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 <i>T</i>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 (<b>BDP-Gd</b>) and Cy7 dye (<b>Cy7-Gd</b>), 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 <b>Cy7-Gd</b> derivatives. On the basis of MR imaging, flow cytometry, and ICP-MS analysis of cells loaded with <b>Cy7-Gd</b> derivatives, highly hydrophobic and nonanionic dyes were effective for enhancing cell permeation of Gd³⁺ complexes. Furthermore, the behavior of these <b>Cy7-Gd</b> 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