MK 677, also known as Ibutamoren, Oratrope, or Nutrobal (molecular weight 528.662), is a ghrelin agonist that acts on the pituitary gland as a human growth hormone secretagogue and is sometimes classified along with selective androgen receptor modulators (SARMs). It differs from Ghrelin itself in that it is not a peptide but rather a spiropiperidine. Since it can cross the blood-brain barrier, MK-677 functions as a neuropeptide in the central nervous system.