Gd complexes of macrocyclic diethylenetriaminepentaacetic acid (DTPA) biphenyl-2,2′-bisamides as strong blood-pool magnetic resonance imaging contrast agents

We report the synthesis of macrocyclic DTPA conjugates of 2,2′-diaminobiphenyl and their Gd complexes of the type [Gd(L)(H ₂O)]•xH₂O (2a,b; L = 1a,b) for use as new MRI blood-pool contrast agents (MRI BPCAs). Pharmacokinetic inertness of 2 compares well with those of analogous Gd-DTPA MRI CAs currently in use. The present system also shows very high stability in human serum. The R₁ relaxivity reaches 10.9 mM^-1 s^-1, which is approximately 3 times as high as that of structurally related Gd-DOTA (R₁ = 3.7 mM^-1 s^-1). The R₁ relaxivity in HSA goes up to 37.2 mM^-1 s^-1, which is almost twice as high as that of MS-325, a leading BPCA, demonstrating a strong blood pool effect. The in vivo MR images of mice obtained with 2b are coherent, showing strong signal enhancement in heart, abdominal aorta, and small vessels. Even the brain tumor is vividly enhanced for an extended period of time. The structural uniqueness of 2 is that it is neutral in charge and thus makes no resort to electrostatic interaction, supposedly one of the essential factors for the blood-pool effect.