Transcript Acetylcholinesterase Inhibitors in the Treatment of Alzheimer*s and

Acetylcholinesterase
Inhibitors
in the Treatment of
Alzheimer’s and Dementia
Pharmaceutical Chemistry II – SSPPS 222
Based on Presentation from : Victor Ramos, Lisa Ferris, and Sarah
Brown
Disease: Alzheimer’s Disease & Stats
Alzheimer’s 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 Alzheimer’s sufferers are women
 6th 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
Disease: Some Alzheimer’s Etiologies
and Possible Therapeutic pathways
Degradation of
Acetylcholine
Name
Etiology
Choliner Alzheimer’s is characterized by an
gic
acetylcholine deficiency due to atrophy
and degeneration of cholinergic neurons
Amyloid Beta-amyloid peptides, partial aggregates
and plaques (or a close relative of βA)
build up in the brain and change synapses,
disrupting communication
Tau
Tau protein is hyperphosphorylated, and
this initiates a cascade in which
neurofibrillary tangles destroy the
transport system inside neurons
Disease/Drugs: History of AZ Drugs
for Different Pathways
 Acetylcholinesterase inhibitors
 1993: Tacrine approved for mild to moderate Alzheimer’s symptoms
 1996: Donepezil approved for mild to severe Alzheimer’s symptoms
 2000: Rivastigmine approved for mild to moderate Alzheimer’s symptoms
 2001: Galantamine approved for mild to moderate Alzheimer’s symptoms
 Namenda (NMDA receptor antagonist)
 2003: Namenda approved for moderate to severe Alzheimer’s symptoms
 2010: Namenda XR approved for moderate to severe Alzheimer’s
symptoms
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
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
Target: Acetylcholinesterase
 Acetylcholinesterase rapidly
hydrolyzes Ach
 Terminates transmission at
cholinergic synapses
 Alzheimer’s 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: 14Å
from anionic subsite
 Enhanced potency if drug can
span both active sites
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
Drugs AZ-AChE: Chemical Properties
Brand Name
Cognex
Aricept
Razadyne
Generic Name
Tacrine
Donepezil
Galantamine
3.5 nM
12 nM
200 nM
Acetylcholinesterase,
Butyrylcholinesterase
Acetylcholinesterase
Acetylcholinesterase,
Butyrylcholinesterase
Molecular Structure
Salt
Ionization/Delivery
Kd
Reversible or
Covalent?/timing
Drug Target
Drug Molecules
Tacrine
Donepezil
Galantamine
 Tacrine has no chiral centers
 Galantamine has three chiral centers and the (S,R,S) conformer
is the naturally occurring form
 Donepezil’s two stereoisomers show activity but its Renantiomer has more activity
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
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 π–π 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
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
inhibitor’s 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
Polar and Non-Polar Characteristics
Tacrine
Galantamine
Donepezil
H-bond donors
2
1
0
H-bond
acceptors
2
4
4
Polar Atoms
2
4
4
Non-Polar Atoms
13
17
24
Polar Surface
Area
38.91 Å2
41.93 Å2
38.77 Å2
Physiological
Charge
+1
+1
+1
Water Solubility
0.136 g/l
1.70 g/l
0.00291 g/L
logP
3.13
1.8
4.14
Pharmacokinetic Properties
Tacrine
Galantamine
Donepezil
Administration
20 mg, 4x daily
8-12 mg, 2x daily
10 mg, 1x daily
Tmax
0.5 - 2 hours
0.5 – 1.5 hours
3 – 5 hours
AUC
83.2 +/- 26.7 μg
h/L
N/R
357.7 +/- 64.0
μg h/L
Bioavailability
17-24%
85-100%
100%
Volume of
Distribution
3.7 – 5.0 L/kg
0.83 – 2.75 L/kg
12 L/kg
Half-life
2-4 hours
7 hours
70 hours
Protein binding
75%
18%
96%
Metabolic
Elimination
CYP2D6, CYP1A2 CYP2D6, CYP3A4
CYP2D6, CYP3A4
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

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
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
Alzheimer’s
 NSAIDs have had variable effects
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 Alzheimer’s and
does not address the underlying pathophysiology of the
disease state
 New treatments will likely target other factors involved
in Alzheimer’s – drugs targeting amyloid-beta plaques
and tau proteins are currently being developed
 Combination therapies
References
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