In vitro binding of [(11)C]raclopride with ultrahigh specific activity in rat brain determined by homogenate assay and autoradiography

Pharmacological Characterization of [18F]-FNM and Evaluation of NMDA Receptors Activation in a Rat Brain Injury Model

PURPOSE

NMDA receptors (NMDARs) dysfunction plays a central role in the physiopathology of psychiatric and neurodegenerative disorders whose mechanisms are still poorly understood. The development of a PET (positron emission tomography) tracer able to selectively bind to the NMDARs intra-channel PCP site may make it possible to visualize NMDARs in an open and active state. We describe the in vitro pharmacological characterization of [¹⁸F]-fluoroethylnormemantine ([¹⁸F]-FNM) and evaluate its ability to localize activated NMDA receptors in a rat preclinical model of excitotoxicity.

PROCEDURES

The affinity of the non-radioactive analog for the intra-channel PCP site was determined in a radioligand competition assay using [³H]TCP ([³H]N-(1-[thienyl]cyclohexyl)piperidine) on rat brain homogenates. Selectivity was also investigated by the displacement of specific radioligands targeting various cerebral receptors. In vivo brain lesions were performed using stereotaxic quinolinic acid (QA) injections in the left motor area (M1) of seven Sprague Dawley rats. Each rat was imaged with a microPET/CT camera, 40 min after receiving a dose of 30 MBq + / - 20 of [¹⁸F]-FNM, 24 and 72 h after injury. Nine non-injured rats were also imaged using the same protocol.

RESULTS

FNM displayed IC₅₀ value of 13.0 ± 8.9 µM in rat forebrain homogenates but also showed significant bindings on opioid receptors. In the frontal and left somatosensory areas, [¹⁸F]FNM PET detected a mean of 37% and 41% increase in [¹⁸F]FNM uptake (p < 0,0001) 24 and 72 h after QA stereotaxic injection, respectively, compared to the control group.

CONCLUSIONS

In spite of FNM's poor affinity for NMDAR PCP site, this study supports the ability of this tracer to track massive activation of NMDARs in neurological diseases.

Impact of the molar activity and PSMA expression level on [18F]AlF-PSMA-11 uptake in prostate cancer

This two-part preclinical study aims to evaluate prostate specific membrane antigen (PSMA) as a valuable target for expression-based imaging applications and to determine changes in target binding in function of varying apparent molar activities (MA_app) of [¹⁸F]AlF-PSMA-11. For the evaluation of PSMA expression levels, male NOD/SCID mice bearing prostate cancer (PCa) xenografts of C4-2 (PSMA+++), 22Rv1 (PSMA+) and PC-3 (PSMA−) were administered [¹⁸F]AlF-PSMA-11 with a medium MA_app (20.24 ± 3.22 MBq/nmol). SUV_mean and SUV_max values were respectively 3.22 and 3.17 times higher for the high versus low PSMA expressing tumors (p < 0.0001). To evaluate the effect of varying MA_app, C4-2 and 22Rv1 xenograft bearing mice underwent additional [¹⁸F]AlF-PSMA-11 imaging with a high (211.2 ± 38.9 MBq/nmol) and/or low MA_app (1.92 ± 0.27 MBq/nmol). SUV values showed a significantly increasing trend with higher MA_app. Significant changes were found for SUV_mean and SUV_max between the high versus low MA_app and medium versus low MA_app (both p < 0.05), but not between the high versus medium MA_app (p = 0.055 and 0.25, respectively). The effect of varying MA_app was more pronounced in low expressing tumors and PSMA expressing tissues (e.g. salivary glands and kidneys). Overall, administration of a high MA_app increases the detection of low expression tumors while also increasing uptake in PSMA expressing tissues, possibly leading to false positive findings. In radioligand therapy, a medium MA_app could reduce radiation exposure to dose-limiting organs with only limited effect on radionuclide accumulation in the tumor.

