Silicon-rhodamine functionalized evocalcet probes (EvoSiR) potently and selectively label calcium sensing receptors (CaSR) in vitro, in vivo and ex vivo

The calcium sensing receptor (CaSR) is a ubiquitously expressed G-protein coupled receptor (GPCR) that regulates extracellular calcium signals via the parathyroid glands. CaSR has recently also been implicated in non-calcitropic pathophysiologies like asthma, gut inflammation and cancer. To date, molecular tools that enable the bioimaging of CaSR in tissues are lacking. Based on in silico analyses of available structure-activity relationship data on CaSR ligands, we designed and prepared silicon-rhodamine (SiR) conjugates of the clinically approved drug evocalcet. The new probes EvoSiR4 and EvoSiR6, with differing linker lengths at the evocalcet carboxyl end, both showed a 6-fold and 3-fold increase in potency towards CaSR (EC50<45 nM) compared to evocalcet and the evocalcet-linker conjugate, respectively, in a FLIPR(R)-based cellular functional assay. The specificity of the EvoSiR probes towards CaSR binding and the impact of albumin was evaluated in live cell experiments. Both probes showed strong albumin binding, which facilitated the clearance of nonspecific binding interactions. Accordingly, in zebrafish embryos, EvoSiR4 specifically labelled the high CaSR expressing neuromasts of the lateral line in vivo. EvoSiR4 was also assessed in human parathyroid tissues ex vivo, showing a specific absolute CaSR associated fluorescence compared to parathyroid autofluorescence. In summary, functionalization of evocalcet by SiR led to the preparation of potent and specific fluorescent CaSR probes. EvoSiR4 is a versatile small molecular probe that can be employed in CaSR-related biomedical analyses where antibodies are not applicable.