Synthesis, radiolabeling, and in vivo pharmacokinetic evaluation of the amyloid beta radioligand [11C]AZD4694 in nonhuman primates

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

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.

Translocator Protein-18 kDa (TSPO) Positron Emission Tomography (PET) Imaging and Its Clinical Impact in Neurodegenerative Diseases

In vivo exploration of activated microglia in neurodegenerative diseases is achievable by Positron Emission Tomography (PET) imaging, using dedicated radiopharmaceuticals targeting the translocator protein-18 kDa (TSPO). In this review, we emphasized the major advances made over the last 20 years, thanks to TSPO PET imaging, to define the pathophysiological implication of microglia activation and neuroinflammation in neurodegenerative diseases, including Parkinson’s disease, Huntington’s disease, dementia, amyotrophic lateral sclerosis, multiple sclerosis, and also in psychiatric disorders. The extent and upregulation of TSPO as a molecular biomarker of activated microglia in the human brain is now widely documented in these pathologies, but its significance, and especially its protective or deleterious action regarding the disease’s stage, remains under debate. Thus, we exposed new and plausible suggestions to enhance the contribution of TSPO PET imaging for biomedical research by exploring microglia’s role and interactions with other cells in brain parenchyma. Multiplex approaches, associating TSPO PET radiopharmaceuticals with other biomarkers (PET imaging of cellular metabolism, neurotransmission or abnormal protein aggregates, but also other imaging modalities, and peripheral cytokine levels measurement and/or metabolomics analysis) was considered. Finally, the actual clinical impact of TSPO PET imaging as a routine biomarker of neuroinflammation was put into perspective regarding the current development of diagnostic and therapeutic strategies for neurodegenerative diseases.