Carotid Plaque Characteristics by MRI in AIM-HIGH (Carotid MRI Substudy)

Although studies have suggested that plaque morphology and composition are important determinants of plaque stability, our understanding on plaque tissue components is mainly from histological studies until recent development in MRI technique. A low level of HDL is associated with higher risk of cardiovascular events and increased amount of lipid content in the carotid plaques. Treatment with LDL-lowering plus HDL-raising compared with LDL-lowering alone more effectively protects against atherosclerosis progression. It is widely believed that HDL or its apolipoproteins mediate the removal of excess free cholesterol from peripheral cells and the cholesterol is delivered via either LDL or HDL to the liver for excretion into the bile. However, it has not been tested and approved in human atherosclerotic condition in vivo. The NIH/Abbott-funded multi-center AIM-HIGH trial is designed to compare the clinical efficacy of LDL-lowering alone with statin versus LDL-lowering plus HDL-raising with statin plus nicotinic acid combination therapy in patients with established vascular disease and high triglycerides and low HDL.

We propose to conduct a carotid MRI sub-study in 220 subjects enrolled in AIM-HIGH to investigate the important vascular biological mechanisms of HDL-raising therapy. Image collection will occur at 3 timepoints. The hypotheses and specific aims are:

• (1) To test the primary hypothesis that compared with LDL-lowering alone, intensive LDL-lowering plus HDL-raising therapy decreases the mean plaque lipid composition in carotid arteries assessed by MRI.
• (2) To test the hypothesis that compared with LDL-lowering alone, intensive LDL-lowering plus HDL-raising therapy decreases the plaque burden including volume and wall thickness.
• (3) To test the hypothesis that increased plaque lipid composition or vessel wall thickness by MRI is associated with increased risk of cardiovascular events.
• (4)To test a hypothesis that LDL-lowering plus HDL-raising, compared to LDL-lowering alone, will promote more rapid plaque lipid depletion. And determine the time-course of atherosclerotic plaque lipid depletion during lipid therapy.
• (5) To examine the association of clinical risk factors, lipids, lipoprotein heterogeneity, inflammatory markers and carotid plaque characteristics.

This MRI sub-study offers a unique opportunity to investigate the effectiveness of LDL-lowering plus HDL-raising therapy on human atherosclerotic plaque in vivo, to examine the association of plaque characteristics both lipid composition and volume assessed by MRI and cardiovascular outcome, and to gain novel insights in our understanding of atherosclerotic plaque pathology and the mechanisms of intensive lipid management in preventing cardiovascular events.