Comparison of off-rates from live cells. Daudi (A), Ramos (B), and Raji (C-E) cells were stained with PE-labeled rituximab (▵), veltuzumab (blue filled square), cA20 (red triangle), D101N (green filled circle), 1F5 (purple open circle) or B1 (gray open square). The labeled mAbs were incubated at 37°C with (A-D) or without (E) excess veltuzumab Fab′-NEM, and the cells were analyzed by flow cytometry over time. The off-rate was determined by nonlinear regression (one-phase exponential decay) and P values were generated by F test using GraphPad Prism software.