Radiosynthesis and characterization of [18F]BS224: a next-generation TSPO PET ligand insensitive to the rs6971 polymorphism

Structure-based drug design for TSPO: Challenges and opportunities

The translocator protein 18 kDa (TSPO) is an evolutionarily conserved mitochondrial transmembrane protein implicated in various neuropathologies and inflammatory conditions, making it a longstanding diagnostic and therapeutic target of interest. Despite the development of various classes of TSPO ligand chemotypes, and the elucidation of bacterial and non-human mammalian experimental structures, many unknowns exist surrounding its differential structural and functional features in health and disease. There are several limitations associated with currently used computational methodologies for modelling the native structure and ligand-binding behaviour of this enigmatic protein. In this perspective, we provide a critical analysis of the developments in the uses of these methods, outlining their uses, inherent limitations, and continuing challenges. We offer suggestions of unexplored opportunities that exist in the use of computational methodologies which offer promise for enhancing our understanding of the TSPO.

Radiosynthesis and whole-body distribution in mice of a 18 F-labeled azepino[4,3-b]indole-1-one derivative with multimodal activity for the treatment of Alzheimer's disease

Recently, the azepino[4,3-b]indole-1-one derivative 1 showed in vitro nanomolar inhibition against butyrylcholinesterase (BChE), the ChE isoform that plays a role in the progression and pathophysiology of Alzheimer's disease (AD), and protects against N-methyl- d-aspartate-induced neuronal toxicity. Three 9-R-substituted (R = F, Br, OMe) congeners were investigated. The 9-F derivative (2a) was found more potent as BChE inhibitors (half-maximal inhibitory concentration value = 21 nM) than 2b (9-Br) and 2c (9-OMe), achieving a residence time (38 s), assessed by surface plasmon resonance, threefold higher than that of 1. To progress in featuring the in vivo pharmacological characterization of 2a, herein the 18 F-labeled congener 2a was synthesized, by applying the aromatic 18 F-fluorination method, and its whole-body distribution in healthy mice, including brain penetration, was evaluated through positron emission tomography imaging. [18 F]2a exhibited a rapid and high brain uptake (3.35 ± 0.26% ID g-1 at 0.95 ± 0.15 min after injection), followed by a rapid clearance (t1/2  = 6.50 ± 0.93 min), showing good blood-brain barrier crossing. After a transient liver accumulation of [18 F]2a, the intestinal and urinary excretion was quantified. Finally, ex vivo pharmacological experiments in mice showed that the unlabeled 2a affects the transmitters' neurochemistry, which might be favorable to reverse cognition impairment in mild-to-moderate AD-related dementias.

Translocator Protein 18 kDa (TSPO): A Promising Molecular Target for Image-Guided Surgery of Solid Cancers

The translocator protein 18-kDa (TSPO) is a mitochondrial membrane protein that is previously identified as the peripheral benzodiazepine receptor (PBR). Furthermore, it plays a significant role in a diverse range of biochemical processes, including steroidogenesis, mitochondrial cholesterol transport, cell survival and death, cell proliferation, and carcinogenesis. Several investigations also reported its roles in various types of cancers, including colorectal, brain, breast, prostate, and lung cancers, as well as melanoma. According to a previous study, the expression of TSPO was upregulated in cancer cells, which corresponds to an aggressive phenotype and/or poor prognosis. Consequently, the potential for crafting diagnostic and prognostic tools with a focus on TSPO holds great potential. In this context, several radioligands designed to target this protein have been identified, and some of the candidates have advanced to clinical trials. In recent years, the use of hybrid probes with radioactive and fluorescence molecules for image-guided surgery has exhibited promising results in animal and human studies. This indicates that the approach can serve as a valuable surgical navigator during cancer surgery. The current hybrid probes are built from various molecular platforms, including small molecules, nanoparticles, and antibodies. Although several TSPO-targeted imaging probes have been developed, their development for image-guided surgery of cancers is still limited. Therefore, this review aims to highlight recent findings on the involvement of TSPO in carcinogenesis, as well as provide a new perspective on the potential application of TSPO-targeted hybrid probes for image-guided surgery.

Huang-Qi-Jian-Zhong-Tang accelerates healing of indomethacin-induced gastric ulceration in rats via anti-inflammatory and antioxidant mechanisms

ETHNOPHARMACOLOGICAL RELEVANCE

Huang-Qi-Jian-Zhong-Tang (HQJZT) is a canonical traditional Chinese medicine (TCM) formula that has been widely used in both the prevention and treatment of gastrointestinal diseases, including gastric ulcer, duodenal ulcer, and chronic atrophic gastritis, in China.

AIM OF THE STUDY

In the present study, we investigated the gastroprotective potential of HQJZT in a rat model of indomethacin (IND)-induced gastric ulcer and explained the biochemical, cellular, and molecular mechanisms involved.

