Side-by-side comparison of published small molecule inhibitors against thapsigargin-induced store-operated Ca2+ entry in HEK293 cells

Calcium (Ca2+) is a key second messenger in eukaryotes, with store-operated Ca2+ entry (SOCE) being the main source of Ca2+ influx into non-excitable cells. ORAI1 is a highly Ca2+-selective plasma membrane channel that encodes SOCE. It is ubiquitously expressed in mammals and has been implicated in numerous diseases, including cardiovascular disease and cancer. A number of small molecules have been identified as inhibitors of SOCE with a variety of potential therapeutic uses proposed and validated in vitro and in vivo. These encompass both nonselective Ca2+ channel inhibitors and targeted selective inhibitors of SOCE. Inhibition of SOCE can be quantified both directly and indirectly with a variety of assay setups, making an accurate comparison of the activity of different SOCE inhibitors challenging. We have used a fluorescence based Ca2+ addback assay in native HEK293 cells to generate dose-response data for many published SOCE inhibitors. We were able to directly compare potency. Most compounds were validated with only minor and expected variations in potency, but some were not. This could be due to differences in assay setup relating to the mechanism of action of the inhibitors and highlights the value of a singular approach to compare these compounds, as well as the general need for biorthogonal validation of novel bioactive compounds. The compounds observed to be the most potent against SOCE in our study were: 7-azaindole 14d (12), JPIII (17), Synta-66 (6), Pyr 3 (5), GSK5503A (8), CM4620 (14) and RO2959 (7). These represent the most promising candidates for future development of SOCE inhibitors for therapeutic use.