TY - JOUR T1 - Dual-Receptor–Targeted Radioimmunotherapy of Human Breast Cancer Xenografts in Athymic Mice Coexpressing HER2 and EGFR Using <sup>177</sup>Lu- or <sup>111</sup>In-Labeled Bispecific Radioimmunoconjugates JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 444 LP - 452 DO - 10.2967/jnumed.115.162339 VL - 57 IS - 3 AU - Eva J. Razumienko AU - Jason C. Chen AU - Zhongli Cai AU - Conrad Chan AU - Raymond M. Reilly Y1 - 2016/03/01 UR - http://jnm.snmjournals.org/content/57/3/444.abstract N2 - One mechanism of resistance to trastuzumab in human epidermal growth factor receptor-2 (HER2)–positive breast cancer (BC) is increased epidermal growth factor receptor (EGFR) expression. We have developed 111In-labeled bispecific radioimmunoconjugates (bsRICs) that bind HER2 and EGFR on BC cells by linking trastuzumab Fab fragments through a polyethylene glycol (PEG24) spacer to epidermal growth factor (EGF). We hypothesized that tumors coexpressing HER2 and EGFR could be treated by dual-receptor–targeted radioimmunotherapy with these bsRICs labeled with the β-particle emitter 177Lu or the Auger electron-emitter 111In. Methods: The binding of 177Lu-DOTA-Fab-PEG24-EGF to tumor cells (MDA-MB-231, SK-OV-3, MDA-MB-231/H2N, or TrR1) coexpressing HER2 and EGFR was assessed in competition assays. The clonogenic survival of these cells was measured after exposure to 177Lu-DOTA-Fab-PEG24-EGF or 111In-DTPA-Fab-PEG24-EGF or to monospecific 177Lu- or 111In-labeled trastuzumab Fab or EGF. The tumor and normal tissue biodistribution of 177Lu-DOTA-Fab-PEG24-EGF was studied at 48 h after injection in athymic mice bearing subcutaneous MDA-MB-231/H2N tumors. Radiation-absorbed doses to tumors and normal tissues were estimated and compared for 111In- and 177Lu-labeled bsRICs. The maximum injected amount of 177Lu-DOTA-Fab-PEG24-EGF that caused no observable adverse effects (NOAEL) was identified in BALB/c mice. Athymic CD1 nu/nu mice bearing subcutaneous trastuzumab-sensitive MDA-MB-231/H2N or trastuzumab-resistant TrR1 tumors were treated with 177Lu-DOTA-Fab-PEG24-EGF or 111In-DTPA-Fab-PEG24-EGF at the NOAEL, or with unlabeled immunoconjugates or normal saline. Tumor growth was evaluated over a period of 49 d. Results: 177Lu-DOTA-Fab-PEG24-EGF bound specifically to HER2 and EGFR on tumor cells. Monospecific 177Lu- and 111In-labeled trastuzumab Fab or EGF killed tumor cells that predominantly expressed HER2 or EGFR, respectively, whereas bsRICs were cytotoxic to cells that displayed either HER2 or EGFR or both receptors. bsRICs were more effective than monospecific agents. 177Lu-DOTA-Fab-PEG24-EGF was more cytotoxic than 111In-DTPA-Fab-PEG24-EGF. The tumor uptake of 177Lu-DOTA-Fab-PEG24-EGF was 2-fold greater than 177Lu-DOTA-trastuzumab Fab or 177Lu-DOTA-EGF. The NOAEL for 177Lu-DOTA-Fab-PEG24-EGF was 11.1 MBq (10 μg). Trastuzumab-sensitive MDA-MB-231/H2N and trastuzumab-resistant TrR1 tumors were growth-inhibited by 177Lu-DOTA-Fab-PEG24-EGF or 111In-DTPA-Fab-PEG24-EGF. Unlabeled immunoconjugates had no effect on tumor growth. 177Lu-DOTA-Fab-PEG24-EGF inhibited tumor growth more effectively than 111In-DTPA-Fab-PEG24-EGF because of a 9.3-fold-higher radiation-absorbed dose (55.0 vs. 5.9 Gy, respectively). Conclusion: These results are encouraging for further development of these bsRICs for dual-receptor–targeted radioimmunotherapy of BC coexpressing HER2 and EGFR, including trastuzumab-resistant tumors. ER -