Quantitative Rodent Brain Receptor Imaging

Positron emission tomography (PET) is a non-invasive imaging technology employed to describe metabolic, physiological, and biochemical processes in vivo. These include receptor availability, metabolic changes, neurotransmitter release, and alterations of gene expression in the brain. Since the introduction of dedicated small-animal PET systems along with the development of many novel PET imaging probes, the number of PET studies using rats and mice in basic biomedical research tremendously increased over the last decade. This article reviews challenges and advances of quantitative rodent brain imaging to make the readers aware of its physical limitations, as well as to inspire them for its potential applications in preclinical research. In the first section, we briefly discuss the limitations of small-animal PET systems in terms of spatial resolution and sensitivity and point to possible improvements in detector development. In addition, different acquisition and post-processing methods used in rodent PET studies are summarized. We further discuss factors influencing the test-retest variability in small-animal PET studies, e.g., different receptor quantification methodologies which have been mainly translated from human to rodent receptor studies to determine the binding potential and changes of receptor availability and radioligand affinity. We further review different kinetic modeling approaches to obtain quantitative binding data in rodents and PET studies focusing on the quantification of endogenous neurotransmitter release using pharmacological interventions. While several studies have focused on the dopamine system due to the availability of several PET tracers which are sensitive to dopamine release, other neurotransmitter systems have become more and more into focus and are described in this review, as well. We further provide an overview of latest genome engineering technologies, including the CRISPR/Cas9 and DREADD systems that may advance our understanding of brain disorders and function and how imaging has been successfully applied to animal models of human brain disorders. Finally, we review the strengths and opportunities of simultaneous PET/magnetic resonance imaging systems to study drug-receptor interactions and challenges for the translation of PET results from bench to bedside.

Synthesis and evaluation of NLRP3-inhibitory sulfonylurea [11C]MCC950 in healthy animals

The diaryl sulfonylurea MCC950/CRID3 is a potent NLRP3 inhibitor (IC₅₀ = 8 nM) and, in animal models, MCC950 protects against numerous NLRP3-related neurodegenerative disorders. To evaluate the brain uptake and investigate target engagement of MCC950, we synthesised [¹¹C-urea]MCC950 via carrier added [¹¹C]CO₂ fixation chemistry (activity yield = 237 MBq; radiochemical purity >99%; molar activity = 7 GBq/µmol; radiochemical yield (decay-corrected from [¹¹C]CO₂) = 1.1%; synthesis time from end-of-bombardment = 31 min; radiochemically stable for >1 h). Despite preclinical efficacy in neurodegeneration studies, preclinical positron emission tomography (PET) imaging studies in mouse, rat and rhesus monkey revealed poor brain uptake of low molar activity [¹¹C]MCC950 and rapid washout. In silico prediction tools suggest efflux transporter liabilities for MCC950 at microdoses, and this information should be taken into account when developing next generation NLRP3 inhibitors and/or PET radiotracers.

Evaluation of Enzyme Substrate Radiotracers as PET/MRS Hybrid Imaging Agents

The development of a positron emission tomography (PET)/magnetic resonance spectroscopy (MRS) hybrid imaging agent allows for functional imaging by both methods with a single imaging agent. Enzyme substrates that are cleaved to form two metabolites present an interesting opportunity, as the unique metabolites generated might each be detected by a different modality. To be successful, such enzyme substrates would require administration of doses that (a) reach the in vivo target tissue at concentrations necessary for MRS imaging, (b) do not show substrate inhibition of tissue uptake or enzymatic activity, and (c) provide PET images that still reflect the action of the enzyme. We report in vitro and in vivo proof-of-concept studies of a carbon-11 small molecule substrate for brain monoamine oxidases that, upon enzyme-mediated cleavage, produces two metabolites, one detectable by PET and the other by MRS.

A useful PET probe [11C]BU99008 with ultra-high specific radioactivity for small animal PET imaging of I2-imidazoline receptors in the hypothalamus

INTRODUCTION

A positron emission tomography (PET) probe with ultra-high specific radioactivity (SA) enables measuring high receptor specific binding in brain regions by avoiding mass effect of the PET probe itself. It has been reported that PET probe with ultra-high SA can detect small change caused by endogenous or exogenous ligand. Recently, Kealey et al. developed [¹¹C]BU99008, a more potent PET probe for I₂-imidazoline receptors (I₂Rs) imaging, with a conventional SA (mean 76GBq/μmol) showed higher specific binding in the brain. Here, to detect small change of specific binding for I₂Rs caused by endogenous or exogenous ligand in an extremely small region, such as hypothalamus in the brain, we synthesized and evaluated [¹¹C]BU99008 with ultra-high SA as a useful PET probe for small-animal PET imaging of I₂Rs.

METHODS

[¹¹C]BU99008 was prepared by [¹¹C]methylation of N-desmethyl precursor with [¹¹C]methyl iodide. Biodistribution, metabolite analysis, and brain PET studies were conducted in rats.

