Ibutamoren, catalogued as MK-677 or MK-0677, is not a peptide at all: it is a non-peptide spiropiperidine small molecule of 528.66 g/mol that binds and activates the same receptor the growth hormone releasing peptides target, GHS-R1a. Two properties separate it from every injectable secretagogue on this catalogue. It is orally active, because a small synthetic molecule survives the gut and first-pass metabolism in a way a five-residue peptide does not; and its reported half-life is approximately 24 hours, against minutes for the peptide secretagogues. Together those two facts explain both its research appeal and the interpretive problems it brings.
MyPeptide supplies Ibutamoren (MK-677) Capsules in 12.5 mg × 30 and 25 mg × 30 capsule bottles, listed within the GHRPs and secretagogues range and the oral capsules format. Research use only; not for human or veterinary use.
What is Ibutamoren (MK-677)?
Ibutamoren belongs to the class of growth hormone secretagogues — compounds that stimulate endogenous growth hormone release rather than supplying the hormone itself. Where the GHRP family reproduces a peptide pharmacophore, ibutamoren is a peptidomimetic: a small molecule discovered by structure-based optimisation that occupies the same receptor pocket without sharing any structural resemblance to ghrelin or to the GHRPs.
Its identifiers are CAS 159752-10-0, molecular formula C27H36N4O5S and molecular weight 528.66 g/mol. It is also catalogued as MK-0677 and L-163,191, and is commonly supplied as the mesylate salt. Because it is a small molecule rather than a peptide, it is stable at room temperature as a solid, requires no reconstitution and needs no cold chain — a practical difference from everything else in this product family.
Origin and structure
Ibutamoren came out of Merck's secretagogue programme in the mid-1990s, which set out to convert the peptide GHRP pharmacology into an orally available drug candidate. The chemical series that produced it was built around a spiropiperidine scaffold bearing sulfonamide and indoline elements; the compound that emerged bound the receptor with high affinity, was absorbed after oral administration and persisted long enough for once-daily use. The receptor itself was cloned in 1996 using this compound class as the probe, and the endogenous ligand, ghrelin, was identified in 1999 — so the synthetic molecule arrived before the natural hormone it mimics, an unusual sequence in pharmacology.
How Ibutamoren is thought to work
Ibutamoren is an agonist at the ghrelin receptor GHS-R1a, a Gq-coupled receptor expressed on pituitary somatotrophs and in the hypothalamic arcuate nucleus. Agonism drives phospholipase C, inositol trisphosphate generation and a rise in intracellular calcium, which in pituitary preparations is associated with secretory granule release. Because the receptor also mediates ghrelin's appetite signalling, agonism reported in animal and human studies is accompanied by increased food intake — an on-target consequence of the same receptor, not a side finding.
The pharmacokinetic profile is the mechanistic point that matters most for study design. Endogenous growth hormone secretion is pulsatile, and the peptide secretagogues produce short pulses that broadly preserve that pattern. A compound with a 24-hour half-life produces near-continuous receptor occupancy instead. Published work reports that ibutamoren raises pulse amplitude and 24-hour secretion while maintaining pulsatility, but continuous GHS-R1a occupancy also raises the question of receptor desensitisation, and any interpretation of long-run data has to address it.
Selectivity is imperfect. Human studies reported modest concurrent elevations in cortisol and prolactin, smaller than those seen with the older GHRPs but not absent as with ipamorelin.
What the research has examined
Long-term studies in older adults
The most-cited human study is a two-year randomised placebo-controlled trial in healthy older adults, reported in 2008, which found sustained increases in growth hormone and IGF-1 and an increase in fat-free mass. The same study reported increases in fasting glucose and markers of insulin resistance, and increased appetite. It is the single most informative source on what continuous GHS-R1a agonism does over time in people.
Metabolic and glycaemic findings
The glucose signal is consistent across the literature rather than isolated. Growth hormone is a counter-regulatory hormone, so sustained elevation is expected to reduce insulin sensitivity; studies in obese and older populations reported the same direction of effect. This is the principal reason the compound is of interest in metabolic research and the principal reason its development did not continue.
