Paired-label in vitro internalization of iso-[125I]SGMIB-Nb (gray) and [131I]SGMIB-Nb (black) by BT474 cells. Cells were allowed to take up the radiotracers at 4°C for an hour, brought up to 37°C and processed at 1 h, 2 h, 4 h, 6 h, and 24 h as described in the text. Specific (in the absence of trastuzumab minus in the presence) total cell-associated radioactivity (cell surface-bound + internalized) (A) and intracellularly trapped activity (B).