– RLX-33 is a potent, selective and blood-brain barrier (BBB) penetrant relaxin family peptide 3 (RXFP3) antagonist, also blocks relaxin-3-induced ERK1/2 phosphorylation, with IC50 values of 2.36 ?M for RXFP3, 7.82 and 13.86 ?M for ERK1 and ERK2 phosphorylation, respectively. RLX-33 can block the stimulation of food intake induced by the RXFP3-selective agonist R3/I5 in rats. RLX-33 can be used for the research of metabolic syndrome[1].