Gadolinium chelate with DO3A conjugated 2-(diphenylphosphoryl)-ethyldiphenylphosphonium cation as potential tumor-selective MRI contrast agent

A series of organic cations, such as triphenylphosphonium (TPP), 2-(diphenylphosphoryl)-ethyldiphenylphosphonium (TPEP), represent molecular probes for imaging tumors. These organic cations have been labeled with ⁶⁴Cu radioisotope for imaging tumors by positron emission tomograghy (PET). Among these organic cation ligands, TPEP was selected for extensive evaluation using magnetic resonance imaging (MRI) based on its higher tumor uptake and better Tumor/Background (T/B) ratios. This report presents the development of a new Gd(III) chelate [Gd(DO3A-xy-TPEP)]⁺ as a cationic MRI contrast agent. The contrast agent was synthesized and characterized in vitro and in vivo. In vitro cell viability showed low cytotoxicity at low [Gd] concentrations. Cell uptake experiment shows that the [Gd(DO3A-xy-TPEP)]⁺ has high affinity for tumor cells. The in vitro T₁ relaxivity measured at 9.4 T is about 50% higher than those of contrast agents in clinical use: Gd-DTPA (Magnevist) and Gd-DOTA (Dotarem). In vivo imaging studies in tumor-bearing mice at 7.0 T demonstrated significant signal enhancement at the site of the tumors. [Gd(DO3A-xy-TPEP)]⁺ is a promising tumor-targeting MRI contrast agent for diagnostic imaging.