Bispecific Radioimmunoconjugates for Molecular Imaging and Radioimmunotherapy of HER2 Overexpressing Breast Cancer

The HER2 receptor is overexpressed in approximately 20% of breast cancer (BC) cases, and is the target for multiple therapies. One challenge in treating HER2+ BC is that a large proportion of patients demonstrate resistance to therapy. Upregulation of other members of the HER family, plays an important role in resistance to HER2-targeted therapies. The hypothesis was that bispecific radioimmunoconjugates (bsRICs) developed against HER2 and another member of the HER receptor family would be able to successfully target cells expressing one or more receptor for molecular imaging and radioimmunotherapy of BC.;Trastuzumab Fab fragments were conjugated to heregulin (HRG) or epidermal growth factor (EGF) with a 24-mer polyethylene glycol (PEG) linker, and labeled with 111In or 177Lu. 111In-DTPA-Fab-PEG 24-HRG and 111In-DTPA-Fab-PEG24-EGF demonstrated the ability to specifically bind to one or both receptors on cancer cell lines, and demonstrated higher binding than monospecific agents recognizing HER2, HER3 or EGFR. SPECT imaging was performed on mice bearing tumour xenografts, and optimal tumour uptake was found 48 hr after injection. Biodistribution studies showed tumour: blood uptake ratios of 6-8. The cytotoxicity of 111In-DTPA-Fab-PEG24-EGF and 177Lu- DOTA-Fab-PEG 24-EGF was evaluated through clonogenic assays which revealed specific cell killing. Radiation absorbed doses to tumours and normal tissues were estimated and compared for 111In and 177Lu-labeled bsRICs. The maximum injected amount of 177Lu-DOTA-Fab-PEG 24-EGF which caused no observable adverse effects (NOAEL) was identified to be 11.1 MBq. Mice bearing trastuzumab-sensitive or resistant tumours were treated with 111In and 177Lu-labeled bsRICs and 177Lu-bsRICs were found to inhibit tumour growth more effectively than 111In-bsRICs due to a 9.3-fold higher radiation absorbed dose (55.0 vs. 5.9 Gy, respectively).;These results suggest that bsRICs may be useful for imaging and radioimmunotherapy of HER2-positive BC co-expressing HER3 or EGFR. Co-expression of these receptors in HER2+ tumours is associated with resistance to HER2-targeted therapies.