Background: Oxidatively modified low-density lipoprotein (LDL) is present in atherosclerotic but not normal arteries and plays a crucial role in the pathogenesis and adverse consequences of atherosclerotic lesions. We previously generated a series of monoclonal antibodies (MoAb) against oxidation-specific neo-epitopes formed during the oxidative modification of LDL. MDA2, a prototype MoAb, recognizes malondialdehyde-lysine epitopes (eg, in malondi-aldehyde-modified LDL) within atherosclerotic lesions. We describe the in vivo characteristics of MDA2 and initial noninvasive imaging studies of atherosclerosis in rabbits.
Methods: To assess the in vivo specificity of MDA2 for atherosclerotic lesions, iodine 125-MDA2 was intravenously injected into 7 LDL-receptor deficient Watanabe heritable hyperlipidemic (WHHL) and 2 normal New Zealand white (NZW) rabbits, and the aortic plaque uptake was evaluated 24 hours later. 125I-Halb, an isotype-matched irrelevant MoAb that binds to human albumin, was injected into 5 WHHL and 2 NZW rabbits as a control. Aortic autoradiography was performed, and the mean uptake of MoAbs was measured as the percent injected dose per gram aortic tissue. Gamma camera imaging was then carried out in 7 WHHL rabbits and 2 NZW rabbits with 99mTc-MDA2. Imaging was carried out at 10 minutes and at 12 or 24 hours. Malondialdehyde-LDL was then injected to clear the blood pool signal, and final images were obtained 2 hours later.
Results: Mean uptake of 125I-MDA2 in the entire aorta was 17.4-fold higher in WHHL than in NZW aortas (P < .001), and 2.8-fold higher than 125I-Halb in WHHL aortas. 125I-MDA2 also had higher specificity for lesioned areas than 125I-Halb (plaque/normal ratio 6.3 vs 2.9, P < .001). Autoradiograph of aortas of 125I-MDA2-injected WHHL rabbits revealed uptake in lipid-stained lesions with absence of signal in adjacent normal arterial tissue. Immunostaining of WHHL lesions, which accumulated MDA2 as noted on autoradiography, revealed that uptake was highest in areas with abundant foam cells and in lipid-rich necrotic core areas. Autoradiograph of aortas from NZW rabbits injected with 125I-MDA2 did not yield any visible signal. Planar gamma camera in vivo scintigraphy revealed a visible signal in 4/7 WHHL rabbits, which was confirmed by aortic Sudan staining.
Conclusion: Radiolabeled MDA2 shows excellent in vivo uptake and specificity for atherosclerotic lesions containing abundant oxidation-specific epitopes. The in vivo imaging studies suggest that noninvasive imaging of oxidation-rich atherosclerotic lesions with radiolabeled MDA2 may be feasible in human beings with optimization of the imaging methods.