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Title:Nitroxide Spin Labels as NMR Contrast Agents: Delivery, Metabolism and Relaxation
Author(s):Chan, Hsiao Chang
Doctoral Committee Chair(s):Magin, Richard; Swartz, Harold M.
Department / Program:Physiology and Biophysics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Biology, General
Abstract:In this project, the potential of nitroxides as contrast agents for magnetic resonance imaging (MRI) has been studied. The difficulties involved in using nitroxides as in vivo MRI contrast agents, such as premature metabolic convertion of nitroxides to their nonparamagnetic state, lack of tissue selectivity, and relatively low relaxivity of nitroxides, were, in part, overcome by the use of liposomes as the delivery system.
A positively charged aqueous nitroxide, 2,2,6,6-tetramethyl-piperidine-N-oxyl-4-trimethylammonium (Cat1), which does not cross the cell membrane readily, was encapsulated in liposomes. This protected the nitroxide from reduction by external ascorbate. The metabolic reduction of liposome-delivered Cat1 by cells was studied as a function of oxygen concentration, and the rate was found to be increased under hypoxic conditions. This suggests the use of nitroxides to provide differentiated MRI contrast to diseased areas, which are poorly oxygenated.
Liposomes with different phospholipid compositions were constructed. The physical states, "fluid" or "solid", of these liposomal membranes were found to determine the mechanisms by which the liposomes interact with cells. Other liposome properties, such as membrane permeability and vesicle structure, were also found to have an important influence on nitroxide-induced water relaxation and on in vivo liposome stability and clearance from blood.
To enhance the relaxivity of a nitroxide, it was covalently bound to an albumin molecule using a relatively simple procedure. The effects of protein conformation, water accessibility, and pH on the relaxivity of the albumin-bound nitroxide were examined. Interactions of cells with albumin-nitroxide complex, such as reduction and digestion of the complex, were studied: the effect of these cellular processes on the relaxivity of the nitroxide-albumin complex was pronounced.
Issue Date:1988
Description:137 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
Other Identifier(s):(UMI)AAI8908643
Date Available in IDEALS:2014-12-16
Date Deposited:1988

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