For laboratory research use only. Not for human consumption. All compounds referenced in this article are sold by Explicit SARMs solely as reference materials for in-vitro and animal research by qualified professionals. Nothing here is medical advice, and no statement has been evaluated by the FDA. These materials are not intended to diagnose, treat, cure or prevent any disease.
MK-677, also known as ibutamoren and by its Merck development codes MK-0677 and L-163,191, is frequently listed beside SARMs in research catalogs. Pharmacologically, it does not belong there. MK-677 is not a selective androgen receptor modulator and does not bind the androgen receptor. It is a nonpeptide agonist of the growth hormone secretagogue receptor (GHS-R1a), the same receptor activated by the endogenous hormone ghrelin. Its primary measured effect in published studies is increased pulsatile secretion of growth hormone (GH) and a downstream rise in insulin-like growth factor 1 (IGF-1).
Getting that classification right matters for study design, because the endpoints, controls and confounders for a ghrelin mimetic are very different from those for an androgen receptor ligand. Explicit SARMs supplies Ibutamoren MK-677 Capsules and MK-677 Liquid strictly as laboratory reference material.
What Is MK-677?
- Common names: MK-677, MK-0677, ibutamoren, ibutamoren mesylate, L-163,191
- Class: nonpeptide growth hormone secretagogue (ghrelin mimetic)
- Primary target: growth hormone secretagogue receptor type 1a (GHS-R1a, the ghrelin receptor)
- Not a SARM; no androgen receptor binding
- Originator: Merck Research Laboratories
- Regulatory status: not approved by the FDA or any other regulator for any indication
Mechanism of Action
GHS-R1a is a G protein-coupled receptor expressed in the pituitary and hypothalamus, as well as other tissues. It was cloned by a Merck team in 1996 using MK-0677 and related compounds as probes, before its natural ligand was known. [2] In 1999, Kojima and colleagues identified ghrelin, an acylated peptide produced mainly in the stomach, as the endogenous ligand. [3]
When MK-677 activates GHS-R1a in the pituitary and hypothalamus, it increases the amplitude of growth hormone pulses. GH then acts on the liver and other tissues to raise IGF-1. Because MK-677 works through the body’s own GH axis rather than supplying GH, the pattern of release remains pulsatile, and the response is still subject to feedback by somatostatin and IGF-1.
Ghrelin-receptor activation has effects beyond GH release. The receptor is involved in appetite signaling, and ghrelin mimetics can raise cortisol and prolactin modestly in some studies. These off-GH effects are important for interpreting data.
Research History
Merck designed MK-0677 in the early 1990s as part of a program to find non-injectable, nonpeptide alternatives to GH-releasing peptides such as GHRP-6. Its design and biological activity were published in PNAS in 1995. [1] The compound became an essential tool in the discovery of the ghrelin receptor itself. [2]
Merck and academic investigators then ran a series of clinical studies in older adults, in patients recovering from hip fracture, and in Alzheimer disease, among other settings. None led to approval.
What Published Studies Report
Endocrine studies in older adults
In a 1996 study in the Journal of Clinical Endocrinology and Metabolism, Chapman and colleagues reported that daily MK-677 increased 24-hour GH secretion and serum IGF-1 in healthy elderly subjects. [4]
A two-year randomized, placebo-controlled trial published in Annals of Internal Medicine in 2008 by Nass and colleagues (NCT00474279) studied MK-677 in healthy older adults. The investigators reported that GH and IGF-1 rose to levels typical of young adults and that fat-free mass increased relative to placebo, but this did not translate into changes in strength or function. They also reported a rise in fasting blood glucose, decreased insulin sensitivity, increased cortisol, and, as the most frequent side effects, increased appetite and transient mild edema and muscle pain. [5]
Hip fracture
A multicenter phase IIb trial in older patients recovering from hip fracture, published in 2011, reported a significant rise in IGF-1, but that rise was not matched by improvement in most functional measures. The trial was terminated early because of a safety signal of congestive heart failure in a limited number of patients, and the authors concluded that MK-0677 had an unfavorable safety profile in that population. [6]
Alzheimer disease
A 12-month randomized trial in Neurology (2008) found that MK-677 raised IGF-1 but had no effect on the progression of Alzheimer disease. [7]
Summary of the clinical record
Across these studies, MK-677 reliably increased GH and IGF-1. The recurring safety-related findings in the literature were increased appetite, edema, rises in fasting glucose, reduced insulin sensitivity and increased cortisol, plus a congestive heart failure safety signal that ended the hip fracture trial early. [5][6]
Research Considerations
- Classify correctly. MK-677 is a ghrelin-receptor agonist. Designs that borrow SARM endpoints, such as androgen-dependent tissue weights or HPG-axis suppression, measure the wrong biology.
