Gadolinium Complexes as Contrast Agent for Cellular NMR Spectroscopy

Aqua Gd(3+) and Gd-DOTA (gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacete) complexes were studied as a contrast agent in cellular NMR (nuclear magnetic resonance) spectroscopy for distinguishing between intracellular and extracellular spaces. The contrast agents for this purpose should provide strong paramagnetic relaxation enhancement and localize in the extracellular space without disturbing biological functions. Cell membrane permeability to Gd complexes was evaluated from the concentrations of gadolinium complexes in the inside and outside of E. coli cells measured by the (1)H-NMR relaxation. The site-specific binding of the complexes to E. coli cells was also analyzed by high-resolution solid-state (13)C-NMR. The aqua Gd(3+) complex did not enhance T(1) relaxation in proportion to the amount of added Gd(3+). This Gd(3+) concentration dependence and the (13)C-NMR indicated that its strong cytotoxicity should be due to the binding of the paramagnetic ions to cellular components especially at the lipid membranes. In contrast, Gd-DOTA stayed in the solution states and enhanced relaxation in proportion to the added amount. This agent exhibited strong T(1) contrast between the intra- and extracellular spaces by a factor of ten at high concentrations under which the cells were viable over a long experimental time of days. These properties make Gd-DOTA suitable for selectively contrasting the living cellular space in NMR spectroscopy primarily owing to its weak interaction with cellular components.