Synthesis and preliminary evaluation of piperidinyl and pyrrolidinyl iodobenzoates as imaging agents for butyrylcholinesterase

A method for the efficient evaluation of substrate-based cholinesterase imaging probes for Alzheimer's disease

Cholinesterase (ChE) enzymes have been identified as diagnostic markers for Alzheimer disease (AD). Substrate-based probes have been synthesised to detect ChEs but they have not detected changes in ChE distribution associated with AD pathology. Probes are typically screened using spectrophotometric methods with pure enzyme for specificity and kinetics. However, the biochemical properties of ChEs associated with AD pathology are altered. The present work was undertaken to determine whether the Karnovsky-Roots (KR) histochemical method could be used to evaluate probes at the site of pathology. Thirty thioesters and esters were synthesised and evaluated using enzyme kinetic and KR methods. Spectrophotometric methods demonstrated all thioesters were ChE substrates, yet only a few provided staining in the brain with the KR method. Esters were ChE substrates with interactions with brain ChEs. These results suggest that the KR method may provide an efficient means to screen compounds as probes for imaging AD-associated ChEs.

Synthesis and Initial Characterization of a Selective, Pseudo-irreversible Inhibitor of Human Butyrylcholinesterase as PET Tracer

The enzyme butyrylcholinesterase (BChE) represents a promising target for imaging probes to potentially enable early diagnosis of neurodegenerative diseases like Alzheimer's disease (AD) and to monitor disease progression in some forms of cancer. In this study, we present the design, facile synthesis, in vitro and preliminary ex vivo and in vivo evaluation of a morpholine-based, selective inhibitor of human BChE as a positron emission tomography (PET) tracer with a pseudo-irreversible binding mode. We demonstrate a novel protecting group strategy for 18 F radiolabeling of carbamate precursors and show that the inhibitory potency as well as kinetic properties of our unlabeled reference compound were retained in comparison to the parent compound. In particular, the prolonged duration of enzyme inhibition of such a morpholinocarbamate motivated us to design a PET tracer, possibly enabling a precise mapping of BChE distribution.

Longitudinal evaluation of a novel BChE PET tracer as an early in vivo biomarker in the brain of a mouse model for Alzheimer disease

Purpose: The increase in butyrylcholinesterase (BChE) activity in the brain of Alzheimer disease (AD) patients and animal models of AD position this enzyme as a potential biomarker of the disease. However, the information on the ability of BChE to serve as AD biomarker is contradicting, also due to scarce longitudinal studies of BChE activity abundance. Here, we report ¹¹C-labeling, in vivo stability, biodistribution, and longitudinal study on BChE abundance in the brains of control and 5xFAD (AD model) animals, using a potent BChE selective inhibitor, [¹¹C]4, and positron emission tomography (PET) in combination with computerised tomography (CT). We correlate the results with in vivo amyloid beta (Aβ) deposition, longitudinally assessed by [¹⁸F]florbetaben-PET imaging.

Methods: [¹¹C]4 was radiolabelled through ¹¹C-methylation. Metabolism studies were performed on blood and brain samples of female wild type (WT) mice. Biodistribution studies were performed in female WT mice using dynamic PET-CT imaging. Specific binding was demonstrated by ex vivo and in vivo PET imaging blocking studies in female WT and 5xFAD mice at the age of 7 months. Longitudinal PET imaging of BChE was conducted in female 5xFAD mice at 4, 6, 8, 10 and 12 months of age and compared to age-matched control animals. Additionally, Aβ plaque distribution was assessed in the same mice using [¹⁸F]florbetaben at the ages of 2, 5, 7 and 11 months. The results were validated by ex vivo staining of BChE at 4, 8, and 12 months and Aβ at 12 months on brain samples.

Results: [¹¹C]4 was produced in sufficient radiochemical yield and molar activity for the use in PET imaging. Metabolism and biodistribution studies confirmed sufficient stability in vivo, the ability of [¹¹C]4 to cross the blood brain barrier (BBB) and rapid washout from the brain. Blocking studies confirmed specificity of the binding. Longitudinal PET studies showed increased levels of BChE in the cerebral cortex, hippocampus, striatum, thalamus, cerebellum and brain stem in aged AD mice compared to WT littermates. [¹⁸F]Florbetaben-PET imaging showed similar trend of Aβ plaques accumulation in the cerebral cortex and the hippocampus of AD animals as the one observed for BChE at ages 4 to 8 months. Contrarily to the results obtained by ex vivo staining, lower abundance of BChE was observed in vivo at 10 and 12 months than at 8 months of age.

Conclusions: The BChE inhibitor [¹¹C]4 crosses the BBB and is quickly washed out of the brain of WT mice. Comparison between AD and WT mice shows accumulation of the radiotracer in the AD-affected areas of the brain over time during the early disease progression. The results correspond well with Aβ accumulation, suggesting that BChE is a promising early biomarker for incipient AD.

