Synthesis and cholinesterase activity of fluorinated donepezil derivatives

Abstract

Alzheimer’s disease (AD) is the most common form of dementia and leads to an irreversible brain disorder slowly destroying memory and other mental functions. Although there are no treatments currently available for AD, cholinesterase inhibitors (Aricept, Exelon, Razadyne) have been developed as an approach to slow the cognitive decline. Aricept is the brand name for donepezil and this project was to evaluate several options for the introduction of fluorine into the structure. The incorporation of fluorine into bioactive molecules is not only an established approach to modify PK/PD properties but also facilitates clinical translation through the use of in vivo diagnostics, for example by using fluorine-18 in PET imaging. The positions considered in this thesis, for the introduction of fluorine, were unexplored benzylic positions and the more tradition aromatic sites of the N-benzyl group. A parallel methodology study in the research group found that α-fluorobenzylamines were not viable as the functionality was not stable due to loss of fluoride leading to the formation of an iminium ion which, on subsequent hydrolysis rapidly resulting in loss of the benzyl group so the other benzylic position associated with the indanone group was the initial focus. Although a range of Fe(acac)2 catalysed approaches to the benzylic fluorination of a number of model compounds were successful this was not the outcome for indanone derived substrates where fluorination alpha to the ketone was preferred. To directly access the desired position we utilised a Mn-salen catalysed approach however fluorination still occurred at both types of C-H group in the cyclic ketone albeit with increased levels of benzylic fluorination. Modification of the salen ligand was investigated to optimise this selectivity and, in this case, it was found that Mn[salen-(Cl2)2]Cl was very selective for the benzylic site in indanone based model ii substrates. Application to donepezil itself retained this selectivity providing the first route to direct fluorination of this position. Although this novel approach was successful, the yield currently remains low and the use of an electrophilic source of fluorine is not ideal for PET imaging due to the low specific activity of the products produced. 2-[ 19F]FDP, 3-[ 19F]FDP and 4-[ 19F]FDP were also considered as they were accessible using a nucleophilic source of fluorine and thus would allow the production of the corresponding radiotracers; 2-[ 18F]FDP, 3-[ 18F]FDP and 4-[ 18F]FDP as potential imaging agents to determine the biodistribution and levels of cholinesterase activity in vivo. Diaryliodonium salts were proposed as radiolabelling precursors as this is one of the few technologies that would use the same approach to incorporate 18F into all three of the aromatic positions. Two main types of cholinesterase are present in the body – acetylcholinesterase (AChE) and butyryl cholinesterase (BuChE), yet their relative importance in vivo is not clear. To determine the effect of fluorination of the AChE/BuChE selectivity the fluorobenzyl derivatives of donepezil were prepared and their AChE and BuChE activity determined. In all cases the incorporation of fluorine resulted in reduced inhibition of AChE whereas in the case of the 2-fluoro derivative the potency and selectivity for BuChE was markedly increased whereas fluorination at the 3- or 4- position made little difference. In summary, this suggests that a range of fluorinated derivatives of donepezil are possible however their biological activity may not be directly comparable to donepezil itself. This is particularly important in the in vivo evaluation of AChE and/or BuChE distribution from PET imaging data and that a complimentary pair of imaging agents is needed – selective for either AChE or BuChE.