Densely packed Gd(III)-chelates with fast water exchange on a calix[4]arene scaffold: a potential MRI contrast agent.

A pyridine-N-oxide functionalized DOTA analogue has been conjugated to a calix[4]arene and the corresponding Gd-complex was characterized with respect to its suitability as MRI contrast agent. The compound forms spherical micelles in water with a cmc of 35 microM and a radius of 8.2 nm. The relaxivity of these aggregates is 31.2 s(-1) mM(-1) at 25 degrees C and 20 MHz, which corresponds to a molecular relaxivity of 125 s(-1) mM(-1). The high relaxivity mainly originates from the short tau(M) (72.7 ns) and the size of the micelles. The interaction with bovine serum albumin (BSA) was studied and an observed relaxivity of up to 40.8 s(-1) mM(-1) (163.2 s(-1) mM(-1) per binding place) at 20 MHz and 37 degrees C was found in the presence of 2.0 mM protein.