Macromolecular agents composed of serum albumins or linear polymers have MRI contrast enhancement factors less than those predicted for rigid molecules of comparable size. To obviate this deficiency, MRI contrast agents based upon dendrimers, wherein terminal primary amines are modified with chelating agents that subsequently form Gd(III) complexes, have been developed in our laboratories. These reagents possess a molar relaxivity up to 6 times that of Gd(III)DTPA, currently employed in the clinic, and better than twice that of other macromolecular agents. Excellent conventional MR imaging and 3D T-O-F MR angiograms have been obtained. Studies have been expanded to thoroughly explore the utility of these agents. Ongoing experiments in a murine model system have demonstrated outstanding detailed imaging of the complete, whole-body vasculature and as blood-pool agents when using the G6 generation dendrimer construct. These reagents have also been evaluated for their ability to provide information concerning the tumor vasculature in murine models. As a complementary application, these reagents are being evaluated as Neutron Capture Therapy targets due to the high density of Gd(III) per molecule and their ability to cross into cells. The development of novel ligand systems for producing MRI contrast agents continues to proceed. The recent success of demonstrating that a Gd(III) calix[4]arene complex could induce and enhanced relaxivity by hydrophilic interaction with serum albumin continues to be pursued. Plans include the preparation and evaluation of more stable Gd(III) that also can take advantage of the hydrophilic interaction capacity of calix[4]arenes. A CT contrast agent based upon albumin conjugated to iopanoic acid continues to be evaluated in pre-clinical models for use in quantifying flow and dispersion of therapeutics being injected inter-cranially. This material is being compared to a MRI albumin-based reagent and preparations are underway to prepare the iodine-based reagent for use in clinical trials. Use of dendrimers to prepare a completely synthetic, and hence a reproducible analog of the CT albumin reagent has been initiated. Dendrimers of the G4 generation approach the MW of the albumin reagent, yet possess significantly greater numbers of reactive functional groups by which poly-iodinated CT contrast agents might be conjugated. A water soluble analog of iopanoic acid was prepared for this purpose and has been conjugated to the G4 dendrimeer. Characterization studies continue for this macromolecule to define the targeted products prior to any in vivo evaluation versus conventional low molecular weight CT contrast agents.
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