- Glucose homeostasis. Include fasting glucose, insulin and HbA1c or glucose tolerance testing in animal protocols.
- Feeding behavior. Because GHS-R1a regulates appetite, pair-feeding or careful food-intake monitoring helps separate direct GH effects from intake effects.
- GH pulsatility. Single GH measurements are noisy. IGF-1 is the more stable readout, and frequent sampling is needed to characterize GH pulses.
- Comparators. Peptide GH secretagogues such as ipamorelin act on the same receptor and are useful comparators, while GHRH analogs act on a different receptor.
Why MK-677 Is Often Mislabeled as a SARM
MK-677 is sold by many research suppliers next to SARMs, and some online content describes it as a SARM or claims that it acts on androgen receptors. Both claims are incorrect. The confusion appears to come from catalog grouping, not pharmacology.
The distinctions are concrete:
- Receptor type. GHS-R1a is a G protein-coupled receptor on the cell surface. The androgen receptor is a nuclear receptor that acts as a transcription factor.
- Signaling chain. MK-677 acts upstream of GH and IGF-1. SARMs act directly on androgen-responsive genes.
- Hormonal axis. MK-677 is not reported to suppress the hypothalamic-pituitary-gonadal axis in the way androgen receptor agonists do.
- WADA section. Ibutamoren is listed under S2 (peptide hormones, growth factors, related substances and mimetics), not S1.2 with the SARMs. [8]
Keeping that classification straight avoids misdesigned studies and inaccurate summaries of the literature.
WADA Status
Ibutamoren (MK-677) is named in WADA section S2.2.4 as a growth hormone secretagogue and is prohibited at all times. [8]
Storage, Purity and COA
- Store sealed and protected from light, heat and moisture. MK-677 is commonly supplied as the mesylate salt, which is hygroscopic in powder form.
- Keep the liquid format tightly capped to prevent solvent evaporation.
- Record lot numbers and receipt dates.
- Review the lot-specific Certificate of Analysis, which should report HPLC purity, identity by mass spectrometry, and whether the material is the free base or mesylate salt, since molecular weight differs between the two.
For lot documentation, use the contact page.
Related Research Compounds
- MK-777 Acetamoren Capsules, a commercially named N-acetyl derivative of ibutamoren with no published pharmacology (see the MK-777 research overview)
- Ipamorelin 5mg, a peptide GHS-R1a agonist
- Tesamorelin 10mg, a GHRH analog that acts on a different receptor
- SARMs collection
Frequently Asked Questions
Is MK-677 a SARM?
No. MK-677 is a growth hormone secretagogue that activates the ghrelin receptor (GHS-R1a). It does not bind the androgen receptor. It is sold next to SARMs by convention only.
Does MK-677 bind the androgen receptor?
No. Its target is GHS-R1a. Any effect it has on tissues runs through GH and IGF-1 signaling, not androgen receptor activation.
Is MK-677 approved by the FDA?
No. It has been studied in multiple clinical trials but has not been approved by the FDA or any other regulator.
What safety findings appear in the published trials?
Investigators reported increased appetite, rises in fasting glucose and decreased insulin sensitivity, and the hip fracture trial was terminated early because of a congestive heart failure safety signal. [5][6]
Is MK-677 prohibited in sport?
Yes. Ibutamoren is named in WADA section S2.2.4 and is prohibited at all times. [8]
How is MK-677 related to ghrelin?
MK-677 mimics ghrelin at the GHS-R1a receptor. Historically, the receptor was identified using MK-0677 before ghrelin itself was discovered. [2][3]
References
- Patchett AA, et al. Proc Natl Acad Sci U S A. 1995. PubMed 7624358
- Howard AD, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996. PubMed 8688086
- Kojima M, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999. PubMed 10604470
- Chapman IM, et al. J Clin Endocrinol Metab. 1996. PubMed 8954023
- Nass R, et al. Ann Intern Med. 2008. PubMed 18981485; trial record NCT00474279
- Adunsky A, 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. PubMed 21067829
- Sevigny JJ, et al. Growth hormone secretagogue MK-677: no clinical effect on AD progression in a randomized trial. Neurology. 2008. PubMed 19015485
- World Anti-Doping Agency. 2026 Prohibited List. wada-ama.org
For laboratory research use only. Not for human consumption. All compounds referenced in this article are sold by Explicit SARMs solely as reference materials for in-vitro and animal research by qualified professionals. Nothing here is medical advice, and no statement has been evaluated by the FDA. These materials are not intended to diagnose, treat, cure or prevent any disease.