Ibutamoren (INN) (developmental code names MK-677, MK-0677, L-163,191; former tentative brand name Oratrope) is a potent, long-acting, orally-active, selective, and non-peptide agonist of the ghrelin receptor and a growth hormone secretagogue, mimicking the growth hormone (GH)-stimulating action of the endogenous hormone ghrelin. It has been shown to increase the secretion of several hormones including GH and insulin-like growth factor 1 (IGF-1) and produces sustained increases in the plasma levels of these hormones without affecting cortisol levels.
Ibutamoren has been shown to sustain activation of the GH–IGF-1 axis and to increase lean body mass with no change in total fat mass or visceral fat. It is under investigation as a potential treatment for reduced levels of these hormones, such as in children or elderly adults with growth hormone deficiency, and human studies have shown it to increase both muscle mass and bone mineral density, making it a promising potential therapy for the treatment of frailty in the elderly. As of June 2017, ibutamoren is in the preclinical stage of development for growth hormone deficiency.
Since MK-677 is still a Investigational New Drug, it has not yet been approved to be marketed for consumption by humans in the United States. However, it has been used experimentally by some in the bodybuilding community. Since it chemically mimics the hormone ghrelin, it functions as a neuropeptide in the central nervous system and crosses the blood-brain-barrier. According to some recent research and discussion, there is a concern that its particularly longer half-life might over-stimulate the ghrelin receptors in the brain and lead to some harmful mental side effects.