What Is MK-677 (Ibutamoren)? Non-Peptide GH Secretagogue Profile

Key takeaways

• MK-677 (ibutamoren, development code MK-0677) is a non-peptide, orally bioavailable growth-hormone secretagogue that acts as an agonist of the ghrelin receptor (GHSR-1a, the growth-hormone secretagogue receptor).
• It is a synthetic small molecule, not a peptide: the free base has the molecular formula C27H36N4O5S and a molecular weight near 528.7 g/mol, and it is commonly supplied as the ibutamoren mesylate (methanesulfonate) salt, which raises the formula weight to roughly 624.8 g/mol.
• Unlike the peptide GH secretagogues, MK-677 has no amino-acid sequence and no peptide bonds; it is a spiro-fused, sulfonamide-containing organic molecule designed to mimic the action of ghrelin at the same receptor.
• It is supplied as a powder (free base or mesylate salt) and stored cold and protected from light; identity is confirmed by mass spectrometry against the expected molecular ion rather than by peptide sequencing.
• Ascend Bio Labs publishes a per-batch Certificate of Analysis with independent third-party HPLC purity and LC-MS identity data, linked by a unique batch ID printed on every vial.

MK-677 is frequently filed alongside peptides such as ipamorelin and CJC-1295 in research catalogs under the umbrella of "growth-hormone secretagogues." That grouping is functional rather than structural, and it hides the single most important fact about MK-677 as a research material: it is not a peptide at all. It is a synthetic small organic molecule. Treating it like a peptide leads to the wrong storage assumptions, the wrong identity tests, and the wrong mental model of what is in the vial.

This guide is a structural and handling reference for MK-677, also known as ibutamoren and by its original development code MK-0677. It describes the molecule by its chemical class, formula, molecular weight, salt form, and the receptor it targets, and contrasts that with the peptide secretagogues it is often shelved beside. It makes no claims about biological activity, outcomes, or use in any organism. MK-677 sold by Ascend Bio Labs is a research chemical, labeled for research use only.

Names, codes, and what "non-peptide secretagogue" means

MK-677 carries several identifiers that all point to the same compound. "Ibutamoren" is the international nonproprietary-style name; "MK-0677" (often shortened to MK-677) is the original Merck development code; and the salt form most often sold for research is "ibutamoren mesylate." When reading a Certificate of Analysis, all of these refer to the same chemical entity, but the salt designation matters for molecular weight (covered below).

The phrase "growth-hormone secretagogue" describes what a molecule does at a receptor, not what it is made of. A secretagogue is simply an agent that prompts a gland to release a stored substance. Several structurally unrelated chemical classes can act as GH secretagogues: small peptides, larger peptide analogs, and non-peptide small molecules like MK-677. Grouping them together is a pharmacological convenience, not a statement about shared structure.

That distinction is the whole reason this guide exists separately from the peptide entries. For the peptide side of the family, see What Is Ipamorelin? Pentapeptide GH Secretagogue Specs and What Is CJC-1295? GHRH Analog Structure, With and Without DAC, and for the broader landscape see GH Secretagogue Research Peptides: A Structural Overview.

• Common name: ibutamoren
• Development code: MK-0677 (often written MK-677)
• Common salt form: ibutamoren mesylate (methanesulfonate)
• Class: non-peptide small-molecule growth-hormone secretagogue
• Receptor target: ghrelin receptor (GHSR-1a)

Small-molecule structure: why it has no sequence

A peptide is a chain of amino acids joined by peptide (amide) bonds, and it is fully described by its sequence. MK-677 has no such sequence because it is not built from amino acids. It is a single, branched organic molecule assembled from a small set of fused and pendant ring systems and functional groups, the way most synthetic drugs are, rather than a polymer of residues.

Structurally, ibutamoren is built around a spiro-fused indoline/piperidine core (a spiroindane-type scaffold) bearing a sulfonamide group (the source of the sulfur in its formula) and an O-benzyl-substituted side chain with an amide linkage. It contains one amide bond within that side chain, but a single amide does not make a molecule a peptide; a peptide requires a chain of amino-acid residues, which MK-677 does not have. Because there is no sequence, concepts like "sequence length" and "number of residues" simply do not apply to it.

This is the practical takeaway for a research setting: you cannot describe, verify, or reason about MK-677 using peptide tools. There is no sequence to confirm by Edman degradation, no residue count, and no peptide-mapping step. Its identity is a question of the intact small-molecule structure and its exact mass.

• Not a peptide: no amino-acid residues, no peptide-bond chain, no sequence
• Core scaffold: spiro-fused indoline/piperidine (spiroindane-type) system
• Notable groups: a sulfonamide (the sulfur in the formula) and an O-benzyl side chain with an amide linkage
• "Sequence length" and "residue count" are not applicable to MK-677
• Identity is defined by the intact small-molecule structure and exact mass

Molecular formula, weight, and salt form

The free base of ibutamoren has the molecular formula C27H36N4O5S and a molecular weight of approximately 528.7 g/mol. The presence of sulfur (S) in the formula is itself a structural fingerprint that sets MK-677 apart from the common peptide secretagogues, whose formulas are dominated by carbon, hydrogen, nitrogen, and oxygen and rarely include a sulfonamide sulfur of this kind.

