Recent Updates on the Calcium-Sensing Receptor as a Drug Target

Parathyroid hormone (PTH) secreted from parathyroid cells is a key regulator of circulating levels of calcium ion, which is maintained within a narrow physiological range mainly by the action of a cell surface Ca2+-sensing receptor (CaSR) residing on parathyroid cells. The CaSR belongs to the family C of G-protein-coupled receptors (GPCRs), all members of which have a large extra cellular domain called Venus flytrap, akin to bacterial nutrient sensor. Promiscuity of the CaSR is reflected in its binding to several physiologically relevant di- or polyvalent cations that could elicit cellular signaling, yet the ligand specificity remains poor. This problem has been circumvented by the discovery of calcimimetics, the positive allosteric modulators of the CaSR that activate the CaSR on parathyroid cells, immediately suppressing PTH secretion. Out of several small molecules that act as calcimimetics, cinacalcet is the most extensively studied. The discovery of the CaSR and these specific agonists provides important insights into the discovery of new drugs that will be discussed in this review from the perspective of their structure-activity relationship. The calcimimetic compound cinacalcet has obtained regulatory approval for the treatment of hyperparathyroidism and is the first positive allosteric modulator of any GPCR to reach the market. It has other important uses in dialysis patients with end-stage renal disease, in whom it decreases Ca P product. Pharmaceutically converse to calcimimetics are calcilytic compounds, which antagonize parathyroid CaSR and stimulate PTH secretion. Antagonism of the CaSR has the potential to yield an anabolic therapy for osteoporosis that awaits clinical validation.