The trisubstituted pyrimidine 1 was identified through high-throughput screening as a novel calcium-sensing receptor (CaSR) antagonist. Small molecule CaSR antagonists and/or negative allosteric modulators have the potential to act as an anabolic agent for the treatment of osteoporosis. The investigation of structure-activity relationships around 1 resulted in the identification of 18c and 18d, which showed efficacy at promoting PTH release in vivo and exhibited improved potency and solubility over the original lead 1.