A novel calcium-sensing receptor antagonist transiently stimulates parathyroid hormone secretion in vivo

Circulating calcium ( Ca2+) is a primary regulator of bone homeostasis through its action on PTH secretion. Extracellular Ca2+ modulates PTH secretion through a cell surface G protein-coupled receptor, the calcium- sensing receptor ( CaR). The expression of the CaR suggests a critical role in cellular regulation by calcium in various organs, including parathyroid gland, bone, and kidney. Despite an obvious pharmacological utility for CaR antagonists in the treatment of disease, only a limited number of such classes of compounds exist. We have identified a novel class of small molecules with specific activity at the CaR. This class of compounds is represented by compound 1. It possesses potent antagonist activity at the human CaR with IC50 values of 64 nM and 230 nM in inhibiting intracellular Ca2+ flux and inositol phosphate generation in vitro, respectively. When administered to male rats in vivo, compound 1 robustly increased serum PTH levels. The stimulation of PTH secretion was rapid and transient when administered either iv or orally. The pharmacokinetic profile of compound 1 after oral administration revealed that maximal plasma levels of compound were reached within 1 h and the half- life of the compound to be approximately 2 h in rats. These data describe a representative compound of a novel chemical class than previously described allosteric modulators that offer a new avenue for the development of improved treatments of osteoporosis.