Noninvasive Assessment of Hypoxia in Rabbit Advanced Atherosclerosis Using 18 F-fluoromisonidazole Positron Emission Tomographic Imaging

Abstract

Background— Hypoxia is an important microenvironmental factor influencing atherosclerosis progression by inducing foam-cell formation, metabolic adaptation of infiltrated macrophages, and plaque neovascularization. Therefore, imaging plaque hypoxia could serve as a marker of lesions at risk. Methods and Results— Advanced aortic atherosclerosis was induced in 18 rabbits by atherogenic diet and double balloon endothelial denudation. Animals underwent 18 F-fluoromisonidazole positron emission tomographic and 18 F-fluorodeoxyglucose positron emission tomographic imaging after 6 to 8 months (atherosclerosis induction) and 12 to 16 months (progression) of diet initiation. Four rabbits fed standard chow served as controls. Radiotracer uptake of the abdominal aorta was measured using standardized uptake values. After imaging, plaque hypoxia (pimonidazole), macrophages (RAM-11), neovessels (CD31), and hypoxia-inducible factor-1α were assessed by immunohistochemistry. 18 F-fluoromisonidazole uptake increased with time on diet (standardized uptake value mean, 0.10±0.01 in nonatherosclerotic animals versus 0.20±0.03 [ P =0.002] at induction and 0.25±0.03 [ P <0.001] at progression). Ex vivo positron emission tomographic imaging corroborated the 18 F-fluoromisonidazole uptake by the aorta of atherosclerotic rabbits. 18 F-fluorodeoxyglucose uptake also augmented in atherosclerotic animals, with an standardized uptake value mean of 0.43±0.02 at induction versus 0.35±0.02 in nonatherosclerotic animals ( P =0.031) and no further increase at progression. By immunohistochemistry, hypoxia was mainly located in the macrophage-rich areas within the atheromatous core, whereas the macrophages close to the lumen were hypoxia-negative. Intraplaque neovessels were found predominantly in macrophage-rich hypoxic regions (pimonidazole + /hypoxia-inducible factor-1α + /RAM-11 + ). Conclusions— Plaque hypoxia increases with disease progression and is present in macrophage-rich areas associated with neovascularization. 18 F-fluoromisonidazole positron emission tomographic imaging emerges as a new tool for the detection of atherosclerotic lesions.