Development of a novel fluorine-18 labeled deuterated fluororasagiline ([18F]fluororasagiline-D2) radioligand for PET studies of monoamino oxidase B (MAO-B)

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

Toward a Nanoencapsulated EPR Imaging Agent for Clinical Use

PURPOSE

Progress toward developing a novel radiocontrast agent for determining pO2 in tumors in a clinical setting is described. The imaging agent is designed for use with electron paramagnetic resonance imaging (EPRI), in which the collision of a paramagnetic probe molecule with molecular oxygen causes a spectroscopic change which can be calibrated to give the real oxygen concentration in the tumor tissue.

PROCEDURES

The imaging agent is based on a nanoscaffold of aluminum hydroxide (boehmite) with sizes from 100 to 200 nm, paramagnetic probe molecule, and encapsulation with a gas permeable, thin (10-20 nm) polymer layer to separate the imaging agent and body environment while still allowing O2 to interact with the paramagnetic probe. A specially designed deuterated Finland trityl (dFT) is covalently attached on the surface of the nanoparticle through 1,3-dipolar addition of the alkyne on the dFT with an azide on the surface of the nanoscaffold. This click-chemistry reaction affords 100% efficiency of the trityl attachment as followed by the complete disappearance of the azide peak in the infrared spectrum. The fully encapsulated, dFT-functionalized nanoparticle is referred to as RADI-Sense.

RESULTS

Side-by-side in vivo imaging comparisons made in a mouse model made between RADI-Sense and free paramagnetic probe (OX-071) showed oxygen sensitivity is retained and RADI-Sense can create 3D pO2 maps of solid tumors CONCLUSIONS: A novel encapsulated nanoparticle EPR imaging agent has been described which could be used in the future to bring EPR imaging for guidance of radiotherapy into clinical reality.

Electrochemical C-H deuteration of pyridine derivatives with D2O

Herein, we develop a straightforward, metal-free, and acid-/base-free electrochemical C4-selective C - H deuteration of pyridine derivatives with economic and convenient D2O at room temperature. This strategy features an efficient and environmentally friendly approach with high chemo- and regioselectivity, affording a wide range of D-compounds, such as pyridines, quinolones, N-ligands and biorelevant compounds. Notably, the mechanistic experiments and cyclic voltammetry (CV) studies demonstrate that N-butyl-2-phenylpyridinium iodide is a crucial intermediate during the electrochemical transformation, which provides a general and efficient way for deuteration of pyridine derivatives.

Deuterated driven new chemical entities: An optimistic way to improve therapeutic efficacy

In organic chemistry, the use of deuterium exchange as a tool to study the mechanism of chemical reaction has been well explored. Since two decades, the research focus on deuterated bioactive molecules has been gaining attention for investigating the therapeutic potential of deuterium replacement in a chemical structure. Recently, Food Drug Administration (FDA) approved the first deuterium-labeled drug "deutetrabenazine", and notified the deuterated drugs as new chemical entities (NCEs). Henceforth, the deuterium substitution driven structure activity relationship, preclinical pharmacokinetics, and toxicity studies were much initiated. Deuteration of a bioactive molecule often results in improved therapeutic efficacy due to the altered pharmacokinetic profile. This review provides a conceptual framework on the importance of deuterium atom in chemical structure of a drug, and its biological value in improved physiochemical properties, pharmacokinetics, biological target interaction, diagnosis, and toxicity. In addition, this review concisely updated the recent deuteration methods, chemical stability, challenges in drug development, deuterium-based imaging in diagnosis, and selected synthetic scheme of deuterated molecules.

