Development of a 18F-Labeled Radiotracer with Improved Brain Kinetics for Positron Emission Tomography Imaging of Translocator Protein (18 kDa) in Ischemic Brain and Glioma

Synthesis and biological profile of benzoxazolone derivatives

The benzoxazolone nucleus is an ideal scaffold for drug design, owing to its discrete physicochemical profile, bioisosteric preference over pharmacokinetically weaker moieties, weakly acidic behavior, presence of both lipophilic and hydrophilic fragments on a single framework, and a wider choice of chemical modification on the benzene and oxazolone rings. These properties apparently influence the interactions of benzoxazolone-based derivatives with their respective biological targets. Hence, the benzoxazolone ring is implicated in the synthesis and development of pharmaceuticals with a diverse biological profile ranging from anticancer, analgesics, insecticides, anti-inflammatory, and neuroprotective agents. This has further led to the commercialization of several benzoxazolone-based molecules and a few others under clinical trials. Nevertheless, the SAR exploration of benzoxazolone derivatives for the identification of potential "hits" followed by the screening of "leads" provides a plethora of opportunities for further exploration of the pharmacological profile of the benzoxazolone nucleus. In this review, we aim to present the biological profile of different derivatives based on the benzoxazolone framework.

Acetamidobenzoxazolone scaffold as a promising translocator protein (18 kDa, TSPO) marker for neuroinflammation imaging: Advancement in last decennial period

Inflammation has been linked to the onset and progression of a wide range of neuropathological disorders. The well-conserved outer mitochondrial membrane 18 kDa translocator protein (TSPO) is perceived as an in vivo neuroinflammation marker. A dearth of a reference region, genetic disparity influencing the ligand's affinity for TSPO, and a substantial signal in the endothelium of the brain veins contributes toward complications in quantifying TSPO positron emission tomography (PET) image. Up to the present time several radiotracers based on different pharmacophore such as (R)[¹¹ C]PK11195, [¹⁸ F]DPA714, [¹¹ C]PBR28, [¹¹ C]ER176, and many more have been recognized for envisaging the prominent TSPO level observed in neurological conditions. Recently acetamidobenzoxazolone (ABO) scaffold, a bicyclic ring system composed of a phenyl ring fused to a carbamate and its substituted radiolabelled analogues especially at C-5 position has evidenced encouraging outcomes as next generation of TSPO PET ligands. Diverse ABO framework-based TSPO ligands have been designed embracing imperative aspects such as lipophilicity, metabolic profile, and capability to penetrate the blood-brain barrier apart from least effect of polymorphism (rs6971). Over the years numerous systematic literature reviews compiling different structural class of TSPO ligands characterized on the grounds of their binding affinity and metabolite profile have been reported but none is especially focused toward a fascinating benzoxazolone scaffold. This review exclusively bestows an overview of the recent advancements on ABO derivatives with neuroinflammation imaging potential and emphases on the structural features accountable for visualizing TSPO in-vivo with collation of published reports during last 10 years.

Interrogating Glioma-Associated Microglia and Macrophage Dynamics Under CSF-1R Therapy with Multitracer In Vivo PET/MRI

Glioma-associated microglia and macrophages (GAMMs) are key players in creating an immunosuppressive microenvironment. They can be efficiently targeted by inhibiting the colony-stimulating factor 1 receptor (CSF-1R). We applied noninvasive PET/CT and PET/MRI using ¹⁸F-fluoroethyltyrosine (¹⁸F-FET) (amino acid metabolism) and N,N-diethyl-2-[4-(2-¹⁸F-fluoroethoxy)phenyl]-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide (¹⁸F-DPA-714) (translocator protein) to understand the role of GAMMs in glioma initiation, monitor in vivo therapy-induced GAMM depletion, and observe GAMM repopulation after drug withdrawal. Methods: C57BL/6 mice (n = 44) orthotopically implanted with syngeneic mouse GL261 glioma cells were treated with different regimens using the CSF-1R inhibitor PLX5622 (6-fluoro-N-((5-fluoro-2-methoxypyridin-3-yl)methyl)-5-((5-methyl-1H-pyrrolo[2,3-b]pyridin-3-yl)methyl)pyridin-2-amine) or vehicle, establishing a preconditioning model and a repopulation model, respectively. The mice underwent longitudinal PET/CT and PET/MRI. Results: The preconditioning model indicated similar tumor growth based on MRI (44.5% ± 24.8%), ¹⁸F-FET PET (18.3% ± 11.3%), and ¹⁸F-DPA-714 PET (16% ± 19.04%) volume dynamics in all groups, suggesting that GAMMs are not involved in glioma initiation. The repopulation model showed significantly reduced ¹⁸F-DPA-714 uptake (-45.6% ± 18.4%), significantly reduced GAMM infiltration even after repopulation, and a significantly decreased tumor volume (-54.29% ± 8.6%) with repopulation as measured by MRI, supported by a significant reduction in ¹⁸F-FET uptake (-50.2% ± 5.3%). Conclusion: ¹⁸F-FET and ¹⁸F-DPA-714 PET/MRI allow noninvasive assessment of glioma growth under various regimens of CSF-1R therapy. CSF-1R-mediated modulation of GAMMs may be of high interest as therapy or cotherapy against glioma.

