SB-423557 is an orally active calcium-sensing receptor (CaR) antagonist (IC₅₀=520 nM), precursor of SB-423562 (IC₅₀=73 nM). SB-423557 is well tolerated in human and increases plasma concentrations of exogenous parathyroid hormone (PTH) and stimulates bone formation^[1]. IC50 & Target: IC50: 520 nM (Calcium-sensing receptor)^[1] In Vivo: SB-423557 (oral administration; 10, 30 or 100 μM/kg; once daily; 12 weeks) elicits transient increases in plasma concentrations of PTH, resulting in increased bone mass and strength in both vertebrae and long bones compared to that of OVX controls^[1].