MK-677 (Ibutamoren) Is Not a Peptide: A Research Review of the Non-Peptide Ghrelin-Receptor Agonist, the Spiropiperidine Secretagogue Lineage, Its Clinical-Literature Record and the Catalogue Vocabulary Behind "mk 677 peptides", "mk667 peptide" and "is mk677 a steroid or peptide" Labels

Peptide Insider Research Desk · Published 2026-09-21 · Compound review; companion to the somatotropic-axis nomenclature review and the catalogue-formats review in which the string "is mk 677 a peptide" was first logged · All content is strictly for research reference; not for human or veterinary use.

MK-677, the Merck development code for ibutamoren (L-163,191; ibutamoren mesylate, MK-0677), is the most searched compound in the research-reagent catalogue that is not a peptide, and the query log shows the confusion directly: is mk 677 a peptide, is mk677 a steroid or peptide, mk 677 peptides, peptides mk677, mk peptides, the digit-error variants mk667 peptide and mk77 peptide, the dual-referent mk2 peptide, the regulatory question is mk 677 a ped and the vendor-anchored peptide sciences mk 677. Every one of these strings points at, or is mistaken for, a small synthetic molecule that was deliberately engineered to behave like a peptide without being one. This review answers the class question first, then places ibutamoren in the secretagogue lineage that begins with the hexapeptide GHRP-6, summarises its unusually deep clinical-literature record, and closes with a research-use-only crosswalk that resolves each query variant to a canonical identifier.

Is MK 677 a Peptide?

Direct answer. No. MK-677 (ibutamoren) is a non-peptide spiropiperidine small molecule. A peptide is a chain of α-amino acids joined by amide (peptide) bonds; ibutamoren has a single amide from O-benzyl-D-serine but no amino-acid chain, no N-terminus and no C-terminus. It is a ghrelin-receptor agonist that mimics the peptide GHRP-6 without sharing its chemistry [1, 5].

The question is not pedantic. Chemical class determines everything a researcher does with a reagent: how it is dissolved, how it is characterised, what degradation pathways to monitor and which analytical method confirms identity. Peptides such as GHRP-6, ipamorelin and ghrelin are assembled by solid-phase synthesis, characterised by RP-HPLC and mass spectrometry against a sequence, and are vulnerable to hydrolysis, oxidation of Met/Trp residues and adsorption to plastics, as detailed in the BPC-157 stability and handling review. Ibutamoren is a conventional organic small molecule: it is characterised by NMR and single-mass identity, has a defined melting point as the mesylate salt, is orally bioavailable, and none of the peptide-handling literature applies to it. Patchett and colleagues were explicit that their goal was to abandon peptide chemistry altogether: starting from a spiroindanylpiperidine "privileged structure" that had appeared in unrelated GPCR ligands, they built a molecule that reproduced the GH-releasing activity of GHRP-6 in rat pituitary cells and in beagle dogs after oral administration [1]. The compound was named a peptidomimetic precisely because it is not a peptide.

Is MK677 a Steroid or a Peptide?

Direct answer. Neither. Ibutamoren contains no steroid nucleus (the fused four-ring cyclopentanoperhydrophenanthrene system of testosterone, oestradiol and cortisol) and no peptide backbone. It belongs to a third class, non-peptidyl growth hormone secretagogues, which act at the ghrelin receptor GHS-R1a rather than at nuclear steroid receptors [2, 5].

The steroid confusion arises from co-marketing rather than chemistry. Ibutamoren is frequently catalogued alongside selective androgen receptor modulators such as MK-2866 (ostarine) because both carry Merck-style "MK-" development codes and both are sold as powders to the same audience. Ostarine is an aryl-propionamide that binds the androgen receptor; ibutamoren is a spiropiperidine that binds a G-protein-coupled receptor on pituitary somatotrophs. They share a code prefix and nothing else. Smith's 2005 review of GHS development in Endocrine Reviews traces the lineage cleanly: enkephalin-derived peptides (Bowers, 1980s) → GHRP-6 → benzolactam L-692,429 → spiropiperidine L-163,191/MK-0677 [5]. Steroids never appear in that lineage.

