MK-677 (Ibutamoren): Complete Research Guide & Chemical Profile

MK-677, also known as ibutamoren mesylate (L-163,191), is a non-peptide, orally active growth hormone secretagogue (GHS) that mimics the GH-stimulating action of the endogenous peptide hormone ghrelin. Unlike peptide-based GH secretagogues such as GHRP-6 or GHRP-2, MK-677 is a small molecule with full oral bioavailability that activates the growth hormone secretagogue receptor type 1a (GHS-R1a, also known as the ghrelin receptor). Developed by Merck Research Laboratories, MK-677 produces sustained elevation of GH and IGF-1 levels over a 24-hour period following a single oral dose.

• Generic Name: Ibutamoren mesylate (MK-677, MK-0677, L-163,191)
• IUPAC: 2-Amino-2-methyl-N-[1-(1-methylsulfonylspiro[2H-indene-1,4'-piperidine]-6-yl)oxy]-1-oxo-3-(phenylmethoxy)propyl)-2-methylpropanamide methanesulfonate
• CAS Registry Number: 159752-10-0 (mesylate salt)
• Molecular Formula: C27H36N4O5S (free base)
• Molecular Weight: 528.67 Da (free base); 624.77 Da (mesylate salt)
• Chemical Class: Spiroindoline, non-peptide GH secretagogue
• Target Receptor: GHS-R1a (ghrelin receptor)
• Oral Bioavailability: High (small molecule)
• Classification: Non-peptide growth hormone secretagogue, ghrelin receptor agonist

MK-677 features a spiroindoline scaffold that was developed through systematic medicinal chemistry optimization of earlier benzolactam-derived GHS leads at Merck. The molecule contains a unique spirocyclic ring system in which an indene ring and a piperidine ring share a common spiro carbon atom, creating a rigid three-dimensional framework that presents pharmacophoric groups in the optimal spatial orientation for GHS-R1a binding.

The molecular architecture includes an alpha-aminoisobutyric acid (Aib)-derived moiety, a benzyloxymethyl group, and the signature spiroindoline-piperidine core. The methane sulfonate salt form (mesylate) is used for improved aqueous solubility and crystallinity. McKee et al. (1997) cloned and characterized the rat GHS-R and demonstrated that [35S]MK-0677 binds with high affinity (Kd = 0.7 nM), confirming the receptor specificity of this non-peptide ligand.

MK-677 activates the GHS-R1a receptor, a 366-amino-acid class A G protein-coupled receptor that is the same target engaged by the endogenous 28-amino-acid peptide hormone ghrelin. Upon MK-677 binding, GHS-R1a couples to Gq/11, activating phospholipase C and generating inositol trisphosphate (IP3) and diacylglycerol (DAG). The resulting intracellular calcium mobilization in pituitary somatotrophs triggers GH vesicle exocytosis.

MK-677 produces GH release through a mechanism distinct from and additive to GHRH-R activation. The GHS-R1a pathway operates synergistically with the GHRH pathway while being partially independent of somatostatin inhibition. A defining characteristic of MK-677 is its ability to produce sustained 24-hour elevation of GH pulsatility and IGF-1 levels after a single oral dose, preserving the endogenous pulsatile pattern of GH secretion rather than producing a monophasic GH peak. This sustained activity reflects the compound's pharmacokinetic profile and the amplification of endogenous GH pulse amplitude.

The key pharmacological advantage of MK-677 over peptide-based GH secretagogues is its full oral bioavailability. As a small molecule (MW 528.67), MK-677 is absorbed from the gastrointestinal tract and achieves systemic exposure sufficient for GHS-R1a activation without requiring parenteral administration.

• Administration: oral (tablet or capsule formulation in clinical trials)
• Oral bioavailability: high (non-peptide small molecule)
• Typical research dose: 25 mg once daily (clinical trial standard)
• Onset of GH release: within 1-2 hours of oral administration
• Duration of effect: 24 hours of elevated GH pulsatility from a single dose
• IGF-1 elevation: sustained increase over days to weeks of administration; 65% greater increase vs. placebo in clinical trials (Nass et al., 2008)
• Tmax: approximately 1-2 hours (peak plasma concentration)
• Protein binding: not fully characterized in published literature
• Metabolism: hepatic; metabolites not fully elucidated
• Elimination half-life: sufficient for once-daily dosing

MK-677 has been evaluated in multiple clinical and preclinical research contexts, primarily as a pharmacological tool for sustained GHS-R1a activation and GH/IGF-1 axis stimulation.

