Calix[4]arenes as Molecular Platforms for Magnetic Resonance Imaging (MRI) Contrast Agents

An amphiphilic conjugate 1 of a calix[4]arene with four Gd-DOTA chelates (DOTA=1,4,7,10-tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane) was prepared and the properties relevant to its application as a magnetic resonance imaging (MRI) contrast agent were investigated by NMR, dynamic light scattering (DLS), and cryo-electron microscopy (cryo-TEM). The compound aggregates in water; its critical micelle concentration (cmc) is 0.21 mm (or 0.84 mM for Gd) at 37 degrees C. The relaxivity of the aggregates at 37 degrees C and 20 MHz (18.3 s(-1) per mM Gd or 73.2 s(-1) per mM 1) is about twice that of the monomer. Nuclear magnetic relaxation dispersion (NMRD) profiles show the relaxivity of the monomer to be almost independent of the magnetic field strength up to 60 MHz. At higher concentrations, the NMRD profiles exhibit a maximum at about 20 MHz, which is typical for high molecular volumes. The average water residence life-time is 1.20 mu s at 298 K as determined by O-17 NMR. The rotational correlation time of the monomer (390 ns at 37 degrees C) is very close to the optimal value predicted for high-field contrast agents. Monomer as well as micelles are very rigid systems with negligible local contributions to the overall rotational dynamics. The binding to human serum albumin (HSA) is significant (K-A = 1.2 x 10(3) M-1) and the relaxivity of the HSA adduct at 20 MHz is 24.6 s(-1) per mm Gd or 98.5 s(-1) per mm 1.