H2 antagonists
Histamine
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Imidazole ring
Tele N and pi N - N pi is closest to the alkyl amine side chain and N tele is furthest away
Ethyl amine side chain
Tautomerism
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Going from lowest pH – for example in the stomach
To higher pH blood at pH7.4 – major form at pH 7.4
In the Stomach of the gastric parietal cell, the major form is the dication
Histamine tautomerisation
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Important for H1-receptor binding
Gastric Parietal Cells
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H2 antagonists inhibit gastric acid secretion elicited by histamine in a manner that is dosedependent and competitive. They also inhibit gastric acid secretion elicited by gastrin and,
to a lesser extent, by ACh H2 antagonists inhibit basal, nocturnal and (to some extent) foodstimulated acid secretion.
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The use of H2 antagonists in the treatment of gastroesophageal reflux disease (GERD) is
feasible, but limited by a relatively short duration of action, and incomplete inhibition of
food-stimulated gastric acid secretion. H2 antagonists also decrease secretion of intrisic
factor, but not to the extent that will affect vitamin B12 absorption.
H2 antagonists: Cimetidine
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Histamine was known to stimulate gastric acid production in vitro, however all known
antihistamines of this time failed to block this action of histamine
SKF researches proposed that there may be 2 types of histamine receptors and that suitably
designed antagonists would be able to discriminate between them and inhibit gastric acid
secretion.
Early 1960’s in the search for a cure for ulcers, research team at SKF decided to look for an
antihistamine that would block the secretion of gastric acid as a treatment for ulcers.
Discovered in 1972 by Black et al
Gastric parietal cells, guinea pig atria, uterus
Control release of gastric acid from gastric parietal cells
H2 agonists
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Title of the slide should say H2 Agonists not H2 Antagonists
No known receptor, just a postulated receptor, only compound that was known to interact
with the proposed receptor was histamine itself, carried out structure activity studies using
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analogues of histamine to find out if they caused the same physiological effects as
histamine, ie release of gastric acid.
From this they found that the structural requirements for H1 and H2 activity were slightly
different
H1 – does not require imidazole ring
Design of an antagonist
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After screening 200 compounds
Having obtained some info about the requirements for H2 receptors, now need to design a
molecule that would bind to a receptor but not activate it, to do this you need to alter the
way that the molecule binds to the receptor.
Guanylhistamine
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A partial agonist with very weak antagonist activity, ie it activates the H2-receptor, but not
as much as histamine, if it is bound to the receptor it prevents histamine from binding
therefore acting as antagonist.
Guanidino group is basic and is therefore protonated at pH 7.4, so that it has a positive
charge similar to histamine, but positive charge is delocalised through the three planar
nitrogens. The positive charge can also be further away from the imidazole group meaning
that the +ve charge may be interacting with a site on the receptor that histamine can’t reach
Guanidine- Resonance Rules
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Thinking an anchoring cation was essential in the structure of H2 antagonists, medicinal
chemists first tried the guanidine group for R3, but it was not effective due to its excessive
basicity. It has a pKa of 13 and, and even at pH 7.4, exists almost exclusively as its cationic
conjugate acid.
So why is guanidine such a strong base? The answer is again …Resonance! Check out the
three guanidinium resonance forms that stabilize (weaken) the conjugate acid form of this
strong base. Remember….the weaker one species is, the stronger its conjugate will be. Since
this acid form is extremely weak due to extensive resonance stabilization, the conjugate
base form (guanidine free base) will be strong, and will readily accept proton to form the
stable conjugate acid. Therefore, the cationic acid form will predominate at physiologic pH.
In fact, Henderson and Hasselbalch tell us that, at pH 7.4, the i/u ratio would be a whopping
106/1 (or close to it). Just for fun, calculate what the i/u ratio of this functional group would
be at pH 2 (i.e., close to gastric pH 1.5).
