MK-677
An oral non-peptide ghrelin-receptor agonist studied as a growth-hormone secretagogue; raised IGF-1 in trials but showed metabolic drawbacks and no demonstrated functional benefit.
Reviewed by Mei Li, PhD, Chief Science Officer
Overview
MK-677, also called ibutamoren, is a synthetic, orally active small molecule that was investigated by Merck beginning in the mid-1990s as a growth-hormone secretagogue. Unlike most compounds discussed in the peptide space, it is technically not a peptide; it is grouped with GH secretagogues because it mimics the action of ghrelin. Published research has examined how it raises circulating growth hormone (GH) and insulin-like growth factor 1 (IGF-1). The most cited human data come from a two-year randomized controlled trial in healthy older adults (Nass et al., Annals of Internal Medicine, 2008), which reported that daily oral administration restored GH and IGF-1 toward young-adult levels and increased fat-free (lean) mass, while also increasing appetite and modestly raising fasting glucose with reduced insulin sensitivity. Notably, that trial found no improvement in muscle strength or physical function despite the biomarker changes. Merck pursued multiple indications, including age-related muscle loss, hip-fracture recovery and Alzheimer's disease, but the program was discontinued and MK-677 was never approved by the FDA for any use. It remains unscheduled and research-use-only.
Mechanism of action
In the published literature, MK-677 acts as a non-peptide agonist at the growth-hormone secretagogue receptor (GHSR-1a), the same receptor targeted by the endogenous hormone ghrelin. Cryo-EM structural work on the human ghrelin receptor bound to ibutamoren (Nature Communications, 2021) helped clarify how the molecule engages this target. By activating GHSR-1a in the hypothalamus and pituitary, it stimulates pulsatile release of growth hormone, which in turn raises hepatic IGF-1 production; phase 2 trials documented roughly 60-90% increases in serum IGF-1. Because it works through the physiological GH-releasing pathway rather than delivering exogenous GH, the described effect is amplification of the body's own GH/IGF-1 axis. Ghrelin-receptor agonism also drives the appetite stimulation observed across trials. Its oral bioavailability and a roughly 24-hour half-life supporting once-daily administration distinguish it mechanistically from injectable peptide secretagogues.
What published research has examined
Research on MK-677 has centered on conditions tied to declining GH/IGF-1 signaling. Merck-era clinical programs examined its effects on body composition and physical function in healthy older adults (Nass et al., 2008), recovery in elderly patients after hip fracture (Adunsky et al., 2011), and cognition in Alzheimer's disease (Sevigny et al., 2008). Additional published studies have explored GH-deficient adults and children and catabolic states. A recurring theme across this literature is a disconnect between robust biomarker changes and clinical outcomes: the compound reliably elevated IGF-1 and increased lean mass, yet trials did not demonstrate corresponding functional benefit, such as improved strength or slowed cognitive decline. More recent investigator-initiated work has examined metabolic contexts, including a registered trial studying its impact on nonalcoholic fatty liver disease. All of this is preclinical or investigational; MK-677 is not approved for any indication.
Safety considerations in the literature
The published literature documents several safety concerns that are relevant context, not guidance. Because it amplifies GH/IGF-1 signaling and mimics ghrelin, trials consistently reported increased appetite, fluid retention, and metabolic effects, most importantly a rise in fasting glucose and reduced insulin sensitivity in the two-year Nass trial, raising a signal for impaired glucose metabolism. A phase 2b hip-fracture study in elderly patients (Adunsky et al., 2011) was stopped early after more participants receiving MK-677 developed congestive heart failure than those on placebo, and researchers concluded the safety profile was unfavorable in that frail population. Elevated IGF-1 itself is a theoretical concern in the literature given IGF-1's associations. MK-677 was never approved by the FDA and remains an unscheduled research-use-only compound; it is not part of the FDA compounding-bulks review and is not on the July 2026 PCAC docket.
Key published studies
| Study | Journal | Year | Finding |
|---|---|---|---|
| Effects of an Oral Ghrelin Mimetic on Body Composition and Clinical Outcomes in Healthy Older Adults: A Randomized Trial | Annals of Internal Medicine | 2008 | Two-year RCT (Nass et al.) in adults aged 60-81; oral administration restored GH/IGF-1 toward young-adult levels and increased fat-free mass, but raised fasting glucose, reduced insulin sensitivity, and produced no strength or functional gain. Human clinical trial, highest-quality evidence for the compound. |
| MK-0677 (ibutamoren mesylate) for the treatment of patients recovering from hip fracture: a multicenter, randomized, placebo-controlled phase IIb study | Clinical Nutrition | 2011 | Phase 2b trial (Adunsky et al.) in elderly hip-fracture patients halted early after a congestive heart failure signal; investigators judged the safety profile unfavorable in that population. Human clinical trial. |
| Structural basis of human ghrelin receptor signaling by ghrelin and the synthetic agonist ibutamoren | Nature Communications | 2021 | Cryo-EM structural study resolving how ibutamoren engages the human ghrelin receptor (GHSR-1a), clarifying its non-peptide agonist mechanism. Preclinical structural/mechanistic evidence. |
Regulatory status
Current status: Unscheduled / RUO. Never FDA approved. PepSense tracks FDA filings and updates this record as the regulatory picture changes. See the FDA PCAC tracker for the July 2026 vote and what happens next.