The Discovery of a Novel Phosphodiesterase (PDE) 4B-Preferring Radioligand for Positron Emission Tomography (PET) Imaging

[11C]ZTP-1: An Effective Short-Lived Radioligand for PET of Rat and Monkey Brain Phosphodiesterase Type 4 Subtype B

Phosphodiesterase type 4 subtype B (PDE4B) selectively hydrolyzes cyclic adenosine monophosphate to enact numerous downstream signaling events. PDE4B is widely expressed in the brain and is implicated in several neuropsychiatric disorders. Moreover, PDE4B inhibition shows antiinflammatory and antidepressant-like effects in animal studies. [18F]PF-06445974 has been developed to image human brain PDE4B using PET, thereby providing a tool for pathophysiologic studies and drug development. However, a radioligand labeled with shorter-lived 11C would be an alternative for studies that require more than 1 administration into the same imaging subject on a single day. Methods: 8-Cyclopropyl-10-(3,5-difluoro-4-(methoxy)phenyl)-7,8-dihydropyrido[2',3':4,5]pyrrolo[1,2-a]pyrazin-9(6H)-1 (ZTP-1) was identified as possessing many favorable properties for development as a 11C-labeled PET radioligand, including high PDE4B inhibitory potency, moderate computed lipophilicity, and a methoxy group as a potential labeling site. Here, [11C]ZTP-1 was readily obtained by 11C methylation of a synthesized O-desmethyl precursor. PET imaging of rat and rhesus monkey brains was performed with [11C]ZTP-1 at baseline and after administration of PDE4B- and PDE4D-selective inhibitors. Radiometabolite profiles for [11C]ZTP-1 were also determined ex vivo in rat plasma and brains. Results: [11C]ZTP-1 was obtained in a high activity yield and with high molar activity. Rat and monkey PET imaging showed high whole-brain radioactivity uptake with subsequent gradual washout. Challenge experiments verified a high and PDE4B-selective PET signal in rat and monkey brains. Ex vivo rat brain uptake of [11C]ZTP-1 showed less than 1% radiometabolite contamination at 30 min. Total distribution volume measures in monkey brains quickly reached stability. Conclusion: [11C]ZTP-1 is a promising, shorter-lived alternative to [18F]PF-06445974 for quantifying brain PDE4B in rodents and nonhuman primates with PET and warrants further investigation in humans.

Dialkyloxyphenyl hybrids as PDE4B inhibitors: Design, synthesis, in vitro/in vivo anti-inflammatory activity and in silico insights

A series of novel dialkyloxyphenyl hybrids 11a-11h and 12a-12c were designed and synthesized as PDE4 inhibitors with anti-inflammatory activity. All compounds demonstrated nanomolar-range inhibitory activity against both PDE4B and PDE4D isoforms with notable selectivity for PDE4B. The 3,4-dimethoxyphenyl derivative 11e exhibited superior PDE4B inhibitory activity (IC50 = 2.82 nM), with nine-fold selectivity compared to 1.5 of Rolipram. In TNF-α inhibition assays, 11e demonstrated remarkable potency (IC50 = 7.20 nM), comparable to roflumilast, followed by 11d (IC50 = 15.54 nM) and 11b (IC50 = 28.52 nM). In vivo evaluation using LPS-induced sepsis model revealed that 11e achieved the highest inhibition of both TNF-α (52.19 %) and neutrophilia (56.47 %) compared to reference compounds. Molecular docking and dynamics studies revealed that hybrids 11b, 11d, and 11e exhibit a characteristic binding mode within the PDE4 active sites, rationalizing their activity through specific interactions, and demonstrating higher stability in the active site compared to Roflumilast.