Extended analogues
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replaced NH sith S, so that +ve charge is only delocalised through terminal 2 nitrogens, less
likely to interact with “agonist binding site” - it was found to be a stronger antagonist, but
still showed partial agonist activity. Other compounds in which one of the terminal NH2
groups wes replaced by methyl or methylthio groups showed that terminal amino grps were
required for antagonist activity – propsed that charged guanidino group was interacting with
a charged carboxylate group on receptor protein
Agonist vs Antagonist
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Proposed that the extended guanidino could bind to the antagonist site in one
conformation, but could also bind to the agonist site – still believed that the +ve charge was
necessary
At first, chemists thought that the strongly basic character of the guanidine group would be
great. They envisioned this highly cationic species competing successfully with histamine’s
cationic nitrogen (which has a pKa of 9.4 and would be less extensively cationic at any pH
than the guanidine nitrogen). They erroneously thought that, once bound to the H2 receptor
though this super-strong ion-ion bond, the rest of the structure would provide the
antagonist activity they wanted. But they soon found out that these compounds were still
H2 agonists because of that cationic group.
Burimamide
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Needed to remove ALL agonist activity.
The requirement for the antagonist to be charged was investigate - thiourea analogue
(neutral not basic) was found to be a weak antagonist devoid of agonist activity.
Thiourea group is very similar to guanidio group apart from pKa,
both planar, similar in size and can take part in H-bonding.
The thiourea group is neutral because the C=S has an electron withdrawing effect, pulling
electrons away from the neighbouring nitrogens and therefore making them less attractive
to H+, similar to amide nitrogens. This functional group is essentially neutral (and unionized)
at pH 7.4.
The activity of this compound suggests that the agonist binding site involves ionic bonds but
the antagonist site involves co-valent bonds.
Final chain extension - moved thiourea group closer to antagonist binding site
Addition of a methyl to the terminal nitrogen increased hydrophobicity leading to
burimamide – a highly specific competitive H2-histamine receptor antagonist.
However activity was still too low for oral administration.
Antagonist
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Focus turned to the imidazole group in an attempt to improve activity,
pKa of imidazole ring in burimamide was found to be much higher than histamine
More basic and therefore more likely to be ionised at physiological pH – if we want an
antagonist to bind at the same site then it should be as close as possible in properties to
agonist.
Side chain of histamine has a –ve inductive effect – pulls electrons away from imidazole ring
towards the protonated N
Side chain of burimamide has no protonated N and therefore has a +ve inductive effect on
imidazole ring - pushes electrons in to it – this means that it is more attractive to protons
and more easily protonated.
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First isostere investigated was sulfur – it is electron withdrawing but about the same size as
a CH2, similar bond angle – replace the carbon one away from imidazole ring – simply
because a synthetic route was available – resulted in increased antagonist activity.
Has more of an effect on N because it is closer, enhances this nitrogen’s desire for
electrons, and it clings to that fickle imidazole double bond. The net result is promotion of
the desired Nt-H tautomer, and high affinity for the H2 receptor.
Antagonist
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In an attempt to increase the desired Nt-H tautomer further a methyl group which as a +ve
inductive effect was added close to the Nt - it pushes electrons in to the ring through 
bonds which causes the “tautomerizable” double bond to move in the direction of the
electron flow. It is “caught in the current” of electron movement, and “lays down” towards
N. Since N is forced to take the double bond, N must take the hydrogen.
CH3 is most commonly found at this position and has been shown to provide selectivity for
H2 receptors when added to the structure of the nonselective agonist histamine. If larger
groups are used in H2 antagonists, the potency of the agent drops, possibly due to steric
hindrance to receptor binding.
The methyl did cause an increase in pKa to 6.6 – which is the same as imidazole itself
indicating that the effects of the two side chains cancel each other out.
The pHa of 6.8 means that the imidazole ring is 20 % protonated at pH 7.4, however the
increase in the percentage of the tele tautomer outweighs the detrimental effct of the
increase in pH.
In effect the compound is “locked” in the form that is essential for receptor recognition but
prohibited from forming the tautomeric form needed for receptor activation. They bind
tightly to the H2 receptor, but can’t stimulate it. And this is just what we want in an H2
receptor antagonist.
