Affinity, potency, efficacy, and selectivity of neurokinin A analogs at human recombinant NK2 and NK1 receptors.

A series of peptide NK2 receptor agonists was evaluated for affinity, potency, efficacy, and selectivity at human recombinant NK2 and NK1 receptors expressed in CHO cells to identify compounds with the greatest separation between NK2 and NK1 receptor agonist activity. Binding studies were performed using displacement of [125I]-NKA binding to NK2 receptors and displacement of [3H]-Septide binding to NK1 receptors expressed in CHO cells. Functional studies examining the increase in intracellular calcium levels and cyclic AMP stimulation were performed using the same cell lines. A correlation was demonstrated between binding affinities (Ki) and potency to increase intracellular calcium (EC50) for NK2 and NK1 receptors. Ranking compounds by their relative affinity (Ki) or potency (EC50) at NK2 or NK1 receptors indicated that the most selective NK2 agonists tested were [Lys5,MeLeu9,Nle10]-NKA(4-10) (NK1/NK2 Ki ratio = 674; NK1/NK2 EC50 ratio = 105) and [Arg5,MeLeu9,Nle10]-NKA(4-10) (NK1/NK2 Ki ratio = 561; NK1/NK2 EC50 ratio = 70). The endogenous peptide, NKA, lacked selectivity with an NK1/NK2 Ki ratio = 20 and NK1/NK2 EC50 ratio = 1. Of the compounds selected for evaluation in cyclic AMP stimulation assays, [ß-Ala8]-NKA(4-10) had the greatest selectivity for activation of NK2 over NK1 receptors (NK1/NK2 EC50 ratio = 244), followed by [Lys5,MeLeu9,Nle10]-NKA(4-10) (ratio = 74), and NKA exhibited marginal selectivity (ratio = 2.8).

Identification and characterization of novel NMDA receptor antagonists selective for NR2A- over NR2B-containing receptors.

NR1/NR2A is a subtype of N-methyl-d-aspartate receptors (NMDARs), which are glutamate and glycine-gated Ca(2+)-permeable channels highly expressed in the central nervous system. A high-throughput screening (HTS) campaign using human osteosarcoma (U-2 OS) cells transiently transduced with NR1/NR2A NMDAR subunits, tested in a specifically designed fluorometric imaging plate reader (FLIPR)/Ca(2+) assay, identified sulfonamide derivative series, exemplified by 3-chloro-4-fluoro-N-[(4-{[2-(phenylcarbonyl)hydrazino]carbonyl}phenyl)methyl]benzenesulfonamide (compound 1) and thiodiazole derivative N-(cyclohexylmethyl)-2-({5-[(phenylmethyl)amino]-1,3,4-thiadiazol-2-yl}thio)acetamide (compound 13) as novel NR1/NR2A receptor antagonists. Compounds 1 and 13 displayed submicromolar and micromolar potency at NR1/NR2A receptor, respectively, although they did not show activity at NR2B-containing receptor up to 50 µM concentration. Addition of 1 mM glycine, but not 1 mM l-glutamate, was able to surmount compound 1 and 13 inhibitory effects in FLIPR NR1/NR2A assay. However, compounds 1 and 13 displaced a glutamate site antagonist [(3)H]d,l-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid ([(3)H]CGP 39653) to a greater extent than the glycine site antagonist [(3)H]3-[(E)-2-carboxy-2-phenylethenyl]-4,6-dichloro-1H-indole-2-carboxylic acid ([(3)H]MDL 105,519), in rat brain cortex binding assay. Results of FLIPR cell-based, electrophysiological, and biochemical binding assays suggest that compounds 1 and 13 are the prototypes of novel classes of NMDAR ligands, which to the best of our knowledge are the first selective antagonists at NR1/NR2A over NR1/NR2B receptor, and might constitute useful tools able to elucidate the relative role of the NR2A subunit in physiological and pathological conditions.