Carbocyanine labeled LDL for optical imaging of tumors 1

Abstract

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 B max and dissociation constants K D of DiI-LDL in B16 melanoma (B16) and hepatoblastoma G 2 (HepG 2) 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). B max and K D for B16 cells were 6.311 ng LDL/mg cell protein and 60.38 μg protein/mL (117 nM), respectively. B max and K D were 7.573 ng LDL/mg cell protein and 26.79 μg protein/mL (52 nM) for HepG 2 cells, respectively. Confocal microscopic images showed specific uptake of DiI-LDL throughout the cytoplasm in the B16/HepG 2 cells. Cryo-imaging demonstrated preferential accumulations of DiI-LDL in the viable tumor regions of both B16 and HepG 2 tumors compared with their adjacent normal tissues and corresponding necrotic tumor regions. In addition, uptake of DiI-LDL by the HepG 2 tumor was much higher than that of the B16 tumor, consistent with the fact that the probe binding affinity for LDLrs of HepG 2 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.