PROTON PUMP INHIBITORS
At the end of the lecture students will be able to
Understand where and how drugs work
Understand principles of how drugs are designed
Understand process of how drugs are designed
Illustrate drug design and development of proton pump inhibitors
using examples
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Learning Outcomes
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CO2H2O H2CO3 H+HCO3
+Carbonic anhydrase
Fig. Enzyme catalysed generation of protons in the parietal cells
Gastric juices consists of digestive enzymes and hydrochloric acid
designed to break down food.
Hydrochloric acid is secreted from parietal cells and the stomach
secrets a layer of mucus to protect itself from its own gastric juices
The protons required for HCl are generated from water and carbon
dioxide catalysed by an enzyme called carbonic anhydrase
Parietal Cells and the Proton Pump
Once proton have been generated, they have to be exported out of the
cells rather than stored because of two reasons:
build up of acids within cells would be harmful to the cell
Enzyme catalysed reaction which generates protons is reversible
The export of protons from the parietal cells is achieved by an enzyme
complex called the proton pump or H+/ K+-ATPase
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Lumen of the stomach
Proton pump
Receptors
Ion channels
Cck2H2
M3
ATP ADP + Pi
Canaliculus
Parietal Cells and the Proton Pump
H+ +K
HCl
Cl-
Histamine Acetylcholine Gastrin
Parietal cell
When the parietal cells are actively secreting HCl into the stomach,
they form invaginations called canaliculus.
Proton pump is present in the canalicular membrane of parietal cells
Pumps protons out of the parietal cell and potassium ions back in
Requires energy - provided by hydrolysis of ATP to ADP, catalysed by
ATPase
The proton pump is also called H+/K+-ATPase
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Parietal Cells and the Proton Pump
Proton pump is not responsible for the efflux of chloride ion
Chloride ions depart through a separate ion channel
HCl is formed in the canaliculus
The potassium ions exit the parietal cell as counterions for the
chloride ions and are then pumped back in
A separate potassium ion channel is used for K+ ions leaving the cell
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The acetylcholine (neurotransmitter), histamine (hormone) and the
gastrin (hormone) activate the cholinergic receptor, H2-receptor and
gastrin-receptor of the parietal cells which in turn activates proton
pump leading to the release of gastric acid into stomach.
The trigger for this process is provided by the sight, smell, or even
thought of food
Thus gastric acid is released before food has even entered the stomach
Release of gastric acid can be inhibited by antagonists blocking either
the cholinergic receptor or the receptor for gastrin.
Unfortunately these antagonists also block the acetylcholine receptors
at other part of the body and cause unwanted side effects
Similarly the histamine antagonists have proved to be important antiulcer
drug but they have now largely been suppresed by PPIs which blocks the
mechanism by which HCl is released from paietal cells
Parietal Cells and the Proton Pump
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It was seen by Swedish scientists that local
anaestetics related to lignocaine reduce gastric
acid secretion in man when taken orally
Then the Swedish scientists attended a
conference where they discovered that a
potential antiviral drug called
pyridylthioacetamide slowed down gastric
secretion as a side effect
This drug was toxic due to thioacetate group
To overcome this other S-C-N groups were
investigated and H 77/67 was discovered which
had antisecretory activity
Design and Development of Proton Pump Inhibitors
N
H
CH3
CH3
O
Lignocane
N
Pyridylthioacetamide
S
NH2
N
H 77/67
S
NH
N
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Structural variants of H77/67 were prepared in
the quest for a better compound
This work culminated in the discovery of the
benzimidazole compound H 124/26
Metaboilsm studies showed that H 124/26
formed a more potent sulphoxide called
timoprazole
It turned out that timoprazole prevented
uptake of iodine by the thiroid
N
H 124/26
S
NH
N
NTimoprazole
S
NH
NO
N Picoprazole
S
NH
NOCH3CO2CH3
CH3
More analogues were prepared and picoprazole was found to be free
from side effects
N
H 77/67
S
NH
N
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More potent analogues were sought
This was achieved by increasing the pKa of the pyridine ring by placing
electron donating groups on it
A promising, but unstable, analogue H 159/69 was formed
This led to the discovery of its analogue omeprazole
OM Launched in 1988 by Astra - World’s biggest selling drug
N H 159/69
S
NH
NOCH3CO2CH3
CH3
H3CO
H3C
N Omeprazole
S
NH
NOCH3OCH3H3CO
H3C
Effect of Substituents on the pyridine ring
N
MeMeO
Me R
N
MeMeO
Me R
N
MeMeO
Me R
N
MeMeO
Me R
Substituents which increase the basicity of the pyridine ring are
good for activity
Promotes the mechanism of activation
Methyl substituents at the meta position have an inductive effect
Methoxy substituent are more effective at para position than meta
position
Resonance effect increases electron density on the nitrogen
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In this ionized form OM cannot diffuse back out of the cell.
This is known as “Ion Trapping”
The result is a build up in concentration of OM.