Material sold for research is frequently the mesylate (methanesulfonate) salt, ibutamoren mesylate, with the formula C27H36N4O5S · CH4O3S. Adding one molecule of methanesulfonic acid raises the formula weight to roughly 624.8 g/mol. This is why two COAs for "MK-677" can list different molecular weights: one is reporting the free base near 528.7 g/mol and the other the mesylate salt near 624.8 g/mol. Neither is wrong; they describe different supplied forms.

When verifying material, the practical check is identity by mass spectrometry against the expected molecular ion of the free base (or the appropriate adduct), with the salt accounted for separately. LC-MS confirms the intact small molecule is present; HPLC reports chromatographic purity. Always read which form the batch COA is quoting before comparing a molecular-weight figure to anything else.

• Free base: C27H36N4O5S, ~528.7 g/mol
• Mesylate salt: C27H36N4O5S · CH4O3S, ~624.8 g/mol
• The MW gap (~528.7 vs ~624.8) reflects free base versus mesylate salt, not a discrepancy
• Sulfur (sulfonamide) in the formula is a structural marker distinct from common peptide secretagogues
• Confirm identity by LC-MS against the expected molecular ion; defer to the batch COA for the quoted form

Receptor class: ghrelin receptor (GHSR-1a) agonist

MK-677 is classified as a ghrelin-receptor agonist. The relevant receptor is GHSR-1a, the growth-hormone secretagogue receptor type 1a, a G-protein-coupled receptor for which the endogenous peptide ghrelin is a natural ligand. Describing MK-677 as a "ghrelin mimetic" captures the idea that a non-peptide molecule can engage the same receptor that an endogenous peptide hormone normally activates.

This places MK-677 in the same receptor family as several of the peptide secretagogues that are also GHSR-1a agonists, such as ipamorelin and the GHRP-class peptides. That shared receptor target is exactly why these compounds are catalogued together despite their structural differences. By contrast, GHRH analogs like CJC-1295 act at a different receptor (the GHRH receptor), which is one reason researchers distinguish the GHRP/ghrelin-mimetic group from the GHRH-analog group.

Stating the receptor class is a structural and pharmacological description of the molecule, not a claim about any physiological result. What MK-677 does at GHSR-1a in a model system, and any downstream consequences, are research questions and are outside the scope of this descriptive profile.

• Mechanistic class: ghrelin-receptor (GHSR-1a) agonist / ghrelin mimetic
• Receptor: growth-hormone secretagogue receptor type 1a, a GPCR
• Shares the GHSR-1a target with peptide GHRPs (e.g., the ipamorelin / GHRP group)
• Distinct from GHRH-receptor agonists such as the CJC-1295 GHRH-analog class
• Receptor classification is descriptive, not an outcome claim

Physical form, handling, and storage

MK-677 is supplied as a solid powder, either the free base or, more commonly, the mesylate salt. Because it is a small molecule rather than a lyophilized peptide, the handling logic is a little different: there is no fragile peptide-bond backbone to protect, but the material is still kept cold and protected from light and moisture to preserve purity and prevent degradation over time. Sealed, desiccated, frozen, and dark is the conservative default for long-term storage of the powder.

For making a working stock, MK-677 is dissolved rather than "reconstituted" in the peptide sense. Solubility depends on the salt form and the chosen solvent, and the resulting concentration is set by the mass of compound dissolved relative to solvent volume, just as with any prepared stock. Where the supplied form matters, the COA and label should state whether the quoted mass refers to the free base or the salt.

For general cold-chain and prep principles that apply across research compounds, the same discipline used for peptides, careful labeling, cold storage, light protection, and minimizing freeze-thaw of solutions, applies here as well; see the peptide-focused entries linked above for the analogous handling discussion.

• Supplied form: solid powder (free base or mesylate salt)
• Long-term storage: sealed, desiccated, frozen, protected from light
• Prepared as a dissolved stock; concentration set by mass dissolved per solvent volume
• Confirm whether the labeled/quoted mass is free base or salt
• Minimize freeze-thaw and moisture exposure for prepared solutions

Verifying an MK-677 batch: COA, HPLC, and LC-MS

For a non-peptide small molecule, the documentation that travels with the material is what lets a lab trust the contents of the vial. A complete Certificate of Analysis for MK-677 pairs an HPLC chromatogram (chromatographic purity) with an LC-MS spectrum (molecular identity, confirming the intact ibutamoren molecule by mass). The batch identifier on the COA should match the identifier printed on the physical vial, and the COA should make clear whether figures refer to the free base or the mesylate salt.

Ascend Bio Labs publishes a per-batch COA for every compound, with independent third-party HPLC purity testing and LC-MS identity confirmation, and links each report to a unique batch ID on the vial. Synthesis, testing, storage, and shipping are fully US-domestic, with no overseas transshipment, and material ships insulated and tracked. Several other vendors in this space also publish testing documentation; the table below compares verifiable, publicly stated attributes so a buyer can confirm specifics directly with each source.

The neutral rule when comparing vendors is to read each supplier's own COA and verify the batch ID against the vial. Where an attribute is not publicly listed, treat it as something to confirm with the vendor rather than as a deficiency.

Related research notes

• What Is CJC-1295? GHRH Analog Structure, With and Without DAC
• What Is Ipamorelin? Pentapeptide GH Secretagogue Specs
• GH Secretagogue Research Peptides: A Structural Overview