PET radiotracers: crossing the blood-brain barrier and surviving metabolism

Tracer development for PET imaging of proteinopathies

This review outlines small molecule radiotracers developed for positron emission tomography (PET) imaging of proteinopathies, neurodegenerative diseases characterised by accumulation of malformed proteins, over the last two decades with the focus on radioligands that have progressed to clinical studies. Introduction provides a short summary of proteinopathy targets used for PET imaging, including vastly studied proteins Aβ and tau and emerging α-synuclein. In the main section, clinically relevant Aβ and tau radioligand classes and their properties are discussed, including an overview of lead compounds and radioligand candidates studied as α-synuclein imaging agents in the early discovery and preclinical development phase. Lastly, the specific challenges and future directions in proteinopathy radioligand development are summarized.

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.

A positron emission tomography imaging probe selectively targeting the BD1 bromodomain and extra-terminal domain

The two tandem bromodomains of BET (bromodomain and extra-terminal domain) proteins (BD1 and BD2) may play distinct and critical roles in neurological diseases. To better understand the underlying mechanisms of the BD1 bromodomain and facilitate brain permeable domain-selective inhibitor development, we describe here the development of the first BET BD1 positron emission tomography (PET) radioligand [¹¹C]1a. Compound 1a was tested to possess potent binding affinities and good selectivity (>20-fold over BD2) for BD1 bromodomains of BRD2 (K_d = 25 nM), BRD3 (K_d = 24 nM), and BRD4 (K_d = 19 nM). Physicochemical characterization of 1a indicated the brain permeability and specific binding. [¹¹C]1a was radiosynthesized in a good radiochemical yield (RCY: 25-30%) and molar activity (258 GBq μmol^-1). The PET imaging studies of [¹¹C]1a in mice showed moderate brain uptake (with peak SUV = 0.7) and binding specificity. Furthermore, [¹¹C]1a demonstrated translational potential in the non-human primate (NHP) PET imaging study, which sets the stage for clinical translation.

Comparison of the Blood-Brain Barrier Transport and Vulnerability to P-Glycoprotein-Mediated Drug-Drug Interaction of Domperidone versus Metoclopramide Assessed Using In Vitro Assay and PET Imaging

Domperidone and metoclopramide are widely prescribed antiemetic drugs with distinct neurological side effects. The impact of P-glycoprotein (P-gp)-mediated efflux at the blood−brain barrier (BBB) on brain exposure and BBB permeation was compared in vitro and in vivo using positron emission tomography (PET) imaging in rats with the radiolabeled analogs [11C]domperidone and [11C]metoclopramide. In P-gp-overexpressing cells, the IC50 of tariquidar, a potent P-gp inhibitor, was drastically different using [11C]domperidone (221 nM [198−248 nM]) or [11C]metoclopramide (4 nM [2−8 nM]) as the substrate. Complete P-gp inhibition led to a 1.8-fold higher increase in the cellular uptake of [11C]domperidone compared with [11C]metoclopramide (p < 0.0001). Brain PET imaging revealed that the baseline brain exposure (AUCbrain) of [11C]metoclopramide was 2.4-fold higher compared with [11C]domperidone (p < 0.001), consistent with a 1.8-fold higher BBB penetration (AUCbrain/AUCplasma). The maximal increase in the brain exposure (2.9-fold, p < 0.0001) and BBB penetration (2.9-fold, p < 0.0001) of [11C]metoclopramide was achieved using 8 mg/kg of tariquidar. In comparison, neither 8 nor 15 mg/kg of tariquidar increased the brain exposure of [11C]domperidone (p > 0.05). Domperidone is an avid P-gp substrate that was in vitro compared with metoclopramide. Domperidone benefits from a lower brain exposure and a limited risk for P-gp-mediated drug−drug interaction involving P-gp inhibition at the BBB.

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.

In Vivo Evaluation of 6 Analogs of 11C-ER176 as Candidate 18F-Labeled Radioligands for 18-kDa Translocator Protein

Because of its excellent ratio of specific to nondisplaceable uptake, the radioligand ¹¹C-ER176 can successfully image 18-kDa translocator protein (TSPO), a biomarker of inflammation, in the human brain and accurately quantify target density in homozygous low-affinity binders. Our laboratory sought to develop an ¹⁸F-labeled TSPO PET radioligand based on ER176 with the potential for broader distribution. This study used generic ¹¹C labeling and in vivo performance in the monkey brain to select the most promising among 6 fluorine-containing analogs of ER176 for subsequent labeling with longer-lived ¹⁸F. Methods: Six fluorine-containing analogs of ER176-3 fluoro and 3 trifluoromethyl isomers-were synthesized and labeled by ¹¹C methylation at the secondary amide group of the respective N-desmethyl precursor. PET imaging of the monkey brain was performed at baseline and after blockade by N-butan-2-yl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide (PK11195). Uptake was quantified using radiometabolite-corrected arterial input function. The 6 candidate radioligands were ranked for performance on the basis of 2 in vivo criteria: the ratio of specific to nondisplaceable uptake (i.e., nondisplaceable binding potential [BP_ND]) and the time stability of total distribution volume (V_T), an indirect measure of lack of radiometabolite accumulation in the brain. Results: Total TSPO binding was quantified as V_T corrected for plasma free fraction (V_T/f_P) using Logan graphical analysis for all 6 radioligands. V_T/f_P was generally high at baseline (222 ± 178 mL·cm^-3) and decreased by 70%-90% after preblocking with PK11195. BP_ND calculated using the Lassen plot was 9.6 ± 3.8; the o-fluoro radioligand exhibited the highest BP_ND (12.1), followed by the m-trifluoromethyl (11.7) and m-fluoro (8.1) radioligands. For all 6 radioligands, V_T reached 90% of the terminal 120-min values by 70 min and remained relatively stable thereafter, with excellent identifiability (SEs < 5%), suggesting that no significant radiometabolites accumulated in the brain. Conclusion: All 6 radioligands had good BP_ND and good time stability of V_T Among them, the o-fluoro, m-trifluoromethyl, and m-fluoro compounds were the 3 best candidates for development as radioligands with an ¹⁸F label.

Facts and Fictions About [18F]FDG versus Other Tracers in Managing Patients with Brain Tumors: It Is Time to Rectify the Ongoing Misconceptions

MRI is the first-choice imaging technique for brain tumors. Positron emission tomography can be combined together with multiparametric MRI to increase diagnostic confidence. Radiolabeled amino acids have gained wide clinical acceptance. The reported pooled specificity of [¹⁸F]FDG positron emission tomography is high and [¹⁸F]FDG might still be the first-choice positron emission tomography tracer in cases of World Health Organization grade 3 to 4 gliomas or [¹⁸F]FDG-avid tumors, avoiding the use of more expensive and less available radiolabeled amino acids. The present review discusses the additional value of positron emission tomography with a focus on [¹⁸F]FDG and radiolabeled amino acids.

In Silico Approaches for Addressing Challenges in CNS Radiopharmaceutical Design

Positron emission tomography (PET) is a highly sensitive and versatile molecular imaging modality that leverages radiolabeled molecules, known as radiotracers, to interrogate biochemical processes such as metabolism, enzymatic activity, and receptor expression. The ability to probe specific molecular and cellular events longitudinally in a noninvasive manner makes PET imaging a particularly powerful technique for studying the central nervous system (CNS) in both health and disease. Unfortunately, developing and translating a single CNS PET tracer for clinical use is typically an extremely resource-intensive endeavor, often requiring synthesis and evaluation of numerous candidate molecules. While existing in vitro methods are beginning to address the challenge of derisking molecules prior to costly in vivo PET studies, most require a significant investment of resources and possess substantial limitations. In the context of CNS drug development, significant time and resources have been invested into the development and optimization of computational methods, particularly involving machine learning, to streamline the design of better CNS therapeutics. However, analogous efforts developed and validated for CNS radiotracer design are conspicuously limited. In this Perspective, we overview the requirements and challenges of CNS PET tracer design, survey the most promising computational methods for in silico CNS drug design, and bridge these two areas by discussing the potential applications and impact of computational design tools in CNS radiotracer design.

