Synthesis of (R,S)-isoproterenol, an inhibitor of tau aggregation, as an 11C-labeled PET tracer via reductive alkylation of (R,S)-norepinephrine with [2-11C]acetone

Selection of lansoprazole from an FDA-approved drug library to inhibit the Alzheimer's disease seed-dependent formation of tau aggregates

The efficacy of current treatments is still insufficient for Alzheimer's disease (AD), the most common cause of Dementia. Out of the two pathological hallmarks of AD amyloid-β plaques and neurofibrillary tangles, comprising of tau protein, tau pathology strongly correlates with the symptoms of AD. Previously, screening for inhibitors of tau aggregation that target recombinant tau aggregates have been attempted. Since a recent cryo-EM analysis revealed distinct differences in the folding patterns of heparin-induced recombinant tau filaments and AD tau filaments, this study focused on AD seed-dependent tau aggregation in drug repositioning for AD. We screened 763 compounds from an FDA-approved drug library using an AD seed-induced tau aggregation in SH-SY5Y cell-based assay. In the first screening, 180 compounds were selected, 72 of which were excluded based on the results of lactate dehydrogenase assay. In the third screening with evaluations of soluble and insoluble tau, 38 compounds were selected. In the fourth screening with 3 different AD seeds, 4 compounds, lansoprazole, calcipotriene, desogestrel, and pentamidine isethionate, were selected. After AD seed-induced real-time quaking-induced conversion, lansoprazole was selected as the most suitable drug for repositioning. The intranasal administration of lansoprazole for 4 months to AD seed-injected mice improved locomotor activity and reduced both the amount of insoluble tau and the extent of phosphorylated tau-positive areas. Alanine replacement of the predicted binding site to an AD filament indicated the involvement of Q351, H362, and K369 in lansoprazole and C-shaped tau filaments. These results suggest the potential of lansoprazole as a candidate for drug repositioning to an inhibitor of tau aggregate formation in AD.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

PET Agents in Dementia: An Overview

This article presents an overview of imaging agents for PET that have been applied for research and diagnostic purposes in patients affected by dementia. Classified by the target which the agents visualize, seven groups of tracers can be distinguished, namely radiopharmaceuticals for: (1) Misfolded proteins (ß-amyloid, tau, α-synuclein), (2) Neuroinflammation (overexpression of translocator protein), (3) Elements of the cholinergic system, (4) Elements of monoamine neurotransmitter systems, (5) Synaptic density, (6) Cerebral energy metabolism (glucose transport/ hexokinase), and (7) Various other proteins. This last category contains proteins involved in mechanisms underlying neuroinflammation or cognitive impairment, which may also be potential therapeutic targets. Many receptors belong to this category: AMPA, cannabinoid, colony stimulating factor 1, metabotropic glutamate receptor 1 and 5 (mGluR1, mGluR5), opioid (kappa, mu), purinergic (P2X7, P2Y12), sigma-1, sigma-2, receptor for advanced glycation endproducts, and triggering receptor expressed on myeloid cells-1, besides several enzymes: cyclooxygenase-1 and 2 (COX-1, COX-2), phosphodiesterase-5 and 10 (PDE5, PDE10), and tropomyosin receptor kinase. Significant advances in neuroimaging have been made in the last 15 years. The use of 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG) for quantification of regional cerebral glucose metabolism is well-established. Three tracers for ß-amyloid plaques have been approved by the Food and Drug Administration and European Medicines Agency. Several tracers for tau neurofibrillary tangles are already applied in clinical research. Since many novel agents are in the preclinical or experimental stage of development, further advances in nuclear medicine imaging can be expected in the near future. PET studies with established tracers and tracers for novel targets may result in early diagnosis and better classification of neurodegenerative disorders and in accurate monitoring of therapy trials which involve these targets. PET data have prognostic value and may be used to assess the response of the human brain to interventions, or to select the appropriate treatment strategy for an individual patient.

Brain pharmacokinetics and biodistribution of 11C-labeled isoproterenol in rodents

INTRODUCTION

Isoproterenol is a non-selective β receptor agonist, which is a drug approved for bradycardia and bronchial asthma in many countries. Recently, isoproterenol has been reported to have the potential as a drug for the treatment of Alzheimer's disease by inhibiting the aggregation of tau protein. Isoproterenol is a highly potent drug causing increases in heart rates even when its plasma concentration is very low. Thus, it is critical to know if potentially effective therapeutic levels of isoproterenol can be achieved, maintaining safe plasma levels without any untoward pharmacological effects. The purpose of the study is to investigate the brain pharmacokinetics and biodistribution of ¹¹C-labeled isoproterenol in rodents.

METHODS

We performed positron emission tomography (PET) brain imaging and biodistribution studies of [¹¹C]isoproterenol. 120-min scans with arterial blood sampling were performed in rats. Additionally, plasma and brain homogenates were analyzed with radio-HPLC to characterize its metabolite profiles. As a measure of [¹¹C]isoproterenol brain uptake, total distribution volumes were determined by a pharmacokinetic compartment model. Biodistribution of [¹¹C]isoproterenol was investigated in mice at six-time points from 1-min to 90-min after injection.

RESULTS

We found a modest brain uptake of [¹¹C]isoproterenol. Its brain pharmacokinetics showed that the concentration of isoproterenol in the brain at equilibrium was about two-fold higher than in the plasma (total distribution volumes 2.0 ± 0.2 cm³/mL). Only unmetabolized isoproterenol was detected in the brain at 30 min after injection, although isoproterenol was rapidly metabolized in plasma. The biodistribution study showed that isoproterenol and its metabolite are excreted mainly via the urinary system. CONCLUSIONS, ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: In this study, we have shown that rat brain concentrations of isoproterenol are only two-fold of that in plasma at equilibrium. If the brain pharmacokinetics are similar in the human brain, it may be difficult to achieve potentially therapeutic levels of this drug safely in humans. Further studies appear warranted to investigate the brain pharmacokinetics in humans with PET using [¹¹C]isoproterenol.