Anti-Neurodegenerative Benefits of Acetylcholinesterase Inhibitors in Alzheimer’s Disease: Nexus of Cholinergic and Nerve Growth Factor Dysfunction

Abstract

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Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that is increasingly viewed as a complex multi-dimensional disease without effective treatments. Recent randomized, placebo-controlled studies have shown volume losses of ~0.7% and ~3.5% per year, respectively, in the basal cholinergic forebrain (CBF) and hippocampus in untreated suspected prodromal AD. One year of donepezil treatment reduced these annualized rates of atrophy to about half of untreated rates. Similar positive although variable results have also been found in volumetric measurements of the cortex and whole brain in patients with mild cognitive impairment as well as more advanced AD stages after treatments with all three currently available acetylcholinesterase (AChE) inhibitors (donepezil, rivastigmine, and galantamine). Here we review the anti-neurodegenerative benefits of AChE inhibitors and the expected parallel disease-accelerating impairments caused by anticholinergics, within a framework of the cholinergic hypothesis of AD and AD-associated loss of nerve growth factor (NGF). Consistent with the “loss of trophic factor hypothesis of AD,” we propose that AChE inhibitors enhance acetylcholine-dependent release and uptake of NGF, thereby sustaining cholinergic neuronal viability and thus slowing AD-associated degeneration of the CBF, to ultimately delay dementia progression. We propose that improved cholinergic therapies for AD started early in asymptomatic persons, especially those with risk factors, will delay the onset, progression, or emergence of dementia. The currently available competitive and pseudo- irreversible AChE inhibitors are not CNS-selective and thus induce gastrointestinal toxicity that limits cortical AChE inhibition to ~30% (ranges from 19% to 41%) as measured by in vivo PET studies in patients undergoing therapy. These levels of inhibition are marginal relative to what is required for effective symptomatic treatment of dementia or slowing AD-associated neurodegeneration. In contrast, because of the inherently slow de novo synthesis of AChE in the CNS (about one-- tenth the rate of synthesis in peripheral tissues), irreversible AChE inhibitors produce significantly higher levels of inhibition in the CNS than in peripheral tissues. For example, methanesulfonyl fluoride, an irreversible inhibitor reduces CNS AChE activity by ~68% in patients undergoing therapy and ~80% in cortical biopsies of non-human primates. The full therapeutic benefits of AChE inhibitors, whether for symptomatic treatment of dementia or disease-slowing, thus would benefit by producing high levels of CNS inhibition. One way to obtain such higher levels of CNS AChE inhibition would be by using irreversible inhibitors.