Tri-peptide type compound operating a melanocortin receptor

PURPOSE: A tri-peptide type compound operating a melanocortin receptor is provided, which compound is useful as an accelerator or antagonist for a melanocortin receptor. CONSTITUTION: The tri-peptide type compound operating a melanocortin receptor is represented by formula(1) or (2), wherein a is 0 or 1, provided that a is 0 when b is 1 and a is 1 when b is 0; W is C1-10 alkyl; E1 is amino acid or its derivative represented by formula(3) or (4) wherein A1 and A2 is the same or different alkyl, and A3 and A4 are the same or different alkyl; E2 is amino acid or its derivative represented by formula(5) or (6) wherein n is 0, 1 or 2, A5 and A6 are linked together, each is C1-3 alkyl, and A7 and A8 is the same or different alkyl; E3 is amino acid or its derivative represented by formula(7) wherein A9 and A10 are any one of hydrogen and the other of C1-5 alkyl; and E4 is amino acid or its derivative represented by formula(8) or (9) wherein A11 and A12 are any one of hydrogen and the other of C1-3 alkyl.