Imaging of Reactive Astrogliosis by Positron Emission Tomography

Many neurodegenerative diseases are neuropathologically characterized by neuronal loss, gliosis, and the deposition of misfolded proteins such as β-amyloid (Aβ) plaques and tau tangles in Alzheimer’s disease (AD). In postmortem AD brains, reactive astrocytes and activated microglia are observed surrounding Aβ plaques and tau tangles. These activated glial cells secrete pro-inflammatory cytokines and reactive oxygen species, which may contribute to neurodegeneration. Therefore, in vivo imaging of glial response by positron emission tomography (PET) combined with Aβ and tau PET would provide new insights to better understand the disease process, as well as aid in the differential diagnosis, and monitoring glial response disease-specific therapeutics. There are two promising targets proposed for imaging reactive astrogliosis: monoamine oxidase-B (MAO-B) and imidazoline2 binding site (I2BS), which are predominantly expressed in the mitochondrial membranes of astrocytes and are upregulated in various neurodegenerative conditions. PET tracers targeting these two MAO-B and I2BS have been evaluated in humans. [18F]THK-5351, which was originally designed to target tau aggregates in AD, showed high affinity for MAO-B and clearly visualized reactive astrocytes in progressive supranuclear palsy (PSP). However, the lack of selectivity of [18F]THK-5351 binding to both MAO-B and tau, severely limits its clinical utility as a biomarker. Recently, [18F]SMBT-1 was developed as a selective and reversible MAO-B PET tracer via compound optimization of [18F]THK-5351. In this review, we summarize the strategy underlying molecular imaging of reactive astrogliosis and clinical studies using MAO-B and I2BS PET tracers.

Platinum on Carbon-Catalysed Site-Selective H-D Exchange Reaction of Allylic Alcohols Using Alkyl Amines as a Hydrogen Source

We developed platinum on carbon-catalysed deuteration reaction of tert-allylic alcohols using deuterium oxide as a deuterium source. Amylamine was dehydrogenated by platinum on carbon to generate an appropriate amount of...

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.

The Pharmaceutical Technology Approach on Imaging Innovations from Italian Research

Many modern therapeutic approaches are based on precise diagnostic evidence, where imaging procedures play an essential role. To date, in the diagnostic field, a plethora of agents have been investigated to increase the selectivity and sensitivity of diagnosis. However, the most common drawbacks of conventional imaging agents reside in their non-specificity, short imaging time, instability, and toxicity. Moreover, routinely used diagnostic agents have low molecular weights and consequently a rapid clearance and renal excretion, and this represents a limitation if long-lasting imaging analyses are to be conducted. Thus, the development of new agents for in vivo diagnostics requires not only a deep knowledge of the physical principles of the imaging techniques and of the physiopathological aspects of the disease but also of the relative pharmaceutical and biopharmaceutical requirements. In this scenario, skills in pharmaceutical technology have become highly indispensable in order to respond to these needs. This review specifically aims to collect examples of newly developed diagnostic agents connoting the importance of an appropriate formulation study for the realization of effective products. Within the context of pharmaceutical technology research in Italy, several groups have developed and patented promising agents for fluorescence and radioactive imaging, the most relevant of which are described hereafter.

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.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

Preclinical evaluation of [18F]D3FSP, deuterated AV-45, for imaging of β-amyloid in the brain

INTRODUCTION

Since the approval of three 18F labeled β-amyloid-targeting PET imaging agents, Amyvid (florbetapir f18, AV-45), Neuraceq (florbetaben f18, AV-1) and Vizamyl (flutemetamol f18, F-PIB), they have increasingly been employed to assist differential diagnosis of Alzheimer's disease in patients with dementia. Also, they are frequently used in selecting patients participating drug trials aiming to reduce β-amyloid (Aβ) plaques in the brain. The first approved tracer in this class was [18F]AV-45, which is metabolized rapidly in blood and some of its N-demethylated metabolites cross the blood brain barrier and resulted in lowering the image contrast. To improve metabolic stability of [18F]AV-45, we hypothesized that substituting N-CH3 with N-CD3 at the metabolically labile position, creating [18F]D3FSP, may reduce in vivo N-demethylation. We report the preclinical evaluation of [18F]D3FSP as an Aβ imaging agent.

