MK-677, also known as Ibutamoren or L-163,191, is a potent orally active non-peptide ghrelin receptor agonist and growth hormone secretagogue. Developed by Merck, it has been studied extensively for its ability to increase GH pulse amplitude and sustain IGF-1 elevation without suppressing the hypothalamic-pituitary axis feedback in the same manner as exogenous GH administration.
MK-677 mimics ghrelin by binding to GHSR-1a in the pituitary and hypothalamus, stimulating GH release via the phospholipase C/IP₃ pathway. Unlike GHRH analogues, MK-677 amplifies endogenous GH pulses while preserving the natural pulsatile release pattern. IGF-1 elevation is typically sustained throughout a 24-hour period, distinguishing it from short-acting peptide secretagogues. Notably, its activity is not abolished by somatostatin, which increases GH pulse amplitude rather than frequency.
MK-677 is orally bioavailable with high stability in aqueous solution. Researchers should note that it also weakly activates ghrelin receptors in peripheral tissues, which may confound metabolic endpoints in long-duration studies. Co-administration with GHRH analogues such as Sermorelin has been studied for synergistic GH release via complementary receptor pathways.
For in vitro laboratory research only. Not for human consumption. Not FDA approved.
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