reverses mitochondrial dysfunction and heart failure in mice.” Nature Cardiovascular Research. 2024;3(7):760-774.
- Deblois G, Giguère V. “Oestrogen-related receptors in breast cancer: control of cellular metabolism and beyond.” Nature Reviews Cancer. 2013;13(1):27-36.