[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.

Cyclooxygenases as Potential PET Imaging Biomarkers to Explore Neuroinflammation in Dementia

The most frequently studied target of neuroinflammation using PET is 18-kDa translocator protein, but its limitations have spurred the molecular imaging community to find more promising targets. This article reviews the development of PET radioligands for cyclooxygenase (COX) subtypes 1 and 2, enzymes that catalyze the production of inflammatory prostanoids in the periphery and brain. Although both isozymes produce the same precursor compound, prostaglandin H₂, they have distinct functions based on their differential cellular localization in the periphery and brain. For example, COX-1 is located primarily in microglia, a resident inflammatory cell in the brain whose role in producing inflammatory cytokines is well documented. In contrast, COX-2 is located primarily in neurons and can be markedly upregulated by inflammatory and excitatory stimuli, but its functions are poorly understood. This article reviews these 2 isozymes as biomarkers of neuroinflammation, as well as the radioligands that have recently been developed to image them in animals and humans. To place this work into context, the properties of COX-1 and COX-2 are compared with 18-kDa translocator protein, with special consideration of their application in Alzheimer disease as a representative neurodegenerative disorder.

Novel 18F-Labeled PET Tracers Specific to Aromatase: Design, Synthesis, and Biological Evaluation

The abnormal expression of aromatase is associated with the occurrence and development of a variety of neurological diseases and tumors. A series of ¹⁸F-labeled and ⁶⁸Ga-labeled potential aromatase-binding candidate compounds were designed and synthesized based on the structures of aromatase inhibitors. Competitive inhibition experiments in vitro and molecular docking showed that BIBD-069 and BIBD-071 have high affinity for aromatase. The radiolabeling conditions of [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 were simple, and the yields were high. Biodistribution and in vivo inhibition experiments confirmed that [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 specifically bind to aromatase. [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 selectively imaged the amygdala and nucleus of the stria terminalis, which is similar to the imaging result of [¹¹C]vorozole. Radiometabolites of [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 did not bind to aromatase and interfered with brain imaging. MicroPET-CT imaging further confirmed that [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 can specifically bind to aromatase and were not defluorinated in vivo. Given that [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 exhibit excellent aromatase binding affinities, mild radiolabeling conditions, and good pharmacokinetics, they can be important tools for the diagnosis and treatment of aromatase-related diseases.

A crosstalk between gut and brain in sepsis-induced cognitive decline

BACKGROUND

Sepsis is a potentially fatal disease characterized by acute organ failure that affects more than 30 million people worldwide. Inflammation is strongly associated with sepsis, and patients can experience impairments in memory, concentration, verbal fluency, and executive functioning after being discharged from the hospital. We hypothesize that sepsis disrupts the microbiota-gut-brain axis homeostasis triggering cognitive impairment. This immune activation persists during treatment, causing neurological dysfunction in sepsis survivors.

METHODS

To test our hypothesis, adult Wistar rats were subjected to cecal-ligation and perforation (CLP) or sham (non-CLP) surgeries. The animals were subjected to the [11C]PBR28 positron emission tomography (PET)/computed tomography (CT) imaging at 24 h and 10 days after CLP and non-CLP surgeries. At 24 h and 10 days after surgery, we evaluated the gut microbiome, bacterial metabolites, cytokines, microglia, and astrocyte markers. Ten days after sepsis induction, the animals were subjected to the novel object recognition (NOR) and the Morris water maze (MWM) test to assess their learning and memory.

RESULTS

Compared to the control group, the 24-h and 10-day CLP groups showed increased [11C]PBR28 uptake, glial cells count, and cytokine levels in the brain. Results show that sepsis modulates the gut villus length and crypt depth, alpha and beta microbial diversities, and fecal short-chain fatty acids (SCFAs). In addition, sepsis surviving animals showed a significant cognitive decline compared with the control group.

CONCLUSIONS

Since several pharmacological studies have failed to prevent cognitive impairment in sepsis survivors, a better understanding of the function of glial cells and gut microbiota can provide new avenues for treating sepsis patients.

Water soluble organometallic small molecules as promising antibacterial agents: synthesis, physical-chemical properties and biological evaluation to tackle bacterial infections

The Na[3,3'-Fe(8-I-1,2-C₂B₉H₁₀)₂] and Na[2,2'-M(1,7-C₂B₉H₁₁)] (M = Co³⁺, Fe³⁺) small molecules are synthesized and the X-ray structures of [(H₃O)(H₂O)₅][2,2'-Co(1,7-C₂B₉H₁₁)₂] and [Cs(MeCN)][8,8'-I₂-Fe(1,2 C₂B₉H₁₀)₂], both displaying a transoid conformation of the [M(C₂B₉)₂]^- framework, are reported. Importantly, the supramolecular structure of [(H₃O)(H₂O)₅][2,2'-Co(1,7-C₂B₉H₁₁)₂] presents 2D layers leading to a lamellar arrangement of the anions while the cation layers form polymeric water rings made of six- and four-membered rings of water molecules connected via OH⋯H hydrogen bonds; B-H⋯O contacts connect the cationic and anionic layers. Herein, we highlight the influence of the ligand isomers (ortho-/meta-), the metal effect (Co³⁺/Fe³⁺) on the same isomer, as well as the influence of the presence of the iodine atoms on the physical-chemical and biological properties of these molecules as antimicrobial agents to tackle antibiotic-resistant bacteria, which were tested with four Gram-positive bacteria, five Gram-negative bacteria, and three Candida albicans strains that have been responsible for human infections. We have demonstrated an antimicrobial effect against Candida species (MIC of 2 and 3 nM for Na[3,3'-Co(8-I-1,2-C₂B₉H₁₀)₂] and Na[2,2'-Co(1,7-C₂B₉H₁₁)₂], respectively), and against Gram-positive and Gram-negative bacteria, including multiresistant MRSA strains (MIC of 6 nM for Na[3,3'-Co(8-I-1,2-C₂B₉H₁₀)₂]). The selectivity index for antimicrobial activity of Na[3,3'-Co(1,2-C₂B₉H₁₁)₂] and Na[3,3'-Co(8-I-1,2-C₂B₉H₁₀)₂] compounds is very high (165 and 1180, respectively), which reveals that these small anionic metallacarborane molecules may be useful to tackle antibiotic-resistant bacteria. Moreover, we have demonstrated that the outer membrane of Gram-negative bacteria constitutes an impermeable barrier for the majority of these compounds. Nonetheless, the addition of two iodine groups in the structure of the parent Na[3,3'-Co(1,2-C₂B₉H₁₁)₂] had an improved effect (3-7 times) against Gram-negative bacteria. Possibly the changes in their physical-chemical properties make the meta-isomers and the ortho-di-iodinated small molecules more permeable for crossing this barrier. It should be emphasized that the most active metallabis(dicarbollide) small molecules are both transoid conformers in contrast to the ortho- [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- that is cisoid. The fact that these small molecules cross the mammalian membrane and have antimicrobial properties but low toxicity for mammalian cells (high selectivity index, SI) represents a promising tool to treat infectious intracellular bacteria. Since there is an urgent need for antibiotic discovery and development, this study represents a relevant advance in the field.