MATERIALS AND METHODS

Observations were conducted at the macroscopic level to ascertain the ulcer index (UI) and the curative index (CI). Histopathological examinations were conducted, and a microscopic score (MS) was computed. The gastric juice volume, total acidity, pH value, and pepsin activity were quantified. Antioxidant and oxidative parameters were assessed, namely GSH, CAT, SOD, and MDA content. The RFLSI Pro instrument was employed to measure the blood flow within the gastric mucosa continuously. The mRNA levels of the inflammatory cytokines were assessed using droplet digital PCR (ddPCR). Molecular docking was employed to examine the interaction between representative active components of HQJZT and the binding sites associated with the NF-κB and STAT signaling pathways. The protein expression and localization of p-JAK, p-STAT, p-IκBβ, and p-NF-κB were evaluated through immunofluorescence analysis.

RESULTS

The administration of HQJZT treatment demonstrated a significant reduction in gastric lesions induced by IND, leading to a notable decrease in the UI. Additionally, HQJZT treatment significantly decreased gastric juice volume, acidity, and pepsin activity, accompanied by increased pH value. IND-treated stomachs exhibited severe hemorrhagic necrosis, submucosal edema, and epithelial cell destruction. However, the administration of HQJZT effectively counteracted these pathological changes. Furthermore, HQJZT administration significantly increased blood flow to the gastric mucosa. HQJZT enhanced antioxidant defenses and modulated oxidative stress by increasing SOD, CAT, and GSH activities while reducing MDA levels. Moreover, HQJZT reversed IND-induced increases in mRNA expression levels of inflammatory cytokines. Molecular docking analysis revealed that the representative active components of HQJZT could bind to the NF-κB and STAT signaling pathways. In addition, immunofluorescence microscopy revealed that HQJZT markedly attenuated the phosphorylation of IκΒβ, NF-κB, JAK, and STAT.

CONCLUSIONS

The therapeutic and protective effect of HQJZT on gastric ulcers is attributed to its ability to suppress gastric acid secretion, enhance antioxidative defenses and blood flow, mitigate proinflammatory cytokines, and inhibit the activation of NF-κB and STAT signaling pathways.

Radiotracers for Imaging of Inflammatory Biomarkers TSPO and COX-2 in the Brain and in the Periphery

Inflammation involves the activation of innate immune cells and is believed to play an important role in the development and progression of both infectious and non-infectious diseases such as neurodegeneration, autoimmune diseases, pulmonary and cancer. Inflammation in the brain is marked by the upregulation of translocator protein (TSPO) in microglia. High TSPO levels are also found, for example, in macrophages in cases of rheumatoid arthritis and in malignant tumor cells compared to their relatively low physiological expression. The same applies for cyclooxgenase-2 (COX-2), which is constitutively expressed in the kidney, brain, thymus and gastrointestinal tract, but induced in microglia, macrophages and synoviocytes during inflammation. This puts TSPO and COX-2 in the spotlight as important targets for the diagnosis of inflammation. Imaging modalities, such as positron emission tomography and single-photon emission tomography, can be used to localize inflammatory processes and to track their progression over time. They could also enable the monitoring of the efficacy of therapy and predict its outcome. This review focuses on the current development of PET and SPECT tracers, not only for the detection of neuroinflammation, but also for emerging diagnostic measures in infectious and other non-infectious diseases such as rheumatic arthritis, cancer, cardiac inflammation and in lung diseases.

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Quantitative assessment of translocator protein (TSPO) in the non-human primate brain and clinical translation of [18F]LW223 as a TSPO-targeted PET radioligand

OBJECTIVES

Translocator protein 18 kDa (TSPO) positron emission tomography (PET) can be harnessed for the non-invasive detection of macrophage-driven inflammation. [¹⁸F]LW223, a newly reported TSPO PET tracer which was insensitive to rs6971 polymorphism, showed favorable performance characteristics in a recent imaging study involving a rat myocardial infarction model. To enable quantitative neuroimaging with [¹⁸F]LW223, we conducted kinetic analysis in the non-human primate (NHP) brain. Further, we sought to assess the utility of [¹⁸F]LW223-based TSPO imaging in a first-in-human study.

METHODS

Radiosynthesis of [¹⁸F]LW223 was accomplished on an automated module, whereas molar activities, stability in formulation, lipophilicity and unbound free fraction (f_u) of the probe were measured. Brain penetration and target specificity of [¹⁸F]LW223 in NHPs were corroborated by PET-MR imaging under baseline and pre-blocking conditions using the validated TSPO inhibitor, (R)-PK11195, at doses ranging from 5 to 10 mg/kg. Kinetic modeling was performed using one-tissue compartment model (1TCM), two-tissue compartment model (2TCM) and Logan graphical analyses, using dynamic PET data acquisition, arterial blood collection and metabolic stability testing. Clinical PET scans were performed in two healthy volunteers (HVs). Regional brain standard uptake value ratio (SUVr) was assessed for different time intervals.

