Preclinical evaluation of (S)-[18F]GE387, a novel 18-kDa translocator protein (TSPO) PET radioligand with low binding sensitivity to human polymorphism rs6971

Abstract

Purpose Positron emission tomography (PET) studies with radioligands for 18-kDa translocator protein (TSPO) have been instrumental in increasing our understanding of the complex role neuroinflammation plays in disorders affecting the brain. However, (R)-[11C]PK11195, the first and most widely used TSPO radioligand has limitations, while the next-generation TSPO radioligands have suffered from high interindividual variability in binding due to a genetic polymorphism in the TSPO gene (rs6971). Herein, we present the biological evaluation of the two enantiomers of [18F]GE387, which we have previously shown to have low sensitivity to this polymorphism. Methods Dynamic PET scans were conducted in male Wistar rats and female rhesus macaques to investigate the in vivo behaviour of (S)-[18F]GE387 and (R)-[18F]GE387. The specific binding of (S)-[18F]GE387 to TSPO was investigated by pre-treatment with (R)-PK11195. (S)-[18F]GE387 was further evaluated in a rat model of lipopolysaccharide (LPS)-induced neuroinflammation. Sensitivity to polymorphism of (S)-GE387 was evaluated in genotyped human brain tissue. Results (S)-[18F]GE387 and (R)-[18F]GE387 entered the brain in both rats and rhesus macaques. (R)-PK11195 blocked the uptake of (S)-[18F]GE387 in healthy olfactory bulb and peripheral tissues constitutively expressing TSPO. A 2.7-fold higher uptake of (S)-[18F]GE387 was found in the inflamed striatum of LPS-treated rodents. In genotyped human brain tissue, (S)-GE387 was shown to bind similarly in low affinity binders (LABs) and high affinity binders (HABs) with a LAB to HAB ratio of 1.8. Conclusion We established that (S)-[18F]GE387 has favourable kinetics in healthy rats and non-human primates and that it can distinguish inflamed from normal brain regions in the LPS model of neuroinflammation. Crucially, we have reconfirmed its low sensitivity to the TSPO polymorphism on genotyped human brain tissue. Based on these factors, we conclude that (S)-[18F]GE387 warrants further evaluation with studies on human subjects to assess its suitability as a TSPO PET radioligand for assessing neuroinflammation.

[18F]LW223 has low non-displaceable binding in murine brain, enabling high sensitivity TSPO PET imaging

“…and 78% parent in brain (1 hour p.i.) (mice); 26 [ 11 C]PBR28 – 50% parent in plasma (5 minutes p.i.) (rats); 27 (R)- [ 11 C]PK11195 – 50% parent in plasma (21 minutes p.i.)…”

The 18-kDa Translocator Protein PET Tracers as a Diagnostic Marker for Neuroinflammation: Development and Current Standing

“…Third generation PET tracers have more of an advantage than previous generations due to minimum susceptibility to human rs6971 polymorphism, but their evaluation still needs further comparisons and clinical experiments to examine. 142,143 [ 18 F]BS224, 2-(-2-(4-[ 18 F]fuorophenyl)-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)-N,N-dipropylaceta-mide is the newest 18 F-labeled imidazo[1,2-a]pyridine analog-based TSPO tracer, which possesses a high binding efficiency (K i = 0.51 nM) and specificity for TSPO. 144 [ 18 F]BS224 was prepared with the use of boronic acid pinacol ester via aromatic 18 F-fuorination (with 39 ± 6.8% radiochemical yield) by using 18-crown-6/CsHCO 3 as a phase transfer catalyst.…”

“…Third generation PET tracers have more of an advantage than previous generations due to minimum susceptibility to human rs6971 polymorphism, but their evaluation still needs further comparisons and clinical experiments to examine. , …”

Advances in Radiopharmaceutical Sciences for Vascular Inflammation Imaging: Focus on Clinical Applications

“…In a study that analyzed culprit carotid plaques associated with stroke or transient ischemic attack in patients who underwent PET imaging with the TSPO radioligand [ 11 C]PK11195, macrophage-rich plaque where identified by the radiotracer [ 30 ]. Such PET radioligand suffers from high interindividual variability in binding due to a genetic polymorphism in the TSPO gene (rs6971) and alternatives PET probes with low sensitivity to TSPO polymorphism are under development [ 31 ]. The turnover of macrophages is rapid in inflamed tissue and these cells have a relatively short life span, implicating intense DNA replication.…”