NF-56-EJ40 hydrochloride is a potent, high-affinity, and highly selective human SUCNR1 (GPR91) antagonist with an IC 50 of 25 nM and a K i of 33 nM, and shows almost no activity towards rat SUCNR1. NF-56-EJ40 hydrochloride has high affinity for humanized rat SUCNR1 with a K i value of 17.4 nM.