Electron-donating effect of methyl group is more significant at Nt
Increases basicity of Nt
Favours tautomer I over tautomer II
Increase in pKa to 6.80
Increase in ionisation to 20%
Increase in the population of tautomer (I) outweighs the increase in population of the
ionised structures (III)
Side chain effect on pka
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The pKa of the imidazole ring of histamine is 5.74, ie the ring is a weak base and mostly
unionised at pH 7.4. The pKa of imidazole itself is 6.8 and burimamide is 7.25. Therefore
burimamide is more likely to be ionised at physiological pH. This is because the side chain of
histamine is electron withdrawing, resulting in a decrease in the pKa of the imidazole ring
compared to the imidazole itself, where as the side chain of burimamide is slightly electron
donating which results in an increase in the pKa of the imidazole ring.
Therefore, the side chain of burimamide needed to be made more electron withdrawing, so
that the proportion of unionised form is more similar to that of histamine – insertion of the
electron withdrawing S lead to thiaburimamide.
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In histamine at physiological pH the major tautomer has the t-nitrogen protonated. To
enhance the amount of tautomer I present in the antagonist, a methyl group was placed
next to the N-t. The methyl group is electron donating and therefore makes the
N-t more basic, but the Me group is also small enough not to cause steric interference with
the receptor
Histamine is approx 3% ionised at physiological pH, burimamide is 40% ionised at
physiological pH
Inductive side chain effects
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Since the tele tautomer is important for histamine activity, it is reasonable to assume that
the tele tautomer is also important for H2 antagonist activity, it has been suggested that H2antagonists undergo electronic rearrangement which induces a conformational change in
the receptor
The optimal position for the sulfur atom is attached to the first CH2 group of the moiety. If
sulfur was attached directly to the imidazole ring, one of the lone pairs would orient itself in
a perfectly parallel way with the  electrons of the ring. In other words, there would be
conjugation between the sulfur atom and the ring. If we push electrons into the imidazole
ring from the R2 side, we’re blowing the electronic wind in the wrong direction. The
“tautomerizable” double bond would be blown back toward N, and the NH tautomer
would start to predominate. In other words, we’d start to lose the tautomeric form we need
for receptor recognition and receptor binding. Antihistaminic activity would, therefore, also
be lost.
So the CH2 group that stands between the sulfur atom and the imdazole ring serves as the
“brick wall” to resonance, but still keeps the sulfur atom close enough to the ring for its
negative inductive effects to be felt by the double bond. It’s as good as it’s going to get in
this system and, luckily for the patient, it’s good enough.
Antagonist
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Urea and guanidino both less active than metiamide less active than the guanidino
But we know that the guanidino group is protonated as physiological pH – however with the
4 atom chain, the guanidino had only antagonist activity – possibly the longer chain length
meant that the +ve charge was too far away from agonist binding site.
Thought that toxicity was due to thiourea grp since this group is not naturally occuring, tried
urea analogue but this was inactive, tried guanidine analogue, which was less active that
metiamide. Although antag activity was weak, decided to look in to this further since
guanidino group occurs in natural amino acids and therefore less likely to be toxic.
Needed to find a way to make the basic guanidino group neutral to stop it being ionised at
pH 7.4
Graph
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In the case of guanidinos the pKa was found to be inversely proportional to the electonn
withdrawing power of a substitutent.
Very strong EWG such as nitrile and nitro pull the electrons away from the guanidine so that
they are not available to attract a proton, decreasing the pKa
Cimetidine
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The aromatic imidazole ring binds to H2 receptors through Van der Waals binding. It has
basic character (pKa 6.8) due to the doubly bonded nitrogen, N. Cimetidine’s i:u ratio
would be approximately 2x105:1 at pH 1.5, ensuring effective ion-ion anchoring to the
anionic ASP98 of the H2 receptors in the parietal cell. At pH 7.4 the i:u ratio would be 1:4,
ensuring good transport through biological membranes to reach those gastric H2 receptors..