The Secretagogue Lineage: From GHRP-6 to a Non-Peptide Mimetic

The GHS story begins with a peptide. Bowers, Momany and colleagues reported in 1984 that a synthetic hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂ (GHRP-6), released GH from rat pituitary cells in vitro and in vivo through a mechanism distinct from GHRH [4]. Its receptor was unknown for twelve years. Merck's chemists, unable to make GHRP-6 orally active, pursued non-peptide surrogates: the benzolactam L-692,429 came first, and the spiropiperidine L-163,191 (MK-0677) followed in 1995 with roughly a hundred-fold gain in potency and full oral activity in dogs [1, 5]. Using MK-0677 and GHRP-6 as tools, Howard and colleagues expression-cloned the receptor from swine and human pituitary in 1996, showing a previously orphan GPCR that both compounds activated with Gq-coupled calcium mobilisation [2]. Three years later Kojima and colleagues purified the endogenous ligand from rat stomach: ghrelin, a 28-residue peptide whose serine-3 carries an n-octanoyl group essential for receptor binding [3]. The nomenclature settled accordingly: the receptor became GHS-R1a, the peptide agonists (GHRP-6, GHRP-2, hexarelin, ipamorelin) were re-described as ghrelin mimetics, and ibutamoren became the archetypal non-peptide ghrelin mimetic. The peptide members of this family are reviewed on this site in the GHRP-6 review, the ipamorelin review and the somatotropic-axis nomenclature review; the GHRH-receptor side of the axis (sermorelin, CJC-1295, tesamorelin) is covered in the GHRH analogue ladder review.

Mechanism at GHS-R1a

GHS-R1a is a class-A GPCR expressed on pituitary somatotrophs and in hypothalamic arcuate neurones [2]. Agonist binding activates Gq/11, phospholipase C and inositol-trisphosphate-dependent calcium release, depolarising somatotrophs and triggering GH exocytosis; in the hypothalamus it increases GHRH release and antagonises somatostatin tone, so that the net effect on pulsatile GH is amplification of pulse amplitude rather than creation of a new secretory rhythm [2, 5]. Because the receptor is upstream of GH, the downstream biomarker is hepatic IGF-1, which rises over days as GH exposure accumulates; this is the read-out every human study of ibutamoren has used [6, 7, 13]. The receptor also displays high constitutive activity, and ghrelin itself has orexigenic and gastric-motility actions mediated by the same receptor outside the pituitary, which is why appetite variables appear as secondary end-points in the trial literature [13].

Two mechanistic features distinguish ibutamoren from its peptide relatives in a research setting. First, oral bioavailability and a plasma half-life measured in hours mean that a single daily administration in the trial literature produced sustained receptor exposure, whereas GHRP-6 and ipamorelin are cleared within minutes to a few hours after parenteral administration [5, 6]. Second, the GH response to GHS-R1a agonists is subject to feedback: IGF-1 and GH themselves suppress GHRH and enhance somatostatin, so the pulsatile pattern is preserved and IGF-1 tends to plateau rather than rise indefinitely [6, 7]. Both features are literature observations relevant to interpreting model data; they are not use guidance.

Evidence Table: The Published Record

Unusually for a compound sold as a research chemical, ibutamoren has a peer-reviewed clinical-trial history spanning from 1996 to 2011, including a two-year randomised trial in Annals of Internal Medicine and two negative or terminated outcome trials. The table records research model and signal vector without reproducing any dose or regimen; the intention is to let researchers locate the primary sources, not to guide any use.

Study	Research model	Signal vector	Interpretive note
Patchett et al. 1995 [1]	Rat pituitary cell assay; in vivo GH release in beagle dogs (oral)	GH release potency and oral activity of L-163,191	Structure-activity design from spiroindanylpiperidine 'privileged structure'
Howard et al. 1996 [2]	Expression cloning, human/swine pituitary and hypothalamus	Cloned GHS-R (now GHS-R1a); MK-677 and GHRP-6 activate the same orphan GPCR	Receptor identity established before the endogenous ligand
Kojima et al. 1999 [3]	Rat stomach extract; GHS-R-expressing cells; Ca2+ mobilisation	Ghrelin identified as the endogenous 28-residue acylated ligand	Defines MK-677 as a ghrelin mimetic
Copinschi et al. 1996 [6]	Healthy young men; 7-day randomised crossover; 24-h GH profiling	Increased GH pulse amplitude; IGF-1 rise; cortisol profile examined	Earliest human pharmacodynamic record
Chapman et al. 1996 [7]	Healthy elderly subjects; randomised, placebo-controlled	GH and IGF-1 axis stimulation sustained with daily oral administration	Age-related axis responsiveness
Copinschi et al. 1997 [8]	Young and older men; polysomnography	Sleep-stage changes reported alongside GH secretion	Secondary neuroendocrine read-out
Svensson et al. 1998 [9]	Obese adult volunteers; 2-month randomised, placebo-controlled	GH secretion, fat-free mass and energy-expenditure variables	Body-composition literature record only; no claim inferred
Murphy et al. 1998 [10]	Healthy volunteers under caloric restriction; nitrogen balance	Diet-induced negative nitrogen balance reversed in the treated arm	Anabolic read-out under controlled catabolic stimulus
Svensson et al. 1998 [11]	Obese young males; bone turnover biomarkers	Bone-formation and bone-resorption markers both increased	Coupled turnover signal
Bach et al. 2004 [12]; Adunsky et al. 2011 [15]	Older adults after hip fracture; randomised, placebo-controlled	IGF-1 increase; functional-recovery endpoints mixed; phase IIb terminated	Outcome data did not support development
Nass et al. 2008 [13]	Healthy older adults; 2-year randomised trial (Ann Intern Med)	GH/IGF-1 restored toward young-adult range; body-composition and adverse-event data reported	Longest controlled exposure in the literature
Sevigny et al. 2008 [14]	Mild-to-moderate Alzheimer disease; 12-month randomised trial	No clinical effect on disease progression	Negative trial; important for balanced reading

