Novel biomedical imaging agents and the in vitro self -assembly of dendrimers: A strategy for amplification of delivery to extracellular targets

This thesis describes the design, development and examination of biomedical imaging agents and a novel strategy used to amplify the sensitivity of an imaging agent.;Chapter 1. describes the examination of Fe8 (C36H120O23N18Fe 8Br8) for its potential as a contrast agent for magnetic resonance imaging. The compound was prepared, isolated, and shown to exhibit superparamagnetism (S = 10). The structure of Fe8 in aqueous solution was probed by several NMR techniques. Relaxivity of aqueous solutions of Fe8 as well as that of comparison compounds FeCl 3, FeCl3(tacn) (tacn = 1,4,7-triazacyclononane), and Gd(DTPA) (DTPA = diethylenetriamine pentaacetic acid) were determined by T 1 and T2 inversion recovery and CPMG sequences. In contrast to a previous report, the relaxivity of Fe 8 solutions was found to be weak compared to Gd(DTPA). However, an interesting dependence upon pH was observed. The influence of environmental factors on relaxivity is of interest in the burgeoning field of molecular imaging.;Chapter 2. describes the development of a nanoparticle magnetic resonance imaging (MRI) contrast agent by conjugation of more than 500 gadolinium chelate groups onto a viral capsid. The high density of paramagnetic centers and slow tumbling rate of modified MS2 capsids provided enhanced T 1 relaxivities up to 7200 mM-1s-1 per particle. A bimodal imaging agent was generated by sequential conjugation of fluorescein and Gd3+ chelate. These results illustrate the potential for engineering natural protein assemblies to address bionanotechnology applications.;Chapter 3. discusses a novel strategy to assemble nanometer-scale architectures on cell surface receptors from smaller macromolecular constituents. This assembly generated a large amplification of fluorescence. Dendrimers containing aldehyde or amine functional groups capable of self-assembly on the surface of human pancreatic cancer cells were prepared by organic and bioconjugate synthesis. A targeted dendrimer was synthesized from a cystamine core G4 PAMAM dendrimer, and contained a humanized anti-BrE3 monoclonal antibody as the targeting group, several fluorophores and an average of 12 aldehyde moieties as complementary bio-orthogonal reactive sites for the covalent assembly. A cargo dendrimer, derived from a PAMAM G4 dendrimer, contained several fluorophores as the cargo for delivery and five hydrazine moieties as complimentary bio-orthogonal reactive sites. The BrE3 Ab targeting group recognizes a 400kD epitope of breast epithelial mucin (BrE3) that is abundant and very prevalent in human breast and pancreatic carcinomas. The system, however, is designed to be flexible and allow for facile incorporation of a variety of targeting ligands.;Chapter 4. describes the synthesis and characterization of novel bimodal biomedical imaging agents. The targeted agents contain both fluorophores for optical imaging, and Gd chelates for MRI. The synthesis of these agents and their characterization is discussed. The MR properties of these agents was probed by T1 inversion recovery sequences on a 9.4 T spectrometer. The agents showed significant increases in relaxivities, up to 997 mM-1s-1 per particle as a result of the high density of the paramagnetic Gd chelates.