Assessment of metabolism of native and oxidized low density lipoprotein in vivo: insights from animal positron emission tomography (PET) studies

Oxidative modification of low density lipoprotein (LDL) is regarded as a crucial event in athergenesis. Data concerning the role of circulating oxidized LDL (oxLDL) in the development of atherosclerosis are scarce. One reason for this is the shortage of methaods for direct assessment of metabolism of oxLDL in vivo. We reprot an improved methodology for labelling of both native LDL (nLDL) and oxLDL with fluorine-18 (¹⁸F) by N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) and the use of LDL-[¹⁸]FB-conjugates in dynamic PET studies in Wistar rats. For labelling experiments, pools of chemically well characterized human nLDL and oxLDL, respectively, were used. Preparation of [¹⁸F]SFB was achieved within 40 min with radiochemical yields of 50±5% and purity of >95% using O-(N-succinimidyl)-N-N,N´,N´-tetramethyluronium tetrafluoroborate (TSTU) as activating reagent. LDL labelling with [¹⁸F]SFB resulted in radiochemical yields of 30±10%. The method was evaluated with respect to uptake of FB-conjugated nLDL in HepG2 cells and of FB-conjugated oxLDL in primary human macrophages, respectively. Biodistribution studies revealed high in vivo stability for the LDL-[¹⁸F]FB conjugates. The metabolic fate of LDL-[¹⁸F]FB conjugates in vivo was delineated by PET using a dedicated small animal tomograph (microPET; spatial resolution of 2 mm). In conclusion, [¹⁸F]SFB-labelling of LDL and the use of PET provide a valuable tool for assessment of metabolism of nLDL and oxLDL in vivo.