Radiolabeled Pegylated Exosomes For In Vivo Pet Imaging And Enhanced Tumor Retention

349 Objectives: Exosomes, the naturally secreted lipid vesicles carrying myriad cellular proteins and genetic information, have recently attracted tremendous attention. Due to its extremely high biocompatibility and role in in cancer progression, exosomes have been considered as a promising natural nanoplatform for drug delivery and immunotherapy of cancer. However, a majority of the exosomes injected systemically are cleared by the liver with suboptimal tumor retention, impeding their potential application and clinical translation in cancer treatment. The goal of this study is to design radiolabeled PEGylated exosomes to improve tumor retention and systematically delineate their biodistribution via PET imaging.\n Methods: Exosomes were generated from 4T1 murine triple negative breast cancer cells by centrifuging conditioned cell culture medium at different velocities. As-prepared exosome was then reacted with polyethylene glycol (PEG) to form exosome-PEG, and characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) and UV-VIS spectrometry. Exosome and exosome-PEG were radiolabeled and their radiolabeling efficiency and stability were measured by thin-layer chromatography (TLC). Radiolabeled exosome and exosome-PEG were injected into 4T1 tumor-bearing mice for in vivo PET imaging and biodistribution studies. The imaging results were validated by histological analysis with fluorescence-labeled exosomes.\n Results: 4T1-derived exosome and exosome-PEG were generated with size ~70 nm in diameter. After radiolabeling, 95.6 ± 0.3 % and 97.7 ± 0.2 % of the nuclides were successfully labeled on exosome and exosome-PEG, respectively, suggesting excellent labeling efficiency. After incubation in serum for 24 h, greater than 80% and 90% of the nuclides remained stable on exosome or exosome-PEG, indicating their excellent stability in physiological environment. Serial PET imaging and biodistribution studies revealed a significantly higher tumor uptake and decreased liver sequestration of exosome-PEG, in comparison with exosome without PEG decoration, demonstrating enhanced tumor retention and reduced hepatic clearance after appropriate surface modification. Histological images of fluorescence-labeled exosome-PEG displayed that exosomes was able to extravasate from the vessels.\nConclusion: Radiolabeled PEGylated exosomes were successfully prepared, which exhibited significantly enhanced tumor uptake and capacity for in vivo PET imaging of tumor. Surface modification with PEG reduced hepatic clearance of exosomes, which promises potential applications of exosomes in drug delivery and immunotherapy of cancer. The important insights provided by in vivo PET imaging of PEGylated exosomes are expected to guide future development of exosome-based drug delivery system.\nAcknowledgment: Supported in part by John S. Dunn Foundation.