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

Abstract

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 μM and a radius of 8.2 nm. The relaxivity of these aggregates is 31.2 s⁻¹ mM⁻¹ at 25 °C and 20 MHz, which corresponds to a molecular relaxivity of 125 s⁻¹ mM⁻¹. The high relaxivity mainly originates from the short τ_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 °C was found in the presence of 2.0 mM protein.