Development of Alzheimer's disease imaging agents for clinical studies

Insight into the Molecular Imaging of Alzheimer's Disease

Alzheimer's disease is a complex neurodegenerative disease affecting millions of individuals worldwide. Earlier it was diagnosed only via clinical assessments and confirmed by postmortem brain histopathology. The development of validated biomarkers for Alzheimer's disease has given impetus to improve diagnostics and accelerate the development of new therapies. Functional imaging like positron emission tomography (PET), single photon emission computed tomography (SPECT), functional magnetic resonance imaging (fMRI), and proton magnetic resonance spectroscopy provides a means of detecting and characterising the regional changes in brain blood flow, metabolism, and receptor binding sites that are associated with Alzheimer's disease. Multimodal neuroimaging techniques have indicated changes in brain structure and metabolic activity, and an array of neurochemical variations that are associated with neurodegenerative diseases. Radiotracer-based PET and SPECT potentially provide sensitive, accurate methods for the early detection of disease. This paper presents a review of neuroimaging modalities like PET, SPECT, and selected imaging biomarkers/tracers used for the early diagnosis of AD. Neuroimaging with such biomarkers and tracers could achieve a much higher diagnostic accuracy for AD and related disorders in the future.

Radiosynthesis and in vivo evaluation of fluorinated huprine derivates as PET radiotracers of acetylcholinesterase

INTRODUCTION

Developing positron emission tomography (PET) radiotracers for non-invasive study of the cholinergic system is crucial to the understanding of neurodegenerative diseases. Although several acetylcholinesterase (AChE) PET tracers radiolabeled with carbon-11 exist, no fluorinated radiotracer is currently used in clinical imaging studies. The purpose of the present study is to describe the first fluorinated PET radiotracer for this brain enzyme.

METHODS

Three structural analogs of huprine, a specific AChE inhibitor presenting high affinity towards AChE in vitro, were synthesized and labeled with fluorine-18 via a mesylate/fluoro-nucleophilic aliphatic substitution: ([(18)F]-FHUa, [(18)F]-FHUb and [(18)F]-FHUc). Initial biological evaluation included in vitro autoradiography in rat with competition with an AChE inhibitor at different concentrations, and microPET-scan on anesthetized rats. In vivo PET studies in anesthetized cat focused on [(18)F]-FHUa.

RESULTS AND CONCLUSIONS

Although radiosynthesis of these huprine analogs was straightforward, they showed poor brain penetration potential, partially reversed after pharmacological inhibition of P-glycoprotein. These results indicated that current huprine analogs are not suitable for PET mapping of brain AChE receptors, but require physicochemical modulation in order to increase brain penetration.

Synthesis and evaluation of new radioligands [(11)C]A-833834 and [(11)C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors

INTRODUCTION

α7-nicotinic acetylcholine receptor (α7-nAChR) is one of the major neuronal nAChR subtypes. α7-nAChR is involved in variety of neuronal processes and disorders including schizophrenia and Alzheimer's disease. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be discovered.

METHODS

High binding affinity α7-nAChR ligands A-833834 and A-752274 were radiolabeled with (11)C. Baseline and blockade biodistribution studies in the mouse brain of [(11)C]A-833834 (5-(6-(5-[(11)C]methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole) and [(11)C]A-752274 (2-(6-[(11)C]methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one) were performed. [(11)C]A-752274 was evaluated in a baseline baboon PET study.

RESULTS

[(11)C]A-833834 and [(11)C]A-752274 were synthesized by radiomethylation of corresponding des-methyl precursors. The radioligands were prepared with radiochemical yield of 12%-32%, high specific radioactivity (330-403GBq/μmol) and radiochemical purity>95%. Dissection studies with [(11)C]A-833834 demonstrated low specific α7-nAChR binding in the mouse brain. [(11)C]A-752274 specifically (~50%) labeled α7-nAChR in the mouse thalamus. However, [(11)CA-752274 exhibited low brain uptake in baboon (%SUV<100).

