The development of potential new fluorine-18 labelled radiotracers for imaging the GABAA receptor

GABAa receptor density alterations revealed in a mouse model of early moderate prenatal ethanol exposure using [18F]AH114726

INTRODUCTION

Prenatal ethanol exposure (PEE) has been shown to alter the level and function of receptors in the brain, one of which is GABA_a receptors (GABA_aR), the major inhibitory ligand gated ion channels that mediate neuronal inhibition. High dose PEE in animals resulted in the upregulation of GABA_aR, but the effects of low and moderate dose PEE at early gestation have not been investigated. This study aimed at examining GABA_aR density in the adult mouse brain following PEE during a period equivalent to the first 3 to 4 weeks in human gestation. It was hypothesized that early moderate PEE would cause alterations in brain GABA_aR levels in the adult offspring.

METHODS

C57BL/6J mice were given 10% v/v ethanol during the first 8 gestational days. Male offspring were studied using in-vivo Positron Emission Tomography (PET)/Magnetic Resonance Imaging (MRI), biodistribution, in-vitro autoradiography using [¹⁸F]AH114726, a novel flumazenil analogue with a high affinity for the benzodiazepine-binding site, and validated using immunohistochemistry.

RESULTS

In vivo PET and biodistribution did not detect alteration in brain tracer uptake. In vitro radiotracer studies detected significantly reduced GABA_aR in the olfactory bulbs. Immunohistochemistry detected reduced GABA_aR in the cerebral cortex, cerebellum and hippocampus, while Nissl staining showed that cell density was significantly higher in the striatum following PEE.

CONCLUSION

Early moderate PEE may induce long-term alterations in the GABA_aR system that persisted into adulthood.

Development of an automated, GMP compliant FASTlab™ radiosynthesis of [18 F]GE-179 for the clinical study of activated NMDA receptors

N-(2-chloro-5-(S-2-[18 F]fluoroethyl)thiophenyl)-N'-(3-thiomethylphenyl)-N'-methylguanidine, ([18 F]GE-179), has been identified as a promising positron emission tomography (PET) ligand for the intra-channel phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor. The radiosynthesis of [18 F]GE-179 has only been performed at low radioactivity levels. However, the manufacture of a GMP compliant product at high radioactivity levels was required for clinical studies. We describe the development of a process using the GE FASTlab™ radiosynthesis platform coupled with HPLC purification. The radiosynthesis is a two-step process, involving the nucleophilic fluorination of ethylene ditosylate, 11, followed by alkylation to the deprotonated thiol precursor, N-(2-chloro-5-thiophenol)-N'-(3-thiomethylphenyl)-N'-methyl guanidine, 8. The crude product was purified by semi-preparative HPLC to give the formulated product in an activity yield (AY) of 7 ± 2% (n = 15) with a total synthesis time of 120 minutes. The radioactive concentration (RAC) and radiochemical purity (RCP) were 328 ± 77 MBq/mL and 96.5 ± 1% respectively and the total chemical content was 2 ± 1 μg. The final formulation volume was 14 mL. The previously described radiosynthesis of [18 F]GE-179 was successfully modified to deliver an process on the FASTlab™ that allows the manufacture of a GMP quality product from high starting radioactivitity (up to 80 GBq) and delivers a product suitable for clinical use.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

Alternative approaches for PET radiotracer development in Alzheimer's disease: imaging beyond plaque

Alzheimer's disease (AD) and related dementias show increasing clinical prevalence, yet our understanding of the etiology and pathobiology of disease-related neurodegeneration remains limited. In this regard, noninvasive imaging with radiotracers for positron emission tomography (PET) presents a unique tool for quantifying spatial and temporal changes in characteristic biological markers of brain disease and for assessing potential drug efficacy. PET radiotracers targeting different protein markers are being developed to address questions pertaining to the molecular and/or genetic heterogeneity of AD and related dementias. For example, radiotracers including [(11) C]-PiB and [(18) F]-AV-45 (Florbetapir) are being used to measure the density of Aβ-plaques in AD patients and to interrogate the biological mechanisms of disease initiation and progression. Our focus is on the development of novel PET imaging agents, targeting proteins beyond Aβ-plaques, which can be used to investigate the broader mechanism of AD pathogenesis. Here, we present the chemical basis of various radiotracers which show promise in preclinical or clinical studies for use in evaluating the phenotypic or biochemical characteristics of AD. Radiotracers for PET imaging neuroinflammation, metal ion association with Aβ-plaques, tau protein, cholinergic and cannabinoid receptors, and enzymes including glycogen-synthase kinase-3β and monoamine oxidase B amongst others, and their connection to AD are highlighted.

