Carbocyanine labeled LDL for optical imaging of tumors

Li H, Zhang Z, Blessington D, Nelson DS, Zhou R, Lund-Katz S, Chance B, Glickson JD & Zheng G
Academic Radiology, 2004

Rationale and objectives: The purpose of this study was to define and characterize carbocyanine labeled low-density lipoprotein (LDL) to be used in the optical imaging of LDL receptor (LDLr)-overexpressing tumor models. Materials and methods: 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) was used to label LDL (DiI-LDL). Scatchard plots were generated to determine the maximum binding capacity Bmax and dissociation constants KD of DiI-LDL in B16 melanoma (B16) and hepatoblastoma G2 (HepG2) cell lines. Selective uptake of DiI-LDL into both tumor cells and corresponding subcutaneous tumors in mice were demonstrated by confocal microscopy and three-dimensional Cryo-imaging, respectively. Results: The labeling efficiency of DiI-LDL was 61 ng DiI/μg LDL protein (34 mol DiI/mol LDL protein). Bmax and KD for B16 cells were 6.311 ng LDL/mg cell protein and 60.38 μg protein/mL (117 nM), respectively. Bmax and KD were 7.573 ng LDL/mg cell protein and 26.79 μg protein/mL (52 nM) for HepG2 cells, respectively. Confocal microscopic images showed specific uptake of DiI-LDL throughout the cytoplasm in the B16/HepG2 cells. Cryo-imaging demonstrated preferential accumulations of DiI-LDL in the viable tumor regions of both B16 and HepG2 tumors compared with their adjacent normal tissues and corresponding necrotic tumor regions. In addition, uptake of DiI-LDL by the HepG2 tumor was much higher than that of the B16 tumor, consistent with the fact that the probe binding affinity for LDLrs of HepG2 cells is 2.3 times that of B16 cells. Conclusion: This study suggested that carbocyanine labeled LDL could be used for optical imaging of tumors overexpressing LDLr.