Gadolinium MRI contrast agents based on triazine dendrimers: relaxivity and in vivo pharmacokinetics

Abstract

Four gadolinium (Gd)-based macromolecular contrast agents, G3-(Gd-DOTA)(24), G5-(Gd-DOTA)(96), G3-(Gd-DTPA)(24), and G5-(Gd-DTPA)(96), were prepared that varied in the size of dendrimer (generation three and five), the type of chelate group (DTPA or DOTA), and the theoretical number of metalated chelates (24 and 96). Synthesis relied on a dichlorotriazine derivatized with a DOTA or DTPA ligand that was incorporated into the dendrimer and ultimately metalated with Gd ions. Paramagnetic characteristics and in vivo pharmacokinetics of all four contrast agents were investigated. The DOTA-containing agents, G3-(Gd-DOTA)(24) and G5-(Gd-DOTA)(96), demonstrated exceptionally high r1 relaxivity values at off-peak magnetic fields. Additionally, G5-(Gd-DOTA)(96) showed increased r1 relaxivity in serum compared to that in PBS, which was consistent with in vivo images. While G3-(Gd-DOTA)(24) and G3-(Gd-DTPA)(24) were rapidly excreted into the urine, G5-(Gd-DOTA)(96) and G5-(Gd-DTPA)(96) did not clear as quickly through the kidneys. Molecular simulation of the DOTA-containing dendrimers suggests that a majority of the metalated ligands are accessible to water. These triazine dendrimer-based MRI contrast agents exhibit several promising features such as high in vivo r1 relaxivity, desirable pharmacokinetics, and well-defined structure.

Figure 1

¹H NMR spectra and molecular weights of the dendrimers, 6, 7, 9, and 10, containing DOTA or DTPA ligands.

Figure 2

Radial distribution functions of dendrimer (black solid line) and metallated chelates (black dotted line) and water molecules (blue line) of G3-(Gd-DOTA)₂₄ (left) and G5-(Gd-DOTA)₉₆ (right).

Figure 3

r1 (a) and r2 (b) relaxivity profiles of triazine dendrimer-based conjugates compared with low molecular weight Gd-chelates (Gd-DTPA and Gd-DOTA) below 63 MHz magnetic field at 23°C.

Figure 4

Early (4 min) and late (60 min) whole body MIP MR images of mice injected with 0.03 mmol/kg of G3-(Gd-DOTA)₂₄ (a), G3-(Gd-DTPA)₂₄ (b), G5-(Gd-DOTA)₉₆ (c), and G5-(Gd-DTPA)₉₆ (d) are shown. Entire dynamic MR images, which were taken every 2 min, are shown as Supplemental movies 1 to 4. (1: G3-(Gd-DOTA)₂₄; 2: G3-(Gd-DTPA)₂₄; 3: G5-(Gd-DOTA)₉₆; 4: G5-(Gd-DTPA)₉₆). Both G3 agents show enhanced kidneys (arrows) as well as blood pool in image 4 min after injection, however, show only the urine in the bladder 60 min after injection. In contrast, both G5 agents show blood pool up to 60 min after injection with minimal enhancement in the kidney and the bladder.

Figure 5

Serial dynamic T1 signals of the blood pool (a), the kidney (b) and the liver (c) are plotted. T1 signals are consistent with the dynamic image shown in Figure 4 and Supplemental videos. Image 1 is Pre-contrast and image 2 is obtained after contrast agent administration. Images were obtained every 2 min after injection (images 2-32).

Scheme 1

Synthesis of the Triazine Dendrimers (1—4) Containing DOTA or DTPA Ligands (G3 and G5 Dendrimer Platforms Depicted as Black Dot Spheres)

Chart 1

Representation of the two triazine dendrimers (left) and the metallated chelates that appear on each peripheral piperazine.

Chart 2

Equilibrated configurations of G3-(Gd-DOTA)₂₄ (a) and G5-(Gd-DOTA)₉₆ (b). The dendritic scaffold is colored in black, DOTA chelates are colored by atoms (C: grey, N: blue, O: red and H: white) and Gd atoms are colored in green. The center of the dendrimer is colored in pink. Explicit water molecules and Cl^- and Na⁺ atoms are not shown for clarity.