The calcium-sensing receptor (CaSR) is a class C GPCR that is essential for life. The CaSR is widely expressed in humans and is particularly abundant in the parathyroid glands and kidneys. In these organs, the CaSR responds to elevated calcium (Ca2+) by inhibiting parathyroid hormone (PTH) secretion (consequently inhibiting calcium reabsorption from the kidneys, intestine, and bone) and promoting renal excretion of calcium and other salts. Given its central role in calcium homeostasis and PTH secretion and more recently asthma and pulmonary arterial hypertension pathology, the CaSR is an important drug target.
Currently, there are three CaSR positive allosteric modulators (PAMs) available on the market for the treatment of secondary hyperparathyroidism (SHPT). However, their uses are limited as all three PAMs tend to cause hypocalcaemia. Most of the current negative allosteric modulators (NAMs) were developed with the intention of treating osteoporosis. Due to the weak cooperativity and failure to show clinical efficacy, no NAMs are approved for therapeutic use. Therefore, novel PAMs and NAMs need to be developed to improve the clinical outcomes of CaSR-related diseases.
Most of the current CaSR PAMs and NAMs are targeting the seven-transmembrane (7TM) domain (~250 residues), and only etelcalcetide (one of the approved PAMs) acts on the extracellular domain (ECD) of the CaSR (~600 residues). In addition, X-ray crystallography1 and cryo-EM structures2 confirmed that L-aromatic amino acids (L-aa) and TNCA (an L-tryptophan derivative) bind to CaSR ECD. In vitro data support that L-aa potentiate CaSR activity in the presence of Ca2+, thereby indicating that the ECD is a feasible drug target to modulate CaSR function.
Herein, we report a primary fragment library screening study to explore novel CaSR hit compounds targeting the ECD, by using biophysical approaches such as saturation transfer difference nuclear magnetic resonance (STD NMR) and surface plasmon resonance (SPR).