GPS1573 is a newly described potent and dose-dependent peptide antagonist of ACTH-stimulated melanocortin type 2 receptor (MC2R) activity in vitro (IC50 = 66 nM). GPS1573 antagonizes MC4R with IC50 of 950 nM, therefore showing selectivity for MC2R. The binding of ACTH to MC2R regulates steroidogenesis leading to metabolic diseases such as Cushing's disease in patients with pituitary dysfunctions.H-Nle-Pro-(D-Phe)-Arg-(D-Trp)-Phe-Lys-Ala-Val-Gly-Lys-Lys-Arg-Arg-NH2 or H-Nle-PfRwFKAVGKKRR-NH2