Nonapeptide-1 (Melanostatine-5) acetate salt, a peptide hormone, is a selective antagonist of MC1R (Ki: 40 nM). Nonapeptide-1 acetate salt is a competitive α-MSH antagonist that potently inhibits intracellular cAMP and melanosome dispersion induced by α-MSH in melanocytes (IC50: 2.5 nM and 11 nM, respectively). Nonapeptide-1 acetate salt inhibits melanin synthesis, and can be used in the research of skin pigmentation and regulation of steroid production in the adrenal gland, skin cancer.