Paramagnetic nano-materials for use as magnetic resonance imaging (MRI) contrast agents have higher relaxivity than conventional low molecular weight MRI agents. However, the biocompatibility and pharmacokinetics of such nano-materials will strongly affect the likelihood of clinical approval. We synthesized MRI contrast agents based on biocompatible lysine-dendri-grafts: Gd-BzDTPAlysineG2 and Gd-BzDTPA-lysineG3. The relaxivity of Gd-BzDTPA-lysineG2 and Gd-BzDTPAlysineG3 increased with sample temperature, while the relaxivity of Gd-BzDTPA-PAMAMG4 decreased with increasing sample temperature. The increase in relaxivity with increasing temperature may be attributed to inaccessibility of water to the internal Gd chelates with lysine-dendri-grafts, which does not occur with PAMAM dendrimers. Gd-BzDTPA-lysineG3 had a long intravascular half life but were largely excreted by the kidneys. Therefore, Gd-BzDTPA-lysineG3 enhanced the blood vessels for longer periods than Gd-BzDTPA-PAMAMG4, but was still excreted through the kidney. Because of their biocompatibility, desirable magneto-physical characteristics and favorable pharmacokinetics, which are derived from different interior structures rather than the physical size, lysine-dendri-graft MR contrast agents may be feasible for clinical use.
Two DTPA derivatives (PIP-DTPA and AZEP-DTPA) as potential contrast enhancement agents in MRI are synthesized. The T1 and T2 relaxivities of their corresponding Gd(III) complexes are reported. At clinically relevant field strengths, the relaxivities of the complexes are comparable to that of the contrast agent, Gd(DTPA) which is in clinical use. The serum stability of the (153)Gd-labeled complexes is assessed by measuring the release of (153)Gd from the ligands. The radiolabeled Gd chelates are found to be kinetically stable in human serum for up to at least 14 days without any measurable loss of radioactivity.
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