Several cholinesterase inhibitors have also been radiolabell

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 iwr 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.
Results
Discussion Current attempts to obtain a definitive diagnosis of AD through specific amyloid neuroimaging fall short since β-amyloid is frequently found in brains of cognitively normal older adults.12, 13 In contrast, using available neuroimaging techniques to detect the characteristic association of cholinesterases with AD pathology may provide a more definitive diagnosis during life. The development of ligands that bind tightly to cholinesterases and that are also amenable to efficient radiolabelling, with isotopes like 18F for PET imaging, may offer new opportunities to reveal AD pathology in the living brain and enable more timely diagnosis and monitoring of treatment for the disease. Trifluoroacetophenones are good candidates for such ligands. Certain acetophenones have been shown to inhibit AChE through covalent interaction between the carbonyl functionality of the ketone inhibitor and the active site serine of the enzyme (Scheme 6). This interaction is facilitated by enhancing the electrophilicity of the ketone carbonyl carbon with adjacent electron-withdrawing groups such as a trifluoromethyl group (e.g., Compounds 1 and 2; Fig. 1). Replacement of hydrogen atoms with small electronegative fluorine atoms produces a bioisostere with comparable structure but enhanced ability to form a hemiketal intermediate that blocks substrate access to the enzyme. Trifluoromethyl ketones are known to be potent inhibitors of AChE.41, 42, 43 The nucleophile reacting with the ketone is the activated alkoxide of the catalytic serine residue of the enzyme (Ser203 in human AChE), that results in the formation of the hemiketal-enzyme complex (Scheme 6).44, 45