An improved, regioselective synthesis of the radiolabelling precursor for the translocator protein targeting positron emission tomography imaging radiotracer [18F]GE-180

[18F]BIBD-239: 18F-Labeled ER176, a Positron Emission Tomography Tracer Specific for the Translocator Protein

[¹¹C]ER176 has adequate sensitivity to image the human brain translocator protein (TSPO) in all three genotypes by positron emission tomography (PET). However, its clinical application is limited by the short half-life of ¹¹C (20.38 min). To overcome the deficiency of [¹¹C]ER176 and keep the pharmacophore features of ER176 to the maximum extent, we designed four fluorine-labeled ER176 derivatives using the deuterium method. In vitro competition binding confirmed that the designed compounds had high affinity for TSPO. Biodistribution experiments showed that tissues with high expression of TSPO had high uptake of these compounds, as well as that the compound showed high brain penetration and mild defluorination in vivo. Therefore, [¹⁸F]BIBD-239 with simple synthesis conditions was selected for further biological evaluation. Theoretical simulations showed that BIBD-239 and ER176 have similar binding modes and sites to Ala147-TSPO and Thr147-TSPO, which indicated that the tracers may have consistent sensitivity to the three affinity genotypes. In vitro autoradiography and in vivo PET studies of the ischemic rat brain showed dramatically higher uptake of [¹⁸F]BIBD-239 on the lesion site compared to the contralateral side with good brain kinetics. Additionally, [¹⁸F]BIBD-239 provided clear tumor PET images in a GL261 glioma model. Importantly, PET imaging and liquid chromatography-high-resolution mass spectrometry (LC-HRMS) results showed that in vivo defluorination and other metabolites of [¹⁸F]BIBD-239 did not interfere with brain imaging. Conclusively, [¹⁸F]BIBD-239, similar to ER176 with low polymorphism sensitivity, has simple labeling conditions, high labeling yield, high affinity, and high specificity for TSPO, and it is planned for further evaluation in higher species.

Radiosynthesis of (R,S)-[18 F]GE387: A Potential PET Radiotracer for Imaging Translocator Protein 18 kDa (TSPO) with Low Binding Sensitivity to the Human Gene Polymorphism rs6971

Translocator protein (TSPO) is a biomarker of neuroinflammation, which is a hallmark of many neurodegenerative diseases and has been exploited as a positron emission tomography (PET) target. Carbon-11-labelled PK11195 remains the most applied agent for imaging TSPO, despite its short-lived isotope and low brain permeability. Second-generation radiotracers show variance in affinity amongst subjects (low-, mixed-, and high-affinity binders) caused by the genetic polymorphism (rs6971) of the TSPO gene. To overcome these limitations, a new structural scaffold was explored based on the TSPO pharmacophore, and the analogue with a low-affinity binder/high-affinity binder (LAB/HAB) ratio similar (1.2 vs. 1.3) to that of (R)-[¹¹ C]PK11195 was investigated. The synthesis of the reference compound was accomplished in six steps and 9 % overall yield, and the precursor was prepared in eight steps and 8 % overall yield. The chiral separation of the reference and precursor compounds was performed using supercritical fluid chromatography with >95 % ee. The absolute configuration was determined by circular dichroism. Optimisation of reaction conditions for manual radiolabelling revealed acetonitrile as a preferred solvent at 100 °C. Automation of this radiolabelling method provided R and S enantiomers in respective 21.3±16.7 and 25.6±7.1 % decay-corrected yields and molar activities of 55.8±35.6 and 63.5±39.5 GBq μmol^-1 (n=3). Injection of the racemic analogue into a healthy rat confirmed passage through the blood-brain barrier.