Chemical conversion of OM then occurs !!
OM is absorbed from the duodenum into the circulation
OM has a pKa of about 4.0
So in blood stream (pH 7.6), it remains in unionized form and
undergoes ionization only in strongly acidic condition where pH is <4.
In neutral form OM has log P of 2.23
Because it is unionized and lipophilic in nature, it is able to cross the
cell membrane of the parietal cell into strongly acidic condition of
canaliculus where the pH is 1-2.
This causes OM to be completely ionized and become polar.
N Omeprazole(Unionised form)
S
NH
NOCH3OCH3H3CO
H3C
N Omeprazole(Ionised form)
S
NH
NOCH3OCH3H3CO
H3C
HCl
HCl
H
Cl
HN
NOMe
SO
N
Me
OMe
Me
H
H+
N
N OMe
SO
N
Me
OMe
Me
H
H NH
N
OMe
H
N
S
O
Me Me
OMe
Spiro intermediate
H
-H+
N
N
OMe
N
S OH
MeMeO
Me
H
Sulfenic acid intermediate
-H2O
N
MeMeO
Me
S
N
N
OMe
Pyridinium sulfenamide structure
HS Protonpump
H
N
MeMeO
Me
NH
N
OMe
S SProtonpump
Mechanism of Proton Pump Inhibition by Omeprazole
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The imidazole nitrogen of benzimidazole ring is first protonated.
Very rapidly the lone pair of e- on pyridine N attacks e- deficient 2nd
carbon atom of the benzimidazole ring to form a spiro-intermediate.
As a result, the aromatic character of imidazole portion of
benzimidazole ring is lost and ring system wants to regain aromaticity.
The aromaticity is achieved by a lone pair of electrons from nitrogen
reforming the double bond & cleaving S-C bond to form sulphenic acid.
Sulphenic acid is highly reactive to nucleophiles involving an
intramolecular attack by NH group of the benzimidazole on sulphenic
acid to displace OH group (loss of water) and forms the sulphenamide.
This cationic, tetracyclic pyridinium sulphenamide acts as an
irreversible enzyme inhibitor by forming a covalent disulphide bond to
accessible cysteine residue/s on proton pump.
There are 3 such cysteine residues: Cys813, Cys892, and Cys821.
Different PPIs bind to different Cys: OM binds to Cys813 and Cys892.
Mechanism of Proton Pump Inhibition by Omeprazole
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Sulphenamide reacts with thiol groups of cysteine residue in
H+/K+- ATPase enzyme which forms a stable disulphide complex.
No more acid is produced until new enzyme is made which results in
long duration of inhibition of gastric acid production
Mechanism of Proton Pump Inhibition by Omeprazole
As acid conditions are required to activate PPIs, they are most active
when parietal cells are actively secreting HCl and they show little
activity when parietal cells are in resting stage.
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Mechanism of Proton Pump Inhibition by Omeprazole
OM used to treat duodenal ulcers and erosive oesophagitis.
OM is formulated in hard gelatin capsules (enteric coated) to prevent
conversion to the sulphenamide whilst in the stomach.
Any sulphenamide formed would react with thiols in food and gastric
mucus and be charged rendering it unavailable.
Uncharged OM is absorbed from the small intestine into the circulation
and then diffuses into the parietal cells.
High does (80mg) of OM can almost completely abolish gastric acid
production for at least 4 hours.
Patients are given 20mg doses daily for 2-4 weeks (duodenal ulcer) or up
to 8 weeks (gastric ulcer).
Omeprazole is a chiral compound and
has an asymmetric centre
The S-enantiomer has better potency
and pharmacokinetic profile
Pantoprazole
HN
N
OCHF2
S
O
N
OMeMeO
pyridine
methylsulfinyl'linker'
benzamidazole
Omeprazole
HN
N
OMe
S
O
N
MeMeO
Me
Lansoprazole
HN
N
S
O
N
MeO
F3CRabeprazole
NaN
N
S
O
N
MeO
MeO
Act as prodrugs
Activated by strongly acidic conditions found in the canaliculae of parietal
cells.
Proton Pump Inhibitors
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OM localizes in parietal cells where stomach acid is produced.
It inhibits H+ /K+- ATPase enzyme which catalyses the final step of stomach
acid production.
In its unionised form it is absorbed into the blood from the duodenum.
As it has a pKa = 4.0 it remains unionized in the blood (pH = 7.6) with a logP
= 2.23
Having a logP = 2.23 allows OM to diffuse though the fatty parietal cell
where there is a pH = 1-2.
OM then ionizes and becomes “trapped” and a build up in concentration of
the drug occurs.
Once in the ionized form, a chemical conversion occurs which turns OM into
the suphenamide (active form).
Sulphenamide reacts with H+/K+-ATPase enzyme and stops acid production.
As no more acid can be produced until more enzyme is generated, OM has a
long lasting action.
OM is a chiral molecule with the S-enantiomer being the active stereoisomer
which was introduced to the clinic.
Summary