Evaluation of the Effect of Alzheimer's Disease and Related Disorders FDA-Approved Drugs on Amyloid-beta Brain and Hepatic Clearance

In Alzheimer's disease (AD), accumulation of brain amyloid-beta (Abeta) occurs due to imbalance between its production and clearance. Transport across the blood-brain barrier (BBB) is a primary pathway for removal of Abeta from brain to blood, which acts in concert with peripheral sink created by the liver to maintain Abeta brain homeostasis. The incidence of AD increases with age and failure of Abeta brain and peripheral clearance correlates with AD. Memantine and the cholinesterase inhibitors (ChEIs), donepezil, galantamine and rivastigmine, are U.S. Food and Drug Administration (FDA) approved drugs that are used to ease symptoms of dementia associated with AD and related disorders (ADRD). Tacrine was approved by FDA for AD, but currently has a limited clinical use because of its reported hepatotoxicity. Mounting preclinical evidence reported that ADRD-FDA drugs provide non-classical (non-cholinergic) effects that may alter Amyloid-beta (Abeta)-related pathology; however, the mechanism(s) by which these drugs demonstrate such effect is not fully understood. In this project, we explored other mechanisms by which ADRD-FDA drugs provide effects independent from their classical effect, including modulation of Abeta brain and hepatic clearance, Abeta brain level and Abeta-mediated brain pathology. Since Abeta hepatic clearance has not been characterized, our first project was to investigate the putative transport proteins that contribute to Abeta hepatic clearance. Using sandwich-cultured primary hepatocytes, we discovered that low density lipoprotein receptor related protein (LRP1) and receptor for advanced glycation end products (RAGE) are involved in Abeta sinusoidal uptake, with major contribution to LRP1. We report for the first time that P-glycoprotein (P-gp) mediates Abeta canalicular efflux into bile compartment. Role of P-gp and LRP1 in Abeta hepatic clearance was further confirmed by induction studies with rifampicin which increased Abeta clearance by 40% as a result of both transport proteins up-regulation. Therefore, inducing Abeta clearance via up-regulating P-gp and LRP1 in the liver could be a useful therapeutic approach to restore peripheral hepatic sink that is reduced with aging and in AD. Next in project II, after characterizing Abeta hepatic clearance, we explored the effect of ADRD-FDA drugs on both Abeta brain and hepatic clearance. Initial findings from in vitro studies demonstrated that ADRD-FDA approved drugs modulate Abeta brain and hepatic clearance differently, with enhancement in Abeta clearance that was most pronounced with donepezil and rivastigmine. Both drugs were further examined in young and aged rats and showed enhancement in 125I-Abeta 40 brain and hepatic clearance, and were concomitant with up-regulation in the expression of major transport proteins involved in A? clearance, P-gp and LRP1. In addition, rivastigmine rescued the reduction in 125I-Abeta 40 clearance caused by aging, and was associated with significant decline in brain levels of endogenous Abeta determined by ELISA. These findings suggest that donepezil and rivastigmine mediate Abeta brain removal through up-regulation of P-gp and LRP1. Alteration in the expression level of P-gp in the brain has recently been suggested to play a key role in the etiology and pathogenesis of neurologic disorders such as AD. In humans, Abeta brain level in elderly and AD is inversely correlated with P-gp expression in brain microvessels. Accordingly, in project- III, we extend our work to understand the role of P-gp in mediating rivastigmine non classical effects, including reducing Abeta brain load, neuroprotective and anti-inflammatory effects in transgenic APP mouse model that express different expression levels of P-gp. As expected, rivastigmine enhanced acetylcholine brain level which was more pronounced in P-gp wild type mice. In addition, rivastigmine treatment decreased Abeta brain load in APP, P-gp wild type mice, while its effect was less pronounced in APP, P-gp null mice. Similarly, rivastigmine showed anti-inflammatory effect by reducing hippocampal astrogliosis by 50% and interleukin-1beta brain level by 43% in APP, P-gp wild type mice and was less pronounced in P-gp null mice. Moreover, rivastigmine showed significant neuroprotective effect by increasing the expression of synaptic markers. Collectively, our findings provide a novel mechanism for donepezil and rivastigmine to modulate Abeta brain levels and reduce Abeta-related pathology. Understanding ADRD-FDA approved drugs mechanism of action will help optimizing their clinical use and future drug development by providing new drug targets and possible mechanisms involved in AD pathology. Our results add further support for the involvement of the ABC transport protein P-gp in the clearance of Abeta across the BBB as well as in modulating the therapeutic effects of ADRD-FDA approved drugs.