Preclinical Evaluation of [ 18 F]P4B-2412 as Phosphodiesterase 4B Radioligand for Positron Emission Tomography Imaging

Phosphodiesterase 4B (PDE4B) plays a critical role in cAMP hydrolysis and is highly expressed in brain regions associated with neuroinflammation and central nervous system (CNS) disorders. Selective PDE4B radioligands hold significant potential for elucidating disease mechanisms, such as those in Parkinson's disease and schizophrenia, and enabling target occupancy measurements. In this study, we developed [ 18 F]P4B-2412, a novel PDE4B-selective radioligand, and evaluated its utility for positron emission tomography imaging (PET). [ 18 F]P4B-2412 was synthesized in high radiochemical yield (27.2%), excellent radiochemical purity (99%), and favorable molar activity (66.2 ± 2.5 GBq/μmol. In vitro autoradiography and dynamic PET imaging demonstrated high specificity for PDE4B in rodent brain regions, with blocking studies confirming negligible interaction with PDE4D. [ 18 F]P4B-2412 also exhibited robust in vitro and in vivo metabolic stability. These results establish [ 18 F]P4B-2412 as a promising PET imaging agent for visualizing PDE4B activity, offering a valuable tool for investigating neuroinflammation and advancing CNS drug development.

PDE4D inhibitors: Opening a new era of PET diagnostics for Alzheimer's disease

As the incidence of Alzheimer's disease (AD) continues to rise, the need for an effective PET radiotracer to facilitate early diagnosis has become more pressing than ever before in modern medicine. Phosphodiesterase (PDE) is closely related to cognitive impairment and neuroinflammatory processes in AD. Current research progress shows that specific PDE4D inhibitors radioligands can bind specifically to the PDE4D enzyme in the brain, thereby showing pathology-related signal enhancement in AD animal models, indicating the potential of these ligands as effective radiotracers. At the same time, we need to pay attention to the important role computer aided drug design (CADD) plays in advancing AD drug design and PET imaging. Future research will verify the potential of these ligands in clinical applications through computer simulation techniques, providing patients with timely intervention and treatment, which is of great significance.

PET imaging in rat brain shows opposite effects of acute and chronic alcohol exposure on phosphodiesterase-4B, an indirect biomarker of cAMP activity

The cyclic adenosine monophosphate (cAMP) cascade is thought to play an important role in regulating alcohol-dependent behaviors, with potentially opposite effects following acute versus chronic administration. Phosphodiesterase 4 (PDE4) is the primary brain enzyme that metabolizes cAMP, thereby terminating its signal. Radioligand binding to PDE4 serves as an indirect biomarker of cAMP activity, as cAMP-protein kinase A (PKA)-mediated phosphorylation of PDE4 increases its affinity for radioligand binding ~10-fold. Of the four PDE4 subtypes, PDE4B polymorphisms are known to be strongly associated with alcohol and substance use disorders. This study imaged rats with the PDE4B-preferring positron emission tomography (PET) radioligand [18F]PF-06445974 following acute and chronic ethanol administration, aiming to explore the potential of PDE4B PET imaging for future human studies. Compared to the control group treated with saline, acute alcohol administration (i.p. ethanol 0.5 g/kg) significantly increased whole brain uptake of [18F]PF-06445974 as early as 30 minutes post-exposure. This effect persisted at 2 hours, peaked at 4 hours, and diminished at 6 hours and 24 hours post-exposure. In contrast, in a rat model of alcohol dependence, [18F]PF-06445974 brain uptake was significantly reduced at 5 hours post-exposure and was normalized by 3 days. This reduction may reflect long-term adaptation to repeated alcohol-induced activation of cAMP signaling with chronic exposure. Taken together, the results suggest that PET imaging of PDE4B in individuals with alcohol use disorder (AUD) should be considered in conjunction with ongoing trials of PDE4 inhibitors to treat alcohol withdrawal and reduce alcohol consumption.

Protective effect of PDE4B subtype-specific inhibition in an App knock-in mouse model for Alzheimer's disease

Meta-analysis of genome-wide association study data has implicated PDE4B in the pathogenesis of Alzheimer's disease (AD), the leading cause of senile dementia. PDE4B encodes one of four subtypes of cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase-4 (PDE4A-D). To interrogate the involvement of PDE4B in the manifestation of AD-related phenotypes, the effects of a hypomorphic mutation (Pde4bY358C) that decreases PDE4B's cAMP hydrolytic activity were evaluated in the AppNL-G-F knock-in mouse model of AD using the Barnes maze test of spatial memory, 14C-2-deoxyglucose autoradiography, thioflavin-S staining of β-amyloid (Aβ) plaques, and inflammatory marker assay and transcriptomic analysis (RNA sequencing) of cerebral cortical tissue. At 12 months of age, AppNL-G-F mice exhibited spatial memory and brain metabolism deficits, which were prevented by the hypomorphic PDE4B in AppNL-G-F/Pde4bY358C mice, without a decrease in Aβ plaque burden. RNA sequencing revealed that, among the 531 transcripts differentially expressed in AppNL-G-F versus wild-type mice, only 13 transcripts from four genes - Ide, Btaf1, Padi2, and C1qb - were differentially expressed in AppNL-G-F/Pde4bY358C versus AppNL-G-F mice, identifying their potential involvement in the protective effect of hypomorphic PDE4B. Our data demonstrate that spatial memory and cerebral glucose metabolism deficits exhibited by 12-month-old AppNL-G-F mice are prevented by targeted inhibition of PDE4B. To our knowledge, this is the first demonstration of a protective effect of PDE4B subtype-specific inhibition in a preclinical model of AD. It thus identifies PDE4B as a key regulator of disease manifestation in the AppNL-G-F model and a promising therapeutic target for AD.

