MicroSPECT/CT imaging of co-expressed HER2 and EGFR on subcutaneous human tumor xenografts in athymic mice using ¹¹¹In-labeled bispecific radioimmunoconjugates.

Epidermal growth factor receptors (EGFR) form heterodimers with HER2 in breast cancer, and increased EGFR expression has been found in HER2-positive tumors resistant to trastuzumab (Herceptin). Our objective was to synthesize bispecific radioimmunoconjugates (bsRICs) that recognize HER2 and EGFR and evaluate their ability to image tumors in athymic mice that express one or both receptors by microSPECT/CT. Bispecific radioimmunoconjugates were constructed by conjugating maleimide-derivatized trastuzumab Fab fragments that bind HER2 to a thiolated form of EGF with an intervening 24 mer polyethylene glycol (PEG24) spacer. Bispecific radioimmunoconjugates were derivatized with diethylenetriaminepentaacetic acid for labeling with (111)In. The ability of (111)In-bsRICs to bind HER2 or EGFR was determined in competition assays using cells expressing one or both receptors. Tumor and normal tissue uptake were examined in CD1 athymic mice bearing subcutaneous tumor xenografts that expressed HER2, EGFR, or both receptors, with or without pre-administration of Fab or EGF to determine specificity. HER2 and EGFR binding and displacement of binding by competitors were found for (111)In-bsICs. The highest uptake of (111)In-bsRICs [7.3 ± 3.5 %ID/g] in 231-H2N human breast cancer xenografts (HER2+/EGFR+) occurred at 48 h post-injection. Pre-administration of trastuzumab Fab decreased uptake in SK-OV-3 (HER2+/EGFR-) human ovarian cancer xenografts from 7.1 ± 1.2 to 2.4 ± 1.5 %ID/g. Pre-administration of excess EGF decreased uptake in MDA-MB-231 (HER2-/EGFR+) human breast cancer xenografts from 5.9 ± 0.5 to 2.0 ± 0.1 %ID/g. All tumors were imaged by microSPECT/CT. We conclude that (111)In-bsRICs composed of trastuzumab Fab and EGF exhibited specific binding in vitro to tumor cells displaying HER2 or EGFR, and were taken up specifically in vivo in tumors expressing one or both receptors, permitting tumor visualization by microSPECT/CT. These agents may ultimately be useful for imaging heterodimerized HER2-EGFR complexes since their bivalent properties permit more avid binding to these complexes.