[18F]BIBD-181: A Novel Positron Emission Tomography Tracer Specific for Synaptic Vesicle Glycoprotein 2A

Dysfunction or decreased expression of synaptic vesicle glycoprotein 2A (SV2A) is closely related to the progression of neurodegenerative diseases and psychiatric disorders. The development of positron emission tomography (PET) tracers targeting SV2A can provide a strong imaging basis for the diagnosis and treatment of these diseases. Herein we report the synthesis of the novel radiotracer [¹⁸F]BIBD-181 and its preclinical evaluation. The absolute configuration of BIBD-181 was confirmed by the single-crystal structure of its precursor. The in vitro binding assay of BIBD-181 showed high SV2A binding affinity. Compared with previously reported tracers, [¹⁸F]BIBD-181 has mild labeling conditions, simple operation, and high yield. The in vivo metabolism of [¹⁸F]BIBD-181 is similar to that of UCB derivatives, and the metabolites do not interfere with brain PET imaging. Biodistribution and PET studies showed that [¹⁸F]BIBD-181 has high brain uptake and good pharmacokinetics. Autoradiography and PET inhibition studies indicated that [¹⁸F]BIBD-181 specifically binds SV2A. Because [¹⁸F]BIBD-181 exhibits excellent properties, it may be a reliable probe of quantities for SV2A-related disease diagnosis.

Alzheimer's Disease: A Silent Pandemic - A Systematic Review on the Situation and Patent Landscape of the Diagnosis

BACKGROUND

Alzheimer's disease (AD) is characterized by cognitive impairment, tau protein deposits, and amyloid beta plaques. AD impacted 44 million people in 2016, and it is estimated to affect 100 million people by 2050. AD is disregarded as a pandemic compared with other diseases. To date, there is no effective treatment or diagnosis.

OBJECTIVE

We aimed to discuss the current tools used to diagnose COVID-19, to point out their potential to be adapted for AD diagnosis, and to review the landscape of existing patents in the AD field and future perspectives for AD diagnosis.

METHOD

We carried out a scientific screening following a research strategy in PubMed; Web of Science; the Derwent Innovation Index; the KCI-Korean Journal Database; SciELO; the Russian Science Citation index; and the CDerwent, EDerwent, and MDerwent index databases.

RESULTS

A total of 326 from 6,446 articles about AD and 376 from 4,595 articles about COVID-19 were analyzed. Of these, AD patents were focused on biomarkers and neuroimaging with no accurate, validated diagnostic methods, and only 7% of kit development patents were found. In comparison, COVID-19 patents were 60% about kit development for diagnosis; they are highly accurate and are now commercialized.

CONCLUSION

AD is still neglected and not recognized as a pandemic that affects the people and economies of all nations. There is a gap in the development of AD diagnostic tools that could be filled if the interest and effort that has been invested to tackle the COVID-19 emergency could also be applied for innovation.

PET Molecular Imaging in Drug Development: The Imaging and Chemistry Perspective

Positron emission tomography with selective radioligands advances the drug discovery and development process by revealing information about target engagement, proof of mechanism, pharmacokinetic and pharmacodynamic profiles. Positron emission tomography (PET) is an essential and highly significant tool to study therapeutic drug development, dose regimen, and the drug plasma concentrations of new drug candidates. Selective radioligands bring up target-specific information in several disease states including cancer, cardiovascular, and neurological conditions by quantifying various rates of biological processes with PET, which are associated with its physiological changes in living subjects, thus it reveals disease progression and also advances the clinical investigation. This study explores the major roles, applications, and advances of PET molecular imaging in drug discovery and development process with a wide range of radiochemistry as well as clinical outcomes of positron-emitting carbon-11 and fluorine-18 radiotracers.

Synthesis of [11C]carbonyl-labeled cyclohexyl (5-(2-acetamidobenzo[d]thiazol-6-yl)-2-methylpyridin-3-yl)carbamate ([11C-carbonyl]PK68) as a potential PET tracer for receptor-interacting protein 1 kinase

Background

Receptor-interacting protein 1 kinase (RIPK1) is a key enzyme in the regulation of cellular necroptosis. Recently, cyclohexyl (5-(2-acetamidobenzo[d]thiazol-6-yl)-2-methylpyridin-3-yl)carbamate (PK68, 5) has been developed as a potent inhibitor of RIPK1. Herein, we synthesized [¹¹C]carbonyl-labeled PK68 ([¹¹C-carbonyl]PK68, [¹¹C]PK68) as a potential PET tracer for imaging RIPK1 and evaluated its brain uptake in vivo.

Results

We synthesized [¹¹C]PK68 by reacting amine precursor 14 with [¹¹C]acetyl chloride. At the end of synthesis, we obtained [¹¹C]PK68 of 1200–1790 MBq with a radiochemical yield of 9.1 ± 5.9% (n = 10, decay-corrected to the end of irradiation) and radiochemical purity of > 99%, and a molar activity of 37–99 GBq/μmol starting from 18–33 GBq of [¹¹C]CO₂. The fully automated synthesis took 30 min from the end of irradiation. In a small-animal PET study, [¹¹C]PK68 was rapidly distributed in the liver and kidneys of healthy mice after injection, and subsequently cleared from their bodies via hepatobiliary excretion and the intestinal reuptake pathway. Although there was no obvious specific binding of RIPK1 in the PET study, [¹¹C]PK68 demonstrated relatively high stability in vivo and provided useful structural information further candidate development.

Conclusions

In the present study, we successfully radiosynthesized [¹¹C]PK68 as a potential PET tracer and evaluated its brain uptake. We are planning to optimize the chemical structure of [¹¹C]PK68 and conduct further PET studies on it using pathological models.

Development and Biological Evaluation of the First Highly Potent and Specific Benzamide-Based Radiotracer [18F]BA3 for Imaging of Histone Deacetylases 1 and 2 in Brain

The degree of acetylation of lysine residues on histones influences the accessibility of DNA and, furthermore, the gene expression. Histone deacetylases (HDACs) are overexpressed in various tumour diseases, resulting in the interest in HDAC inhibitors for cancer therapy. The aim of this work is the development of a novel 18F-labelled HDAC1/2-specific inhibitor with a benzamide-based zinc-binding group to visualize these enzymes in brain tumours by positron emission tomography (PET). BA3, exhibiting high inhibitory potency for HDAC1 (IC50 = 4.8 nM) and HDAC2 (IC50 = 39.9 nM), and specificity towards HDAC3 and HDAC6 (specificity ratios >230 and >2080, respectively), was selected for radiofluorination. The two-step one-pot radiosynthesis of [18F]BA3 was performed in a TRACERlab FX2 N radiosynthesizer by a nucleophilic aliphatic substitution reaction. The automated radiosynthesis of [18F]BA3 resulted in a radiochemical yield of 1%, a radiochemical purity of >96% and a molar activity between 21 and 51 GBq/µmol (n = 5, EOS). For the characterization of BA3, in vitro and in vivo experiments were carried out. The results of these pharmacological and pharmacokinetic studies indicate a suitable inhibitory potency of BA3, whereas the applicability for non-invasive imaging of HDAC1/2 by PET requires further optimization of the properties of this compound.

Arene radiofluorination enabled by photoredox-mediated halide interconversion

Positron emission tomography (PET) is a powerful imaging technology that can visualize and measure metabolic processes in vivo and/or obtain unique information about drug candidates. The identification of new and improved molecular probes plays a critical role in PET, but its progress is somewhat limited due to the lack of efficient and simple labelling methods to modify biologically active small molecules and/or drugs. Current methods to radiofluorinate unactivated arenes are still relatively limited, especially in a simple and site-selective way. Here we disclose a method for constructing C-¹⁸F bonds through direct halide/¹⁸F conversion in electron-rich halo(hetero)arenes. [¹⁸F]F^- is introduced into a broad spectrum of readily available aryl halide precursors in a site-selective manner under mild photoredox conditions. Notably, our direct ¹⁹F/¹⁸F exchange method enables rapid PET probe diversification through the preparation and evaluation of an [¹⁸F]-labelled O-methyl tyrosine library. This strategy also results in the high-yielding synthesis of the widely used PET agent L-[¹⁸F]FDOPA from a readily available L-FDOPA analogue.