The Bifunctional Dimer Caffeine-Indan Attenuates α-Synuclein Misfolding, Neurodegeneration and Behavioral Deficits after Chronic Stimulation of Adenosine A1 Receptors

We previously found that chronic adenosine A1 receptor stimulation with N6-Cyclopentyladenosine increased α-synuclein misfolding and neurodegeneration in a novel α-synucleinopathy model, a hallmark of Parkinson's disease. Here, we aimed to synthesize a dimer caffeine-indan linked by a 6-carbon chain to cross the blood-brain barrier and tested its ability to bind α-synuclein, reducing misfolding, behavioral abnormalities, and neurodegeneration in our rodent model. Behavioral tests and histological stains assessed neuroprotective effects of the dimer compound. A rapid synthesis of the 18F-labeled analogue enabled Positron Emission Tomography and Computed Tomography imaging for biodistribution measurement. Molecular docking analysis showed that the dimer binds to α-synuclein N- and C-termini and the non-amyloid-β-component (NAC) domain, similar to 1-aminoindan, and this binding promotes a neuroprotective α-synuclein "loop" conformation. The dimer also binds to the orthosteric binding site for adenosine within the adenosine A1 receptor. Immunohistochemistry and confocal imaging showed the dimer abolished α-synuclein upregulation and aggregation in the substantia nigra and hippocampus, and the dimer mitigated cognitive deficits, anxiety, despair, and motor abnormalities. The 18F-labeled dimer remained stable post-injection and distributed in various organs, notably in the brain, suggesting its potential as a Positron Emission Tomography tracer for α-synuclein and adenosine A1 receptor in Parkinson's disease therapy.

Developing Low Molecular Weight PET and SPECT Imaging Agents

Imaging agents for positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) have shown their utility in many situations, answering clinical questions related to drug development and medical considerations. The discovery and development of imaging agents follow a well-understood process, with variations related to available starting points and to the envisaged imaging application. This article describes the general development path leading from the expression of an imaging need and project initiation to a clinically usable imaging agent. The definition of the project rationale, the design and optimization of early leads, and the assessment of the imaging potential of an imaging agent candidate are followed by preclinical and clinical development activities that differ from those required for therapeutic agents. These include radiolabeling with a positron emitter and first-in-human clinical studies, to rapidly evaluate the ability of a new imaging agent to address the questions it was designed to answer.

Robust Quantification of Phosphodiesterase-4D in Monkey Brain with PET and 11C-Labeled Radioligands That Avoid Radiometabolite Contamination

Phosphodiesterase-4D (PDE4D) has emerged as a significant target for treating neuropsychiatric disorders, but no PET radioligand currently exists for robustly quantifying human brain PDE4D to assist biomedical research and drug discovery. A prior candidate PDE4D PET radioligand, namely [11C]T1650, failed in humans because of poor time stability of brain PDE4D-specific signal (indexed by total volume of distribution), likely due to radiometabolites accumulating in brain. Its nitro group was considered to be a source of the brain radiometabolites. Methods: We selected 5 high-affinity and selective PDE4D inhibitors, absent of a nitro group, from our prior structure-activity relationship study for evaluation as PET radioligands. Results: All 5 radioligands were labeled with 11C (half-time, 20.4 min) in useful yields and with high molar activity. All displayed sizable PDE4D-specific signals in rhesus monkey brain. Notably, [11C]JMJ-81 and [11C]JMJ-129 exhibited excellent time stability of signal (total volume of distribution). Furthermore, as an example, [11C]JMJ-81 was found to be free of radiometabolites in ex vivo monkey brain, affirming that this radioligand can provide robust quantification of brain PDE4D with PET. Conclusion: Given their high similarity in structures and metabolic profiles, both [11C]JMJ-81 and [11C]JMJ-129 warrant further evaluation in human subjects. [11C]JMJ-129 shows a higher PDE4D specific-to-nonspecific binding ratio and will be the first to be evaluated.