METHODS

Preclinical pharmacology of [18F]D3FSP was evaluated using in vitro autoradiography and competitive binding assay. Biodistribution of [18F]D3FSP was evaluated in wild-type CD-1 mice. In vivo metabolism in mice and in vitro microsomal metabolism were analyzed by HPLC. Single dose acute toxicity of D3FSP was also performed in rats.

RESULTS

[18F]D3FSP showed high binding affinity to β-amyloid plaques (Ki = 3.44 ± 1.22 nM, a value similar as AV-45 (Ki = 4.02 ± 0.22 nM)), and displayed excellent β-amyloid binding in AD brain sections consistent with known Aβ regional distribution. After an iv injection it exhibited good initial brain uptake and fast washout in wild-type CD-1 mice. In vitro microsomal metabolism and in vivo metabolism in mice did not result in any significant differences between [18F]D3FSP and [18F]AV-45. No treatment-related mortality or any adverse effects were observed in single dose acute toxicity studies administered at hundred-folds of maximum human dose.

CONCLUSION

A new small molecule, [18F]D3FSP, was prepared and tested as an alternative to [18F]AV-45 to reduce N-demethylation in vivo. This strategy did not lead to better in vivo stability. However, [18F]D3FSP displayed very similar Aβ targeting property comparable to [18F]AV-45. Preclinical studies suggest that [18F]D3FSP is useful as a β-amyloid-targeting PET imaging agent.

18F-SMBT-1: A Selective and Reversible PET Tracer for Monoamine Oxidase-B Imaging

Reactive astrocytes play a key role in the pathogenesis of various neurodegenerative diseases. Monoamine oxidase-B (MAO-B) is one of the promising targets for the imaging of astrogliosis in the human brain. A novel selective and reversible MAO-B tracer, (S)-(2-methylpyrid-5-yl)-6-[(3-¹⁸F-fluoro-2-hydroxy)propoxy]quinoline (¹⁸F-SMBT-1), was successfully developed via lead optimization from the first-generation tau PET tracer ¹⁸F-THK-5351. Methods: SMBT-1 was radiolabeled with ¹⁸F using the corresponding precursor. The binding affinity of radiolabeled compounds to MAO-B was assessed using saturation and competitive binding assays. The binding selectivity of ¹⁸F-SMBT-1 to MAO-B was evaluated by autoradiography of frozen human brain tissues. The pharmacokinetics and metabolism were assessed in normal mice after intravenous administration of ¹⁸F-SMBT-1. A 14-d toxicity study after the intravenous administration of ¹⁸F-SMBT-1 was performed using rats and mice. Results: In vitro binding assays demonstrated a high binding affinity of ¹⁸F-SMBT-1 to MAO-B (dissociation constant, 3.7 nM). In contrast, it showed low binding affinity to MAO-A and protein aggregates such as amyloid-β and tau fibrils. Autoradiographic analysis showed higher amounts of ¹⁸F-SMBT-1 binding in the Alzheimer disease brain sections than in the control brain sections. ¹⁸F-SMBT-1 binding was completely displaced with the reversible MAO-B inhibitor lazabemide, demonstrating the high selectivity of ¹⁸F-SMBT-1 for MAO-B. Furthermore, ¹⁸F-SMBT-1 showed a high uptake by brain, rapid washout, and no radiolabeled metabolites in the brain of normal mice. ¹⁸F-SMBT-1 showed no significant binding to various receptors, ion channels, or transporters, and no toxic effects related to its administration were observed in mice and rats. Conclusion: ¹⁸F-SMBT-1 is a promising and selective MAO-B PET tracer candidate, which would be useful for quantitative monitoring of astrogliosis in the human brain.

