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Acetylcholinesterase Inhibitors in the Treatment of Alzheimers and Dementia Pharmaceutical Chemistry II SSPPS 222 Based on Presentation from : Victor Ramos, Lisa Ferris, and Sarah Brown

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Disease: Alzheimers Disease & Stats Alzheimers is a form of dementia 2012 Statistics 5.4 million citizens (5.2 million 65 and older) One in eight older Americans By 2025, 6.7 million (30% increase) 2/3 of Alzheimers sufferers are women 6 th leading cause of death in the United States Payments for care are estimated to exceed $200 billion 80% of care is delivered by family (valued at over $210 billion) http://www.alz.org/downloads/facts_figures_2012.pdf

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Disease: Some Alzheimers Etiologies and Possible Therapeutic pathways NameEtiology Choliner gic Alzheimers is characterized by an acetylcholine deficiency due to atrophy and degeneration of cholinergic neurons AmyloidBeta-amyloid peptides, partial aggregates and plaques (or a close relative of A) build up in the brain and change synapses, disrupting communication TauTau protein is hyperphosphorylated, and this initiates a cascade in which neurofibrillary tangles destroy the transport system inside neurons Degradation of Acetylcholine

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Disease/Drugs: History of AZ Drugs for Different Pathways Acetylcholinesterase inhibitors 1993: Tacrine approved for mild to moderate Alzheimers symptoms 1996: Donepezil approved for mild to severe Alzheimers symptoms 2000: Rivastigmine approved for mild to moderate Alzheimers symptoms 2001: Galantamine approved for mild to moderate Alzheimers symptoms Namenda (NMDA receptor antagonist) 2003: Namenda approved for moderate to severe Alzheimers symptoms 2010: Namenda XR approved for moderate to severe Alzheimers symptoms

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Target 1: AChE: Mechanism of Action Acetylcholinesterase breaks down Ach into choline and an acetate through hydrolysis Acetylcholinesterase inhibitors block this reaction in several regions of the brain There is a significant correlation between acetylcholinesterase inhibition and observed cognitive improvement

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Target: Acetylcholinesterase 2 general classes of molecular forms Simple homomeric oligomers of catalytic subunits Founds as soluble species in cell Exported Heteromeric associations of catalytic subunits with structural subunits Found in neuronal synapses Tetramer of catalytic subunits disulfide linked to a 20kDa lipid-linked subunit Outer surface of cell membrane

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Target: Acetylcholinesterase Acetylcholinesterase rapidly hydrolyzes Ach Terminates transmission at cholinergic synapses Alzheimers may involve depletion of Ach Inhibition of acetylcholinesterase could help symptoms Active Site Esteratic subsite: catalytic machinery Anionic subsite: binds quaternary group of Ach Peripheral anionic subsite: 14 from anionic subsite Enhanced potency if drug can span both active sites

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Target: Acetylcholinesterase Site Contains catalytic triad Located at bottom of aromatic gorge Deep, narrow cavity 40% lined by rings of 14 aromatic amino acids Primary site of interaction between quaternary group of Ach and acetylcholinesterase is aromatic ring of Trp-84 Trp-84 and Phe-330 part of anionic subsite Trp-275 part of peripheral anionic subsite

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Drugs AZ-AChE: Chemical Properties Brand NameCognexAriceptRazadyne Generic NameTacrineDonepezilGalantamine Molecular Structure Salt Ionization/Delivery Kd3.5 nM12 nM200 nM Reversible or Covalent?/timing Drug TargetAcetylcholinesterase, Butyrylcholinesterase AcetylcholinesteraseAcetylcholinesterase, Butyrylcholinesterase

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Drug Molecules Donepezil Tacrine Galantamine Tacrine has no chiral centers Galantamine has three chiral centers and the (S,R,S) conformer is the naturally occurring form Donepezils two stereoisomers show activity but its R- enantiomer has more activity

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Drug: Tacrine Normally, phenyl ring of Phe-330 lies parallel to surface of gorge When tacrine binds, it makes contact with the bound ligand Ring of Phe-330 is rotated about both X1 and X2 Tacrine is thus sandwiched between between the rings of Phe-330 and Trp-84 Recall Trp-84 is primary site of interaction between Ach and acetylcholinesterase

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Drug Groups: Donepezil Three segments of Donepezil, all interact with Acetylcholinesterase gorge Dimethoxyindanone Inandone ring has pi-pi interactions with indole ring of Trp279 Piperidine Cation-pi interaction with Phe330 Ring N makes H bonds with water which makes H bonds with Tyr121 Benzyl Parallel stacking with the Trp84 indole, Makes an aromatic H-bonds with water molecules that H-bond to the residues of the oxyanion hole, namely with Gly118 N, Gly119, Gly201 N, and Ser200 Occupies the binding site for quaternary ligands such a ACh

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Drug Groups: Galantamine The inhibitor spans the active site gorge, including the acyl binding site Hydrogen bonding Two H-bonds form between the hydroxyl of the inhibitor and Glu-199 and Ser-200 and the inhibitors oxygen molecule Water molecules Rest of interactions are Non-Polar Notable that galantamine lacks the characteristic cation-pi interaction with Phe-330 Pi-stacking occurs between the double bonds in the cyclohexene ring of GAL and the indole ring of Trp-84 No charge-charge interactions