Preclinical Evaluation of Novel Positron Emission Tomography (PET) Probes for Imaging Leucine-Rich Repeat Kinase 2 (LRRK2)

Parkinson's disease (PD) is one of the most highly debilitating neurodegenerative disorders, which affects millions of people worldwide, and leucine-rich repeat kinase 2 (LRRK2) mutations have been involved in the pathogenesis of PD. Developing a potent LRRK2 positron emission tomography (PET) tracer would allow for in vivo visualization of LRRK2 distribution and expression in PD patients. In this work, we present the facile synthesis of two potent and selective LRRK2 radioligands [11C]3 ([11C]PF-06447475) and [18F]4 ([18F]PF-06455943). Both radioligands exhibited favorable brain uptake and specific bindings in rodent autoradiography and PET imaging studies. More importantly, [18F]4 demonstrated significantly higher brain uptake in the transgenic LRRK2-G2019S mutant and lipopolysaccharide (LPS)-injected mouse models. This work may serve as a roadmap for the future design of potent LRRK2 PET tracers.

Selective PDE4B and PDE4D inhibitors produce distinct behavioral responses to ethanol and GABAergic drugs in mice

Apremilast is a phosphodiesterase (PDE) type 4 inhibitor that is nonselective at subtypes PDE4A-D. It modulates ethanol and GABAergic responses via protein kinase A (PKA) phosphorylation of specific GABAA receptor subunits and has opposite effects on ethanol-induced ataxia in wild-type and GABAA β3-S408/409A knock-in mice. We hypothesized that these different effects are due to preferential actions at different PDE4 subtypes. To test this hypothesis, we compared effects of selective PDE4 inhibitors on responses to ethanol and GABAergic drugs in male and female C57BL/6J mice. The PDE4B inhibitor A33 accelerated recovery from ataxia induced by ethanol and diazepam but did not alter ataxia induced by propofol. The PDE4D inhibitor D159687 accelerated recovery from diazepam-induced ataxia but prolonged recovery from ethanol- and propofol-induced ataxia. A33 shortened, while D159687 prolonged, the sedative-hypnotic effects of ethanol. Both drugs shortened diazepam's sedative-hypnotic effects. The modulatory effects of A33 and D159687 were completely prevented by the PKA inhibitor H89. Only D159687 prevented development of acute functional tolerance to ethanol-induced ataxia. D159687 transiently reduced two-bottle choice drinking in male and female mice that had consumed ethanol for 3 weeks and transiently reduced two-bottle choice, every-other-day drinking in male mice. A33 did not alter ethanol drinking in either procedure. Neither drug altered binge-like ethanol consumption or blood ethanol clearance. Thus, D159687 produced behavioral effects similar to apremilast, although it produced a more transient and smaller reduction in drinking. These results indicate that PDE4D inhibition contributes to apremilast's ability to reduce ethanol drinking, whereas PDE4B inhibition is not involved.

Computational Chemistry for the Identification of Lead Compounds for Radiotracer Development

The use of computer-aided drug design (CADD) for the identification of lead compounds in radiotracer development is steadily increasing. Traditional CADD methods, such as structure-based and ligand-based virtual screening and optimization, have been successfully utilized in many drug discovery programs and are highlighted throughout this review. First, we discuss the use of virtual screening for hit identification at the beginning of drug discovery programs. This is followed by an analysis of how the hits derived from virtual screening can be filtered and culled to highly probable candidates to test in in vitro assays. We then illustrate how CADD can be used to optimize the potency of experimentally validated hit compounds from virtual screening for use in positron emission tomography (PET). Finally, we conclude with a survey of the newest techniques in CADD employing machine learning (ML).