Development of High Brain-Penetrant and Reversible Monoacylglycerol Lipase PET Tracers for Neuroimaging

Monoacylglycerol lipase (MAGL) is one of the key enzymes in the endocannabinoid system. Inhibition of MAGL has been proposed as an attractive approach for the treatment of various diseases. In this study, we designed and successfully synthesized two series of piperazinyl pyrrolidin-2-one derivatives as novel reversible MAGL inhibitors. (R)-[¹⁸F]13 was identified through the preliminary evaluation of two carbon-11-labeled racemic structures [¹¹C]11 and [¹¹C]16. In dynamic positron-emission tomography (PET) scans, (R)-[¹⁸F]13 showed a heterogeneous distribution and matched the MAGL expression pattern in the mouse brain. High brain uptake and brain-to-blood ratio were achieved by (R)-[¹⁸F]13 in comparison with previously reported reversible MAGL PET radiotracers. Target occupancy studies with a therapeutic MAGL inhibitor revealed a dose-dependent reduction of (R)-[¹⁸F]13 accumulation in the mouse brain. These findings indicate that (R)-[¹⁸F]13 ([¹⁸F]YH149) is a highly promising PET probe for visualizing MAGL non-invasively in vivo and holds great potential to support drug development.

Recent Developments in Positron Emission Tomography Tracers for Proteinopathies Imaging in Dementia

An early detection and intervention for dementia represent tremendous unmet clinical needs and priorities in society. A shared feature of neurodegenerative diseases causing dementia is the abnormal accumulation and spreading of pathological protein aggregates, which affect the selective vulnerable circuit in a disease-specific pattern. The advancement in positron emission tomography (PET) biomarkers has accelerated the understanding of the disease mechanism and development of therapeutics for Alzheimer's disease and Parkinson's disease. The clinical utility of amyloid-β PET and the clinical validity of tau PET as diagnostic biomarker for Alzheimer's disease continuum have been demonstrated. The inclusion of biomarkers in the diagnostic criteria has introduced a paradigm shift that facilitated the early and differential disease diagnosis and impacted on the clinical management. Application of disease-modifying therapy likely requires screening of patients with molecular evidence of pathological accumulation and monitoring of treatment effect assisted with biomarkers. There is currently still a gap in specific 4-repeat tau imaging probes for 4-repeat tauopathies and α-synuclein imaging probes for Parkinson's disease and dementia with Lewy body. In this review, we focused on recent development in molecular imaging biomarkers for assisting the early diagnosis of proteinopathies (i.e., amyloid-β, tau, and α-synuclein) in dementia and discussed future perspectives.

Synthesis, Physicochemical and Biological Study of Gallium-68- and Lutetium-177-Labeled VEGF-A165/NRP-1 Complex Inhibitors Based on Peptide A7R and Branched Peptidomimetic

Neuropilin-1 (NRP-1) is a surface receptor found on many types of cancer cells. The overexpression of NRP-1 and its interaction with vascular endothelial growth factor-165 (VEGF₁₆₅) are associated with tumor growth and metastasis. Therefore, compounds that block the VEGF₁₆₅/NRP-1 interaction represent a promising strategy to image and treat NRP-1-related pathologies. The aim of the presented work was to design and synthesize radioconjugates of two known peptide-type inhibitors of the VEGF₁₆₅/NRP-1 complex: A7R peptide and its shorter analog, the branched peptidomimetic Lys(hArg)-Dab-Pro-Arg. Both peptide-type inhibitors were coupled to a radionuclide chelator (DOTA) via a linker (Ahx) and so radiolabeled with Ga-68 and Lu-177 radionuclides, for diagnostic and therapeutic uses, respectively. The synthesized radioconjugates were tested for their possible use as theranostic-like radiopharmaceuticals for the imaging and therapy of cancers that overexpress NRP-1. The obtained results indicate good efficiency of the radiolabeling reaction and satisfactory stability, at least 3t_1/2 for the ⁶⁸Ga- and 1t_1/2 for the ¹⁷⁷Lu-radiocompounds, in solutions mimicking human body fluids. However, enzymatic degradation of both the studied inhibitors caused insufficient stability of the radiocompounds in human serum, indicating that further modifications are needed to sufficiently stabilize the peptidomimetics with inhibitory properties against VEGF₁₆₅/NRP-1 complex formation.

Tau PET following acute TBI: Off-target binding to blood products, tauopathy, or both?

Repeated mild Traumatic Brain Injury (TBI) is a risk factor for Chronic Traumatic Encephalopathy (CTE), characterized pathologically by neurofibrillary tau deposition in the depths of brain sulci and surrounding blood vessels. The mechanism by which TBI leads to CTE remains unknown but has been posited to relate to axonal shear injury leading to release and possibly deposition of tau at the time of injury. As part of an IRB-approved study designed to learn how processes occurring acutely after TBI may predict later proteinopathy and neurodegeneration, we performed tau PET using 18F-MK6240 and MRI within 14 days of complicated mild TBI in three subjects. PET radiotracer accumulation was apparent in regions of traumatic hemorrhage in all subjects, with prominent intraparenchymal PET signal in one young subject with a history of repeated sports-related concussions. These results are consistent with off-target tracer binding to blood products as well as possible on-target binding to chronically and/or acutely-deposited neurofibrillary tau. Both explanations are highly relevant to applying tau PET to understanding TBI and CTE. Additional study is needed to assess the potential utility of tau PET in understanding how processes occurring acutely after TBI, such as release and deposition of tau and blood from damaged axons and blood vessels, may relate to development CTE years later.

Comparative vulnerability of PET radioligands to partial inhibition of P-glycoprotein at the blood-brain barrier: A criterion of choice?

Only partial deficiency/inhibition of P-glycoprotein (P-gp, ABCB1) function at the blood-brain barrier (BBB) is likely to occur in pathophysiological situations or drug-drug interactions. This raises questions regarding the sensitivity of available PET imaging probes to detect moderate changes in P-gp function at the living BBB. In vitro, the half-maximum inhibitory concentration (IC₅₀) of the potent P-gp inhibitor tariquidar in P-gp-overexpressing cells was significantly different using either [¹¹C]verapamil (44 nM), [¹¹C]N-desmethyl-loperamide (19 nM) or [¹¹C]metoclopramide (4 nM) as substrate probes. In vivo PET imaging in rats showed that the half-maximum inhibition of P-gp-mediated efflux of [¹¹C]metoclopramide, achieved using 1 mg/kg tariquidar (in vivo IC₅₀ = 82 nM in plasma), increased brain exposure by 2.1-fold for [¹¹C]metoclopramide (p < 0.05, n = 4) and 2.4-fold for [¹¹C]verapamil (p < 0.05, n = 4), whereby cerebral uptake of the "avid" substrate [¹¹C]N-desmethyl-loperamide was unaffected (p > 0.05, n = 4). This comparative study points to differences in the "vulnerability" to P-gp inhibition among radiolabeled substrates, which were apparently unrelated to their "avidity" (maximal response to P-gp inhibition). Herein, we advocate that partial inhibition of transporter function, in addition to complete inhibition, should be a primary criterion of evaluation regarding the sensitivity of radiolabeled substrates to detect moderate but physiologically-relevant changes in transporter function in vivo.

Cobaltabis(dicarbollide) ([o-COSAN]-) as Multifunctional Chemotherapeutics: A Prospective Application in Boron Neutron Capture Therapy (BNCT) for Glioblastoma

PURPOSE

The aim of our study was to assess if the sodium salt of cobaltabis(dicarbollide) and its di-iodinated derivative (Na[o-COSAN] and Na[8,8'-I2-o-COSAN]) could be promising agents for dual anti-cancer treatment (chemotherapy + BNCT) for GBM.