Metal-Free C-C Coupling of an Allenyl Sulfone with Picolyl Amides to Access Vinyl Sulfones via Pyridine-Initiated In Situ Generation of Sulfinate Anion

Vinyl sulfones are privileged motifs known for their biological activity and synthetic utility. Synthetic transformations to efficiently access high-value compounds with these motifs are desired and sought after. Herein, a new procedure is described to form vinyl sulfone-containing compounds by selective functionalization of the C(sp³)-H bond adjacent to the pyridine ring of pharmacologically prevalent picolyl amides with an allenyl sulfone, 1-methyl-4-(propa-1,2-dien-1-ylsulfonyl)benzene. The reaction conditions are mild with no metal catalyst or additives required and display good functional group tolerance. Mechanistic studies for this unusual transformation suggest that the reaction operates via a rare pyridine-initiated and p-toluenesulfinate anion-mediated activation of the allenyl sulfone analogous to phosphine-triggered reactions.

Silver-catalyzed regioselective deuteration of (hetero)arenes and α-deuteration of 2-alkyl azaarenes

A simple silver-catalyzed regioselective deuteration of (hetero)arenes and α-deuteration of 2-alkyl azaarenes has been described. This strategy provides an efficient and practical avenue to access various deuterated electron-rich arenes, azaarenes and α-deuterated 2-alkyl azaarenes with good to excellent deuterium incorporation utilizing D₂O as the source of deuterium atoms.

A practical silver-catalyzed regioselective deuteration of (hetero)arenes and α-deuteration of 2-alkyl azaarenes utilizing D₂O as a deuterium source has been developed.

Reduction and Reductive Deuteration of Tertiary Amides Mediated by Sodium Dispersions with Distinct Proton Donor-Dependent Chemoselectivity

A practical and scalable single electron transfer reduction mediated by sodium dispersions has been developed for the reduction and reductive deuteration of tertiary amides. The chemoselectivity of this method highly depends on the nature of the proton donor. The challenging reduction via C-N bond cleavage has been achieved using Na/EtOH, affording alcohol products, while the use of Na/NaOH/H₂O leads to the formation of amines via selective C-O scission. Sodium dispersions with high specific surface areas are crucial to obtain high yields and good chemoselectivity. This new method tolerates a range of tertiary amides. Moreover, the corresponding reductive deuterations mediated by Na/EtOD- d₁ and Na/NaOH/D₂O afford useful α,α-dideuterio alcohols and α,α-dideuterio amines with an excellent deuterium content.

Transition-Metal-Free, Selective Reductive Deuteration of Terminal Alkynes with Sodium Dispersions and EtOD- d1

A transition-metal-free single electron transfer reaction has been developed for the synthesis of [D₃]-alkenes from terminal alkynes using sodium dispersions as the electron donor and EtOD- d₁ as the deuterium source. Both reagents are cost-effective and bench-stable. This practical method exhibits remarkable terminal alkyne selectivity and exclusive alkene selectivity. Excellent deuterium incorporations and yields were achieved across a broad range of terminal alkynes without olefin isomerization. Of note, this reaction is highly solvent dependent. n-Hexane provides unique enhancement to this reductive deuteration process.

Deuterium- and Tritium-Labelled Compounds: Applications in the Life Sciences

Hydrogen isotopes are unique tools for identifying and understanding biological and chemical processes. Hydrogen isotope labelling allows for the traceless and direct incorporation of an additional mass or radioactive tag into an organic molecule with almost no changes in its chemical structure, physical properties, or biological activity. Using deuterium-labelled isotopologues to study the unique mass-spectrometric patterns generated from mixtures of biologically relevant molecules drastically simplifies analysis. Such methods are now providing unprecedented levels of insight in a wide and continuously growing range of applications in the life sciences and beyond. Tritium (³ H), in particular, has seen an increase in utilization, especially in pharmaceutical drug discovery. The efforts and costs associated with the synthesis of labelled compounds are more than compensated for by the enhanced molecular sensitivity during analysis and the high reliability of the data obtained. In this Review, advances in the application of hydrogen isotopes in the life sciences are described.

