Targeted Delivery of Molecular Probes for in vivo Electron Paramagnetic Resonance Imaging

Abstract

With recent advances in electron paramagnetic resonance imaging (EPRI), in vivo visualization of a physiologically distinct tissue (e.g., a tumor) has become a real possibility. EPRI could be a powerful imaging modality to detect metastatic lesions and report tissue-specific physiological information. Approximately 25–30% of breast tumors overexpress the Human Epidermal Growth Factor Receptor 2 (HER2). HER2-overexpressing breast tumors are proliferative, metastatic, and have poor clinical prognoses. We have developed a novel mechanism for selective in vivo delivery of “spin probes” (molecular probes for EPRI) to Hc7 cells, which are MCF7 breast cancer cells engineered to overexpress HER2. Spin probes can be encapsulated in anti-HER2 immunoliposomes at high concentration (>100 mM). At such concentrations, the spectroscopic signal of spin probes is severely “quenched”—a process analogous to the self-quenching of fluorophores. This makes the intact immunoliposomes spectroscopically “dark” and thus invisible by EPRI. Tumor-specific contrast is generated after selective endocytosis of anti-HER2 immunoliposomes. Intracellular degradation of endocytosed liposomes liberates the spin probes from the liposomal lumen. Once de-quenched by dilution into the much-larger cellular volume, the spin probes regain their spectral signal and make the cells visible by EPRI. Through uptake of immunoliposomes, Hc7 cells can achieve an intracellular spin probe concentration of ~750 μM. Using imaging phantoms, we verify that this concentration of spin probes is easily imageable by EPRI. We have optimized immunoliposomes for in vivo applications by increasing their persistence in circulation to maxi mize tumor targeting. Through near-infrared fluorescence imaging of tumor-bearing animals, we demonstrate that optimized anti-HER2 immunoliposomes selectively target Hc7 tumors in vivo, enabling high-contrast imaging with minimal background. This work lays the foundation for imaging Hc7 tumors with EPRI.