Essential Principles and Recent Progress in the Development of TSPO PET Ligands for Neuroinflammation Imaging

The translocator protein 18kDa (TSPO) is expressed in the outer mitochondrial membrane and is implicated in several functions, including cholesterol transport and steroidogenesis. Under normal physiological conditions, TSPO is present in very low concentrations in the human brain but is markedly upregulated in response to brain injury and inflammation. This upregulation is strongly associated with activated microglia. Therefore, TSPO is particularly suited for assessing active gliosis associated with brain lesions following injury or disease. For over three decades, TSPO has been studied as a biomarker. Numerous radioligands for positron emission tomography (PET) that target TSPO have been developed for imaging inflammatory progression in the brain. Although [11C]PK11195, the prototypical first-generation PET radioligand, is still widely used for in vivo studies, mainly now as its single more potent R-enantiomer, it has severe limitations, including low sensitivity and poor amenability to quantification. Second-generation radioligands are characterized by higher TSPO specific signals but suffer from other drawbacks, such as sensitivity to the TSPO single nucleotide polymorphism (SNP) rs6971. Therefore, their applications in human studies have the burden of needing to genotype subjects. Consequently, recent efforts are focused on developing improved radioligands that combine the optimal features of the second generation with the ability to overcome the differences in binding affinities across the population. This review presents essential principles in the design and development of TSPO PET ligands and discusses prominent examples among the main chemotypes.

Glioblastoma: Current Status, Emerging Targets, and Recent Advances

Glioblastoma (GBM) is a highly malignant brain tumor characterized by a heterogeneous population of genetically unstable and highly infiltrative cells that are resistant to chemotherapy. Although substantial efforts have been invested in the field of anti-GBM drug discovery in the past decade, success has primarily been confined to the preclinical level, and clinical studies have often been hampered due to efficacy-, selectivity-, or physicochemical property-related issues. Thus, expansion of the list of molecular targets coupled with a pragmatic design of new small-molecule inhibitors with central nervous system (CNS)-penetrating ability is required to steer the wheels of anti-GBM drug discovery endeavors. This Perspective presents various aspects of drug discovery (challenges in GBM drug discovery and delivery, therapeutic targets, and agents under clinical investigation). The comprehensively covered sections include the recent medicinal chemistry campaigns embarked upon to validate the potential of numerous enzymes/proteins/receptors as therapeutic targets in GBM.

[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.

Quantification of monoacylglycerol lipase and its occupancy by an exogenous ligand in rhesus monkey brains using [18F]T-401 and PET

Monoacylglycerol lipase (MAGL) is a cytosolic serine hydrolase that cleaves monoacylglycerols into fatty acids and is a potential target for the novel treatment of CNS disorders related to the endocannabinoid system and neuroinflammation. We have developed [¹⁸F]T-401 as a selective Positron emission tomography (PET) imaging agent for MAGL. In this study, we determined an analytical method to quantify MAGL availability and its occupancy by an exogenous inhibitor in rhesus monkey brains using [¹⁸F]T-401-PET. In rhesus monkeys, regional time-activity curves were described well when using an extended 2-tissue compartment model that accommodated the formation of a radiometabolite in the brain. This model yielded reliable estimates of the total distribution volume (V_T), and the rank order of V_T was consistent with known regional activity of MAGL enzyme in primates. The pretreatment of monkeys with JW642 resulted in a dose-dependent reduction of [¹⁸F]T-401 retentions in the brain, and V_T. Lassen's graphical analysis indicated a V_ND of 0.69 mL/cm³ and a plasma JW642 concentration of 126 ng/mL for inhibiting the specific binding by 50%. [¹⁸F]T-401 and the method established can be used for quantification of MAGL in healthy brain and in disease conditions, and is suitable for evaluations of target engagement at cerebral MAGL.

