TH1177, Clemastine, and Benzhydryl Derivatives Act as Novel Extracellular Blockers of ORAI Channels

ORAI channels are store-operated Ca²⁺ channels, which play important roles in physiology and pathophysiology. ORAI channel modulators are useful for developing pharmacological interventions. Here, we aimed to investigate the benzhydryl-derived analogues as novel ORAI channel blockers. The effects of benzhydryl-derived analogues TH1177, clemastine, cloperastine, and diphenhydramine were investigated in HEK-293 cells overexpressing individual ORAI isoforms and STIM1. The ORAI currents were recorded by patch-clamp, and cytosolic Ca²⁺ was measured by a fluorescent Ca²⁺ dye. TH1177 inhibited ORAI1, ORAI2, and ORAI3 currents with IC₅₀ values of 18.3 μM, 4.8 μM, and 35.6 μM, respectively. Clemastine inhibited ORAI1 and ORAI2, but showed no effect on ORAI3. Both cloperastine and diphenhydramine, without a pyrrolidine ring structure, only showed minimal inhibition of ORAI1. Intracellular application of TH1177 and outside-out patch demonstrated that TH1177 blocked ORAI3 currents via the extracellular surface. TH1177 did not affect cytosolic STIM1 clustering and movement. Moreover, TH1177 potently inhibited cell proliferation with IC₅₀ values of 2.6 µM for HAEC, 5.2 µM for EA.hy926, and 29.4 µM for HK-2 cells and induced apoptotic cell death. It is concluded that TH1177 is a pan-inhibitor for ORAI1-3 channels, while clemastine inhibits ORAI1 and ORAI2 but shows little effect on ORAI3. The pyrrolidine ring structure in the compounds of TH1177 and clemastine is critical for their inhibitory activity. The two benzhydryl derivatives (cloperastine and diphenhydramine) without the pyrrolidine ring only showed minimal inhibition of ORAI1. Our findings give a new pharmacological profile of ORAI channels.