Fe-HBED Analogs: A Promising Class of Iron-Chelate Contrast Agents for Magnetic Resonance Imaging.

Contrast-enhanced magnetic resonance imaging is an essential tool for disease diagnosis and management; all marketed clinical magnetic resonance imaging (MRI) contrast agents (CAs) are gadolinium (Gd) chelates and most are extracellular fluid (ECF) agents. After intravenous injection, these agents rapidly distribute to the extracellular space and are also characterized by low serum protein binding and predominant renal clearance. Gd is an abiotic element with no biological recycling processes; low levels of Gd have been detected in the central nervous system and bone long after administration. These observations have prompted interest in the development of new MRI contrast agents based on biotic elements such as iron (Fe); Fe-HBED (HBED = N,N'-bis(2-hydroxyphenyl)ethylenediamine-N,N'-diacetic acid), a coordinatively saturated iron chelate, is an attractive MRI CA platform suitable for modification to adjust relaxivity and biodistribution. Compared to the parent Fe-HBED, the Fe-HBED analogs reported here have lower serum protein binding and higher relaxivity as well as lower relative liver enhancement in mice, comparable to that of a representative gadolinium-based contrast agent (GBCA). Fe-HBED analogs are therefore a promising class of non-Gd ECF MRI CA.

A multimodal contrast agent for preoperative MR Imaging and intraoperative tumor margin delineation.

We have constructed a multimodal contrast agent suitable for near-infrared, NIR, fluorescent imaging as well as magnetic resonance imaging, MRI. This class of agents may be useful for preoperative tumor localization and tumor functional evaluation and for intraoperative delineation of tumor margins. We have covalently attached dyes of the cyanine family to a previously described polymeric contrast agent, Gd-DTPA-polylysine, of an extended, uncoiled conformation. The dual modality agent is as effective in imaging tumors by MRI as the parent compound provided that the dye loading on the polymer is such that it does not eliminate all the available free-lysine groups on the parent Gd-DTPA-polylysine polymers. NIR fluorescence from preclinical subcutaneous and orthotopic mammary gland tumors could be detected with a signal to background ratio of as high as 4.5 at 12 hours post agent injection at a dye dose of 125 nmole/kg. For intraoperative delineation of tumor margins, a wide-field illumination camera system was devised giving high signal to background NIR fluorescent images of surgically exposed orthotopic mammary gland tumors. Histologic microscopy confirmed the location of the dual modality agent at the boundary of the tumor with a margin distance of about 0.3 mm from labeled tumor cells.

Conformation and structure of polymeric contrast agents for medical imaging.

The structure of Gd-DTPA-polylysine, Gd-DOTA-polylysine, Gd-SCN-Bz-DOTA-polylysine, and Gd-DTPA-poly(glu:lys) was investigated with circular dichroism, gel permeation chromatography, low angle light scattering, and proton longitudinal relaxivity. Molecular modeling calculations were performed and predicted helical secondary structure for charged Gd-chelator residues, i.e., Gd-DTPA, when the DTPA conjugation levels reached 90% and higher. This helical secondary structure was observed with circular dichroism. The conformational transition from coiled to extended linear was observed also by gel permeation chromatography and by proton relaxivity measurements. The helical secondary structure was not observed when the chelator was changed to DOTA. The residue charge interactions were eliminated in this case since the Gd-DOTA complex had no net charge. For this construct, the gel permeation and relaxivity measurements indicated a coiled conformation. An extended linear conformation was regained when the chelator complex was changed to Gd-SCN-Bz-DOTA, which had a net negative charge. The functional aspects of these structures were investigated by MR imaging of an animal tumor model. The linear extended polymer constructs gave 10-fold higher tumor signals then the coiled-collapsed constructs, indicating a much higher degree of trans-endothelial transport in the tumors.