Abstract

Calcimimetic compounds, which activate the parathyroid cell Ca²⁺ receptor (CaR) and inhibit parathyroid hormone (PTH) secretion, are under experimental study as a treatment for hyperparathyroidism. This report describes the salient pharmacodynamic properties, using several test systems, of a new calcimimetic compound, cinacalcet HCl. Cinacalcet HCl increased the concentration of cytoplasmic Ca²⁺ ([Ca²⁺]_i) in human embryonic kidney 293 cells expressing the human parathyroid CaR. Cinacalcet HCl (EC₅₀ = 51 nM) in the presence of 0.5 mM extracellular Ca²⁺ elicited increases in [Ca²⁺]_i in a dose- and calcium-dependent manner. Similarly, in the presence of 0.5 mM extracellular Ca²⁺, cinacalcet HCl (IC₅₀ = 28 nM) produced a concentration-dependent decrease in PTH secretion from cultured bovine parathyroid cells. Using rat medullary thyroid carcinoma 6-23 cells expressing the CaR, cinacalcet HCl (EC₅₀ = 34 nM) produced a concentration-dependent increase in calcitonin secretion. In vivo studies in rats demonstrated cinacalcet HCl is orally bioavailable and displays approximately linear pharmacokinetics over the dose range of 1 to 36 mg/kg. Furthermore, this compound suppressed serum PTH and blood-ionized Ca²⁺ levels and increased serum calcitonin levels in a dose-dependent manner. Cinacalcet was about 30-fold more potent at lowering serum levels of PTH than it was at increasing serum calcitonin levels. The S-enantiomer of cinacalcet (S-AMG 073) was at least 75-fold less active in these assay systems. The present findings provide compelling evidence that cinacalcet HCl is a potent and stereoselective activator of the parathyroid CaR and, as such, might be beneficial in the treatment of hyperparathyroidism.