METHODS

The biological activities of the small molecules were evaluated in vitro with glioblastoma cells lines U87 and T98G in 2D and 3D cell models and in vivo in the small model animal Caenorhabditis elegans (C. elegans) at the L4-stage and using the eggs.

RESULTS

Our studies indicated that only spheroids from the U87 cell line have impaired growth after treatment with both compounds, suggesting an increased resistance from T98G spheroids, contrary to what was observed in the monolayer culture, which highlights the need to employ 3D models for future GBM studies. In vitro tests in U87 and T98G cells conclude that the amount of 10B inside the cells is enough for BNCT irradiation. BNCT becomes more effective on T98G after their incubation with Na[8,8'-I2-o-COSAN], whereas no apparent cell-killing effect was observed for untreated cells.

CONCLUSIONS

These small molecules, particularly [8,8'-I2-o-COSAN]-, are serious candidates for BNCT now that the facilities of accelerator-based neutron sources are more accessible, providing an alternative treatment for resistant glioblastoma.

Validation of SV2A-Targeted PET Imaging for Noninvasive Assessment of Neuroendocrine Differentiation in Prostate Cancer

Neuroendocrine prostate cancer (NEPC) is an aggressive and lethal variant of prostate cancer (PCa), and it remains a diagnostic challenge. Herein we report our findings of using synaptic vesicle glycoprotein 2 isoform A (SV2A) as a promising marker for positron emission tomography (PET) imaging of neuroendocrine differentiation (NED). The bioinformatic analyses revealed an amplified SV2A gene expression in clinical samples of NEPC versus castration-resistant PCa with adenocarcinoma characteristics (CRPC-Adeno). Importantly, significantly upregulated SV2A protein levels were found in both NEPC cell lines and tumor tissues. PET imaging studies were carried out in NEPC xenograft models with ¹⁸F-SynVesT-1. Although ¹⁸F-SynVesT-1 is not a cancer imaging agent, it showed a significant uptake level in the SV2A⁺ tumor (NCI-H660: 0.70 ± 0.14 %ID/g at 50–60 min p.i.). The SV2A blockade resulted in a significant reduction of tumor uptake (0.25 ± 0.03 %ID/g, p = 0.025), indicating the desired SV2A imaging specificity. Moreover, the comparative PET imaging study showed that the DU145 tumors could be clearly visualized by ¹⁸F-SynVesT-1 but not ⁶⁸Ga-PSMA-11 nor ⁶⁸Ga-DOTATATE, further validating the role of SV2A-targeted imaging for noninvasive assessment of NED in PCa. In conclusion, we demonstrated that SV2A, highly expressed in NEPC, can serve as a promising target for noninvasive imaging evaluation of NED.

Preclinical Evaluation of [11C]YC-72-AB85 for In Vivo Visualization of Heat Shock Protein 90 in Brain and Cancer with Positron Emission Tomography

Aberrant Hsp90 has been implied in cancer and neurodegenerative disorders. The development of a suitable Hsp90 Positron emission tomography (PET) probe can provide in vivo quantification of the expression levels of Hsp90 as a biomarker for diagnosis and follow-up of cancer and central nervous system (CNS) disease progression. In this respect, [¹¹C]YC-72-AB85 was evaluated as an Hsp90 PET probe in B16.F10 melanoma bearing mice and its brain uptake was determined in rats and nonhuman primate. In vitro binding of [¹¹C]YC-72-AB85 to tissue slices of mouse B16.F10 melanoma, PC3 prostate carcinoma, and rodent brain was evaluated using autoradiography. Biodistribution of [¹¹C]YC-72-AB85 was evaluated in healthy and B16.F10 melanoma mice. In vivo brain uptake was assessed by μPET studies in rats and a rhesus monkey. In vitro binding was deemed Hsp90-specific by blocking studies with heterologous Hsp90 inhibitors onalespib and SNX-0723. Saturable Hsp90 binding was observed in brain, tumor, blood, and blood-rich organs in mice. In combined pretreatment and displacement studies, reversible and Hsp90-specific binding of [¹¹C]YC-72-AB85 was observed in rat brain. Dynamic μPET brain scans in baseline and blocking conditions in a rhesus monkey indicated Hsp90-specific binding. [¹¹C]YC-72-AB85 is a promising PET tracer for in vivo visualization of Hsp90 in tumor and brain. Clear differences of Hsp90 binding to blood and blood-rich organs were observed in tumor vs control mice. Further, we clearly demonstrate, for the first time, binding to a saturable Hsp90 pool in brain of rats and a rhesus monkey.

Design, Synthesis, and Evaluation of Thienodiazepine Derivatives as Positron Emission Tomography Imaging Probes for Bromodomain and Extra-Terminal Domain Family Proteins

To better understand the role of bromodomain and extra-terminal domain (BET) proteins in epigenetic mechanisms, we developed a series of thienodiazepine-based derivatives and identified two compounds, 3a and 6a, as potent BET inhibitors. Further in vivo pharmacokinetic studies and analysis of in vitro metabolic stability of 6a revealed excellent brain penetration and reasonable metabolic stability. Compounds 3a and 6a were radiolabeled with fluorine-18 in two steps and utilized in positron emission tomography (PET) imaging studies in mice. Preliminary PET imaging results demonstrated that [¹⁸F]3a and [¹⁸F]6a have good brain uptake (with maximum SUV = 1.7 and 2, respectively) and binding specificity in mice brains. These results show that [¹⁸F]6a is a potential PET radiotracer that could be applied to imaging BET proteins in the brain. Further optimization and improvement of the metabolic stability of [¹⁸F]6a are still needed in order to create optimal PET imaging probes of BET family members.

Dynamic 11C-Methionine PET-CT: Prognostic Factors for Disease Progression and Survival in Patients with Suspected Glioma Recurrence

Simple Summary

Recurrence after initial treatments is an expected event in glioma patients, particularly for high-grade glioma, with a median progression-free survival of 8–11 weeks. The prognostic evaluation of disease is a crucial step in the planning of therapeutic strategies, in both the primary and recurrence stages of disease. The aim of our retrospective study was to assess the prognostic value of ¹¹C-methionine PET-CT dynamic and semiquantitative parameters in patients with suspected glioma recurrence at MR, in terms of progression-free survival and overall survival. In a population of sixty-seven consecutive patients, both static and kinetic analyses provided parameters (i.e., tumour-to-background ratio and SUVmax associated with time-to-peak, respectively) able to predict both progression-free and overall survival in the whole population and in the high-grade glioma subgroup of patients. Dynamic ¹¹C-methionine PET-CT can be a useful diagnostic tool, in patients with suspicion of glioma recurrence, able to produce significant prognostic indices.

