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

Thank you……..

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