Imaging Leucine-Rich Repeat Kinase 2 In Vivo with 18F-Labeled Positron Emission Tomography Ligand

Leucine-rich repeat kinase 2 (LRRK2) has been demonstrated to be closely involved in the pathogenesis of Parkinson's disease (PD), and pharmacological blockade of LRRK2 represents a new opportunity for therapeutical treatment of PD and other related neurodegenerative conditions. The development of an LRRK2-specific positron emission tomography (PET) ligand would enable a target occupancy study in vivo and greatly facilitate LRRK2 drug discovery and clinical translation as well as provide a molecular imaging tool for studying physiopathological changes in neurodegenerative diseases. In this work, we present the design and development of compound 8 (PF-06455943) as a promising PET radioligand through a PET-specific structure-activity relationship optimization, followed by comprehensive pharmacology and ADME/neuroPK characterization. Following an efficient 18F-labeling method, we have confirmed high brain penetration of [18F]8 in nonhuman primates (NHPs) and validated its specific binding in vitro by autoradiography in postmortem NHP brain tissues and in vivo by PET imaging studies.

An Empirical Quantitative Structure-Activity Relationship Equation Assists the Discovery of High-Affinity Phosphodiesterase 4D Inhibitors as Leads to PET Radioligands

A positron emission tomography (PET) radioligand for imaging phosphodiesterase 4D (PDE4D) would benefit drug discovery and the investigation of neuropsychiatric disorders. The most promising radioligand to date, namely, [11C]T1650, has shown unstable quantification in humans. Structural elaboration of [11C]T1650 was therefore deemed necessary. High target affinity in the low nM range is usually required for successful PET radioligands. In our PDE4D PET radioligand development, we formulated and optimized an empirical equation (log[IC50 (nM)] = P1 + P2 + P3 + P4) that well described the relationship between binding affinity and empirically derived values (P1-P4) for the individual fragments in four subregions commonly composing each inhibitor (R2 = 0.988, n = 62). This equation was used to predict compounds that would have high inhibitory potency. Fourteen new compounds were obtained with IC50 of 0.3-10 nM. Finally, eight compounds were judged to be worthy of future radiolabeling and evaluation as PDE4D PET radioligands.

Apremilast-induced increases in acute ethanol intoxication and decreases in ethanol drinking in mice involve PKA phosphorylation of GABAA β3 subunits

We previously showed that apremilast, an FDA-approved PDE4 inhibitor, selectively alters behavioral responses to ethanol and certain GABAergic drugs in a PKA-dependent manner in C57BL6/J mice. Here, we investigated if PKA phosphorylation of β3 GABAA receptor subunits is involved in apremilast regulation of ethanol, propofol, or diazepam responses. Apremilast prolonged rotarod ataxia and loss of the righting reflex by ethanol and propofol in wild-type mice, but not in β3-S408A/S409A knock-in mice. In contrast, apremilast hastened recovery from the ataxic and sedative effects of diazepam in both genotypes. These findings suggest that apremilast modulation of ethanol and propofol behaviors in wild-type mice is mediated by β3 subunit phosphorylation, whereas its actions on diazepam responses involve a different mechanism. The PKA inhibitor H-89 prevented apremilast modulation of ethanol-induced ataxia. Apremilast sensitized wild-type males to ethanol-induced ataxia and decreased acute functional tolerance (AFT) in females but had no effect in β3-S408A/S409A mice of either sex. These results could not be attributed to genotype differences in blood ethanol clearance. There were also no baseline genotype differences in ethanol consumption and preference in two different voluntary drinking procedures. However, the ability of apremilast to reduce ethanol consumption was diminished in β3-S408A/S409A mice. Our results provide strong evidence that PKA-dependent phosphorylation of β3 GABAA receptor subunits is an important mechanism by which apremilast increases acute sensitivity to alcohol, decreases AFT, and decreases ethanol drinking.