Abstract

Purpose: The prognostic evaluation of glioma recurrence patients is important in the therapeutic management. We investigated the prognostic value of ¹¹C-methionine PET-CT (MET-PET) dynamic and semiquantitative parameters in patients with suspected glioma recurrence. Methods: Sixty-seven consecutive patients who underwent MET-PET for suspected glioma recurrence at MR were retrospectively included. Twenty-one patients underwent static MET-PET; 46/67 underwent dynamic MET-PET. In all patients, SUVmax, SUVmean and tumour-to-background ratio (T/B) were calculated. From dynamic acquisition, the shape and slope of time-activity curves, time-to-peak and its SUVmax (SUVmax_TTP) were extrapolated. The prognostic value of PET parameters on progression-free (PFS) and overall survival (OS) was evaluated using Kaplan–Meier survival estimates and Cox regression. Results: The overall median follow-up was 19 months from MET-PET. Recurrence patients (38/67) had higher SUVmax (p = 0.001), SUVmean (p = 0.002) and T/B (p < 0.001); deceased patients (16/67) showed higher SUVmax (p = 0.03), SUVmean (p = 0.03) and T/B (p = 0.006). All static parameters were associated with PFS (all p < 0.001); T/B was associated with OS (p = 0.031). Regarding kinetic analyses, recurrence (27/46) and deceased (14/46) patients had higher SUVmax_TTP (p = 0.02, p = 0.01, respectively). SUVmax_TTP was the only dynamic parameter associated with PFS (p = 0.02) and OS (p = 0.006). At univariate analysis, SUVmax, SUVmean, T/B and SUVmax_TTP were predictive for PFS (all p < 0.05); SUVmax_TTP was predictive for OS (p = 0.02). At multivariate analysis, SUVmax_TTP remained significant for PFS (p = 0.03). Conclusion: Semiquantitative parameters and SUVmax_TTP were associated with clinical outcomes in patients with suspected glioma recurrence. Dynamic PET-CT acquisition, with static and kinetic parameters, can be a valuable non-invasive prognostic marker, identifying patients with worse prognosis who require personalised therapy.

Drug Penetration into the Central Nervous System: Pharmacokinetic Concepts and In Vitro Model Systems

Delivery of most drugs into the central nervous system (CNS) is restricted by the blood-brain barrier (BBB), which remains a significant bottleneck for development of novel CNS-targeted therapeutics or molecular tracers for neuroimaging. Consistent failure to reliably predict drug efficiency based on single measures for the rate or extent of brain penetration has led to the emergence of a more holistic framework that integrates data from various in vivo, in situ and in vitro assays to obtain a comprehensive description of drug delivery to and distribution within the brain. Coupled with ongoing development of suitable in vitro BBB models, this integrated approach promises to reduce the incidence of costly late-stage failures in CNS drug development, and could help to overcome some of the technical, economic and ethical issues associated with in vivo studies in animal models. Here, we provide an overview of BBB structure and function in vivo, and a summary of the pharmacokinetic parameters that can be used to determine and predict the rate and extent of drug penetration into the brain. We also review different in vitro models with regard to their inherent shortcomings and potential usefulness for development of fast-acting drugs or neurotracers labeled with short-lived radionuclides. In this regard, a special focus has been set on those systems that are sufficiently well established to be used in laboratories without significant bioengineering expertise.

[18F]ALX5406: A Brain-Penetrating Prodrug for GlyT1-Specific PET Imaging

Selective inhibition of glycine transporter 1 (GlyT1) has emerged as a potential approach to alleviate N-methyl-d-aspartate receptor (NMDAR) hypofunction in patients with schizophrenia and cognitive decline. ALX5407 is a potent and selective inhibitor of GlyT1 derived from the metabolic intermediate sarcosine (N-methylglycine) that showed antipsychotic potential in a number of animal models. Whereas clinical application of ALX5407 is limited by adverse effects on motor performance and respiratory function, a suitably radiolabeled drug could represent a promising PET tracer for the visualization of GlyT1 in the brain. Herein, [¹⁸F]ALX5407 and the corresponding methyl ester, [¹⁸F]ALX5406, were prepared by alcohol-enhanced copper mediated radiofluorination and studied in vitro in rat brain slices and in vivo in normal rats. [¹⁸F]ALX5407 demonstrated accumulation consistent with the distribution of GlyT1 in in vitro autoradiographic studies but no brain uptake in μPET experiments in naı̈ve rats. In contrast, the methyl ester [¹⁸F]ALX5406 rapidly entered the brain and was enzymatically transformed into [¹⁸F]ALX5407, resulting in a regional accumulation pattern consistent with GlyT1 specific binding. We conclude that [¹⁸F]ALX5406 is a promising and easily accessible PET probe for preclinical in vivo imaging of GlyT1 in the brain.

BLZ945 derivatives for PET imaging of colony stimulating factor-1 receptors in the brain

BACKGROUND

The kinase colony stimulating factor-1 receptor (CSF-1R) has recently been identified as a novel therapeutic target for decreasing tumor associated macrophages and microglia load in cancer treatment. In glioblastoma multiforme (GBM), a high-grade cancer in the brain with extremely poor prognosis, macrophages and microglia can make up to 50% of the total tumor mass. Currently, no non-invasive methods are available for measuring CSF-1R expression in vivo. The aim of this work is to develop a PET tracer for imaging of CSF-1R receptor expression in the brain for future GBM patient selection and treatment monitoring.

METHODS

BLZ945 and a derivative that potentially allows for fluorine-18 labeling were synthesized and evaluated in vitro to determine their affinity towards CSF-1R. BLZ945 was radiolabeled with carbon-11 by N-methylation of des-methyl-BLZ945 using [11C]CH3I. Following administration to healthy mice, metabolic stability of [11C]BLZ945 in blood and brain and activity distribution were determined ex vivo. PET scanning was performed at baseline, efflux transporter blocking, and CSF-1R blocking conditions. Finally, [11C]BLZ945 binding was evaluated in vitro by autoradiography on mouse brain sections.

RESULTS

BLZ945 was the most potent compound in our series with an IC50 value of 6.9 ± 1.4 nM. BLZ945 was radiolabeled with carbon-11 in 20.7 ± 1.1% decay corrected radiochemical yield in a 60 min synthesis procedure with a radiochemical purity of >95% and a molar activity of 153 ± 34 GBq·μmol-1. Ex vivo biodistribution showed moderate brain uptake and slow wash-out, in addition to slow blood clearance. The stability of BLZ945 in blood plasma and brain was >99% at 60 min post injection. PET scanning demonstrated BLZ945 to be a substrate for efflux transporters. High brain uptake was observed, which was shown to be mostly non-specific. In accordance, in vitro autoradiography on brain sections revealed high non-specific binding.

CONCLUSIONS

[11C]BLZ945, a CSF-1R PET tracer, was synthesized in high yield and purity. The tracer has high potency for the target, however, future studies are warranted to address non-specific binding and tracer efflux before BLZ945 or derivatives could be translated into humans for brain imaging.

Alpha-Synuclein PET Tracer Development-An Overview about Current Efforts

Neurodegenerative diseases such as Parkinson’s disease (PD) are manifested by inclusion bodies of alpha-synuclein (α-syn) also called α-synucleinopathies. Detection of these inclusions is thus far only possible by histological examination of postmortem brain tissue. The possibility of non-invasively detecting α-syn will therefore provide valuable insights into the disease progression of α-synucleinopathies. In particular, α-syn imaging can quantify changes in monomeric, oligomeric, and fibrillic α-syn over time and improve early diagnosis of various α-synucleinopathies or monitor treatment progress. Positron emission tomography (PET) is a non-invasive in vivo imaging technique that can quantify target expression and drug occupancies when a suitable tracer exists. As such, novel α-syn PET tracers are highly sought after. The development of an α-syn PET tracer faces several challenges. For example, the low abundance of α-syn within the brain necessitates the development of a high-affinity ligand. Moreover, α-syn depositions are, in contrast to amyloid proteins, predominantly localized intracellularly, limiting their accessibility. Furthermore, another challenge is the ligand selectivity over structurally similar amyloids such as amyloid-beta or tau, which are often co-localized with α-syn pathology. The lack of a defined crystal structure of α-syn has also hindered rational drug and tracer design efforts. Our objective for this review is to provide a comprehensive overview of current efforts in the development of selective α-syn PET tracers.