TSPO imaging in animal models of brain diseases

Over the last 30 years, the 18-kDa TSPO protein has been considered as the PET imaging biomarker of reference to measure increased neuroinflammation. Generally assumed to image activated microglia, TSPO has also been detected in endothelial cells and activated astrocytes. Here, we provide an exhaustive overview of the recent literature on the TSPO-PET imaging (i) in the search and development of new TSPO tracers and (ii) in the understanding of acute and chronic neuroinflammation in animal models of neurological disorders. Generally, studies testing new TSPO radiotracers against the prototypic [11C]-R-PK11195 or more recent competitors use models of acute focal neuroinflammation (e.g. stroke or lipopolysaccharide injection). These studies have led to the development of over 60 new tracers during the last 15 years. These studies highlighted that interpretation of TSPO-PET is easier in acute models of focal lesions, whereas in chronic models with lower or diffuse microglial activation, such as models of Alzheimer's disease or Parkinson's disease, TSPO quantification for detection of neuroinflammation is more challenging, mirroring what is observed in clinic. Moreover, technical limitations of preclinical scanners provide a drawback when studying modest neuroinflammation in small brains (e.g. in mice). Overall, this review underlines the value of TSPO imaging to study the time course or response to treatment of neuroinflammation in acute or chronic models of diseases. As such, TSPO remains the gold standard biomarker reference for neuroinflammation, waiting for new radioligands for other, more specific targets for neuroinflammatory processes and/or immune cells to emerge.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Radiosynthesis and evaluation of acetamidobenzoxazolone based radioligand [11C]N′-MPB for visualization of 18 kDa TSPO in brain

Modified acetamidobenzoxazolone radioligand [11C]N′-MPB for visualization of 18 kDa TSPO.

Smart diagnostic nano-agents for cerebral ischemia

A summary of the latest developments on imaging techniques and smart nano-diagnostics used for ischemic stroke.

Synthesis and in vitro evaluation of new translocator protein ligands designed for positron emission tomography

AIM

Dysregulated levels of the translocator protein TSPO 18 KDa have been reported in several disorders, particularly neurodegenerative diseases. This makes TSPO an interesting target for the development of diagnostic biomarkers. Even though several radioligands have already been developed for in vivo TSPO imaging, the ideal TSPO radiotracer has still not been found.

RESULTS

Here, we report the chemical synthesis of a set of new TSPO ligands designed for future application in positron emission tomography, together with the determination of their biological activity and applied ¹¹C-labeling strategy.

CONCLUSION

The lead compound of our series, (R)-[¹¹C]Me@NEBIQUINIDE, showed very promising results and is therefore proposed to be further evaluated under in vivo settings.

Synthesis of two novel [18F]fluorobenzene-containing radiotracers via spirocyclic iodonium ylides and positron emission tomography imaging of translocator protein (18 kDa) in ischemic brain

Two novel radiotracers, namely, N-(4-[18F]fluorobenzyl)-N-methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([18F]5) and 2-(5-(4-[18F]fluorophenyl)-2-oxobenzo[d]oxazol-3(2H)-yl)-N-methyl-N-phenylacetamide ([18F]6), were developed for positron emission tomography (PET) imaging of translocator protein (18 kDa) (TSPO) in ischemic brain in this study. The two radiotracers with a [18F]fluorobenzene ring were derived from the corresponding [18F]fluoroethyl tracers [18F]7 and [18F]8 which underwent [18F]defluoroethylation in vivo easily. [18F]5 or [18F]6 was synthesized by the radiofluorination of the spirocyclic iodonium ylide precursor 10 or 17 with [18F]F- in 23 ± 10% (n = 7) or 56 ± 9% (n = 7) radiochemical yields (decay-corrected, based on [18F]F-). [18F]5 and [18F]6 showed high in vitro binding affinities (Ki = 0.70 nM and 5.9 nM) for TSPO and moderate lipophilicities (log D = 2.9 and 3.4). Low uptake of radioactivity for both radiotracers was observed in mouse bones. Metabolite analysis showed that the in vivo stability of [18F]5 and [18F]6 was improved in comparison to the parent radiotracers [18F]7 and [18F]8. In particular, no radiolabelled metabolite of [18F]5 was found in the mouse brains at 60 min after the radiotracer injection. PET studies with [18F]5 on ischemic rat brains revealed a higher binding potential (BPND = 3.42) and maximum uptake ratio (4.49) between the ipsilateral and contralateral sides. Thus, [18F]5 was shown to be a useful PET radiotracer for visualizing TSPO in neuroinflammation models.

Comparative evaluation of 99mTc-MBIP-X/11[C] MBMP for visualization of 18 kDa translocator protein

An elevated translocator protein (18 kDa, TSPO) density is observed during inflammation in the brain and peripheral organs making it a viable target for imaging.

Medicinal Chemistry strategies for PET tracer discovery

The detection of gamma rays, resulting from decay of positron emitting isotopes, allows exquisitely sensitive detection of probes radiolabeled with such isotopes. These probes can be designed for high affinity binding to specific molecular targets and be used as tools in the early development of drugs, particularly for neuropsychiatric disorders. Availability of novel tracers requires dedicated resources and selection assays. Many of the selection assays are similar to those used for discovery of clinical compounds, although the distribution and clearance of target specific radioligands requires different in vitro and in vivo methods and new derivatives.