Poly(amidoamine) dendrimer based MRI contrast agents exhibiting enhanced relaxivities derived via metal preligation techniques

Abstract

This report presents the preparation and characterization of three [Gd-C-DOTA](-1)-dendrimer assemblies by way of analysis, NMRD spectroscopy, and photon correlation spectroscopy (PCS). The metal-ligand chelates were preformed in alcohol media prior to conjugation to generation 4, 5, and 6 PAMAM dendrimers. The dendrimer-based agents were purified by Sephadex G-25 column chromatography. The combustion analysis, SE-HPLC, and UV-vis data indicated chelate to dendrimer ratios of 28:1, 61:1 and 115:1, respectively. Molar relaxivity measured at pH 7.4, 22 degrees C, and 3 T (29.6, 49.8, and 89.1 mM(-1) s(-1)) indicated the viability of conjugates as MRI contrast agents. 1/T(1) NMRD profiles were measured at 23 degrees C and indicated that at 22 MHz the 1/T(1) reached a plateau at 60, 85, and 140 mM(-1) s(-1) for the generation 4, 5, and 6 dendrimer conjugates, respectively. The PCS data showed the respective sizes of 5.2, 6.5, and 7.8 nm for G-4, 5, and 6 conjugates.

Figure 3

Molar relaxivity plots of G4-(C-DOTA-Gd)₂₈ (■; 29.6 mM⁻¹s⁻¹), G5-(C-DOTA-Gd)₆₁) (●; 49.8 mM⁻¹s⁻¹), G6-(C-DOTA-Gd)₁₁₅) (▲; 89.1 mM⁻¹s⁻¹) and Magnevist (◆; 4.2 mM⁻¹s⁻¹).

Figure 4

1/T1 NMRD profiles of 1 mM solution of prohance (▲), Magnevist^® (●), G4-(C-DOTA-Gd)₂₈(−), G5-(C-DOTA-Gd)₆₁(◆), G6-(C-DOTA-Gd)₁₁₅(■), as a function of frequencies at 23°C.

Scheme 1

Synthesis of dendrimer conjugates.