ABSTRACT
We show that changing the number and position of nitrogen atoms in the heteroatomic core of a pyrazolopyrimidine acetamide is sufficient to induce complex binding to wild type human TSPO. Only compounds with this complex binding profile lacked intrinsic effect on glioblastoma proliferation but positively modulated the antiproliferative effects of a synthetic TSPO ligand. To the best of our knowledge this is the first demonstration of allosteric-like interaction at the wild type human TSPO.
Subject(s)
Acetamides/pharmacology , Antineoplastic Agents/pharmacology , Cell Proliferation/drug effects , Pyrimidines/pharmacology , Receptors, GABA/metabolism , Acetamides/chemistry , Allosteric Regulation/drug effects , Antineoplastic Agents/chemistry , Cell Line, Tumor , Glioblastoma/drug therapy , Glioblastoma/metabolism , Humans , Protein Binding , Pyrimidines/chemistryABSTRACT
New 1,2-closo- and 7,8-nido-carboranylpyrazolopyrimidines bind to the translocator protein (TSPO) with high affinity, providing the first evidence of a unique two-site binding profile for the closo-carborane derivative. The boron-rich compounds can also deliver boron to human glioma cells far more effectively than clinical agents used in boron neutron capture therapy (BNCT).
Subject(s)
Boron Compounds/administration & dosage , Boron Neutron Capture Therapy , Drug Delivery Systems , Pyrimidines/administration & dosage , Receptors, GABA/metabolism , Antineoplastic Agents/pharmacology , Boron Compounds/chemistry , Brain Neoplasms , Cell Line, Tumor , Glioma , HEK293 Cells , Humans , Isoquinolines/pharmacology , Pyrimidines/chemistryABSTRACT
6-Fluoro-PBR28 (N-(6-fluoro-4-phenoxypyridin-3-yl)-N-(2-methoxybenzyl)acetamide), a fluorinated analogue of the recently developed TSPO 18 kDa ligand PBR28, was synthesized and labelled with fluorine-18. 6-Fluoro-PBR28 and its 6-chloro/6-bromo counterparts were synthesized in six chemical steps and obtained in 16%, 10% and 19% overall yields, respectively. Labelling with fluorine-18 was performed in one single step (chlorine/bromine-for-fluorine heteroaromatic substitution) using a Zymate-XP robotic system affording HPLC-purified, ready-to-inject, 6-[(18)F]fluoro-PBR28 (>95% radiochemically pure). Non-decay-corrected overall yields were 9-10% and specific radioactivities ranged from 74 to 148 GBq/µmol. In vitro binding experiments, dynamic µPET studies performed in a rat model of acute neuroinflammation (unilaterally, AMPA-induced, striatum-lesioned rats) and ex vivo autoradiography on the same model demonstrated the potential of 6-[(18)F]fluoro-PBR28 to image the TSPO 18 kDa using PET.
Subject(s)
Acetamides , Aminopyridines , Positron-Emission Tomography/methods , Radiopharmaceuticals , Receptors, GABA/chemistry , Acetamides/chemical synthesis , Acetamides/chemistry , Aminopyridines/chemical synthesis , Aminopyridines/chemistry , Animals , Corpus Striatum/drug effects , Corpus Striatum/pathology , Disease Models, Animal , Fluorine Radioisotopes , Ligands , Molecular Imaging , Molecular Structure , Radiopharmaceuticals/chemical synthesis , Radiopharmaceuticals/chemistry , Rats , Stereoisomerism , alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic AcidABSTRACT
The translocator protein (TSPO) is expressed at low levels in the healthy human brain and is markedly upregulated in response to brain injury and inflammation. This increase in TSPO expression is correlated to the extent of microglial activation, making the measurement of TSPO density a useful indicator of active brain disease. Several classes of TSPO radioligands have therefore been developed and evaluated for use in PET, to track the progression and severity of neuroinflammatory disease. TSPO is also overexpressed in cancer and peripheral inflammation, making TSPO PET ligands possible candidates for the imaging of a multitude of pathologies. However, we currently possess a limited understanding about the molecular structure of TSPO and about the interaction of ligands with the protein. Furthermore, the incomplete characterization of multiple TSPO binding sites and the role of TSPO polymerization suggest that current interpretation of PET data may require further refinement.
