Synthesis and Evaluation of [¹⁸F]RAGER: A First Generation Small-Molecule PET Radioligand Targeting the Receptor for Advanced Glycation Endproducts

Abstract

The receptor for advanced glycation endproducts (RAGE) is a 35kDa transmembrane receptor that belongs to the immunoglobulin superfamily of cell surface molecules. Its role in Alzheimer’s disease (AD) is complex, but it is thought to mediate influx of circulating amyloid-β into the brain as well as amplify Aβ-induced pathogenic responses. RAGE is therefore of considerable interest as both a diagnostic and a therapeutic target in AD. Herein we report the synthesis and preliminary pre-clinical evaluation of [18F]RAGER, the first small molecule PET radiotracer for RAGE (Kd = 15 nM). Docking studies propose a likely binding interaction between RAGE and RAGER, [18F]RAGER autoradiography showed co-localization with RAGE identified by immunohistochemistry in AD brain samples, and [18F]RAGER microPET confirmed CNS penetration and increased uptake in areas of the brain known to express RAGE. This 1st generation radiotracer represents initial proof-of-concept and a promising first step towards quantifying CNS RAGE activity using PET. However, there were high levels of non-specific [18F]RAGER binding in vitro, likely due to its high logP (experimental logP = 3.5), and rapid metabolism of [18F]RAGER in rat liver microsome studies. Therefore development of 2nd generation ligands with improved imaging properties would be advantageous prior to anticipated translation into clinical PET imaging studies.

Targeting the Receptor for Advanced Glycation Endproducts (RAGE): A Medicinal Chemistry Perspective

“… 18 This binding appeared to be largely dependent on hydrophobic interactions with Pro45, Leu49, Trp51, Pro66, Leu78, and Pro80. 18 …”

“… 18 [ 18 F] 13 was obtained using a trimethylammonium precursor and [ 18 F]fluoride in the presence of Kryptofix-2.2.2 (K2.2.2) in DMF at 130 °C for 30 min. 18 In vitro autoradiography of human brain tissue showed higher uptake of [ 18 F] 13 in the frontal cortex of AD compared with healthy control tissue. However, when studied in vivo, significant nonspecific binding to the white matter was observed, peaking within 3 min post iv injection and followed by virtually complete washout over the duration of the PET scan.…”

“… 17 , 46 12 was used as a chemotype for development of the potential PET radiotracer ([ 18 F] 13 ) with the aim to quantify CNS RAGE in vivo. 18 Further in vivo characterization of this radiotracer will be required to demonstrate the potential utility of [ 18 F] 13 . Molecular docking studies of RAGE protein confirmed that the interaction of 12 , 13 with its conserved residues (Leu49 and Trp51, Figure 7 ) is important for the RAGE–Aβ interaction.…”

Microenvironment Remodeling Micelles for Alzheimer's Disease Therapy by Early Modulation of Activated Microglia

“…[15] Similar to the situation in tumor microenvironment, oxidative stress is leveraged to trigger drug release in ROS-responsive drug delivery systems for AD treatment. [20] Based on the aberrant hyperreactive state of microglia and the associated microenvironment changes in the early stage of AD progression, we herein present a polymeric micelle drug delivery system (Ab-PEG-LysB/CUR) with sequential targeting ability to normalize AD microenvironment via microglia modulation. [8c] The hindrance of blood-brain barrier (BBB) and the locusspecific intracerebral drug distribution should also be taken into consideration for AD drug delivery.…”

Glycation in Huntington’s Disease: A Possible Modifier and Target for Intervention

“…Finally, as described above, RAGE is as an important subject of research [174], and in vitro and in vivo studies have demonstrated the potential of RAGE as a therapeutic target in neurodegeneration [150,151,[175][176][177]. Additional studies will be important to confirm this.…”