Nonapeptide-1 is an antagonist of melanocortin receptor 1 (MC1R; Ki = 40 nM in COS-1 cells expressing human receptors). It is selective for MC1R over MC3R, MC4R, and MC5R (Kis = 470, 1,340, and 2,400 nM, respectively). Nonapeptide-1 inhibits cAMP accumulation and melanosome dispersion induced by α-melanocyte-stimulating hormone (α-MSH) in melanocytes with IC50 values of 2.5 and 11 nM, respectively.
