Imaging of HER2/neu-positive BT-474 human breast cancer xenografts in athymic mice using 111In-trastuzumab (Herceptin) Fab fragments

Trastuzumab (Herceptin) Fab were prepared by digestion of intact IgG with immobilized papain, derivatized with diethylenetriaminepentaacetic acid (DTPA) and radiolabeled with 111In. The dissociation constant (Kd) for binding of Fab to HER2/neu-positive SK-BR-3 human breast cancer cells was two- to threefold higher than for intact IgG (14-36 vs. 8-14 nM). The binding affinity was not significantly decreased after DTPA derivatization (Kd=47 nM). 111In-trastuzumab Fab localized specifically in HER2/neu-positive BT-474 human breast cancer xenografts in athymic mice with tumor uptake of 7.8±0.7% injected dose (ID)/g and tumor/blood ratio of 25.2±1.6 at 72 h postinjection compared with 2.7±0.7% ID/g and 7.0±0.9 for 111In-HuM195 anti-CD33 Fab (significantly different, P<.001). Small (3-5 mm in diameter) BT-474 tumors were imaged with 111In-trastuzumab Fab as early as 24 h postinjection.