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Polar and Non-Polar Characteristics TacrineGalantamineDonepezil H-bond donors210 H-bond acceptors 244 Polar Atoms244 Non-Polar Atoms131724 Polar Surface Area 38.91 241.93 238.77 2 Physiological Charge +1 Water Solubility0.136 g/l1.70 g/l0.00291 g/L logP3.131.84.14

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Pharmacokinetic Properties TacrineGalantamineDonepezil Administration20 mg, 4x daily8-12 mg, 2x daily10 mg, 1x daily Tmax0.5 - 2 hours0.5 1.5 hours3 5 hours AUC83.2 +/- 26.7 g h/L N/R357.7 +/- 64.0 g h/L Bioavailability17-24%85-100%100% Volume of Distribution 3.7 5.0 L/kg0.83 2.75 L/kg12 L/kg Half-life2-4 hours7 hours70 hours Protein binding75%18%96% Metabolic Elimination CYP2D6, CYP1A2CYP2D6, CYP3A4

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Drugs: Side Effects Tacrine Causes elevated hepatic enzymes (CYP1A2) and is hepatotoxic Tacrine metabolite is cytotoxic Off market Galantamine Abdominal pain, diarrhea, nausea related to cholinergic effects Resolve with continued treatment Donepezil Well tolerated at 5 mg/day 13% discontinuation rate at 10 mg/day. Gastrointestinal side effects are most common, related to cholinergic effects All acetylcholinesterase inhibitors act through similar mechanisms, so GI side effects are similar, with severity depending on the dose administered Increased acetylcholine over-stimulates cholinergic receptors in the GI tract to cause secretory and motor activity

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Drug-Drug Interactions CYP34A inhibitors like erythromycin, cimetidine, and saquinavir increase bioavailability of the drugs and lead to increased adverse effects The same is true for CYP2D6 and CYP1A2 inhibitors In contrast, inducers of these metabolic enzymes like phenytoin and rifampicin will decrease bioavailability and lead to limited efficacy of the drugs

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Future Treatments Immunizations that utilize the immune system to attack beta-amyloid plaques This went to clinical trials but was stopped when some participants developed acute brain inflammation Anti-amyloid antibodies derived from other sources infused into the blood via IV Preventing neurofibrillary tangles Reducing chronic neuron inflammation associated with Alzheimers NSAIDs have had variable effects

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Conclusion These drugs effectively inhibit acetylcholinesterase from hydrolyzing acetylcholine into choline and an acetyl group However, this may or may not be effective in prolonging onset or reducing symptom severity in Alzheimers and does not address the underlying pathophysiology of the disease state New treatments will likely target other factors involved in Alzheimers drugs targeting amyloid-beta plaques and tau proteins are currently being developed Combination therapies

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References "2012 Alzheimer's Disease Facts and Figures." Alzheimer's and Dementia 8.2 (2012): 1-67. 2012. Web. 7 Mar. 2013. A. Koster, Hemmung der cholinesterasen in verscheidenen organen durch eserin, galanthamine und tacrin; conzentrations-wirkungsbeziehungen, bedeuting fir die therapeutisch anwendung. Dissertation 1994; Medezinsiche Fakultat der Humboldt Univ zu Berlin. Abagyan, R., Physical Pharmacology. http://xablab.ucsd.edu/ (accessed March 5, 2013). "Alzheimer's Disease Treatments." Alzheimer's Disease Treatments. BrightFocus Foundation, 4 Oct. 2012. Web. 07 Mar. 2013. "Alzheimer's Treatments: What's on the Horizon?" Mayo Clinic. Mayo Foundation for Medical Education and Research, 06 Mar. 2013. Web. 07 Mar. 2013. "Drug Bank: Donepezil." DrugBank. GenomeQuest, 8 Feb. 2013. Web. 7 Mar. 2013. "Drug Bank: Galantamine." DrugBank. GenomeQuest, 8 Feb. 2013. Web. 7 Mar. 2013. "Drug Bank: Tacrine." DrugBank. GenomeQuest, 8 Feb. 2013. Web. 7 Mar. 2013. Greenblatt, H., et al. "Structure of Acetylcholinesterase Complexed with (-)-galanthamine at 2.3 A Resolution." Federation of European Biochemical Societies 463 (1999): 321-26. FEBS Letters, 8 Nov. 1999. Web. 7 Mar. 2013. Harel, M., et al. "Quaternary Ligand Binding to Aromatic Residues in the Active-site Gorge of Acetylcholinesterase." Proc Natl Acad Sci U S A 90.19 (1993): 9031-035. PubMed. Web. 7 Mar. 2013. Kryger, G., et al. "Structure of Acetylcholinesterase Complexed with E2020 (Aricept): Implications for the Design of New Anti-Alzheimer Drugs." Structure 7.3 (1999): 297-307. Elsevier Science Ltd., 1 Mar. 1999. Web. 7 Mar. 2013. Maccioni, R., Perry, G., Current Hypotheses and Research Milestones in Alzheimer's Disease. New York: Springer, 2009. Print. Massouli, J Molecular forms and anchoring of acetylcholinesterase. In, Cholinesterases and Cholinesterase Inhibitors. (Giacobini E, ed) Martin Dunitz, London, 2000 pp. 81-103 Sussman, J.. et al. "Atomic Structure of Acetylcholinesterase from Torpedo Californica: A Prototypic Acetylcholine-Binding Protein." Science 253 (1991): 253-61. Sciencemag.org. 21 Dec. 2006. Web. 7 Mar. 2013.