[11C]CHDI-626, a PET Tracer Candidate for Imaging Mutant Huntingtin Aggregates with Reduced Binding to AD Pathological Proteins

The expanded polyglutamine-containing mutant huntingtin (mHTT) protein is implicated in neuronal degeneration of medium spiny neurons in Huntington's disease (HD) for which multiple therapeutic approaches are currently being evaluated to eliminate or reduce mHTT. Development of effective and orthogonal biomarkers will ensure accurate assessment of the safety and efficacy of pharmacologic interventions. We have identified and optimized a class of ligands that bind to oligomerized/aggregated mHTT, which is a hallmark in the HD postmortem brain. These ligands are potentially useful imaging biomarkers for HD therapeutic development in both preclinical and clinical settings. We describe here the optimization of the benzo[4,5]imidazo[1,2-a]pyrimidine series that show selective binding to mHTT aggregates over Aβ- and/or tau-aggregates associated with Alzheimer's disease pathology. Compound [11C]-2 was selected as a clinical candidate based on its high free fraction in the brain, specific binding in the HD mouse model, and rapid brain uptake/washout in nonhuman primate positron emission tomography imaging studies.

Translation of 11C-labeled tracer synthesis to a CGMP environment as exemplified by [11C]ER176 for PET imaging of human TSPO

Radiotracers labeled with carbon-11 (t_1/2 = 20.4 min) are widely used with positron emission tomography for biomedical research. Radiotracers must be produced for positron emission tomography studies in humans according to prescribed time schedules while also meeting current good manufacturing practice. Translation of an experimental radiosynthesis to a current good manufacturing practice environment is challenging. Here we exemplify such translation with a protocol for the production of an emerging radiotracer for imaging brain translocator protein 18 kDa, namely [¹¹C]ER176. This radiotracer is produced by rapid conversion of cyclotron-produced [¹¹C]carbon dioxide into [¹¹C]iodomethane, which is then used to treat N-desmethyl-ER176 in the presence of base (^tBuOK) at room temperature for 5 min. [¹¹C]ER176 is separated in high purity by reversed-phase HPLC and formulated for intravenous injection in sterile ethanol-saline. The radiosynthesis is reliable and takes 50 min. Quality control takes another 20 min. All aspects of the protocol, including quality control, are discussed.

Understanding the brain uptake and permeability of small molecules through the BBB: A technical overview

The brain is the most important organ in our body requiring its unique microenvironment. By the virtue of its function, the blood-brain barrier poses a significant hurdle in drug delivery for the treatment of neurological diseases. There are also different theories regarding how molecules are typically effluxed from the brain. In this review, we comprehensively discuss how the different pharmacokinetic techniques used for measuring brain uptake/permeability of small molecules have evolved with time. We also discuss the advantages and disadvantages associated with these different techniques as well as the importance to utilize the right method to properly assess CNS exposure to drug molecules. Even though very strong advances have been made we still have a long way to go to ensure a reduction in failures in central nervous system drug development programs.

Positron emission tomographic imaging in drug discovery

Positron emission tomography (PET) is an extensively used nuclear functional imaging technique, especially for central nervous system (CNS) and oncological disorders. Currently, drug development is a lengthy and costly pursuit. Imaging with PET radiotracers could be an effective way to hasten drug discovery and advancement, because it facilitates the monitoring of key facets, such as receptor occupancy quantification, drug biodistribution, pharmacokinetic (PK) analyses, validation of target engagement, treatment monitoring, and measurement of neurotransmitter concentrations. These parameters demand careful analyses for the robust appraisal of newly formulated drugs during preclinical and clinical trials. In this review, we discuss the usage of PET imaging in radiopharmaceutical development; drug development approaches with PET imaging; and PET developments in oncological and cardiac drug discovery.

In vitro and in vivo evaluation of fluorinated indanone derivatives as potential positron emission tomography agents for the imaging of monoamine oxidase B in the brain

Monoamine oxidases (MAOs) play a key role in the metabolism of major monoamine neurotransmitters. In particular, the upregulation of MAO-B in Parkinson's disease, Alzheimer's disease and cancer augmented the development of selective MAO-B inhibitors for diagnostic and therapeutic purposes, such as the anti-parkinsonian MAO-B irreversible binder l-deprenyl (Selegiline®). Herein we report on the synthesis of novel fluorinated indanone derivatives for PET imaging of MAO-B in the brain. Out of our series, the derivatives 6, 8, 9 and 13 are amongst the most affine and selective ligands for MAO-B reported so far. For the derivative 6-((3-fluorobenzyl)oxy)-2,3-dihydro-1H-inden-1-one (6) exhibiting an outstanding affinity (K_iMAO-B = 6 nM), an automated copper-mediated radiofluorination starting from the pinacol boronic ester 17 is described. An in vitro screening in different species revealed a MAO-B region-specific accumulation of [¹⁸F]6 in rats and piglets in comparison to L-[³H]deprenyl. The pre-clinical in vivo assessment of [¹⁸F]6 in mice demonstrated the potential of indanones to readily cross the blood-brain barrier. Nonetheless, parallel in vivo metabolism studies indicated the presence of blood-brain barrier metabolites, thus arguing for further structural modifications. With the matching analytical profiles of the radiometabolite analysis from the in vitro liver microsome studies and the in vivo evaluation, the structure's elucidation of the blood-brain barrier penetrant radiometabolites is possible and will serve as basis for the development of new indanone derivatives suitable for the PET imaging of MAO-B.

Radionuclide Imaging for Neuroscience: Current Opinion and Future Directions

This meeting report summarizes a Consultants Meeting that was held at International Atomic Energy Agency headquarters in Vienna to provide an update on radionuclide imaging for neuroscience applications.

Evaluation of the potassium channel tracer [18F]3F4AP in rhesus macaques

Demyelination causes slowed or failed neuronal conduction and is a driver of disability in multiple sclerosis and other neurological diseases. Currently, the gold standard for imaging demyelination is MRI, but despite its high spatial resolution and sensitivity to demyelinated lesions, it remains challenging to obtain specific and quantitative measures of molecular changes involved in demyelination. To understand the contribution of demyelination in different diseases and to assess the efficacy of myelin-repair therapies, it is critical to develop new in vivo imaging tools sensitive to changes induced by demyelination. Upon demyelination, axonal K⁺ channels, normally located underneath the myelin sheath, become exposed and increase in expression, causing impaired conduction. Here, we investigate the properties of the K⁺ channel PET tracer [¹⁸F]3F4AP in primates and its sensitivity to a focal brain injury that occurred three years prior to imaging. [¹⁸F]3F4AP exhibited favorable properties for brain imaging including high brain penetration, high metabolic stability, high plasma availability, high reproducibility, high specificity, and fast kinetics. [¹⁸F]3F4AP showed preferential binding in areas of low myelin content as well as in the previously injured area. Sensitivity of [¹⁸F]3F4AP for the focal brain injury was higher than [¹⁸F]FDG, [¹¹C]PiB, and [¹¹C]PBR28, and compared favorably to currently used MRI methods.

Synthesis and Characterization of a Positron Emission Tomography Imaging Probe Selectively Targeting the Second Bromodomain of Bromodomain Protein BRD4

Two tandem bromodomains (BD1 and BD2) of bromodomain and extraterminal domain (BET) family proteins have shown distinct roles in mediating gene transcription and expression. Inhibitors that interact with a specific bromodomain may contribute to a specific therapeutic potential with fewer side effects. However, little is known about this disease-related target. Positron emission tomography (PET) imaging could allow us to achieve in-depth knowledge of the BD2 bromodomain. Herein we describe the radiosynthesis and evaluation of [¹¹C]1 as a BRD4 BD2 bromodomain PET imaging radioligand. Our preliminary PET imaging results in rodents demonstrated that [¹¹C]1 had suitable biodistribution in peripheral organs and tissues. Further blocking studies indicated that [¹¹C]1 had good binding specificity toward the BD2 bromodomain. This study may pave the way for the development of a PET radioligand specifically targeting BD1/2 bromodomains as well as for the biological mechanism investigation of BD1/2 bromodomains.

