MK-677 (ibutamoren mesylate, also L-163,191 and MK-0677) is a non-peptide spiroindoline sulfonamide growth hormone secretagogue developed at Merck. Its design and biological activities were reported in 1995, when it was selected for clinical study. It acts at the growth hormone secretagogue receptor (GHSR-1a), the G protein-coupled receptor later identified as the receptor for ghrelin. Published research includes in vitro pituitary cell assays, endocrine studies in dogs and rats, receptor structural biology, rodent neurodegeneration models and registered clinical trials.