Bone, muscle and recovery models
Trials examined bone mineral density markers and functional recovery after hip fracture, and separate work looked at nitrogen balance during dietary restriction. Results were mixed, with biochemical changes more consistent than functional ones.
Sleep architecture
Because growth hormone secretion is coupled to slow-wave sleep, several studies measured polysomnographic variables and reported changes in slow-wave and REM sleep parameters in young and older participants.
Development status
Despite an extensive clinical dataset, ibutamoren was never approved for any indication and development was discontinued. It is best understood as a well-characterised investigational compound with published human pharmacology, not as an unstudied research chemical — and equally not as an approved medicine.
Forms and sizes we supply
| Attribute | Specification |
|---|---|
| Product | Ibutamoren (MK-677) Capsules |
| Also known as | MK-677, MK-0677, L-163,191, ibutamoren mesylate |
| Form | Oral capsules in a sealed bottle with desiccant |
| Available sizes | 12.5 mg × 30 capsules, 25 mg × 30 capsules |
| Purity | ≥98% by HPLC, lot-matched COA available |
| CAS number | 159752-10-0 |
| Molecular formula | C27H36N4O5S |
| Molecular weight | 528.66 g/mol |
| Compound class | Non-peptide spiropiperidine growth hormone secretagogue |
| Molecular target | GHS-R1a (ghrelin receptor) |
| Reported half-life | Approximately 24 hours |
| Intended use | Laboratory research only |
The capsule format removes the reconstitution and cold-chain variables that dominate handling of lyophilized peptides, which is why oral small molecules are attractive in study designs where preparation error is a concern. The trade-offs of oral formats generally are set out in our guide to oral peptides, capsules and troches.
Handling and storage in a lab context
No reconstitution is required — this is a finished capsule product, not a lyophilized powder. Bottles are stored at room temperature in the sealed container with the desiccant intact, protected from light, heat and humidity. Humidity is the practical enemy of a capsule product: gelatin shells soften and clump once the desiccant is exhausted or the bottle is left open, and a clumped capsule is a quantification problem rather than a cosmetic one.
Where a laboratory needs the compound in solution for an in-vitro assay, capsules are the wrong starting material; the free compound is required, since capsule contents include excipients that will confound a cell-based readout. Ibutamoren is poorly water-soluble and is typically taken up in DMSO for culture work, then diluted so the final solvent concentration stays below the tolerance of the cell system. General storage principles are in how to store peptides.
Purity, COA and how to read it
Certificates for a small molecule read differently from peptide certificates. HPLC purity is still the headline, but the identity confirmation is a mass spectrometry result matching 528.66 g/mol, and for a compound of this size an NMR spectrum is a much stronger identity check than mass alone — it distinguishes structural isomers that share a molecular formula. Salt form should be stated, since the mesylate salt and the free base differ in mass and therefore in the actual compound content of a nominal 25 mg capsule. For a capsule product specifically, content uniformity is the specification that matters most: the label figure is a claim about every capsule in the bottle, not an average. Our guide to reading a certificate of analysis covers the general anatomy of these documents.
Regulatory status
Ibutamoren has no marketing authorisation as a medicine anywhere. Clinical development ended without approval, so there is no reference-listed product and no approved labelling, and it is not a dietary supplement — US regulators have taken the position that it does not qualify as a dietary ingredient. Material supplied here is a research-use-only chemical for laboratory investigation by qualified personnel, not for human or veterinary administration. Ibutamoren is explicitly named on the World Anti-Doping Agency prohibited list under growth hormone secretagogues, and is prohibited at all times in and out of competition.
Related compounds and further reading
The injectable counterparts acting at the same receptor are Ipamorelin, the selective pentapeptide, and the blends that pair a secretagogue with a GHRH analogue such as CJC-1295 + Ipamorelin. Where a study needs the peptide comparator rather than the oral small molecule, our ipamorelin explainer is the place to start, and broader orientation is in the muscle growth research overview.