Emerging PET Radiotracers and Targets for Imaging of Neuroinflammation in Neurodegenerative Diseases: Outlook Beyond TSPO

The dynamic and multicellular processes of neuroinflammation are mediated by the nonneuronal cells of the central nervous system, which include astrocytes and the brain's resident macrophages, microglia. Although initiation of an inflammatory response may be beneficial in response to injury of the nervous system, chronic or maladaptive neuroinflammation can have harmful outcomes in many neurological diseases. An acute neuroinflammatory response is protective when activated neuroglia facilitate tissue repair by releasing anti-inflammatory cytokines and neurotrophic factors. On the other hand, chronic neuroglial activation is a major pathological mechanism in neurodegenerative diseases, likely contributing to neuronal dysfunction, injury, and disease progression. Therefore, the development of specific and sensitive probes for positron emission tomography (PET) studies of neuroinflammation is attracting immense scientific and clinical interest. An early phase of this research emphasized PET studies of the prototypical imaging biomarker of glial activation, translocator protein-18 kDa (TSPO), which presents difficulties for quantitation and lacks absolute cellular specificity. Many alternate molecular targets present themselves for PET imaging of neuroinflammation in vivo, including enzymes, intracellular signaling molecules as well as ionotropic, G-protein coupled, and immunoglobulin receptors. We now review the lead structures in radiotracer development for PET studies of neuroinflammation targets for neurodegenerative diseases extending beyond TSPO, including glycogen synthase kinase 3, monoamine oxidase-B, reactive oxygen species, imidazoline-2 binding sites, cyclooxygenase, the phospholipase A2/arachidonic acid pathway, sphingosine-1-phosphate receptor-1, cannabinoid-2 receptor, the chemokine receptor CX3CR1, purinergic receptors: P2X7 and P2Y12, the receptor for advanced glycation end products, Mer tyrosine kinase, and triggering receptor expressed on myeloid cells-1. We provide a brief overview of the cellular expression and function of these targets, noting their selectivity for astrocytes and/or microglia, and highlight the classes of PET radiotracers that have been investigated in early-stage preclinical or clinical research studies of neuroinflammation.

Simultaneous determination of MAO-A and -B activity following first time intake of an irreversible MAO-B inhibitor in patients with Parkinson's disease

We determined monoamine oxidase-A (plasma) and -B (platelets) enzyme activity in chronic levodopa treated patients with Parkinson's disease after first time intake of an irreversible monoamine oxidase-B inhibitor. One patient received 10 mg selegiline and eleven patients took 1 mg rasagiline. A significant decrease of monoamine oxidase-B activity appeared 2 and 4 h following monoamine oxidase-B inhibitor intake in comparison to baseline. We confirm with this design, that rasagiline and selegiline inhibit monoamine oxidase-B but not monoamine oxidase-A after single dosing.

Recent developments on the structure–activity relationship studies of MAO inhibitors and their role in different neurological disorders

Development of MAO inhibitors as effective drug candidates for the management and/or treatment of different neurological disorders.

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography

Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

The MAO-B inhibitor deprenyl reduces the oral tremor and the dopamine depletion induced by the VMAT-2 inhibitor tetrabenazine

