Improving T₁ and T₂ magnetic resonance imaging contrast agents through the conjugation of an esteramide dendrimer to high-water-coordination Gd(III) hydroxypyridinone complexes

Abstract

Commercial gadolinium magnetic resonance imaging (MRI) contrast agents are limited by low relaxivity (r₁) and coordination to only a single water molecule (q = 1). Consequently, gram quantities of these agents must be injected to obtain sufficient diagnostic contrast. In this study, MRI contrast agents for T(1) and T₂ relaxivity were synthesized using hydroxypyridinone and terephthalamide chelators with mesityl and 1,4,7-triazacyclononane capping moieties. When covalently conjugated to a highly biocompatible esteramide dendrimer, T₂ relaxation rates up to 52 mm(-1) s(-1) and T₁ relaxation rates up to 31 mm(-1) s(-1) per gadolinium were observed under clinically relevant conditions. These values are believed to be brought about by using a dendritic macromolecule to decrease the molecular tumbling time of the small molecule complexes. These agents also show high aqueous solubility and low toxicity in vitro. In this study we report six new compounds: three discrete complexes and three dendrimer conjugates.

Figure 1

Previously developed hexadentate oxygen donor Gd(III)MRI contrast agents with fast water exchange and high q values (12). Left: Gd-TACN-tris-(1-Me)-3,2-HOPO (1). Right: Gd-TACN-tris-1,2-HOPO (2).

Figure 2

Previously reported esteramide dendrimer (EA dendrimer, 3) developed for drug delivery. The architecture is designed for stability for the duration of a complete MRI scan with biodegradability as an eventual excretion pathway (25).

Figure 3

Novel TACN capped complexes to increase T₁ and T₂ relaxivity. Gd-TACN-trisHOPO (12) has a q = 3 Gd-TACN-bis-(1-Me)-3,2-HOPO-TAM-ethylamine (left, 4) was conjugated to the esteramide dendrimer and Gd-TACN-bis-(1-Me)-3,2-HOPO-TAM-ethyl (right, 5) was used as a small molecule analogue to compare relaxivity and cytotoxicity.

Figure 4

Gd-Mes-bis(1-Me)-3,2-HOPO-TAM-ethylamine (left, 7) and Gd-Mesbis(1-Me)-3,2-HOPO-TAM-ethylamine-bisethylamine (right, 8) were originally synthesized to capitalize on high relaxivity stemming from a higher q value. However, these compounds were not suitable for MRI contrast agents due to their low solubility (<10 μM in water) and subsequent aggregation (9). Upon conjugation to the esteramide dendrimer, they become suitably soluble and have high relaxivities.

Figure 5

Mesityl capped ligands with branched dendrimer (Gd-Mes-bis-(1-Me)-3,2-HOPO-TAM-Asp-Asp₂-12OH, left, 9) and PEGylated dendrimer (Gd-Mes-bis-(1-Me)-3,2-HOPO-TAM-PEG450, right, 10). The branched dendrimer uses its rigidity to achieve a larger T₁ relaxivity and the PEGylated dendrimer uses its much larger surface area to achieve a larger T₂ relaxivity.

Scheme 1

Conjugation of the Gd-TACN-bis-(1-Me)-3,2-HOPO-TAM-ethylamine complex to the EA dendrimer (6). Conjugations to other ligands were carried out under similar conditions.