Direct (saturation) radioligand binding assays. Direct (saturation) radioligand binding to SKBR-3 human breast cancer cells of (a) 111In-DOTA-trastuzumab Fab or (b) 64Cu-DOTA-trastuzumab Fab, in the absence (total binding; TB) or presence (non-specific binding; NSB) of excess (16 μM) unlabeled trastuzumab IgG. Specific binding (SB) was calculated by subtraction of NSB from TB. Curves were fitted to a 1-site receptor-binding model using Prism Ver. 4.0 software (GraphPad).