Potency, efficacy, and selectivity of GR64349 at human recombinant neurokinin NK2 and NK1 receptors

The affinity, potency, efficacy, and selectivity of the NK2 receptor agonist GR64349 ([Lys³,Gly⁸,-R-γ-lactam-Leu⁹]NKA(3-10)) at human recombinant NK2 and NK1 receptors was examined. In radioligand binding studies, GR64349 displaced [¹²⁵I]-NKA binding to NK2 receptors with high affinity (pKi 7.77 + 0.10) but only weakly displaced [³H]-septide binding to NK1 receptors (pKi <5). In functional studies examining increases in intracellular inositol-1 phosphate (IP-1) accumulation, calcium levels, and cyclic AMP synthesis, GR64349 was a full agonist by reference to the endogenous agonists NKA (NK2 receptors) and substance P (NK1 receptors). GR64349 increased IP-1 accumulation with 1,400-fold greater potency in cells expressing NK2 receptors (pEC₅₀ 9.10 + 0.16) than cells expressing NK1 receptors (pEC₅₀ 5.95 + 0.80). For calcium responses, GR64349 was 500-fold more potent in the assay using NK2 receptors (pEC₅₀ 9.27 + 0.26) than NK1 receptors (pEC₅₀ 6.55 + 0.16). GR64349 also stimulated cyclic AMP synthesis in both cell lines, and was almost 900-fold more potent at NK2 receptors (pEC₅₀ 10.66 + 0.27) than NK1 receptors (pEC₅₀ 7.71 + 0.41). These findings confirm that GR64349 is the most selective NK2 receptor agonist described to date.