PET for the imaging of cerebral α7 acetylcholine receptors: from tracer development to clinical application

Background: Changes in the expression of homomeric α7 nicotinic acetylcholine receptors (α7 nAChR) in the human brain are widely assumed to be associated with neuropsychiatric and neurooncologial processes. Indeed, thoroughly performed studies have shown the ability of α7 nAChR modulators to minimise the extent of cell death as well as to promote synaptic plasticity in different diseases including depression, schizophrenia, stroke and Alzheimer´s disease. Nonetheless, up to date, the clinical meaningful findings obtained with these agents were not always supported by a complete understanding of the downstream effects initiated by α7 nAChR modulators.
Methods: To help understanding these processes an extensive work has been done by our and other groups on the development of positron emission tomography (PET) α7 nAChR agents labeled with the radioisotopes fluorine-18 (18F) and carbon-11 (11C). So far two main classes of α7 nAChR PET tracers have advanced to clinical trials: scaffolds composed of a three-side binding mode to the receptor (e.g., hydrogen bond acceptor, hydrophobic element and a rigid basic amine as the cationic centre), and the scaffolds containing fused functionalities belonging to the interferon inducer tilorone class of derivatives.
Results and Discussion: Structure-activity relationship studies on these two classes have been the subject of continuous research aiming at the development of highly affine and selective α7 nAChR PET tracers with suitable pharmacokinetic properties for an accurate receptor occupancy quantification and distribution of α7 nAChR in the brain. As a result, [18F]NS10743, [18F]NS14490, [11C]NS14992, [18F]DBT10 and its ortho isomer [18F]ASEM emerged as the most promising α7 nAChR PET tracers developed so far. Studies in piglets were done for [18F]NS10743 and [11C]NS14992. Ongoing clinical trials have been reported using [18F]ASEM. Efforts to translate [18F]DBT10 into the clinics have been initiated with its transfer onto an automated synthesis in compliance to clinical production. The results of a successful pre-clinical imaging study, including dosimetry in piglets and evaluation in monkeys suggests the suitability of [18F]DBT10 for imaging α7 nAChR. Very recently a pilot study in a large animal model of ischemic stroke in sheep revealed a high inflammation-related specific uptake of [18F]DBT10 in the stroke border 14 days after permanent middle cerebral artery occlusion.
Conclusion: Among the receptor-specific α7 nAChR PET tracers developed so far, the dibenzothiophene isomers [18F]DBT10 and [18F]ASEM are under continuous investigation due to their suitable pharmacokinetics and high target-specific signal. More proof-of-concept studies are required to support the usefulness of these tracers for sensitive and specific α7 nAChR PET imaging.