In vitro activity: NPS 2143 blocks increases in cytoplasmic Ca2+ concentrations with IC50 of 43 nM elicited by activating the Ca2+ receptor in HEK 293 cells expressing the human Ca2+ receptor. NPS 2143 stimulates parathyroid hormone (PTH) secretion from bovine parathyroid cells with EC50 of 41 nM. Moreover, NPS 214 also blocks the inhibitory effects of calcimimetic NPS R-467 on PTH secretion from bovine parathyroid cells and the inhibitory effects of extracellular Ca2+ on isoproterenol-stimulated increases in cyclic AMP formation. In HEK-293 cells transiently expressing hCaSRs, NPS 2143 significantly suppresses the kokumi taste by effectively inhibiting the activity of both GSH (data not shown) and γ-Glu-Val-Gly. A recent study shows that NPS 2143 treatment suppresses low molecular weight fractions of azuki hydrolysate-induced cholecystokinin (CCK) secretion in CaSR-transfected HEK 293 cells.

Kinase Assay: This clonal cell line, referred to as HEK 293 4.0-7 cells, are used in a high-throughput screening format to detect agonists and allosteric activators of the Ca2+ receptor. Changes in the concentration of cytoplasmic [Ca2+]i provide a quantitative and functional assessment of Ca2+ receptor activity in these cells and the results using this assay parallel those obtained using a homologous expression system of bovine parathyroid cells. On-line continuous measurements of fluorescence in fluo-3- or fura-2-loaded HEK 293 4.0-7 cells are obtained using a custom-built spectrofluorimeter or a fluorescence imaging plate reader instrument. NPS-2143 is incubated with cells for 1 minute before increasing the concentration of extracellular Ca2+ from 1.0 mM to 1.75 mM. NPS-2143 is tested individually at a concentration of 100 μg/mL (20 μM-80 μM) and those causing more than a 40% inhibition of the control response are considered to be biologically active. To determine the potencies (IC50) of NPS-2143 with biological activity, concentration-response curves are obtained and then, as an initial assessment of selectivity, the effects of NPS-2143 on [Ca2+]i evoked by other G protein-coupled receptors are examined at a concentration several times their IC50. Wild-type HEK 293 cells (and HEK 293 4.0-7 cells) express receptors for thrombin, bradykinin, and ATP, which couple to the mobilization of intracellular Ca2+. These responses can be studied to quickly assess any nonselective action of compounds on G protein-coupled receptors. Additional assays for selectivity include HEK 293 cells engineered to express receptors most homologous in sequence and topology to the Ca2+ receptor. These include native or chimeric receptors for various metabotropic glutamate and γ-aminobutyric acid type B receptors (GABABRs). Chimeric receptors are created using partial sequences of metabotropic glutamate receptors and Ca2+ receptors, engineered to couple to activation of phospholipase C and release of intracellular Ca2+ in HEK 293 cells. NPS-2143 lacking pan-activity are then subjected to structural modifications and their potencies and selectivities monitored using these HEK 293 4.0-7 cell assays in an iterative process.

Cell Assay: Calcilytics NPS-2143 has been reported to diminish the sensitivity of the CaSR for [Ca2+]o, resulting in an attenuated signal transduction in vitro and increased secretion of PTH in vivo. In addition, The EC50 values of NPS-2143 are 4.27, 1.56, 1.61, 2.46, 2.07 and 3.15mM for the wild-type CaSR, mutant CaSR (T151R, P221L, E767Q, G830S, and A844T), respectively. Apart from these, in HEK 293T cells expressing CaSR, NPS-2143 has been revealed to inhibit the [Ca2+]o-induced cytosolic calcium signal in a concentration-dependent manner.