RESULTS

[¹⁸F]LW223 was synthesized in non-decay corrected radiochemical yields (n.d.c. RCYs) of 33.3 ± 6.5% with molar activities ranging from 1.8 ± 0.7 Ci/µmol (n = 11). [¹⁸F]LW223 was stable in formulation for up to 4 h and LogD_7.4 of 2.31 ± 0.13 (n = 6) and f_u of 5.80 ± 1.42% (n = 6) were determined. [¹⁸F]LW223 exhibited good brain penetration in NHPs, with a peak SUV value of ca. 1.79 in the whole brain. Pre-treatment with (R)-PK11195 substantially accelerated the washout and attenuated the area under the time-activity curve, indicating in vivo specificity of [¹⁸F]LW223 towards TSPO. Kinetic modeling demonstrated that 2TCM was the most suitable model for [¹⁸F]LW223-based neuroimaging. Global transfer rate constants (K₁) and total volumes of distribution (V_T) were found to be 0.10 ± 0.01 mL/cm³/min and 2.30 ± 0.17 mL/cm³, respectively. Dynamic PET data analyses across distinct time windows revealed that the V_T values were relatively stable after 60 min post-injection. In a preliminary clinical study with two healthy volunteers, [¹⁸F]LW223 exhibited good brain uptake and considerable tracer retention across all analyzed brain regions. Of note, an excellent correlation between SUVr with V_T was obtained when assessing the time interval from 20 to 40 min post tracer injection (SUVr_(20-40 min), R² = 0.94, p < 0.0001), suggesting this time window may be suitable to estimate specific binding to TSPO in human brain.

CONCLUSION

Our findings indicate that [¹⁸F]LW223 is suitable for quantitative TSPO-targeted PET imaging in higher species. Employing state-of-the-art kinetic modeling, we found that [¹⁸F]LW223 was effective in mapping TSPO throughout the NHP brain, with best model fits obtained from 2TCM and Logan graphical analyses. Overall, our results indicate that [¹⁸F]LW223 exhibits favorable tracer performance characteristics in higher species, and this novel imaging tool may hold promise to provide effective neuroinflammation imaging in patients with neurological disease.

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

18F-Radiolabeled Translocator Protein (TSPO) PET Tracers: Recent Development of TSPO Radioligands and Their Application to PET Study

Translocator protein 18 kDa (TSPO) is a transmembrane protein in the mitochondrial membrane, which has been identified as a peripheral benzodiazepine receptor. TSPO is generally present at high concentrations in steroid-producing cells and plays an important role in steroid synthesis, apoptosis, and cell proliferation. In the central nervous system, TSPO expression is relatively modest under normal physiological circumstances. However, some pathological disorders can lead to changes in TSPO expression. Overexpression of TSPO is associated with several diseases, such as neurodegenerative diseases, neuroinflammation, brain injury, and cancers. TSPO has therefore become an effective biomarker of related diseases. Positron emission tomography (PET), a non-invasive molecular imaging technique used for the clinical diagnosis of numerous diseases, can detect diseases related to TSPO expression. Several radiolabeled TSPO ligands have been developed for PET. In this review, we describe recent advances in the development of TSPO ligands, and ¹⁸F-radiolabeled TSPO in particular, as PET tracers. This review covers pharmacokinetic studies, preclinical and clinical trials of ¹⁸F-labeled TSPO PET ligands, and the synthesis of TSPO ligands.

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.

Recent progress on the development of fluorescent probes targeting the translocator protein 18 kDa (TSPO)

The translocator protein 18 kDa (TSPO) was first identified in 1997, and has now become one of the appealing subcellular targets in medicinal chemistry and its related fields. TSPO involves in a variety of diseases, covering neurodegenerative diseases, psychiatric disorders, cancers, and so on. To date, various high-affinity TSPO ligands labelled with single-photon emission computed tomography (SPECT)/positron emission tomography (PET) radionuclides have been reported, with some third-generation radioligands advanced to clinical trials. On the other hand, only a few number of TSPO ligands have been labelled with fluorophores for disease diagnosis. It is noteworthy that the majority of the TSPO fluorescent probes synthesised to date are based on visible fluorophores, suggesting that their applications are limited to in vitro studies, such as in vitro imaging of cancer cells, post-mortem analysis, and tissue biopsies examinations. In this context, the potential application of TSPO ligands can be broadened for in vivo investigations of human diseases by labelling with near-infrared (NIR)-fluorophores or substituting visible fluorophores with NIR-fluorophores on the currently developed fluorescent probes. In this review article, recent progress on fluorescent probes targeting the TSPO are summarised, with an emphasis on development trend in recent years and application prospects in the future.

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.

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

Translocator protein (TSPO, 18 kDa) is an evolutionary, well-preserved, and tryptophan-rich 169-amino-acid protein which localizes on the contact sites between the outer and inner mitochondrial membranes of steroid-synthesizing cells. This mitochondrial protein is implicated in an extensive range of cellular activities, including steroid synthesis, cholesterol transport, apoptosis, mitochondrial respiration, and cell proliferation. The upregulation of TSPO is well documented in diverse disease conditions including neuroinflammation, cancer, brain injury, and inflammation in peripheral organs. On the basis of these outcomes, TSPO has been assumed to be a fascinating subcellular target for early stage imaging of the diseased state and for therapeutic purposes. The main outline of this Review is to give an update on dealing with the advances made in TSPO PET tracers for neuroinflammation, synchronously emphasizing the approaches applied for the design and advancement of new tracers with reference to their structure-activity relationship (SAR).