Development of a lipoprotein based molecular imaging MR contrast agent for the noninvasive detection of early atherosclerotic disease.

Currently there are no clinically available means of noninvasively detecting early atherosclerotic disease because these lesions are characterized by an accumulation of extracellular lipid and foam cells, but a lack of significant wall thickening or architectural distortion. We hypothesize that a paramagnetically labeled low density lipoprotein (LDL) could serve as a functional probe to detect sites of abnormal lipid metabolism in the vessel wall that represent sites of early disease. Isolated LDL was first incubated with manganese-mesoporphyrin, a hydrophobic MR contrast agent (MnMeso). Size exclusion chromatography and absorption mass spectroscopy were performed on the resulting samples to prove that an association between the two occurred. Subsequently, foam cell cultures (n=7) were incubated (10-30 microg/ml for 48 h) with these labeled lipoproteins and the T1 relaxivity of centrifuged pellets of these cells was determined by using an inversion recovery sequence on a 1.5T scanner. These results were compared to control measurements made from foam cell cultures fed unlabeled lipoproteins (n=7). Measured T1 relaxation times of the cells fed the MnMeso-LDL (443.3 +/- 51.8 ms) was significantly different from the T1 relaxivity obtained from cells fed unlabeled lipoproteins (661.3 +/- 60.9 ms). These findings indicate that the amount of contrast bound to the constructed lipoproteins is sufficient to produce measurable MR signal changes noninvasively. The study results support the feasibility of future in vivo MR experiments with labeled lipoproteins to assess lipoprotein kinetics in the vessel wall, which will hopefully provide a means of detecting early atherosclerotic disease.