Synthesis and Evaluation of 18F-Labeled Chalcone Analogue for Detection of α-Synuclein Aggregates in the Brain Using the Mouse Model

In the brains of patients with synucleinopathies such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, α-synuclein (α-syn) aggregates deposit abnormally to induce neurodegeneration, although the mechanism is unclear. Thus, in vivo imaging studies targeting α-syn aggregates have attracted much attention to guide medical intervention against synucleinopathy. In our previous study, a chalcone analogue, [¹²⁵I]PHNP-3, functioned as a feasible probe in terms of α-syn binding in vitro; however, it did not migrate to the mouse brain, and further improvement of brain uptake was required. In the present study, we designed and synthesized two novel ¹⁸F-labeled chalcone analogues, [¹⁸F]FHCL-1 and [¹⁸F]FHCL-2, using a central nervous system multiparameter optimization (CNS MPO) algorithm with the aim of improving blood-brain barrier permeation in the mouse brain. Then, we evaluated their utility for in vivo imaging of α-syn aggregates using a mouse model. In the competitive inhibition assay, both chalcone analogues exhibited high binding affinity for α-syn aggregates (K_i = 2.6 and 3.4 nM, respectively), while no marked amyloid β (Aβ)-binding was observed. The ¹⁸F-labeling reaction was successfully performed. In a biodistribution experiment, brain uptake of both chalcone analogues in normal mice (2.09 and 2.40% injected dose/gram (% ID/g) at 2 min postinjection, respectively) was higher than that of [¹²⁵I]PHNP-3, suggesting that the introduction of ¹⁸F into the chalcone analogue led to an improvement in brain uptake in mice while maintaining favorable binding ability for α-syn aggregates. Furthermore, in an ex vivo autoradiography experiment, [¹⁸F]FHCL-2 showed the feasibility of the detection of α-syn aggregates in the mouse brain in vivo. These preclinical studies demonstrated the validity of the design of α-syn-targeting probes based on the CNS MPO score and the possibility of in vivo imaging of α-syn aggregates in a mouse model using ¹⁸F-labeled chalcone analogues.

Application of positron emission tomography in psychiatry-methodological developments and future directions

Mental disorders represent an increasing source of disability and high costs for societies globally. Molecular imaging techniques such as positron emission tomography (PET) represent powerful tools with the potential to advance knowledge regarding disease mechanisms, allowing the development of new treatment approaches. Thus far, most PET research on pathophysiology in psychiatric disorders has focused on the monoaminergic neurotransmission systems, and although a series of discoveries have been made, the results have not led to any material changes in clinical practice. We outline areas of methodological development that can address some of the important obstacles to fruitful progress. First, we point towards new radioligands and targets that can lead to the identification of processes upstream, or parallel to disturbances in monoaminergic systems. Second, we describe the development of new methods of PET data quantification and PET systems that may facilitate research in psychiatric populations. Third, we review the application of multimodal imaging that can link molecular imaging data to other aspects of brain function, thus deepening our understanding of disease processes. Fourth, we highlight the need to develop imaging study protocols to include longitudinal and interventional paradigms, as well as frameworks to assess dimensional symptoms such that the field can move beyond cross-sectional studies within current diagnostic boundaries. Particular effort should be paid to include also the most severely ill patients. Finally, we discuss the importance of harmonizing data collection and promoting data sharing to reach the desired sample sizes needed to fully capture the phenotype of psychiatric conditions.

Imaging Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation plays a major role in the etiopathology of neurodegenerative diseases, including Alzheimer and Parkinson diseases, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. In vivo monitoring of neuroinflammation using PET is critical to understand this process, and data are accumulating in this regard, thus a review is useful. From PubMed, we retrieved publications using any of the available PET tracers to image neuroinflammation in humans as well as selected articles dealing with experimental animal models or the chemistry of currently used or potential radiotracers. We reviewed 280 articles. The most common PET neuroinflammation target, translocator protein (TSPO), has limitations, lacking cellular specificity and the ability to separate neuroprotective from neurotoxic inflammation. However, TSPO PET is useful to define the amount and location of inflammation in the brain of people with neurodegenerative disorders. We describe the characteristics of TSPO and other potential PET neuroinflammation targets and PET tracers available or in development. Despite target and tracer limitations, in recent years there has been a sharp increase in the number of reports of neuroinflammation PET in humans. The most studied has been Alzheimer disease, in which neuroinflammation seems initially neuroprotective and neurotoxic later in the progression of the disease. We describe the findings in all the major neurodegenerative disorders. Neuroinflammation PET is an indispensable tool to understand the process of neurodegeneration, particularly in humans, as well as to validate target engagement in therapeutic clinical trials.

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.