The balance of this record matters. Pharmacodynamic end-points (GH pulse amplitude, IGF-1) responded consistently across young, elderly, obese and calorically restricted cohorts [6, 7, 9, 10, 13]. Clinical-outcome end-points did not: the hip-fracture programme reported mixed functional results and its phase IIb study was terminated [12, 15], and the Alzheimer disease trial found no effect on progression [14]. Sigalos and Pastuszak's 2018 review of the safety and efficacy of GHS summarises the adverse-event signals reported in these trials, including changes in fasting glucose and insulin sensitivity and increased appetite, and stresses that no GHS, peptide or non-peptide, holds regulatory approval for any body-composition indication [16]. That is the appropriate frame for the string is mk 677 a ped: growth hormone secretagogues are prohibited as a class under section S2 of the WADA Prohibited List, and the compound has no approved medical indication; neither fact is a research outcome.

Query-Orthography and Category-Error Crosswalk

The query variants below follow the error-class method introduced in the FOXO4 / melanotan / BPC-157 orthography review. Each is classified by the mechanism that produced it (category error, digit transposition, dual referent, vendor anchoring), resolved to a canonical referent with a persistent identifier, and assigned an explicit research-use-only scope status. No vendor is assessed or endorsed.

Query string (verbatim)	Error / query class	Canonical referent	Resolution note	Identifier	RUO scope status
is mk 677 a peptide	Question-form (chemical class)	Ibutamoren (MK-0677), non-peptide	No — spiropiperidine small molecule; no amide-linked amino-acid backbone	CAS 159634-47-6; PubChem CID 178024	Recorded — direct answer given
is mk677 a steroid or peptide	Question-form (dual class error)	Ibutamoren, non-peptide, non-steroid	Neither — it is not a steroid (no cyclopentanoperhydrophenanthrene core) and not a peptide	CAS 159634-47-6	Recorded — direct answer given
is mk 677 a ped	Question-form (regulatory class)	Ibutamoren; growth hormone secretagogue (GHS) class	GHS are listed as a class under S2 of the WADA Prohibited List; regulatory status is not a research end-point	WADA Prohibited List, S2.2	Recorded — scope-limited answer
mk 677 peptides	Category error (plural)	Ibutamoren	Catalogue co-listing beside peptides; molecule is non-peptidyl	CAS 159634-47-6	Recorded — terminology
mk peptides	Prefix truncation + category error	Merck 'MK-' code series (MK-0677, MK-2866 and others)	The MK prefix is a Merck compound-code convention, not a peptide family	n/a (code prefix)	Recorded — nomenclature
peptides mk677	Token re-ordering + hyphen loss	Ibutamoren	Same referent as 'mk 677 peptides'	CAS 159634-47-6	Recorded — orthography
mk667 peptide	Digit transposition (677 → 667)	Ibutamoren (probable)	No compound coded MK-667 is catalogued as a research secretagogue; resolved to MK-677	CAS 159634-47-6	Recorded — orthography
mk77 peptide	Digit deletion (677 → 77)	Ibutamoren (probable)	Truncated code; resolved to MK-677 by context of 'peptide' co-listing	CAS 159634-47-6	Recorded — orthography
mk2 peptide	Dual referent	(a) MK-2866 (ostarine, a SARM; non-peptide) or (b) MT-2 (Melanotan II; cyclic heptapeptide)	Unresolvable without context: 'MK-2' is not a catalogue code; MT-2 is a peptide, MK-2866 is not	MK-2866 CAS 841205-47-8; MT-2 CAS 121062-08-6	Recorded — dual referent flagged
peptide sciences mk 677	Vendor-anchored token	Ibutamoren listed by a named vendor	Vendor names are search anchors only; no vendor is assessed or endorsed here	n/a	Recorded — vendor token (no assessment)