CONCLUSION

Two novel α7-nAChR ligands radioligands were synthesized and studied in animals. Specific binding of [(11)C]A-833834 in the mouse brain is low due to the insufficient binding affinity of the radioligand. The very high binding affinity [(11)C]A-752274 exhibited good specific binding in the α7-nAChR-rich mouse brain regions. The low uptake of [(11)C]A-752274 in the baboon brain is due to its high hydrophilicity, rapid metabolism or other properties. Future development of α7-nAChR PET radioligands will be based on compounds with high binding affinities and good blood-brain barrier permeability.

Anomalous PiB enhancement in the superior sagittal and transverse venous sinuses

Pittsburgh compound-B (PiB), an amyloid-binding positron emission tomography (PET) tracer, is widely used for imaging amyloid-β in those with and at risk for Alzheimer disease. Here, we report on an otherwise normal 68-year-old female with abnormally high and very focal PiB retention. Coregistered T1-weighted magnetic resonance imaging and dynamic 2-fluoro-2-deoxy-D-glucose (FDG) images confirmed that the focal PiB enhancement was in the superior sagittal and transverse sinuses, outside of the adjacent cortex. Flow through the venous vasculature was normal as assessed by dynamic FDG PET imaging. These features supported the conclusion that PiB retention was not simply due to a hemodynamic abnormality, but may have represented PiB binding to fibrillar deposits of a β-sheet protein (ie, amyloid), whose nature is currently unclear.

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.

In vivo imaging biomarkers in mouse models of Alzheimer's disease: are we lost in translation or breaking through?

Identification of biomarkers of Alzheimer's Disease (AD) is a critical priority to efficiently diagnose the patients, to stage the progression of neurodegeneration in living subjects, and to assess the effects of disease-modifier treatments. This paper addresses the development and usefulness of preclinical neuroimaging biomarkers of AD. It is today possible to image in vivo the brain of small rodents at high resolution and to detect the occurrence of macroscopic/microscopic lesions in these species, as well as of functional alterations reminiscent of AD pathology. We will outline three different types of imaging biomarkers that can be used in AD mouse models: biomarkers with clear translational potential, biomarkers that can serve as in vivo readouts (in particular in the context of drug discovery) exclusively for preclinical research, and finally biomarkers that constitute new tools for fundamental research on AD physiopathogeny.

Rational design of amyloid binding agents based on the molecular rotor motif

Alzheimer’s disease (AD) is characterized by a progressive loss of cognitive function and constitutes the most common and fatal neurodegenerative disorder.[1 ] Genetic and clinical evidence supports the hypothesis that accumulation of amyloid deposits in the brain plays an important role in the pathology of the disease. This event is associated with perturbations of biological functions in the surrounding tissue leading to neuronal cell death, thus contributing to the disease process. The deposits are comprised primarily of amyloid (Aβ) peptides, a 39–43 amino acid sequence that self aggregates into a fibrillar β-pleated sheet motif. While the exact three-dimensional structure of the aggregated Aβ peptides is not known, a model structure that sustains the property of aggregation has been proposed.[2 ] This creates opportunities for in vivo imaging of amyloid deposits that can not only help evaluate the time course and evolution of the disease, but can also allow the timely monitoring of therapeutic treatments.[3 ]

Molecular Imaging and Magnetic Resonance Imaging in Early Diagnosis of Alzheimer's Disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is the most common cause of dementia in the elderly. Magnetic resonance (MR) or computed tomography (CT) imaging is recommended for routine evaluation of dementias. The development of molecular imaging agents and the new techniques of MR for AD are critically important for early diagnosis, neuropathogenesis studies and assessing treatment efficacy in AD. Neuroimaging using nuclear medicine techniques such as SPECT, PET and MR spectroscopy has the potential to characterize the biomarkers for Alzheimer's disease. The present review summarizes the results of radionuclide imaging and MR imaging in AD.