Tetrabenazine (TBZ) is prescribed for the treatment of chorea associated with Huntington's disease. Via inhibition of the vesicular monoamine transporter (VMAT-2), TBZ blocks dopamine (DA) storage and depletes striatal DA; this drug also has been shown to induce Parkinsonian motor side effects in patients. Recently, TBZ was shown to induce tremulous jaw movements (TJMs) in rats and mice. TJMs are an oral tremor that has many of the characteristics of Parkinsonian tremor in humans. The present study focused upon the ability of the well-established antiparkinsonian agent deprenyl to attenuate the behavioral and neurochemical effects of 2.0mg/kg TBZ. Deprenyl is a selective and irreversible inhibitor of monoamine oxidase-B, and administration of deprenyl produced a dose-related suppression of TBZ-induced TJMs. A second experiment employed in vivo microdialysis to examine extracellular DA levels in the ventrolateral striatum, the neostriatal region most closely associated with the production of TJMs, after administration of TBZ and deprenyl. Consistent with the behavioral data, TBZ alone produced a biphasic effect on extracellular DA, with an initial increases followed by a prolonged decrease during the period in which TJMs are displayed. Co-administration of deprenyl with TBZ increased DA levels compared to rats treated with TBZ alone. These results provide support for use of TBZ as a rodent model of Parkinsonism, and future studies should utilize this model to evaluate putative anti-Parkinsonian agents.

Methods to Increase the Metabolic Stability of (18)F-Radiotracers

The majority of pharmaceuticals and other organic compounds incorporating radiotracers that are considered foreign to the body undergo metabolic changes in vivo. Metabolic degradation of these drugs is commonly caused by a system of enzymes of low substrate specificity requirement, which is present mainly in the liver, but drug metabolism may also take place in the kidneys or other organs. Thus, radiotracers and all other pharmaceuticals are faced with enormous challenges to maintain their stability in vivo highlighting the importance of their structure. Often in practice, such biologically active molecules exhibit these properties in vitro, but fail during in vivo studies due to obtaining an increased metabolism within minutes. Many pharmacologically and biologically interesting compounds never see application due to their lack of stability. One of the most important issues of radiotracers development based on fluorine-18 is the stability in vitro and in vivo. Sometimes, the metabolism of ¹⁸F-radiotracers goes along with the cleavage of the C-F bond and with the rejection of [¹⁸F]fluoride mostly combined with high background and accumulation in the skeleton. This review deals with the impact of radiodefluorination and with approaches to stabilize the C-F bond to avoid the cleavage between fluorine and carbon.

Monoamine oxidase: radiotracer chemistry and human studies

Monoamine oxidase (MAO) oxidizes amines from both endogenous and exogenous sources thereby regulating the concentration of neurotransmitter amines such as serotonin, norepinephrine, and dopamine as well as many xenobiotics. MAO inhibitor drugs are used in the treatment of Parkinson's disease and in depression stimulating the development of radiotracer tools to probe the role of MAO in normal human biology and in disease. Over the past 30 years since the first radiotracers were developed and the first positron emission tomography (PET) images of MAO in humans were carried out, PET studies of brain MAO in healthy volunteers and in patients have identified different variables that have contributed to different MAO levels in brain and in peripheral organs. MAO radiotracers and PET have also been used to study the current and developing MAO inhibitor drugs including the selection of doses for clinical trials. In this article, we describe the following: (1) the development of MAO radiotracers; (2) human studies including the relationship of brain MAO levels to genotype, personality, neurological, and psychiatric disorders; and (3) examples of the use of MAO radiotracers in drug research and development. We will conclude with outstanding needs to improve the radiotracers that are currently used and possible new applications.

Monoamine oxidase A and B substrates: probing the pathway for drug development

Drug-discovery and -development efforts focused on the MAOs have increased at an accelerated rate over the past decade. Since the first crystal structure of human MAO-B was solved in 2002, over 40 additional structures have been reported and have helped define new, or confirm speculative, binding modes of inhibitors. The detailed mechanism of the MAO-catalyzed oxidation of amine substrates has not been fully elucidated, but its significance is central in the development of new mechanism-based inactivators. Novel fungal MAO-N variants derived from directed evolution strategies are enabling the production of new chiral amine products. Robust assays have been established for measuring MAO status in tissue and cells, while improved MAO radioligands are being deployed for PET imaging studies. This review will attempt to highlight the more recent and salient aspects of MAO research in drug discovery and development, with emphasis on substrates 'probing the pathway'.