RESULTS

[¹¹C]BU99008 with ultra-high SA in the range of 5400-16,600GBq/μmol were successfully synthesized (n=7), and had appropriate radioactivity for in vivo study. In the biodistribution study, the mean radioactivity levels in all investigated tissues except for the kidney did not show significant difference between [¹¹C]BU99008 with ultra-high SA and that with conventional SA. In the metabolite analysis, the percentage of unchanged [¹¹C]BU99008 at 30min after the injection of probes with ultra-high and conventional SA was similar in rat brain and plasma. In the PET study of rats' brain, radioactivity level (AUC_{30-60 min}) in the hypothalamus of rats injected with [¹¹C]BU99008 with ultra-high SA (64 [SUV ∙ min]) was significantly higher than that observed for that with conventional SA (50 [SUV ∙ min]). The specific binding of [¹¹C]BU99008 with ultra-high SA (86% of total binding) for I₂R was higher than that of conventional SA (76% of total binding).

CONCLUSION

A PET study using [¹¹C]BU99008 with ultra-high SA would thus contribute to the detection of small changes in or small regions with I₂R expression and hence may be useful in elucidating new functions of I₂R.

Considerations in the Development of Reversibly Binding PET Radioligands for Brain Imaging

The development of reversibly binding radioligands for imaging brain proteins in vivo, such as enzymes, neurotransmitter transporters, receptors and ion channels, with positron emission tomography (PET) is keenly sought for biomedical studies of neuropsychiatric disorders and for drug discovery and development, but is recognized as being highly challenging at the medicinal chemistry level. This article aims to compile and discuss the main considerations to be taken into account by chemists embarking on programs of radioligand development for PET imaging of brain protein targets.

Improved Visualization and Specific Binding for Metabotropic Glutamate Receptor Subtype 1 (mGluR1) Using [11C]ITMM with Ultra-High Specific Activity in Small-Animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial target in the development of new medications to treat central nervous system (CNS) disorders. Recently, we developed N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-[¹¹C]methoxy-N-methyl-benzamide ([¹¹C]ITMM) as a useful positron emission tomography (PET) probe for mGluR1 in clinical studies. Here, we aimed to improve visualization and threshold of specific binding for mGluR1 using [¹¹C]ITMM with ultra-high specific activity (SA) of > 3,500 GBq/μmol in rat brains. A two-tissue compartment model indicated large differences between the two SAs in the constants k₃ and k₄, representing binding ability for mGluR1, while constants K₁ and k₂ showed no differences. The total distribution volume (V_T) values of conventional and ultra-high SA were 9.1 and 11.2 in the thalamus, 7.7 and 9.7 in the striatum, and 6.4 and 8.5 mL/cm³ in the substantia nigra, respectively. The specific binding of [¹¹C]ITMM with ultra-high SA was significantly higher than the conventional SA, especially in the basal ganglia. Parametric PET images scaled with V_T of the ultra-high SA clearly identified regional differences in the rat brain. In conclusion, PET studies using [¹¹C]ITMM with ultra-high SA could sufficiently improve visualization and specific binding for mGluR1, which could help further understanding for mGluR1 functions in CNS disorders.

Radioligands for the dopamine receptor subtypes

The actions of the predominant neurotransmitter in the brain, dopamine, are mediated by the postsynaptic dopamine receptors. The five dopamine receptor subtypes and their regulation have been associated with a large variety of psychiatric diseases. Therefore, positron emission tomography (PET) imaging studies using suitable and selective (18) F-labeled and (11) C-labeled dopamine receptor radioligands could provide valuable knowledge on the impact of receptor density on the pathogenesis and evolvement of neuropsychiatric and neurological diseases. This special issue subchapter provides a summary of the most important (18) F-labeled and (11) C-labeled radioligands for PET imaging of the dopamine receptor subtypes, their radiochemistry, and characteristics from in vitro and in vivo applications, considering not only the already established PET ligands but also the recently published preclinical work.

Synthesis and evaluation of [11C]MMPIP as a potential radioligand for imaging of metabotropic glutamate 7 receptor in the brain

BACKGROUND

Metabotropic glutamate 7 (mGlu7) receptor is a crucial target protein for the development of pharmaceuticals against central nervous system disorders. In the present study, we synthesized [11C]MMPIP, a putative radioligand for mGlu7 (binding constant KB = 30 nM), and evaluated its potential for imaging of mGlu7 via in vitro and in vivo techniques.