The Repertoire of Small-Molecule PET Probes for Neuroinflammation Imaging: Challenges and Opportunities beyond TSPO

Neuroinflammation is an adaptive response of the central nervous system to diverse potentially injurious stimuli, which is closely associated with neurodegeneration and typically characterized by activation of microglia and astrocytes. As a noninvasive and translational molecular imaging tool, positron emission tomography (PET) could provide a better understanding of neuroinflammation and its role in neurodegenerative diseases. Ligands to translator protein (TSPO), a putative marker of neuroinflammation, have been the most commonly studied in this context, but they suffer from serious limitations. Herein we present a repertoire of different structural chemotypes and novel PET ligand design for classical and emerging neuroinflammatory targets beyond TSPO. We believe that this Perspective will support multidisciplinary collaborations in academic and industrial institutions working on neuroinflammation and facilitate the progress of neuroinflammation PET probe development for clinical use.

Artificial Intelligence and the Future of Diagnostic and Therapeutic Radiopharmaceutical Development:: In Silico Smart Molecular Design

Novel diagnostic and therapeutic radiopharmaceuticals are increasingly becoming a central part of personalized medicine. Continued innovation in the development of new radiopharmaceuticals is key to sustained growth and advancement of precision medicine. Artificial intelligence has been used in multiple fields of medicine to develop and validate better tools for patient diagnosis and therapy, including in radiopharmaceutical design. In this review, we first discuss common in silico approaches and focus on their usefulness and challenges in radiopharmaceutical development. Next, we discuss the practical applications of in silico modeling in design of radiopharmaceuticals in various diseases.

Advances in Cyclic Nucleotide Phosphodiesterase-Targeted PET Imaging and Drug Discovery

Cyclic nucleotide phosphodiesterases (PDEs) control the intracellular concentrations of cAMP and cGMP in virtually all mammalian cells. Accordingly, the PDE family regulates a myriad of physiological functions, including cell proliferation, differentiation and apoptosis, gene expression, central nervous system function, and muscle contraction. Along this line, dysfunction of PDEs has been implicated in neurodegenerative disorders, coronary artery diseases, chronic obstructive pulmonary disease, and cancer development. To date, 11 PDE families have been identified; however, their distinct roles in the various pathologies are largely unexplored and subject to contemporary research efforts. Indeed, there is growing interest for the development of isoform-selective PDE inhibitors as potential therapeutic agents. Similarly, the evolving knowledge on the various PDE isoforms has channeled the identification of new PET probes, allowing isoform-selective imaging. This review highlights recent advances in PDE-targeted PET tracer development, thereby focusing on efforts to assess disease-related PDE pathophysiology and to support isoform-selective drug discovery.

Challenges on Cyclic Nucleotide Phosphodiesterases Imaging with Positron Emission Tomography: Novel Radioligands and (Pre-)Clinical Insights since 2016

Cyclic nucleotide phosphodiesterases (PDEs) represent one of the key targets in the research field of intracellular signaling related to the second messenger molecules cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP). Hence, non-invasive imaging of this enzyme class by positron emission tomography (PET) using appropriate isoform-selective PDE radioligands is gaining importance. This methodology enables the in vivo diagnosis and staging of numerous diseases associated with altered PDE density or activity in the periphery and the central nervous system as well as the translational evaluation of novel PDE inhibitors as therapeutics. In this follow-up review, we summarize the efforts in the development of novel PDE radioligands and highlight (pre-)clinical insights from PET studies using already known PDE radioligands since 2016.

Identification and Development of an Irreversible Monoacylglycerol Lipase (MAGL) Positron Emission Tomography (PET) Radioligand with High Specificity

Monoacylglycerol lipase (MAGL), a serine hydrolase extensively expressed throughout the brain, serves as a key gatekeeper regulating the tone of endocannabinoid signaling. Preclinically, inhibition of MAGL is known to provide therapeutic benefits for a number of neurological disorders. The availability of a MAGL-specific positron emission tomography (PET) ligand would considerably facilitate the development and clinical characterization of MAGL inhibitors via noninvasive and quantitative PET imaging. Herein, we report the identification of the potent and selective irreversible MAGL inhibitor 7 (PF-06809247) as a suitable radioligand lead, which upon radiolabeling was found to exhibit a high level of MAGL specificity; this enabled cross-species measurement of MAGL brain expression (B_max), assessment of in vivo binding in the rat, and nonhuman primate PET imaging.