The most consequential row is mk2 peptide. MT-2 (Melanotan II) is a genuine cyclic heptapeptide acting at melanocortin receptors, reviewed in the Melanotan II review; MK-2866 is a non-peptide SARM. A researcher who conflates them will search the wrong literature, apply the wrong analytical method and store the reagent under the wrong conditions. The digit-error strings mk667 peptide and mk77 peptide are lower-risk because no catalogued compound carries either code, so they collapse onto MK-677 by default.

Analytical and Handling Implications of Being a Small Molecule

Because ibutamoren is not a peptide, the certificate-of-analysis conventions reviewed in the vendor-independent quality verification review apply differently. Identity is confirmed by ¹H/¹³C NMR and accurate mass rather than by sequence-matched MS/MS; purity is measured by HPLC against a reference standard, and there is no trifluoroacetate counter-ion issue because the compound is not made by solid-phase peptide synthesis. The mesylate salt is a crystalline solid with aqueous solubility adequate for stock solutions; it does not require the reconstitution media discussed in the bacteriostatic-water review, and the peptide-adsorption concerns described in the laboratory consumables review do not arise. For in-vitro receptor work the relevant reference compounds are ghrelin itself, GHRP-6 and the inverse agonist [D-Arg¹,D-Phe⁵,D-Trp^7,9,Leu¹¹]-substance P; for GHS-R1a expression systems, the cloned human receptor described by Howard and colleagues remains the standard construct [2].

Where to Source for Research

Ibutamoren and the peptide ghrelin mimetics with which it is co-catalogued are stocked as research-use-only reagents by several suppliers. Researchers comparing sources may consult Short Chain Aminos, BioPep, Catalyst Research and Apex Research Services. Availability is noted for research reference only; no supplier is ranked, assessed or endorsed, and the certificate-of-analysis principles discussed above apply irrespective of source.

Frequently Asked Research Questions

Is MK 677 a peptide?

No. MK-677 (ibutamoren) is a non-peptide small molecule of the spiropiperidine class (C27H36N4O5S, 528.7 g/mol). It contains no amide-linked chain of α-amino acids, so it fails the structural definition of a peptide, although it acts at the same ghrelin receptor (GHS-R1a) as the peptide secretagogues GHRP-6 and ipamorelin.

Is MK677 a steroid or peptide?

Neither. Ibutamoren has no steroid nucleus and no peptide backbone. It is a synthetic ghrelin-receptor agonist designed by Merck in the 1990s to reproduce the growth-hormone-releasing activity of the hexapeptide GHRP-6 in an orally active, non-peptidyl scaffold.

Is MK 677 a PED?

Growth hormone secretagogues, the class to which ibutamoren belongs, are listed under section S2 of the World Anti-Doping Agency Prohibited List. That is a regulatory classification for sport, not a research finding; this review records the classification and offers no use guidance.

Why do catalogues list 'mk 677 peptides' if it is not a peptide?

Research vendors group ibutamoren with peptides because both are sold as research-grade lyophilised or powdered reagents to the same audience, and search engines inherit the co-listing. The query strings 'mk 677 peptides', 'peptides mk677' and 'mk peptides' therefore carry a category error rather than a different compound.

What do 'mk667 peptide', 'mk77 peptide' and 'mk2 peptide' refer to?

'mk667 peptide' and 'mk77 peptide' are digit-error variants of MK-677. 'mk2 peptide' is a dual-referent string: it may intend MK-2866 (ostarine, a non-peptide SARM) or MT-2 (Melanotan II, a genuine cyclic heptapeptide); the two are unrelated compounds and cannot be resolved without context.

What receptor does ibutamoren act on?

Ibutamoren is an agonist of the growth hormone secretagogue receptor type 1a (GHS-R1a; gene GHSR), the G-protein-coupled receptor cloned by Howard and colleagues in 1996 and later shown by Kojima and colleagues to be the receptor for the endogenous 28-residue acylated peptide ghrelin.

