Feasibility of poly(ethylene glycol) derivatives as diagnostic drug carriers for tumor imaging

Poly(ethylene glycol) (PEG) is an artificial but biocompatible hydrophilic polymer that has been widely used in clinical products. To evaluate the feasibility of using PEG derivative itself as a tumor imaging carrier via an enhanced permeability and retention (EPR) effect, we prepared indium-111-labeled PEG (In-111-DTPA-PEG) and in-docyanine green (ICG)-labeled PEG (ICG-PEG) with PEG molecular weights of 5-40 kDa and investigated their in vivo biodistribution in colon26 tumor-bearing mice. Thereafter, single-photon emission computed tomography (SPECT) and photoacoustic (PA) imaging studies were performed. The in vivo biodistribution studies demonstrated increased tumor uptake and a prolongation of circulation half-life as the molecular weight of PEG increased. Al-though the observed differences in in vivo biodistribution were dependent on the labeling method (In-111 or ICG), the tumor-to-normal tissue ratios were comparable. Because PEG-based probes with a molecular weight of 20 kDa (PEG20) showed a preferable biodistribution (highest accumulation among tissues excised and relatively high tumor-to-blood ratios), an imaging study using In-111-DTPA-PEG20 and ICG-PEG20 was performed. Colon26 tumors inoculated in the right shoulder were clearly visualized by SPECT 24 h after administration. Furthermore, PA imaging using ICG-PEG20 also detected tumor regions, and the detected PA signals increased in proportion with the injected dose. These results suggest that PEG derivatives (20 kDa) serve as robust diagnostic drug carriers for tumor imaging. (C) 2016 Elsevier B.V. All rights reserved.