High-relaxivity Magnetic Resonance Imaging Contrast Agents Part 2: Optimization of Inner- and Second-sphere Relaxivity

Rationale and Objectives: The observed relaxivity of gadolinium-based contrast agents has contributions from the water molecule(s) that bind directly to the gadolinium ion (inner-sphere water), long-lived water molecules and exchangeable protons that make up the second-sphere of coordination, and water molecules that diffuse near the contrast agent (outer-sphere). Inner- and second-sphere relaxivity can both be increased by optimization of the lifetimes of the water molecules and protons in these coordination spheres, the rotational motion of the complex, and the electronic relaxation of the gadolinium ion. We sought to identify new high-relaxivity contrast agents by systematically varying the donor atoms that bind directly to gadolinium to increase inner-sphere relaxivity and concurrently including substituents that influence the second-sphere relaxivity. Methods: Twenty gadolinium-1,4,7,10-tetraazacyclo-dodecane-N,N',N'',N'''-tetraacetato derivatives were prepared and their relaxivity determined in presence and absence of human serum albumin as a function of temperature and magnetic field. Data was analyzed to extract the underlying molecular parameters influencing relaxivity. Each compound had a common albumin-binding group and an inner-sphere donor set comprising the 4 tertiary amine N atoms from cyclen, an α-substituted acetate oxygen atom, 2 amide oxygen atoms, an inner-sphere water oxygen atom, and a variable donor group. Each amide nitrogen was substituted with different groups to promote hydrogen bonding with second-sphere water molecules. Results: Relativities at 0.47 and 1.4 T, 37°C, in serum albumin ranged from 16.0 to 58.1 mM -1 s -1 and from 12.3 to 34.8 mM -1 s -1 , respectively. The reduction of inner-sphere water exchange typical of amide donor groups could be offset by incorporating a phosphonate or phenolate oxygen atom donor in the first coordination sphere, resulting in higher relaxivity. Amide nitrogen substitution with pendant phosphonate or carboxylate groups increased relaxivity by as much as 88% compared with the N-methyl amide analog. Second-sphere relaxivity contributed as much as 24 and 14 mM -1 s -1 at 0.47 and 1.4 T, respectively. Conclusions: Water/proton exchange dynamics in the inner- and second-coordination sphere can be predictably tuned by choice of donor atoms and second-sphere substituents, resulting in high-relaxivity agents.