Synthesis, fluorine-18 radiolabeling, and in vitro characterization of 1-iodophenyl-N-methyl-N-fluoroalkyl-3-isoquinoline carboxamide derivatives as potential PET radioligands for imaging peripheral benzodiazepine receptor

Palladium-Catalyzed Isoquinoline Synthesis by Tandem C-H Allylation and Oxidative Cyclization of Benzylamines with Allyl Acetate

A novel approach to synthesize 3-methylisoquinolines via a one-pot, two-step, palladium(II)-catalyzed tandem C-H allylation/intermolecular amination and aromatization is reported. A wide series of 3-methylisoquinoline derivatives were obtained directly using this method in moderate to good yields, and we highlight the synthetic importance of this new transformation.

Advances in the Preparation of Fluorinated Isoquinolines: A Decade of Progress

Heterocyclic molecules incorporating fluorinated isoquinoline components are found in many medicinally and agriculturally important bioactive products as well as industrially impactful materials. Within the past decade, a variety of isoquinolinic ring assembly techniques has enabled the introduction of diverse fluorine-containing functionalities which can enhance potential bioactivity and industrial utility. This review examines recent noncatalyzed and transition metal catalyzed synthetic approaches to the assembly of isoquinoline derivatives that are ring-fluorinated and/or result in the incorporation of fluorine-containing functional groups. Specifically, efficient synthetic methods and regioselectivity in the incorporation of functional groups into isoquinoline ring systems are examined.

Translocator protein (TSPO) ligands for the diagnosis or treatment of neurodegenerative diseases: a patent review (2010-2015; part 1)

INTRODUCTION

The translocator protein (TSPO) is an emerging target in diverse neurodegenerative diseases. Up-regulated TSPO in the central nervous system (CNS) appears to be involved in neuroinflammatory processes; therefore, the development of potent TSPO ligands is a promising method for alleviating or imaging patients with neurodegenerative diseases. Areas covered: This review will provide an overview of recently developed TSPO ligands patented from 2010 to 2015. Part 1 will present a summary focusing on TSPO ligands other than indole-based or cholesterol-like compounds, which will be discussed in part 2. Part 1 covers diverse benzodiazepine-derived analogues such as isoquinoline carboxamides and aryloxyanilides. Moreover, bicyclic ring structures such as imidazopyridine, pyrazolopyrimidine, and phenylpurine will be highlighted as promising scaffolds for TSPO ligands. A brief analysis of currently reported TSPO structures will also be covered in part 1. Expert opinion: Although the underlying pharmacological mechanism of TSPO remains to be elucidated, several TSPO ligands have shown therapeutic efficacy in experimental animal models of neurodegenerative diseases. In addition, radioactive TSPO ligands have been extensively studied for the diagnosis of neurodegenerative processes. Thus, further studies on both the basic and applied mechanisms of TSPO are warranted in the pursuit of successful pharmacological applications of TSPO ligands.

A Novel PET Imaging Probe for the Detection and Monitoring of Translocator Protein 18 kDa Expression in Pathological Disorders

A new fluorine-substituted ligand, compound 1 (CB251), with a very high affinity (Ki = 0.27 ± 0.09 nM) and selectivity for the 18-kDa translocator protein (TSPO), is presented as an attractive biomarker for the diagnosis of neuroinflammation, neurodegeneration and tumour progression. To test compound 1 as a TSPO PET imaging agent in vivo, 2-(2-(4-(2-[¹⁸F]fluoroethoxy)phenyl)-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide ([¹⁸F]1; [¹⁸F]CB251) was synthesized by nucleophilic aliphatic substitution in a single-step radiolabelling procedure with a 11.1 ± 3.5% (n = 14, decay corrected) radiochemical yield and over 99% radiochemical purity. In animal PET imaging studies, [¹⁸F]CB251 provided a clearly visible image of the inflammatory lesion with the binding potential of the specifically bound radioligand relative to the non-displaceable radioligand in tissue (BP_ND 1.83 ± 0.18), in a neuroinflammation rat model based on the unilateral stereotaxic injection of lipopolysaccharide (LPS), comparable to that of [¹¹C]PBR28 (BP_ND 1.55 ± 0.41). [¹⁸F]CB251 showed moderate tumour uptake (1.96 ± 0.11%ID/g at 1 h post injection) in human glioblastoma U87-MG xenografts. These results suggest that [¹⁸F]CB251 is a promising TSPO PET imaging agent for neuroinflammation and TSPO-rich cancers.