Evaluation of a novel series of fluorine-18-labeled imidazobenzodiazepines as potential new positron emission tomography radioligands for the GABAA receptor

INTRODUCTION

[(11)C]Flumazenil has been used to study the GABAA receptor in many preclinical and clinical studies, but the short half-life of carbon-11 means that this molecule is restricted to use by investigators with access to on-site cyclotron and radiosynthesis facilities. The radiosynthesis of [(18)F]flumazenil has been evaluated by several groups, but the radiochemical yield can be low and inconsistent. We previously reported a series of fluorine-18-labeled imidazobenzodiazepine-based ligands for the GABAA receptor, which had significantly improved radiosynthesis yields. Here we report the in vivo evaluation and comparison of the distribution, metabolism and specificity of the novel ligands in comparison with [(18)F]flumazenil.

METHODS

In vivo biodistribution studies, at time points up to 90min post-injection, were performed in naïve rats to compare the performance of the novel compounds with particular attention paid to regional brain uptake and clearance. In vivo metabolism studies were carried out to determine the percentage of parent compound remaining in the plasma and brain at selected time points. Blocking studies were carried out, using pre-treatment of the test animals with either bretazenil or unlabeled fluorine-19 test compound, to determine the levels of specific and non-specific binding in selected brain regions.

RESULTS

Two of the 12 new compounds were rejected due to poor biodistribution showing significant bone uptake. Some of the compounds showed insufficient whole brain uptake or limited evidence of differential binding to GABAA-rich brain regions to warrant further investigation. Four of the compounds were selected for in vivo metabolism and blocking studies. Overall, the studies indicated that two compounds 3 and 5 showed comparable or improved performance compared with [(18)F]flumazenil, with respect to distribution, metabolic profile and specific binding.

CONCLUSIONS

These studies have demonstrated that compounds based on [(18)F]flumazenil, but with alterations to allow improved radiosynthesis, can be prepared which have ideal properties and warrant further evaluation as PET agents for the GABAA receptor. In particular, compounds 3 and 5 show very promising profiles with specific binding and in vivo stability comparable to flumazenil.

Novel fluorine-18 PET radiotracers based on flumazenil for GABAA imaging in the brain

INTRODUCTION

Two 7-fluoroimidazobenzodiazepines (AH114726 and GEH120348), analogs of flumazenil, were labeled with fluorine-18 and evaluated as alternative radioligands for in vivo imaging of the GABAA/benzodiazepine receptor by comparing them to [(11)C]flumazenil in rhesus monkey.

METHODS

Radiotracers were prepared from the corresponding nitro-precursors in an automated synthesis module, and primate imaging studies were conducted on a Concorde MicroPET P4 scanner. The brain was imaged for 60 (12 × 5 min frames) or 90 min (18 × 5 min frames), and data was reconstructed using the 3D MAP algorithm. Specificity of [(18)F]AH114726 and [(18)F]GEH120348 was confirmed by displacement studies using unlabeled flumazenil.

RESULTS

[(18)F]GEH120348 and [(18)F]AH114726 were obtained in 13-24% yields (end of synthesis) with high chemical (>95%) and radiochemical (>99%) purities, and high specific activities (2061 ± 985 Ci/mmol). The in vivo pharmacokinetics of [(18)F]AH114726 and [(18)F]GEH120348 were determined in a non-human primate and directly compared with [(11)C]flumazenil. Both fluorine-18 radioligands showed time-dependent regional brain distributions that correlated with the distribution of [(11)C]flumazenil and the known concentrations of GABAA/benzodiazepine receptors in the monkey brain. [(18)F]AH114726 exhibited maximal brain uptake and tissue time-radioactivity curves that were most similar to [(11)C]flumazenil. In contrast, [(18)F]GEH120348 showed higher initial brain uptake but very different pharmacokinetics with continued accumulation of radioactivity into the cortical regions of high GABA/benzodiazepine receptor concentrations and very little clearance from the regions of low receptor densities. Rapid washout of both radiotracers occurred upon treatment with unlabeled flumazenil.

CONCLUSION

The ease of the radiochemical synthesis, together with in vivo brain pharmacokinetics most similar to [(11)C]flumazenil, support that [(18)F]AH114726 is a suitable option for imaging the GABAA receptor.