PET probes for imaging brain acetylcholinesterase

Imaging acetylcholinesterase (AChE) is valuable not only for diagnosing and understanding dementia but also for monitoring the effects of cholinesterase inhibitors used as antidementia drugs and for determining the appropriate clinical dosage of newly developed cholinesterase inhibitors. The distribution of AChE in the living brain can be imaged with two different types of radioprobes, including substrate-type and ligand-type probes. The substrate-type positron emission tomography (PET) probes, N-[(11) C]methylpiperidin-4-yl acetate ([(11) C]MP4A), and its propionate, [(11) C]MP4P, have been widely used in clinical studies of dementia, including Alzheimer's disease. [(11) C]MP4A and [(11) C]MP4P have been used to demonstrate a reduction in AChE activity in the brains of dementia patients, as well as the bioavailability of AChE inhibitors, leading to the subsequent development of the widely available (18) F-labeled derivatives of MP4A. In addition, several radiolabeled cholinesterase inhibitors have been developed as PET probes for AChE mapping in the brain. Herein, we have reviewed the development of PET probes for the imaging of AChE in the brain and described the principles of measuring AChE activity in the brain using PET with substrate-type radioprobes. A discussion of the reagents developed from substrate-type PET probes for the specific measurement of AChE activity in vitro has also been provided.