Fluorinated molecules in the diagnosis and treatment of neurodegenerative diseases

The use of fluorinated molecules as drugs and imaging agents for CNS disorders has been studied extensively over the years. Incorporating a fluorine atom into the structure of a drug changes its physiochemical properties and can thereby lead to much more desirable pharmacokinetic and pharmacodynamic properties. This change can help to facilitate blood-brain barrier permeability, which is a critical matter for drugs intended for CNS activities. Fluorine incorporation into structures of drugs for the treatment of neurodegenerative diseases has been an attractive field for drug discovery. Such incorporation can greatly influence the physicochemical properties, metabolic stability and receptor binding affinity of the resulting molecule. Some studies have shown that when a proton was substituted with fluorine, the binding or inhibitory potency was greatly increased. The fluorine-18 isotope, (18)F, is utilized in detection and diagnosis of neurodegenerative diseases, whereas (19)F compounds are used in the treatment of these diseases and in MRI. (18)F is widely used in PET imaging because it offers the advantage of a longer half-life compared with other radionuclides. It is used for imaging various receptors and transporters that have been linked to neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease and multiple system atrophy. Fluorine plays an important role in the diagnosis and treatment of many CNS diseases, including neurodegenerative disorders. The use of fluorine in the diagnosis and treatment of neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, will be discussed in this review.

Molecular tracers for the PET and SPECT imaging of disease

The development of positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging continues to grow due to the ability of these techniques to allow the non-invasive in vivo visualisation of biological processes at the molecular and cellular levels. As well as finding application for the diagnosis of disease, these techniques have also been used in the drug discovery process. Crucial to the growth of these techniques is the continued development of molecular probes that can bind to the target biological receptor with high selectivity. This tutorial review describes the use of PET and SPECT for molecular imaging and highlights key strategies for the development of molecular probes for the imaging of both cancer and neurological diseases.

New iodinated quinoline-2-carboxamides for SPECT imaging of the translocator protein

With the aim of developing new SPECT imaging agents for the translocator protein (TSPO), a small library of iodinated quinoline-2-carboxamides have been prepared and tested for binding affinity with TSPO. N,N-Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide was found to have excellent affinity (K(i) 12.0 nM), comparable to that of the widely used TSPO imaging agent PK11195.

Single-step high-yield radiosynthesis and evaluation of a sensitive 18F-labeled ligand for imaging brain peripheral benzodiazepine receptors with PET

Elevated levels of peripheral benzodiazepine receptors (PBR) are associated with activated microglia in their response to inflammation. Hence, PBR imaging in vivo is valuable for investigating brain inflammatory conditions. Sensitive, easily prepared, and readily available radioligands for imaging with positron emission tomography (PET) are desirable for this purpose. We describe a new 18F-labeled PBR radioligand, namely [18F]N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([18F]9). [18F]9 was produced easily through a single and highly efficient step, the reaction of [18F]fluoride ion with the corresponding bromo precursor, 8. Ligand 9 exhibited high affinity for PBR in vitro. PET showed that [18F]9 was avidly taken into monkey brain and gave a high ratio of PBR-specific to nonspecific binding. [18F]9 was devoid of defluorination in rat and monkey and gave predominantly polar radiometabolite(s). In rat, a low level radiometabolite of intermediate lipophilicity was identified as [18F]2-fluoro-N-(2-phenoxyphenyl)acetamide ([18F]11). [18F]9 is a promising radioligand for future imaging of PBR in living human brain.