Correlation between in vitro cytotoxicity and cell surface CD19 density. (A) IC₅₀ values (mean of 3 independent determinations) of ADCT-402 against the named cell lines are plotted against the measured cell surface CD19 levels expressed as sites/cell. The line is the regression through all data points. (B) The equivalent analysis performed for the naked PBD dimer warhead SG3199. Linear relationship between indicated variables was calculated by Pearson correlation coefficient r test.