On the Fate of MRI Gd-Based Contrast Agents in Cells. Evidence for Extensive Degradation of Linear Complexes upon Endosomal Internalization

Commercial Gd-containing complexes are often used as MRI reporters in cellular labeling procedures as they are internalized into endosomes by pinocytosis. A methodology has been applied to assess the relative stability of three commercial Gd contrast agents following cellular uptake in fibroblasts and macrophages. It has been found that the acyclic series of Gd MRI contrast agents are degraded much more rapidly than their macrocyclic analogues, following endosomal internalization into living cells. This helps to explain their causal role in the development of nephrogenic systemic fibrosis in renally impaired patients. The methodology has also been applied to assess the fate of Gd-DTPA-BMA-loaded liposomes upon their endosomal internalization. Resistant liposomes prevent the degradation of the complex, whereas liposomes designed to release their payload in the acidic environments show a loss of integrity of Gd-DTPA-BMA analogous to the one observed upon internalization of the free complex

Synthesis of High Relaxivity Gadolinium AAZTA Tetramers as Building Blocks for Bioconjugation

Molecular imaging requires the specific accumulation of contrast agents at the target. To exploit the superb resolution of MRI for applications in molecular imaging, gadolinium chelates, as the MRI contrast agents (CA), have to be conjugated to a specific vector able to recognize the epitope of interest. Several Gd­(III)-chelates can be chemically linked to the same binding vector in order to deliver multiple copies of the CA (multimers) in a single targeting event thus increasing the sensitivity of the molecular probe. Herein three novel bifunctional agents, carrying one functional group for the bioconjugation to targeting vectors and four Gd­(III)-AAZTA chelate functions for MRI contrast enhancement (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid), are reported. The relaxivity in the tetrameric derivatives is 16.4 ± 0.2 mM_Gd^–1 s^–1 at 21.5 MHz and 25 °C, being 2.4-fold higher than that of parent, monomeric Gd­(III)-AAZTA. These compounds can be used as versatile building blocks to insert preformed, high relaxivity, and high density Gd-centers to biological targeting vectors. As an example, we describe the use of these bifunctional Gd­(III)-chelates to label a fibrin-targeting peptide

Synthesis and Preliminary Evaluation in Tumor Bearing Mice of New ¹⁸F‑Labeled Arylsulfone Matrix Metalloproteinase Inhibitors as Tracers for Positron Emission Tomography

New fluorinated, arylsulfone-based matrix metalloproteinase (MMP) inhibitors containing carboxylate as the zinc binding group were synthesized as radiotracers for positron emission tomography. Inhibitors were characterized by <i>K</i>_i for MMP-2 in the nanomolar range and by a fair selectivity for MMP-2/9/12/13 over MMP-1/3/14. Two of these compounds were obtained in the ¹⁸F-radiolabeled form, with radiochemical purity and yield suitable for preliminary studies in mice xenografted with a human U-87 MG glioblastoma. Target density in xenografts was assessed by Western blot, yielding <i>B</i>_max/<i>K</i>_d = 14. The biodistribution of the tracer was dominated by liver uptake and hepatobiliary clearance. Tumor uptake of ¹⁸F-labeled MMP inhibitors was about 30% that of [¹⁸F]­fluorodeoxyglucose. Accumulation of radioactivity within the tumor periphery colocalized with MMP-2 activity (evaluated by in situ zimography). However, specific tumor uptake accounted for only 18% of total uptake. The aspecific uptake was ascribed to the high binding affinity between the radiotracer and serum albumin