Ibutamoren (MK-677)

What is Ibutamoren (MK-677)?#

Ibutamoren (developmental code MK-677, also known as MK-0677 or L-163,191) is a potent, long-acting, orally active, non-peptide agonist of the ghrelin receptor (growth hormone secretagogue receptor type 1a, GHS-R1a). Developed by Merck & Co. in the 1990s, ibutamoren mimics the growth hormone-stimulating action of the endogenous hormone ghrelin, producing sustained increases in growth hormone (GH) and insulin-like growth factor 1 (IGF-1) levels.

Unlike peptide-based growth hormone secretagogues such as GHRP-6 or hexarelin, ibutamoren is a small molecule with a spiroindoline sulfonamide structure, giving it oral bioavailability. This property made it attractive as a potential alternative to injectable GH therapy for conditions involving GH deficiency, sarcopenia, and osteoporosis.

Ibutamoren reached phase 2b clinical trials but was never approved for human use. Development was effectively discontinued due to safety concerns, particularly increased rates of congestive heart failure observed in elderly patient populations and unfavorable effects on glucose metabolism.

Mechanism of Action#

Ibutamoren acts as a selective agonist of the GHS-R1a receptor, the same receptor that binds endogenous ghrelin. By activating this receptor in the anterior pituitary gland and hypothalamus, ibutamoren stimulates growth hormone release through a mechanism distinct from growth hormone-releasing hormone (GHRH).

Growth Hormone Stimulation#

• Pulsatile GH release: MK-677 enhances the amplitude of GH secretory pulses without disrupting the normal pulsatile pattern, mimicking physiological GH secretion rather than producing flat, supraphysiological levels
• IGF-1 elevation: Sustained GH stimulation leads to increased hepatic IGF-1 production, with levels rising to those typical of young adults in elderly subjects
• GH axis restoration: In clinical trials, MK-677 increased GH secretion by up to 97% in elderly subjects, effectively restoring age-related declines

Additional Receptor Effects#

• Appetite stimulation: GHS-R1a activation in the hypothalamus increases appetite, consistent with ghrelin's role as the "hunger hormone"
• Cortisol elevation: Modest increases in cortisol levels have been observed, though typically remaining within the normal physiological range
• Sleep architecture: Some studies suggest improvements in REM sleep duration, consistent with GH's role in sleep regulation

Research Overview#

Ibutamoren has been evaluated in multiple clinical trials spanning growth hormone deficiency, age-related sarcopenia, hip fracture recovery, and body composition in healthy elderly adults. The most significant body of evidence comes from Merck-sponsored trials conducted in the late 1990s and 2000s.

The landmark Nass et al. (2008) trial in healthy older adults demonstrated that daily MK-677 increased fat-free mass by 1.6 kg over 12 months, with sustained GH and IGF-1 elevation. However, the Adunsky et al. hip fracture recovery trial was stopped early due to increased congestive heart failure rates, effectively ending clinical development.

Important Considerations#

• Not approved by any regulatory agency for human use
• Classified as a prohibited substance by WADA (World Anti-Doping Agency) under S2 (peptide hormones, growth factors, and related substances)
• Associated with insulin resistance and elevated fasting glucose
• Heart failure risk identified in vulnerable elderly populations
• Increases appetite, which may be undesirable for some uses
• Not legal as a dietary supplement ingredient in the United States
• Long-term safety has not been established

Key Research Findings#

Effects of an Oral Ghrelin Mimetic on Body Composition and Clinical Outcomes in Healthy Older Adults: A Randomized Trial, published in Annals of Internal Medicine (Nass R et al., 2008; PMID: 18981485):

• The study demonstrated GH secretion increased by of 97% compared to baseline
• The study showed landmark 2-year modified crossover RCT in 65 healthy adults aged 60-81. MK-677 25 mg daily significantly increased fat free mass by 1.6 kg over 12 months and restored GH and IGF-1 levels to those of young adults.

MK-0677 (Ibutamoren Mesylate) for the Treatment of Patients Recovering from Hip Fracture: A Multicenter, Randomized, Placebo-Controlled Phase IIb Study, published in Archives of Gerontology and Geriatrics (Adunsky A et al., 2011; PMID: 21067829):

• The study showed congestive heart failure rate of 6.5% ibutamoren vs 1.7% placebo

Related Reading#

• Ibutamoren (MK-677) research studies and evidence
• Ibutamoren (MK-677) dosing protocols
• Ibutamoren (MK-677) side effects profile
• Sermorelin research guide
• Tesamorelin research guide