Method for Enhancing Cell Penetration of Gd3+-based MRI Contrast Agents by Conjugation with Hydrophobic Fluorescent Dyes

Gadolinium ion (Gd3+) complexes are commonly used as magnetic resonance imaging (MRI) contrast agents to enhance signals in T-1-weighted MR images. Recently, several methods to achieve cell-permeation of Gd3+ complexes have been reported, but more general and efficient methodology is needed. In this report, we describe a novel method to achieve cell permeation of Gd3+ complexes by using hydrophobic fluorescent dyes as a cell-permeability-enhancing unit. We synthesized Gd3+ complexes conjugated with boron dipyrromethene (BDP-Gd) and Cy7 dye (Cy7-Gd), and showed that these conjugates can be introduced efficiently into cells. To examine the relationship between cell permeability and dye structure, we further synthesized a series of Cy7-Gd derivatives. On the basis of MR imaging, flow cytometry, and ICP-MS analysis of cells loaded with Cy7-Gd derivatives, highly hydrophobic and nonanionic dyes were effective for enhancing cell permeation of Gd3+ complexes. Furthermore, the behavior of these Cy7-Gd derivatives was examined in mice. Thus, conjugation of hydrophobic fluorescent dyes appears to be an effective approach to improve the cell permeability of Gd3+ complexes, and should be applicable for further development of Gd3+-based MRI contrast agents.