Radiation Nanomedicine for EGFR-Positive Breast Cancer: Panitumumab-Modified Gold Nanoparticles Complexed to the β-Particle-Emitter, 177Lu

Our objective was to construct a novel radiation nanomedicine for treatment of breast cancer (BC) expressing epidermal growth factor receptors (EGFR), particularly triple-negative tumors (TNBC). Gold nanoparticles (AuNP; 30 nm) were modified with polyethylene glycol (PEG) chains (4 kDa) derivatized with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelators for complexing the β-emitter, 177Lu and with PEG chains (5 kDa) linked to panitumumab for targeting BC cells expressing EGFR. The AuNP were further coated with PEG chains (2 kDa) to stabilize the particles to aggregation. The binding and internalization of EGFR-targeted AuNP (177Lu-T-AuNP) into BC cells was studied and compared to nontargeted 177Lu-NT-AuNP. The cytotoxicity of 177Lu-T-AuNP and 177Lu-NT-AuNP was measured in clonogenic assays using BC cells with widely different EGFR densities: MDA-MB-468 (106 receptors/cell), MDA-MB-231 (105 receptors/cell), and MCF-7 cells (104 receptors/cell). Radiation absorbed doses to the cell nucleus of MDA-MB-468 cells were estimated based on subcellular distribution. Darkfield and fluorescence microscopy as well as radioligand binding assays revealed that 177Lu-T-AuNP were specifically bound by BC cells dependent on their EGFR density whereas the binding and internalization of 177Lu-NT-AuNP was significantly lower. The affinity of binding of 177Lu-T-AuNP to MDA-MB-468 cells was reduced by 2-fold compared to 123I-labeled panitumumab (KD = 1.3 ± 0.2 nM vs 0.7 ± 0.4 nM, respectively). The cytotoxicity of 177Lu-T-AuNP was dependent on the amount of radioactivity incubated with BC cells, their EGFR density and the radiosensitivity of the cells. The clonogenic survival (CS) of MDA-MB-468 cells overexpressing EGFR was reduced to <0.001% at the highest amount of 177Lu-T-AuNP tested (4.5 MBq; 6 × 1011 AuNP per 2.5 × 104-1.2 × 105 cells). 177Lu-T-AuNP were less effective for killing MDA-MB-231 cells or MCF-7 cells with moderate or low EGFR density (CS = 33.8 ± 1.6% and 25.8 ± 1.2%, respectively). Because the β-particles emitted by 177Lu have a 2 mm range, 177Lu-NT-AuNP were also cytotoxic to BC cells due to a cross-fire effect but 177Lu-T-AuNP were significantly more potent for killing MDA-MB-468 cells overexpressing EGFR than 177Lu-NT-AuNP at all amounts tested. The cross-fire effect of the β-particles emitted by 177Lu may be valuable for eradicating BC cells in tumors that have low or moderate EGFR expression or cells that are not targeted by 177Lu-T-AuNP as a consequence of heterogeneous intratumoral distribution. The radiation dose to the nucleus of a single MDA-MB-468 cell was 73.2 ± 6.7 Gy, whereas 177Lu-NT-AuNP delivered 5.6 ± 0.6 Gy. We conclude that 177Lu-T-AuNP is a promising novel radiation nanomedicine with potential application for treatment of TNBC, in which EGFR are often overexpressed.

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