Substituent effects on Gd(III)-based MRI contrast agents: optimizing the stability and selectivity of the complex and the number of coordinated water molecules

Abstract

Hydroxypyridinone (HOPO)-based Gd(III) complexes have previously been shown to exhibit high relaxivity, especially at the high magnetic fields that are clinically relevant for present and future clinical use. This is due to more than one coordinated water molecule exchanging rapidly with bulk solvent. These complexes, however, present poor water solubility. Heteropodal complexes which include a terephthalamide (TAM) moiety maintain the high relaxivity characteristics of the HOPO family and have been functionalized with solubilizing substituents of various charges. The charge of the substituent significantly affects the stability of the Gd(III) complex, with the most stable complex presenting a neutral charge. The solubilizing substituent also moderately affects the affinity of the complex for physiological anions, with the highest affinity observed for the positively charged complex. In any case, only two anions, phosphate and oxalate, measureably bind the Gd(III) complex with weak affinities that are comparable to other q = 1 complexes and much weaker than DO3A, q = 2 based complexes. Furthermore, unlike poly(amino-carboxylate)-based complexes, HOPO-based Gd(III) complexes do not show any noticeable interaction with carbonates. The nature of the substituent can also favorably stabilize the coordination of a third water molecule on the Gd(III) center and lead to a nine-coordinate ground state. Such complexes that attain q = 3 incorporate a substituent beta to the terminal amide of the TAM podand that is a hydrogen-bond acceptor, suggesting that the third water molecule is coordinated to the metal center through a hydrogen-bond network. These substituents include alcohols, primary amines, and acids. Moreover, the coordination of a third water molecule has been achieved without destabilizing the complex.

Figure 1

Functionalized Gd-TREN-bisHOPO-TAM complexes.

Figure 2

Competition titrations of Gd-TREN-bisHOPO-TAM complexes against DTPA. The x-intercepts indicate the difference in pGd between each heteropodal ligand and the competing poly(amino-carboxylate). Experimental conditions: 0.1 M KCl, pH 7.4, 25 °C.

Figure 3

Effect of the charge of the Gd(III) complex on its stability.

Figure 4

Competition titrations of Gd(III) (filled squares), Cu(II) (open circles) and Zn(II) (grey triangles) of L5 complexes against DTPA. The x-intercepts indicate the difference in pM between each heteropodal ligand and the competing poly(amino-carboxylate). Experimental conditions: 0.1 M KCl, pH 7.4, 25 °C.

Figure 5

Titrations of Gd-L5 (filled circles) and Gd-L9 (open circles) with (a) K₂HPO₄ and (b) K oxalate. Experimental conditions: 25°C, pH = 7.40, [Gd complex] = 0.501 mM.

Figure 6

Nuclear Magnetic Resonance Dispersion profile of Gd-L9 in water (filled circles) and in the presence of 200 equivalents of oxalate (open circles). The solid curve through the data of the ternary complex were fitted with the following parameters: r=3.0 Å, q=1, τ_R=145 ps, τ_V=17 ns, Δ²=6.3×10¹⁹ s⁻¹, a = 4.0 Å, D = 2.24×10⁻⁵ cm²s⁻¹. The dotted curve is a simulation of the NMRD profile of the complex in water with a single water molecule. Experimental conditions: 25 °C, pH 7.40.

Figure 7

Proposed water-exchange mechanisms for Gd-L9 and its ternary complex with oxalate. The 8-and 9-coordinate geometries are from the single crystal X-ray structure of La-TREN-1-Me-3,2-HOPO.

Figure 8

¹H Nuclear Magnetic Relaxation Dispersion profiles of 0.25 mM Gd-L5 in water and human serum at 25 °C. The data are corrected for the diamagnetic contributions of water and serum, respectively..

Figure 9

Nuclear Magnetic Relaxation Dispersion profiles of Gd-L4 (open circles; dotted line), Gd-L5 (filled squares), and Gd-L6 (filled triangles). The best fitting curve for Gd-L4 was calculated with a model that considers a contribution of second hydration sphere water molecules (lower dotted line): q′=4, r′=3.9 Å, τ_R′=70 ps. Experimental conditions: 25 °C and pH = 7.4.

Figure 10

Proposed hydrogen-bonding network to a third water molecule resulting in the stabilization of the 9-coordinate geometry at for Gd-L6.

Figure 11

Nuclear Magnetic Relaxation Dispersion profiles of Gd-L1 (open circles), Gd-L2 (filled squares), and Gd-L3 (filled triangles). Experimental conditions: 25 °C and pH = 7.4.

Figure 12

Proposed hydrogen-bonding network to a third water molecule resulting in the stabilization of the 9-coordinate geometry at for (a) Gd-L1 and (b) Gd-L2.

Figure 13

Effect of pH on the longitudinal relaxivity, r_1p, of Gd-L2. Experimental conditions: 25 °C and 20 MHz.

Figure 14

Temperature dependence of the paramagnetic contribution to the water ¹⁷O NMR transverse relaxation rate (R_2p) for Gd-L1 (17 mM; filled circles) and Gd-L2 (14 mM; open circles). Experimental conditions: pH 7.03, 2.1 T.

Figure 15

Rotational correlation time, τ_R, versus molecular weight for Gd-TREN-bisHOPO-TAM derivatives (R=0.981 for the straight line). The data labelled with an asterisk refers to a dendritic complex reported in ref. . Note that the fact that L4 is above the line supports our view that a significant number of water molecules are H-bonded to the PEG chain and contribute to the relaxivity but also result in a higher effective Mw.