Discovery and Preclinical Validation of [(11)C]AZ13153556, a Novel Probe for the Histamine Type 3 Receptor

Pharmacokinetic Modeling of [11C]GSK-189254, PET Tracer Targeting H3 Receptors, in Rat Brain

The histamine H3 receptor has been considered as a target for the treatment of various central nervous system diseases. Positron emission tomography (PET) studies with the radiolabeled potent and selective histamine H3 receptor antagonist [11C]GSK-189254 in rodents could be used to examine the mechanisms of action of novel therapeutic drugs or to assess changes of regional H3 receptor density in animal models of neurodegenerative disease. [11C]GSK-189254 was intravenously administered to healthy Wistar rats (n = 10), and a 60 min dynamic PET scan was carried out. Arterial blood samples were obtained during the scan to generate a metabolite-corrected plasma input function. PET data were analyzed using a one-tissue compartment model (1T2k), irreversible (2T3k) or reversible two-tissue compartment models (2T4k), graphical analysis (Logan and Patlak), reference tissue models (SRTM and SRTM2), and standard uptake values (SUVs). The Akaike information criterion and the standard error of the estimated parameters were used to select the most optimal quantification method. This study demonstrated that the 2T4k model with a fixed blood volume fraction and Logan graphical analysis can best describe the kinetics of [11C]GSK-189254 in the rat brain. SUV40-60 and the reference tissue-based measurements DVR(2T4k), BPND(SRTM), and SUV ratio could also be used as a simplified method to estimate H3 receptor availability in case blood sampling is not feasible.

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

Development of [ Carbonyl-11C]AZ13198083, a Novel Histamine Type-3 Receptor Radioligand with Favorable Kinetics

The histamine subtype-3 receptor (H₃R) is implicated in a range of central nervous system disorders, and several radioligands have been developed for H₃R positron emission tomography imaging. However, a limitation of currently used PET radioligands for H₃R is the slow binding kinetics in high density brain regions. To address this, we herein report the development of three novel candidate H₃R radioligands, namely, [ carbonyl-¹¹C]AZ13153556 ([ carbonyl-¹¹C]4), [ carbonyl-¹¹C]AZD5213([ carbonyl-¹¹C]5), and [ carbonyl-¹¹C]AZ13198083 ([ carbonyl-¹¹C]6), and their subsequent preclinical evaluation in nonhuman primates (NHP). Radioligands [ carbonyl-¹¹C]4-6 were produced and isolated in high radioactivity (>1000 MBq), radiochemical purity (>99%), and moderate molar activity (19-28 GBq/μmol at time of injection) using a palladium-mediated ¹¹C-aminocarbonylation protocol. All three radioligands showed high brain permeability as well as a regional brain radioactivity distribution in accordance with H₃R expression (striatum > cortex > cerebellum). [ Carbonyl-¹¹C]6 displayed the most favorable in vivo kinetics and brain uptake, with an early peak in the striatal time-activity curve followed by a progressive washout from the brain. The specificity and on-target kinetics of [ carbonyl-¹¹C]6 were next investigated in pretreatment and displacement studies. After pretreatment or displacement with 5 (0.1 mg/kg), a uniformly low distribution of radioactivity across the NHP brain was observed. Collectively, this work demonstrates that [ carbonyl-¹¹C]6 is a promising candidate for H₃R imaging in human subjects.

Progress in the development of histamine H3 receptor antagonists/inverse agonists: a patent review (2013-2017)

INTRODUCTION

Since years, ligands blocking histamine H₃ receptor (H₃R) activity (antagonists/inverse agonists) are interesting targets in the search for new cures for CNS disorders. Intensive works done by academic and pharmaceutical company researchers have led to many potent and selective H₃R antagonists/inverse agonists. Some of them have reached to clinical trials.

AREAS COVERED

Patent applications from January 2013 to September 2017 and the most important topics connected with H₃R field are analysed. Espacenet, Patentscope, Pubmed, GoogleScholar or Cochrane Library online databases were principially used to collect all the materials.

EXPERT OPINION

The research interest in histamine H₃R field is still high although the number of patent applications has decreased during the past 4 years (around 20 publications). Complexity of histamine H₃R biology e.g. many isoforms, constitutive activity, heteromerization with other receptors (dopamine D₂, D₁, adenosine A_2A) and pharmacology make not easy realization and evaluation of therapeutic potential of anti-H₃R ligands. First results from clinical trials have verified potential utility of histamine H₃R antagonist/inverse agonists in some diseases. However, more studies are necessary for better understanding of an involvement of the histaminergic system in CNS-related disorders and helping more ligands approach to clinical trials and the market. Lists of abbreviations: hAChEI - human acetylcholinesterase inhibitor; hBuChEI - human butyrylcholinesterase inhibitor; hMAO - human monoamine oxidase; MAO - monoamine oxidase.

11C-Labeling of Aryl Ketones as Candidate Histamine Subtype-3 Receptor PET Radioligands through Pd(0)-Mediated 11C-Carbonylative Coupling

Pd(0)-mediated coupling between iodoarenes, [¹¹C]carbon monoxide and aryltributylstannanes has been used to prepare simple model [¹¹C]aryl ketones. Here, we aimed to label four 2-aminoethylbenzofuran chemotype based molecules ([¹¹C]1–4) in the carbonyl position, as prospective positron emission tomography (PET) radioligands for the histamine subtype 3 receptor (H3R) by adapting this methodology with use of aryltrimethylstannanes. Radiosynthesis was successfully performed on a platform equipped with a mini-autoclave and a liquid handling robotic arm, within a lead-shielded hot-cell. Candidate radioligands were readily formulated in saline containing ethanol (10%, v/v) and ascorbic acid (0.5 mg/10 mL). Yields for preclinical use were in the range of 5–9%, decay-corrected from cyclotron-produced [¹¹C]CO₂ and molar activities were >115 GBq/µmol at end of synthesis. Radiochemical purities exceeded >97%.