In vivo metabolism of glycated and glycoxidized LDL: insights from small animal positron emission tomography (PET) studies

Data concerning the role of circulating glycated (glycLDL) and glycoxidized LDL (glycoxLDL) in atherogenesis and other pathologies are scarce. One reason for this is the lack of suitable radiolabeling methods for direct assessment of metabolic pathways of modified LDL in vivo. We report a novel approach for specific labeling of human native LDL (nLDL), glycLDL, and glycoxLDL with the positron emitter fluorine-18 (¹⁸F) by either N-succinimidyl-4-[¹⁸F]fluorobenzoate or N-[6-(4-[¹⁸F]fluorobenzylidene)-aminooxyhexyl]maleimide (radiochemical yields, 15-40%; specific radioactivity, 150-400 GBq/µmol). Radiolabeling itself caused neither additional oxidative structural modifications of LDL constituents nor adverse alterations of their biological activity and functionality in vitro. The approach was evaluated with respect to binding and uptake of ¹⁸F-nLDL, ¹⁸F-glycLDL, and ¹⁸F-glycoxLDL in cells overexpressing various lipoprotein-recognizing receptors. The metabolic fate of ¹⁸F-labeled nLDL, glycLDL, and glycoxLDL in vivo was delineated by dynamic small animal PET studies in rats. Dynamic PET data demonstrated a significantly delayed catabolism of glycLDL when compared with nLDL. In contrast, glycoxLDL showed an enhanced catabolism when compared with nLDL. The in vivo distribution and kinetics of nLDL, glycLDL, and glycoxLDL correlated well with the anatomical localization and functional expression of LDL receptors, scavenger receptors, and receptors for advanced glycation end products. The study shows that LDL modification in part or fully blocks binding to the LDL receptor, and reroutes modified LDL to various tissue-specific pathways. In this line, ¹⁸F-labeling of LDL and the use of small animal PET provide a valuable tool for imaging and functional characterization of these pathways in animal models in vivo.