Dual-Phase β-Amyloid PET Captures Neuronal Injury and Amyloidosis in Corticobasal Syndrome

Objectives: In recent years several ¹⁸F-labeled amyloid PET (Aβ-PET) tracers have been developed and have obtained clinical approval. There is evidence that Aβ-PET perfusion can provide surrogate information about neuronal injury in neurodegenerative diseases when compared to conventional blood flow and glucose metabolism assessment. However, this paradigm has not yet been tested in neurodegenerative disorders with cortical and subcortical affection. Therefore, we investigated the performance of early acquisition ¹⁸F-flutemetamol Aβ-PET in comparison to ¹⁸F-fluorodeoxyglucose (FDG)-PET in corticobasal syndrome (CBS). Methods: Subjects with clinically possible or probable CBS were recruited within the prospective Activity of Cerebral Networks, Amyloid and Microglia in Aging and Alzheimer's Disease (ActiGliA) observational study and all CBS cases with an available FDG-PET prior to Aβ-PET were selected. Aβ-PET was acquired 0-10 min p.i. (early-phase) and 90-110 min p.i. (late-phase) whereas FDG-PET was recorded statically from 30 to 50 min p.i. Semiquantitative regional values and asymmetry indices (AI) were compared between early-phase Aβ-PET and FDG-PET. Visual assessments of hypoperfusion and hypometabolism were compared between both methods. Late-phase Aβ-PET was evaluated visually for assessment of Aβ-positivity. Results: Among 20 evaluated patients with CBS, 5 were Aβ-positive. Early-phase Aβ-PET and FDG-PET SUVr correlated highly in cortical (mean R = 0.86, range 0.77-0.92) and subcortical brain regions (mean R = 0.84, range 0.79-0.90). Strong asymmetry was observed in FDG-PET for the motor cortex (mean |AI| = 2.9%), the parietal cortex (mean |AI| = 2.9%), and the thalamus (mean |AI| = 5.5%), correlating well with AI of early-phase Aβ-PET (mean R = 0.87, range 0.62-0.98). Visual assessments of hypoperfusion and hypometabolism were highly congruent. Conclusion: Early-phase Aβ-PET facilitates assessment of neuronal injury in CBS for cortical and subcortical areas. Known asymmetries in CBS are captured by this method, enabling assessment of Aβ-status and neuronal injury with a single radiation exposure at a single visit.

Leveraging Open Science Drug Development for PET: Preliminary Neuroimaging of 11C-Labeled ALK2 Inhibitors

Mutations in the gene encoding activin receptor-like kinase 2 (ALK2) are implicated in the pathophysiology of a pediatric brainstem cancer, diffuse intrinsic pontine glioma (DIPG). Inhibitors of ALK2 that cross the blood–brain barrier have been proposed as a method of treatment for DIPG. As part of an open science approach to radiopharmaceutical and drug discovery, we developed ¹¹C-labeled radiotracers from potent and selective lead ALK2 inhibitors to investigate their brain permeability through positron emission tomography (PET) neuroimaging. Four radiotracers were synthesized by ¹¹C-methylation and assessed by dynamic PET imaging in healthy Sprague–Dawley rats. One of the compounds, [¹¹C]M4K2127, showed high initial brain uptake (SUV ∼ 2), including in the region of interest (pons). This data supports the use of this chemotype as a brain penetrant ALK2 inhibitor that permeates evenly into the pons with potential application for the treatment of DIPG.

Multivalent Probes in Molecular Imaging: Reality or Future?

The rapidly developing field of molecular medical imaging focuses on specific visualization of (patho)physiological processes through the application of imaging agents (IAs) in multiple clinical modalities. Although our understanding of the principles underlying efficient IAs design has increased tremendously, many IAs still show poor in vivo imaging performance because of low binding affinity and/or specificity. These limitations can be addressed by taking advantage of multivalency, in which multiple copies of a ligand are employed to strengthen the interaction. We critically address specific challenges associated with the application of multivalent compounds in molecular imaging, and we give directions for a stepwise approach to the design of multivalent imaging probes to improve their target binding and pharmacokinetics (PK) for improved diagnostic potential.

Species Differences in Microsomal Metabolism of Xanthine-Derived A1 Adenosine Receptor Ligands

Tracer development for positron emission tomography (PET) requires thorough evaluation of pharmacokinetics, metabolism, and dosimetry of candidate radioligands in preclinical animal studies. Since variations in pharmacokinetics and metabolism of a compound occur in different species, careful selection of a suitable model species is mandatory to obtain valid data. This study focuses on species differences in the in vitro metabolism of three xanthine-derived ligands for the A₁ adenosine receptor (A₁AR), which, in their ¹⁸F-labeled form, can be used to image A₁AR via PET. In vitro intrinsic clearance and metabolite profiles of 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX), an established A₁AR-ligand, and two novel analogs, 8-cyclobutyl-3-(3-fluoropropyl)-1-propylxanthine (CBX) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine (MCBX), were determined in liver microsomes from humans and preclinical animal species. Molecular mechanisms leading to significant differences between human and animal metabolite profiles were also examined. The results revealed significant species differences regarding qualitative and quantitative aspects of microsomal metabolism. None of the tested animal species fully matched human microsomal metabolism of the three A₁AR ligands. In conclusion, preclinical evaluation of xanthine-derived A₁AR ligands should employ at least two animal species, preferably rodent and dog, to predict in vivo behavior in humans. Surprisingly, rhesus macaques appear unsuitable due to large differences in metabolic activity towards the test compounds.

Design, Synthesis, and Evaluation of (2-Aminocyclopropyl)phenyl Derivatives as Novel Positron Emission Tomography Imaging Agents for Lysine-Specific Demethylase 1 in the Brain

Dysregulation of histone H3 lysine 4 (H3K4) methylation is implicated in the pathogenesis of neurodevelopmental disorders. Lysine-specific demethylase 1 (LSD1) determines the methylation status of H3K4 through flavin adenine dinucleotide (FAD)-mediated histone demethylation. Therefore, LSD1 inhibition in the brain can be a novel therapeutic option for treating these disorders. Positron emission tomography (PET) imaging of LSD1 allows for investigating LSD1 expression levels under normal and disease conditions and validating target engagement of therapeutic LSD1 inhibitors. This study designed and synthesized (2-aminocyclopropyl)phenyl derivatives with irreversible binding to LSD1 as PET imaging agents for LSD1 in the brain. We optimized lipophilicity of the lead compound to minimize the risk of nonspecific binding and identified 1e with high selectivity over monoamine oxidase A and B, which are a family of FAD-dependent enzymes homologous to LSD1. PET imaging in a monkey showed a high uptake of [¹⁸F]1e to regions enriched with LSD1, indicating its specific binding to LSD1.

GluN2B/N-methyl-D-aspartate Receptor Antagonists: Advances in Design, Synthesis, and Pharmacological Evaluation Studies

Selective GluN2B/N-methyl-D-aspartate receptor (NMDAR) antagonists have exposed their clinical effectiveness in a cluster of neurodegenerative diseases, such as epilepsy, Alzheimer's disease, Parkinson's disease, pain, and depression. Hence, GluN2B/NMDARs are considered to be a prospective target for the management of neurodegenerative diseases. Here, we have discussed the current results and significance of subunit selective GluN2B/NMDAR antagonists to pave the way for the establishment of new, safe, and economical drug candidates in the near future. By using summarized data of selective GluN2B/NMDAR antagonists, medicinal chemists are certainly a step closer to the goal of improving the therapeutic and side effect profile of selective antagonists. Outlined summary of designing strategies, synthetic schemes, and pharmacological evaluation studies reinvigorate efforts to identify, modify, and synthesize novel GluN2B/NMDAR antagonists for treating neurodegenerative diseases.