• Body composition research: effects on fat-free mass, lean body mass, and body composition in older adults (Nass et al., 2008)
• Sarcopenia: prevention of age-related decline in fat-free mass and muscle function
• Bone metabolism: effects on bone mineral density and bone remodeling markers
• Hip fracture recovery: functional recovery and IGF-1 restoration in elderly patients (Bach et al., 2004)
• Renal disease: IGF-1 elevation in hemodialysis patients with protein-energy wasting (Campbell et al., 2018)
• GH deficiency: GH-IGF-1 axis stimulation in GH-deficient children (Codner et al., 2001)
• GH isoform research: effects on 22K vs. non-22K GH isoform proportions (Svensson et al., 2003)
• Neurodegeneration: evaluation in AD mouse models for hippocampal neurogenesis effects (Tian et al., 2019)

The most comprehensive clinical dataset for MK-677 comes from a 2-year, double-blind, randomized, placebo-controlled trial in 65 healthy older adults (ages 60-81). Nass et al. (2008) reported that daily administration of MK-677 25 mg significantly increased GH and IGF-1 levels to those of healthy young adults. Fat-free mass increased by 1.1 kg in the MK-677 group versus a decrease of 0.5 kg in the placebo group. Body weight increased by 2.7 kg, and fasting glucose increased by approximately 0.3 mmol/L.

In a multicenter hip fracture trial, Bach et al. (2004) evaluated MK-677 in 161 previously ambulatory patients aged 65 and older. MK-677 increased serum IGF-1 by 84% compared to 17% with placebo, but statistically significant improvements in functional performance measures were not achieved, potentially due to the limitations of outcome measures and small sample sizes. In hemodialysis patients, Campbell et al. (2018) demonstrated a 65% greater increase in IGF-1 with MK-677 versus placebo with minimal adverse effects.

• MK-677 (Ibutamoren): non-peptide, oral, MW 528.67, GHS-R1a selective, 24h GH elevation, once-daily dosing, no injection required
• GHRP-6: hexapeptide, injectable, MW 873.01, GHS-R1a agonist, short-acting GH pulse, appetite stimulation, cortisol/prolactin elevation
• GHRP-2 (Pralmorelin): hexapeptide, injectable, MW 817.97, most potent classical GHRP, approved diagnostic agent in Japan
• Ipamorelin: pentapeptide, injectable, MW 711.85, GHS-R1a selective, minimal cortisol/prolactin effects, clean secretagogue profile
• Hexarelin: hexapeptide, injectable, MW 887.04, potent GH release, significant prolactin/cortisol elevation, cardioprotective properties
• Ghrelin: endogenous 28-amino acid acylated peptide, requires octanoylation for receptor activation, injectable, short half-life

• Solid form: store at room temperature (15-25°C) in airtight container; protect from light and moisture
• Long-term storage: -20°C for maximum stability; stable for 24+ months as anhydrous solid
• Solution preparation: soluble in DMSO (typically 50-100 mM stock); limited aqueous solubility at neutral pH
• Working solutions: prepare fresh in appropriate vehicle; DMSO stocks stable at -20°C for months
• Mesylate salt: hygroscopic; handle under dry conditions; store with desiccant
• Light sensitivity: moderate; store in amber containers or protect from direct light

• HPLC (RP): primary purity assessment; >98% expected for research grade; UV detection at 254 nm
• Mass spectrometry: ESI-MS for molecular weight confirmation (expected 528.67 Da free base, 624.77 Da mesylate)
• 1H and 13C NMR: structural verification of spiroindoline scaffold and functional groups
• Elemental analysis: C, H, N, S content verification for salt form confirmation
• Melting point: characteristic range for mesylate salt
• Karl Fischer titration: water content determination (<0.5% for anhydrous form)
• Radioligand binding: [35S]MK-0677 displacement assay for functional activity confirmation (Kd = 0.7 nM at rat GHS-R1a)

MK-677 remains an investigational compound that has not received regulatory approval for any therapeutic indication. Despite extensive clinical trial data, the translation from GH/IGF-1 elevation to clinically meaningful functional outcomes has been challenging. The 2-year trial by Nass et al. demonstrated that increased fat-free mass did not translate into measurable improvements in strength or function in healthy elderly subjects.

Notable limitations include glucose metabolism effects (fasting glucose elevation, insulin sensitivity reduction), appetite stimulation, and the failure of a large-scale Alzheimer's disease prevention trial. Tian et al. (2019) found that MK-677 fostered hippocampal neurogenesis in a 5xFAD mouse model but did not prevent hippocampal amyloid-beta deposition, synaptic loss, microglial activation, or cognitive impairment. At higher doses, increased mortality was observed in 5xFAD mice. These findings underscore the complexity of GHS-R1a pharmacology and the limitations of GH axis stimulation as a standalone therapeutic strategy.

• Read more: TRH (Thyrotropin-Releasing Hormone): Research Guide & Chemical Profile → https://www.chemverify.com/learn/trh-thyrotropin-releasing-hormone-research-guide
• Read more: Ipamorelin + CJC-1295 (No DAC) Stack: Synergy Research Guide → https://www.chemverify.com/learn/ipamorelin-cjc-1295-no-dac-stack-synergy
• Read more: TP508 (Chrysalin): Research Guide & Chemical Profile → https://www.chemverify.com/learn/tp508-chrysalin-research-guide-chemical-profile
• Read more: Semax for Cognitive Research: ACTH(4-10) Analog Mechanism → https://www.chemverify.com/learn/semax-cognitive-research-acth-mechanism