Repurposing Duloxetine\nas a Potent Butyrylcholinesterase\nInhibitor: Potential Cholinergic Enhancing Benefits for Elderly Individuals\nwith Depression and Cognitive Impairment

Despite the advent of new treatment strategies, cholinesterase\ninhibitors (ChEIs) are still the go-to treatment for dementia disorders.\nChEIs act by inhibiting the main acetylcholine-degrading enzyme, acetylcholinesterase\n(AChE). Nonetheless, accumulating evidence indicates that the impact\nof inhibition of the sister enzyme, butyrylcholinesterase (BChE),\ncould be even broader in older adults due to the multifaceted role\nof BChE in several biological functional pathways. Therefore, we employed\nan <i>in silico</i> modeling-based drug repurposing strategy\nto identify novel potent BChE inhibitors from the FDA drug database.\nThis was followed by <i>in vitro</i> screening and <i>ex vivo</i> enzyme kinetic validation using human plasma samples\nas the source of BChE. The analysis revealed that the antidepressant\ndrug, duloxetine, inhibited BChE with high selectivity in comparison\nto AChE. In contrast, two other antidepressants, namely, citalopram\nand escitalopram exhibited a weak to moderate activity. <i>Ex\nvivo</i> enzyme inhibition kinetic analyses indicated that duloxetine\nacted as a competitive inhibitor of BChE with an inhibition constant\n(<i>K</i><sub>i</sub>) of 210 nM. This <i>K</i><sub>i</sub> value is comparable with 100–400 nM concentration\nof duloxetine following normal dosages in humans, thereby indicating\nthat duloxetine should be able to induce a pharmacologically and biologically\nrelevant <i>in vivo</i> inhibition of BChE. Additionally,\nwe performed the enzyme inhibition kinetic assessment in parallel\nfor ethopropazine, a known potent selective BChE inhibitor, and physostigmine,\na dual inhibitor of AChE and BChE. These analyses indicated that duloxetine\nshould be considered a potent BChE inhibitor since its <i>K</i><sub>i</sub> was comparable with ethopropazine (<i>K</i><sub>i</sub> = 150 nM) but was 4 times smaller than that of physostigmine\n(<i>K</i><sub>i</sub> <i>=</i> 840 nM). In conclusion,\nthis study reports the discovery of duloxetine being a highly potent\nselective competitive BChE inhibitor. This, in turn, indicates that\nduloxetine could be the choice of antidepressive treatment in older\nadults with both depressive and dementia symptoms since it may offer\nadditional clinically beneficial effects <i>via</i> this\nsecondary mode of cholinergic enhancing action.