DEVELOPMENT OF BUTYRYLCHOLINESTERASE LIGANDS FOR THE IMAGING OF NEUROLOGICAL DISORDERS
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Butyrylcholinesterase (BuChE) is a serine hydrolase enzyme that, along with acetylcholinesterase (AChE), catalyzes the hydrolysis of acetylcholine. These enzymes are associated with the pathology of neurologic disorders such as Alzheimer's disease (AD) and multiple sclerosis (MS). In particular, AChE and BuChE accumulate in B- amyloid (AB) plaques and tau neurofibrillary tangles in the AD brain. Thus, imaging cholinesterase activity associated with plaques and tangles in the brain has the potential to provide definitive diagnosis of AD during life. This would be advantageous since, at present, confirmation of AD relies on detecting pathology through post-mortem examination of the brain. In a similar respect, BuChE is associated with the characteristic lesions in MS brain and thus, is a promising target for diagnosis and monitoring of pathology in this disease. It is hypothesized that cholinesterase-binding radiopharmaceuticals can be used in SPECT or PET imaging to visualize these enzymes associated with AD and MS pathology in the living brain. Several classes of cholinesterase ligands were synthesized and exhibited potent binding and specificity towards AChE and BuChE using enzyme kinetic analysis. These compounds were rapidly radiolabelled with 123I and purified. Radiolabelled molecules accumulated in vitro in areas known to contain cholinesterase activity in transgenic AD mice and post-mortem human AD brain tissues, using autoradiography. Furthermore, cholinesterase activity associated with AB plaques was visualized in human and transgenic mouse AD brain tissues. An enzyme kinetic approach was employed to determine critical residues in the BuChE active site gorge for ligand binding. In particular, residues pertaining to the peripheral site of the enzyme were identified and found to be involved in the binding of various ligands. These results are crucial for optimizing the enzyme binding properties of cholinesterase imaging agents. Finally, PET imaging of a transgenic mouse model of AD was performed as a vanguard for pre-clinical evaluation of cholinesterase imaging agents. PET imaging identified similar characteristics between this AD mouse model and the human condition. This is a promising approach for evaluation of cholinesterase imaging agents. Radioligands specific for cholinesterases have the potential to provide a noninvasive means for early diagnosis of neurological diseases using brain scanning.