Targeting butyrylcholinesterase for preclinical single photon emission computed tomography (SPECT) imaging of Alzheimer's disease

INTRODUCTION

Diagnosis of Alzheimer's disease (AD) in vivo, by molecular imaging of amyloid or tau, is constrained because similar changes can be found in brains of cognitively normal individuals. Butyrylcholinesterase (BChE), which becomes associated with these structures in AD, could elevate the accuracy of AD diagnosis by focusing on BChE pathology in the cerebral cortex, a region of scant BChE activity in healthy brain.

METHODS

N-methylpiperidin-4-yl 4-[¹²³I]iodobenzoate, a BChE radiotracer, was injected intravenously into B6SJL-Tg(APPSwFlLon, PSEN1∗M146 L∗L286 V) 6799Vas/Mmjax (5XFAD) mice and their wild-type (WT) counterparts for comparative single photon emission computed tomography (SPECT) studies. SPECT, computed tomography (CT), and magnetic resonance imaging (MRI) enabled comparison of whole brain and regional retention of the BChE radiotracer in both mouse strains.

RESULTS

Retention of the BChE radiotracer was consistently higher in the 5XFAD mouse than in WT, and differences were particularly evident in the cerebral cortex.

DISCUSSION

Cerebral cortical BChE imaging with SPECT can distinguish 5XFAD mouse model from the WT counterpart.

Synthesis and Preliminary Evaluation of Phenyl 4-123I-Iodophenylcarbamate for Visualization of Cholinesterases Associated with Alzheimer Disease Pathology

Acetylcholinesterase and butyrylcholinesterase accumulate with brain β-amyloid (Aβ) plaques in Alzheimer disease (AD). The overall activity of acetylcholinesterase is found to decline in AD, whereas butyrylcholinesterase has been found to either increase or remain the same. Although some cognitively normal older adults also have Aβ plaques within the brain, cholinesterase-associated plaques are generally less abundant in such individuals. Thus, brain imaging of cholinesterase activity associated with Aβ plaques has the potential to distinguish AD from cognitively normal older adults, with or without Aβ accumulation, during life. Current Aβ imaging agents are not able to provide this distinction. To address this unmet need, synthesis and evaluation of a cholinesterase-binding ligand, phenyl 4-(123)I-iodophenylcarbamate ((123)I-PIP), is described.

METHODS

Phenyl 4-iodophenylcarbamate was synthesized and evaluated for binding potency toward acetylcholinesterase and butyrylcholinesterase using enzyme kinetic analysis. This compound was subsequently rapidly radiolabeled with (123)I and purified by high-performance liquid chromatography. Autoradiographic analyses were performed with (123)I-PIP using postmortem orbitofrontal cortex from cognitively normal and AD human brains. Comparisons were made with an Aβ imaging agent, 2-(4'-dimethylaminophenyl)-6-(123)I-iodo-imidazo[1,2-a]pyridine ((123)I-IMPY), in adjacent brain sections. Tissues were also stained for Aβ and cholinesterase activity to visualize Aβ plaque load for comparison with radioligand uptake.

RESULTS

Synthesized and purified PIP exhibited binding to cholinesterases. (123)I was successfully incorporated into this ligand. (123)I-PIP autoradiography with human tissue revealed accumulation of radioactivity only in AD brain tissues in which Aβ plaques had cholinesterase activity. (123)I-IMPY accumulated in brain tissues with Aβ plaques from both AD and cognitively normal individuals.

CONCLUSION

Radiolabeled ligands specific for cholinesterases have potential for use in neuroimaging AD plaques during life. The compound herein described, (123)I-PIP, can detect cholinesterases associated with Aβ plaques and can distinguish AD brain tissues from those of cognitively normal older adults with Aβ plaques. Imaging cholinesterase activity associated with Aβ plaques in the living brain may contribute to the definitive diagnosis of AD during life.

Butyrylcholinesterase radioligands to image Alzheimer's disease brain

Butyrylcholinesterase (BuChE) is found to have a brain distribution pattern that is distinct from that of acetylcholinesterase (AChE). Neurons containing BuChE are particularly located in the amygdala, hippocampal formation and the thalamus, structures involved in the normal functions of cognition and behavior that typically become compromised in Alzheimer's disease (AD). Progress of this disease is thought to result, at least in part, from the accumulation of β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain. These structures characteristically become associated with cholinesterase activity, and are major determinants of AD diagnosis post-mortem. Early definitive AD diagnosis in the living brain could greatly facilitate specific timely treatment of the disorder and the search for novel drugs to preempt progress of this disease. Radioligands have been developed to detect deposition of Aβ plaques in the brain; however, since many cognitively normal individuals also exhibit Aβ plaque deposition, this approach has inherent disadvantages for definitive AD diagnosis during life. The association of BuChE with Aβ plaques appears to be a characteristic of AD. This has prompted the search for radioligands that target BuChE in association with Aβ plaques that accumulate in cortical grey matter, a region normally with very little of this enzyme activity. A number of BuChE radioligands have been synthesized and preliminary testing indicates that some such radioligands enter the brain and accumulate in regions known to contain BuChE. Radioligands targeting unusual BuChE activity in the brain may represent a means for early diagnosis and treatment monitoring of AD.