The cholinesterases acetylcholinesterase AChE and butyrylcho

The cholinesterases, acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), are co-regulators of acetylcholine. These enzymes have been found to co-localize with Aβ plaques that accumulate in various regions of AD brains, especially in the cerebral cortex.15, 16, 17, 18 In the normal human CU CPT 22 BuChE activity is associated with certain populations of neurons distinct from those with AChE activity.19, 20, 21 In AD, there is an association of cholinesterase activity, particularly that of BuChE, with Aβ plaques.15, 16, 17, 18 Imaging of cholinesterase activity in Aβ plaques has the potential to permit differentiation between such plaques in the brains of cognitively normal older adults from those in AD brains. Investigations into the distribution of cholinesterases in the brain have been conducted with radiolabelled ligands. A number of 11C cholinesterase-targeting radioligand substrates have been prepared and tested for the ability to image brain cholinesterase activity. These include N-11C-methylpiperidyl acetate and N-11C-methylpiperidyl propionate24, 25 for detection of AChE and N-11C-methylpiperidyl butyrate24, 26, 27, 28 for detection of BuChE. However, these agents have yielded only partial recapitulation of known cholinesterase distribution in human brains.23, 26, 27, 28 These substrate-radiotracers, however, have provided proof-of-concept that a sufficient amount survives hydrolysis by cholinesterases in the periphery, crosses the blood–brain-barrier, and binds to and is hydrolyzed by cholinesterases, to provide PET images through metabolic trapping. Several cholinesterase inhibitors have also been radiolabelled with 11C as potential cholinesterase imaging agents. These include 11C-donepezil, 11C-methyltacrine, and 11C-physostigmine, which also have had limited success in demonstrating the known histochemically defined cholinesterase distribution in the brain. Thus, there is a need to develop more effective radioligands that can detect, in the living brain, the distribution of cholinesterases characteristic of AD pathology. A number of trifluorinated acetophenone derivatives have been shown to be very potent, time-dependent inhibitors of AChE.29, 30 Foremost among these is N,N,N-trimethyl ammonium trifluoroacetophenone (TMTFA), which has very high affinity for AChE. However, this derivative is a quaternary ammonium cation, a property that may represent a possible impediment to crossing the blood–brain barrier for PET imaging cholinesterase activity in the brain. Somewhat less potent neutral trifluoroacetophenones have also been shown to bind to and inhibit AChE and, being uncharged, should cross the blood–brain barrier more readily with estimated LogP values between 2.7 and 3.5. Such 18F-radiolabelled compounds could potentially be used for PET imaging of cholinesterase-associated pathology in the living brain. Radiolabelling of acetophenones has been reported through electrophilic addition using 18F–19F gas. The focus of the present work is to develop a generally applicable methodology involving nucleophilic substitution of 18F− for chlorine in chlorodifluoro acetophenone derivatives, thereby avoiding the inherent difficulties of handling 18F–19F gas. Herein we report the synthesis of m-(N,N-dimethylamino)trifluoroacetophenone (1) and m-(tert-butyl)trifluoroacetophenone (2) and a comparable synthesis of the chlorodifluoro analogues 3 and 4 as precursors for conversion to 1 and 2 by displacement of a chlorine leaving group by F−, a reaction amenable to incorporation of 18F−. Each of the synthesized acetophenones (1–4, Fig. 1) was tested for its ability to inhibit the enzyme activities of human AChE and BuChE, and all exhibited affinity for both cholinesterases. The reactions of the chlorodifluoromethylketones with AChE have not been reported or quantified previously, and reactions of none of the four compounds with BChE have been previously described. The Weinreb amide strategy for the synthesis of the acetophenone derivatives is consistent with expectations from previous literature reports and represent a valuable practical addition for the preparation of PET tracers. Additional in vivo work is beyond the scope of this report but will be conducted in the future.