Exploration of the impact of stereochemistry on the identification of the novel translocator protein PET imaging agent [(18)F]GE-180

INTRODUCTION

The tricyclic indole compound, [(18)F]GE-180 has been previously identified as a promising positron emission tomography (PET) imaging agent of the translocator protein (TSPO) with the potential to aid in the diagnosis, prognosis and therapy monitoring of degenerative neuroinflammatory conditions such as multiple sclerosis. [(18)F]GE-180 was first identified and evaluated as a racemate, but subsequent evaluations of the resolved enantiomers have shown that the S-enantiomer has a higher affinity for TSPO and an improved in vivo biodistribution performance, in terms of higher uptake in specific brain regions and good clearance (as described previously). Here we describe the additional biological evaluations carried out to confirm the improved performance of the S-enantiomer and including experiments which have demonstrated the stability of the chiral centre to chemical and biological factors.

MATERIALS AND METHODS

GE-180 and the corresponding radiolabelling precursor were separated into single enantiomers using semi-preparative chiral supercritical fluid chromatography (SFC). A detailed comparison of the individual enantiomers and the racemate was carried out in a number of biological studies. TSPO binding affinity was assessed using a radioligand binding assay. Incubation with rat hepatic S9 fractions was used to monitor metabolic stability. In vivo biodistribution studies up to 60 min post injection (PI) in naïve rats were carried out to monitor uptake and clearance. Achiral and chiral in vivo metabolite detection methods were developed to assess the presence of metabolite/s in plasma and brain samples, with the chiral method also determining potential racemisation at the chiral centre.

RESULTS

Evaluation of the chiral stability of the two enantiomers to metabolism by rat S9 fractions, showed no racemisation of enantiomers. There were notable differences in the biodistribution between the racemate and the R- and S-enantiomers. All compounds had similar initial brain uptake between 0.99 and 1.01% injected dose (id) at 2 min PI, with S-[(18)F]GE-180 showing significantly greater retention than the R-enantiomer at 10 and 30 min PI (P<0.05). S-[(18)F]GE-180 uptake to the TSPO-expressing olfactory bulbs was 0.45% id (SD ± 0.17) at 30 min PI in comparison to RS-[(18)F]GE-180 or R-[(18)F]GE-180 levels of 0.41% id ± 0.09 and 0.23% id ± 0.02 respectively, at the same timepoint (P > 0.05). The signal-to-noise ratio (ratio olfactory bulb to striata binding) were similar for both RS-[(18)F]GE-180 and S-[(18)F]GE-180 (3.2 and 3.4 respectively). Initial uptake to the lungs (an organ with high TSPO expression) was more than 3-fold greater with S-[(18)F]GE-180 than R-[(18)F]GE-180, and significantly higher at 10 and 30 min PI (P < 0.05). Furthermore lung uptake of S-[(18)F]GE-180 at 2 and 10 min PI was also significant when compared to the racemate (P < 0.05). The majority of the radioactivity in the rat brain following administration of RS-[(18)F]GE-180 or S-[(18)F]GE-180 was due to the presence of the parent compound (91% ± 1.5 and 94% ± 2.0 of total radioactivity at 60 min PI respectively). In contrast at 60 min PI for the plasma samples, the parent compounds accounted for only 28% ± 1.2 and 21% ± 4.6 of total radioactivity for RS-[(18)F]GE-180 and S-[(18)F]GE-180 respectively. Chiral assessment confirmed that the S-enantiomer was chirally stable in vivo, with no stereochemical conversion in brain and plasma samples up to 60 min PI.

CONCLUSIONS

Developing racemic radiotracers, as for racemic therapeutics, is a considerable challenge due to differences of the enantiomers in pharmacokinetics, efficacy and potential toxicity. We have shown that the enantiomers of the promising racemic PET ligand [(18)F]GE-180 do not share identical performance, with S-[(18)F]GE-180 demonstrating higher TSPO affinity, higher brain uptake and better retention to the high TSPO-expressing lungs. Furthermore, S-[(18)F]GE-180 has also been shown to be enantiomerically stable in vivo, with no observed conversation of the eutomer to the distomer. As a single enantiomer, S-[(18)F]GE-180 retains the beneficial characteristics of the racemate and is a promising imaging agent for imaging neuroinflammation in vivo.