The cell-cell fusion assay was performed as described previously (Yamamoto et al., 2016 (link)). To quantitate the cell-cell fusion, a pair of 293FT-based reporter cells, effector and target cells, that express individual split reporters (DSP1-7 and DSP8-11 proteins) were used, because DSP1-7 and DSP8-11 produce fluorescence and luminescence only when the two proteins form a tight complex (Wang et al., 2014 (link)). The effector cells stably expressing DSP8-11 and S-protein and the target cells stably expressing DSP1-7 together with CD26 and TMPRSS2 were prepared. 2 h before the fusion assay, both cells were treated with 6 μM EnduRen (Promega, Madison, WI, USA), a substrate for Renilla luciferase, to activate EnduRen. 1 mL of each compound dissolved in dimethyl sulfoxide (DMSO) was added to 384-well plates (Greiner Bioscience, Frickenhausen, Germany) using a 12-stage workstation (Biotech, Tokyo, Japan). Next, a Multidrop dispenser (Thermo Scientific, Waltham, MA, USA) was used to add 50 μL of each single cell suspension (1.5×104 effector and target cells) to the wells. Incubation was performed at 37°C for 4 h, then RL activity measurements were obtained with a microplate reader (PHERAStar Plus, BMG Labtech, Cary, NC, USA).