Works Cited

1. Patchett AA, Nargund RP, Tata JR, et al. Design and biological activities of L-163,191 (MK-0677): a potent, orally active growth hormone secretagogue. Proc Natl Acad Sci U S A. 1995;92(15):7001-7005. doi:10.1073/pnas.92.15.7001. PMID: 7624358.
2. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. doi:10.1126/science.273.5277.974. PMID: 8688086.
3. Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656-660. doi:10.1038/45230. PMID: 10604470.
4. Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114(5):1537-1545. doi:10.1210/endo-114-5-1537. PMID: 6714155.
5. Smith RG. Development of growth hormone secretagogues. Endocr Rev. 2005;26(3):346-360. doi:10.1210/er.2004-0019. PMID: 15814848.
6. Copinschi G, Van Onderbergen A, L'Hermite-Balériaux M, et al. Effects of a 7-day treatment with a novel, orally active, growth hormone (GH) secretagogue, MK-677, on 24-hour GH profiles, insulin-like growth factor I, and adrenocortical function in normal young men. J Clin Endocrinol Metab. 1996;81(8):2776-2782. doi:10.1210/jcem.81.8.8768828. PMID: 8768828.
7. Chapman IM, Bach MA, Van Cauter E, et al. Stimulation of the growth hormone (GH)-insulin-like growth factor I axis by daily oral administration of a GH secretagogue (MK-677) in healthy elderly subjects. J Clin Endocrinol Metab. 1996;81(12):4249-4257. doi:10.1210/jcem.81.12.8954023. PMID: 8954023.
8. Copinschi G, Leproult R, Van Onderbergen A, et al. Prolonged oral treatment with MK-677, a novel growth hormone secretagogue, improves sleep quality in man. Neuroendocrinology. 1997;66(4):278-286. doi:10.1159/000127249. PMID: 9349662.
9. Svensson J, Lönn L, Jansson JO, et al. Two-month treatment of obese subjects with the oral growth hormone (GH) secretagogue MK-677 increases GH secretion, fat-free mass, and energy expenditure. J Clin Endocrinol Metab. 1998;83(2):362-369. doi:10.1210/jcem.83.2.4539. PMID: 9467542.
10. Murphy MG, Plunkett LM, Gertz BJ, et al. MK-677, an orally active growth hormone secretagogue, reverses diet-induced catabolism. J Clin Endocrinol Metab. 1998;83(2):320-325. doi:10.1210/jcem.83.2.4551. PMID: 9467534.
11. Svensson J, Ohlsson C, Jansson JO, et al. Treatment with the oral growth hormone secretagogue MK-677 increases markers of bone formation and bone resorption in obese young males. J Bone Miner Res. 1998;13(7):1158-1166. doi:10.1359/jbmr.1998.13.7.1158. PMID: 9661080.
12. Bach MA, Rockwood K, Zetterberg C, et al. The effects of MK-0677, an oral growth hormone secretagogue, in patients with hip fracture. J Am Geriatr Soc. 2004;52(4):516-523. doi:10.1111/j.1532-5415.2004.52156.x. PMID: 15066065.
13. Nass R, Pezzoli SS, Oliveri MC, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med. 2008;149(9):601-611. doi:10.7326/0003-4819-149-9-200811040-00003. PMID: 18981485.
14. Sevigny JJ, Ryan JM, van Dyck CH, Peng Y, Lines CR, Nessly ML; MK-677 Protocol 201 Study Group. Growth hormone secretagogue MK-677: no clinical effect on AD progression in a randomized trial. Neurology. 2008;71(21):1702-1708. doi:10.1212/01.wnl.0000335163.88054.e7. PMID: 19015485.
15. Adunsky A, Chandler J, Heyden N, et al. MK-0677 (ibutamoren mesylate) for the treatment of patients recovering from hip fracture: a multicenter, randomized, placebo-controlled phase IIb study. Arch Gerontol Geriatr. 2011;53(2):183-189. doi:10.1016/j.archger.2010.10.004. PMID: 21067829.
16. Sigalos JT, Pastuszak AW. The Safety and Efficacy of Growth Hormone Secretagogues. Sex Med Rev. 2018;6(1):45-53. doi:10.1016/j.sxmr.2017.02.004. PMID: 28400207.

Research-use-only disclaimer. This article is a literature review compiled strictly for research reference; not for human or veterinary use. It contains no dosing, administration or protocol information, makes no health, performance or body-composition claims, and does not constitute medical, regulatory or purchasing advice. Compounds discussed are research reagents; their regulatory status varies by jurisdiction.