METHODS

[11C]MMPIP was synthesized by the reaction of phenol precursor 3 with [11C]CH3I. In vitro autoradiography using [11C]MMPIP was performed on rat brain sections. To determine in vitro specific binding of [11C]MMPIP with mGlu7, a blocking study was conducted by co-incubation with excess AMN082, a selective antagonist for mGlu7, or unlabeled MMPIP. Positron emission tomography (PET) studies and ex vivo metabolite analysis were carried out on rat brains.

RESULTS

[11C]MMPIP was obtained with two specific activity (SA) levels of average 58 (conventional) and 3,800 (high SA) GBq/μmol, respectively. High radioactive signals derived from conventional [11C]MMPIP in the in vitro autoradiography were seen in the thalamus, medulla oblongata, and striatum, corresponding with comprehensive brain distributions of mGlu7. Co-incubation with ANM082 or unlabeled MMPIP reduced the radioactive signals in the brain sections, respectively. In the PET studies with [11C]MMPIP, no specific uptake relative to mGlu7 was found in the examined brain regions.

CONCLUSION

Despite in vitro specific binding of [11C]MMPIP with mGlu7, visualization of mGlu7 in the living brain using PET was not successful. Development of new ligand candidates with higher affinity for mGlu7 is necessary.

Synthesis and evaluation of novel radioligands for positron emission tomography imaging of metabotropic glutamate receptor subtype 1 (mGluR1) in rodent brain

We designed three novel positron emission tomography ligands, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-4-[(11)C]methoxy-N-methylbenzamide ([(11)C]6), 4-[(18)F]fluoroethoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]7), and 4-[(18)F]fluoropropoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]8), for imaging metabotropic glutamate receptor type 1 (mGluR1) in rodent brain. Unlabeled compound 6 was synthesized by benzoylation of 4-pyrimidinyl-2-methylaminothiazole 10, followed by reaction with isopropylamine. Removal of the methyl group in 6 gave phenol precursor 12 for radiosynthesis. Two fluoroalkoxy analogues 7 and 8 were prepared by reacting 12 with tosylates 13 and 14. Radioligands [(11)C]6, [(18)F]7, and [(18)F]8 were synthesized by O-[(11)C]methylation or [(18)F]fluoroalkylation of 12. Compound 6 showed high in vitro binding affinity for mGluR1, whereas 7 and 8 had weak affinity. Autoradiography using rat brain sections showed that [(11)C]6 binding is aligned with the reported distribution of mGluR1 with high specific binding in the cerebellum and thalamus. PET study with [(11)C]6 in rats showed high brain uptake and a similar distribution pattern to that in autoradiography, indicating the usefulness of [(11)C]6 for imaging brain mGluR1.

Visualization of early infarction in rat brain after ischemia using a translocator protein (18 kDa) PET ligand [11C]DAC with ultra-high specific activity

The aim of this study was to visualize early infarction in the rat brain after ischemia using a translocator protein (TSPO) (18 kDa) PET ligand [(11)C]DAC with ultra-high specific activity (SA) of 3670-4450 GBq/μmol. An infarction model of rat brain was prepared by ischemic surgery and evaluated 2 days after ischemia using small-animal PET and in vitro autoradiography. Early infarction with a small increase of TSPO expression in the brain was visualized using PET with high SA [(11)C]DAC (average 4060 GBq/μmol), but was not distinguished clearly with usually reported SA [(11)C]DAC (37 GBq/μmol). Infarction in the rat brain 4 days after ischemia was visualized using high and usually reported SAs [(11)C]DAC. Displacement experiments with unlabeled TSPO-selective AC-5216 or PK11195 diminished the difference in radioactivity between ipsilateral and contralateral sides, confirming that the increased uptake on the infracted brain was specific to TSPO. In vitro autoradiography with high SA [(11)C]DAC showed that the TSPO expression increased on early infarction in the rat brain. High SA [(11)C]DAC is a useful and sensitive biomarker for the visualization of early infarction and the characterization of TSPO expression which was slightly elevated in the infarcted brain using PET.

How to introduce radioactive chlorine into a benzene ring using [*Cl]Cl-?

The aim of this study was how to introduce radioactive chlorine (*Cl) isotope into a benzene ring to afford *Cl-labeled chlorobenzene ([*Cl]1). Using tributylphenylstannyl compound (2) as a starting material, we determined two routes for introducing *Cl into the benzene ring: reaction of [*Cl]Cl(-) with 2 in the presence of chloramine-T and nucleophilic reaction of [*Cl]Cl(-) with diphenyliodonium tosylate (3). The two routes provide suitable tools to prepare [*Cl]1 without electron-abstracting group in the benzene ring.