SK-BR-3 cells (a HER2-positive cell line) were seeded at 2000 cells/well in a 96-well plate and incubated overnight at 37°C in a humidified atmosphere containing 5% CO2. The cells were treated with serially diluted homogeneous ADCs and incubated for 4 days at 37°C in a humidified atmosphere containing 5% CO2. The cytotoxicity was evaluated using the Cell Counting Kit SF (Nacalai Tesque). Each dose-response curve was fitted by 4-parameter logistic model with same upper/lower asymptotes using GraphPad prism 7 software, and calculated IC50 of homogeneous ADCs