Conjugation effects of various linkers on Gd(III) MRI contrast agents with dendrimers: optimizing the hydroxypyridinonate (HOPO) ligands with nontoxic, degradable esteramide (EA) dendrimers for high relaxivity

Abstract

One essential requirement for more sensitive gadolinium-based MRI contrast agents is to slow the molecular tumbling of the gadolinium(III) ion, which increases the gadolinium's relaxivity (i.e., its ability to speed up the NMR relaxation of nearby water molecules). One route to this is through conjugation to high-molecular-weight polymers such as dendrimers. In this work, amine-functionalized TREN-bis(1,2-HOPO)-TAM-ethylamine and TREN-bis(1-Me-3,2-HOPO)-TAM-ethylamine ligands have been synthesized and attached to biocompatible 40 kDa esteramide (EA)- and poly-l-lysine (PLL)-based dendrimers capable of binding up to eight gadolinium complexes. These conjugates have T(1) relaxivities of up to 38.14 ± 0.02 mM(-1) s(-1) per gadolinium at 37 °C, corresponding to relaxivities of up to 228 mM(-1) s(-1) per dendrimer molecule. This relaxivity expressed on a "per Gd" basis is several times that of the small-molecule complexes and an order of magnitude higher than that of current commercial agents. Because of their high performance and low toxicity, these macromolecules may constitute an attractive complement to currently available gadolinium(III)-based contrast agents.

Figure 1

The Gd-TREN-bis(1-Me-3,2-HOPO)-TAM-ethylamine complexes 1-3 and Gd-TREN-bis(1,2-HOPO)-TAM-ethylamine complex 4.

Figure 2

A comparison of the in vitro per gadolinium relaxivities of several clinical Gd(III) contrast agents and the dendrimer contrast agents investigated. All values for conjugates 5-9 were measured at 37 °C at 60 mHz.

Scheme 1

The conjugation of Gd-TREN-bis(1-Me-3,2-HOPO)-TAM-Ethylamine complexes 1-3 to the EA dendrimer. Conjugates 7 and 8 were synthesized under identical conditions.