Challenges and Opportunities for Consideration of Efavirenz Drug Repurposing for Alzheimer's Disease Therapeutics

Abstract

Therapeutic research and development for Alzheimer's disease (AD) has been an area of intense research to alleviate memory loss and neurodegeneration. There is growing interest in drug repositioning and repurposing strategies for FDA-approved medications as potential candidates that may further advance AD therapeutics. The FDA drug efavirenz has been investigated as a candidate drug for repurposing as an AD medication. The proposed mechanism of action of efavirenz (at low doses) is the activation of the neuron-specific enzyme CYP46A1 that converts excess brain cholesterol into 24-hydroxycholesterol (24-HC) that is exported to the periphery. Efavirenz at a low dose was found to improve memory deficit in the 5XFAD model of AD that was accompanied by elevated 24-HC and reduction in Aβ; furthermore, efavirenz reduced pTau and excess cholesterol levels in human iPSC-derived Alzheimer's neurons. The low dose of efavirenz used in the AD mouse model to increase 24-HC contrasts with the use of more than 100-fold higher doses of efavirenz for clinical treatment of human immunodeficiency virus (HIV) through inhibition of reverse transcriptase. Low doses of efavirenz may avoid neurotoxic adverse effects that occur at high efavirenz doses used for HIV treatment. This review evaluates the drug properties of efavirenz with respect to its preclinical data on regulating memory deficit, pharmacokinetics, pharmacodynamics, metabolites, and genetic variabilities in drug metabolism as well as its potential adverse effects. These analyses discuss the challenges and questions that should be addressed in future studies to consider the opportunity for low dose efavirenz as a candidate for AD drug development.

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Assessment of efavirenz to modulate memory deficit in 5XFAD mouse model of AD. 5XFAD mice were treated with efavirenz at 0.1 mg/kg/day (oral administration via drinking water). Two drug treatment plans were conducted. (a) Efavirenz improves memory deficit in 9 month old 5XFAD mice. Efavirenz treatment was conducted for 8 months during the age of 2 months to 9 months, and memory was evaluated at 9 months of age by the Morris water maze test (trials on days 1–5). (b) Efavirenz compromises memory function in 5 month old 5XFAD mice. Efavirenz treatment was conducted for 4 months during the age of 2 months to 5 months, and memory was assessed at 5 months of age by the Morris water maze test (trials on days 1–5). Permission has been received to reproduce and adapt data from ref , published in 2017 by Elsevier.

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Efavirenz treatment of human iPSC AD neurons lowers p-Tau, increases 24-HC, and maintains viability. Efavirenz treatment (10 μM for 3 days) resulted in decreased pThr231Tau/tTau in human iPSC AD neurons (panel a), increased 24-HC (panel b), and maintained viability of iPSC neurons and astrocytes. This data are reproduced from ref , authored by R. van der Kant and L. Goldstein who are coauthors of this article.

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Efavirenz activation of CYP46A1 and 24-HC for regulation of p-Tau and Aβ of AD. A proposed mechanism for efavirenz targeting of CYP46A1 activation to convert cholesterol to its metabolite 24-HC, resulting in reductions in pTau and Aβ is illustrated. Cholesteryl esters are known to induce accumulation of p-Tau and Aβ in AD. Efavirenz activates CYP46A1 (cholesterol 24-hydroxylase) that metabolizes cholesterol to 24-hydroxycholesterol (24-HC) which, thus, reduces cholesterol and cholesteryl esters (CE). The decreased CE results in reduction of CE-mediated increases in p-Tau and Aβ.

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Efavirenz activation of human CYP46A1 for improved memory function. (a) Activation of human CYP46A1 by efavirenz at different concentrations. Recombinant human CYP46A1 was incubated with efavirenz at different concentrations, and production of 24-HC was measured. This data is reproduced from ref , covered by the Open Access license, Creative Commons Attribution License (CC BY), 2014, Elsevier, by authors Mast et al. (b) CYP46A1 expression improves memory deficit in APP 23 AD mice. The wild-type CYP46A1 gene, as well as inactive mutant CYP46A1, was expressed in APP23 AD mice. Mice were assessed for memory deficit by the Morris water maze test measuring escape latency. This data is reproduced from ref , covered by the Open Access license, Creative Commons Attribution-NonCommericial-NoDerivs (CC BY-NC-ND 4.0), 2010, Molecular Therapy Family of Journals, by authors Hudry et al.

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CYP46A1 gene expression in human brain regions. The distribution of CYP46A1 gene expression is illustrated in human adult brain (age 57, male). Expression was obtained from the Allen Human Brain Atlas (https://human.brain-map.org/). (a) CYP46A1 expression in cortical regions of human brain. The sagittal brain view shows CYP46A1 expression in cortical brain areas. (b) CYP46A1 in brain regions. A cross-section human brain view shows CYP46A1 expression in the temporal lobe, hippocampus, cingulate gyrus, and insula brain regions. (c) Relative expression of CYP46A1 among multiple brain regions shown by a heat map. CYP46A1 expression (relative) in different brain regions is shown in a color-coded manner. The top bar shows the region names indicated by abbreviations: FL, frontal lobe; CgG, cingulate gyrus; HiF, hippocampal formation; OL, occipital lobe; PL, parietal lobe; TL, temporal lobe; BF, basal forebrain; Str, striatum; Cl, claustrum; Hy, hypothalamus; DT, dorsal thalamus; VT, ventral thalamus; MES, mesencephalon; CbCx, cerebellar cortex; and MY, myelencephalon.

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Metabolites of efavirenz generated by CYP2B6, CYP2A6, and UGT. Efavirenz undergoes metabolism by CYP2B6 to generate 8-hydroxyefavienz and by CYP2A6 to generate 7-hydroxyefavirenz. − Secondary metabolites of 8,14-dihydroxyefavirenz and 7,8-dihydroxyefavirenz are generated by CYP2B6. , These metabolites undergo further metabolism by UDP-glucuronyltransferases (UGT) to generate glucuronide-conjugated metabolites. , The metabolite 8,14-dihydroxyefavirenz activates CYP46A1. Metabolites 7-hydroxyefavirenz and 8-hydroxyefavirenz produce neurotoxic effects. SNPs in the CYP2B6 and CYP2A6 metabolizing enzymes largely result in decreased metabolizing activities and, therefore, increases in efavirenz. ,