R1 = CH3: The +Is group promotes the desired Nt-H tautomer that is essential for H2
receptor recognition
R2 = ethylthiomethyl: This -Is group also promotes the Nt-H tautomeric form. It is
approximately the same length as a butyl chain, which maintains the optimal separation of
the aromatic ring and the cyanoguanidine group for high affinity binding to the H2 receptor.
R3 = cyanoguanidine: This group is very polar, but neutral. The electron withdrawing cyano
group destroys the basicity of the guanidine nitrogens, and they are unable to take on
proton.
The group is essentially 100% unionized at any pH, which promotes potent and pure H2
receptor antagonism.
The NH groups of the guanidine are H-donors in H-bonds with ASP186 and ARG257 (or TYR182)
residues on the H2 receptor.
The terminal N-CH3 adds some lipophilic character, which enhances activity
From a toxicity standpoint, cimetidine is the most problematic of all marketed H2
antagonists.
This drug inhibits CYP450, leading to an increased potency and duration of
action of co-administered drugs that are metabolized by the cytochrome enzyme system.
The imidazole ring is responsible for this activity. The basic imidazole nitrogen competes
with a HIS residue of the CYP protein that normally complexes with the cytochrome heme
Fe+2, causing reversible inhibition of several isoforms, including CYP3A4. Several important
drug-drug interactions have been documented. Use cimetidine with caution in patients
taking other drugs metabolized by CYP enzymes.
Cimetidine itself is metabolized by CYP450, with reactions including sulfoxidation and
hydroxylation of the “benzylic”-like C4-CH3 group. All metabolites are inactive.
Cimetidine also induces antiandrogenic side effects (gynecomastia, impotence), and is more
likely than other H2 antagonists to cause CNS side effects (e.g. mental confusion)
Conformations
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Cimetidine has been shown to exist as a mixture of Z,E and E,Z isomers
The double bond Z and E nomenclature is used here because this is a delocalised system and
all the N-C bonds have double bond character.
ZZ and EE forms are not favoured because of the steric interactions
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Believed that the E,Z form is the active isomer – forms 2 H-bonds – one to the left and the
second one downwards
Dipole-dipole interactions
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Explored other functional groups for the polar hydrophobic end – found that the compounds
with the greatest activity had an angle between the direction of the dipole moment and the
NR bond (which is vertical in this diagram) of close to 30 degrees.
Believed to be important for alignment of molecule as it approaches the receptors.
In A – nitroketine aminal group of Famotidine has correct dipole and all functional groups
are aligned with receptor in correct position to form H-bonds.
In B – which is an imidaoline analogue used in the study, when the dipole is aligned the
functional groups are in the wrong position for forming strong binding interactions.
Other H2 antagonists
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Ranitidine
The aromatic furan ring, coupled with the basic dimethylaminomethyl side chain, is
considered isosteric with the aromatic and basic imidazole ring. The pKa of the sidechain
amine is 8.2. Therefore the i;u ratio would be approximately 5x106:1 at gastric pH 1.5, and
6.3:1 at pH 7.4.
Like the imidazole ring of cimetidine, the furan ring of ranitidine binds to the H2 receptor via
Van der Waals forces. The protonated nitrogen of the dimethylaminomethyl sidechain binds
this antagonist to the anionic ASP98 of the H2 receptor.
The ethylthiomethyl connecting group is approximately equivalent in size to a butyl chain,
and maintains the desired distance between the furan ring and the modified guanidine
group. The electron withdrawing character of the ethylthiomethyl group is not important to
us anymore since we don’t have an imidazole ring. The furan ring cannot tautomerize!
R3 is a diaminonitroethene group, which is very polar, but non-ionic. Potent and pure H2
antagonism is observed.
The therapeutic advantages of ranitidine over cimetidine include:
A 10 fold higher potency
A longer duration of action
The lack of an imidazole ring which greatly limits CYP450 inhibition. However ranitidine can
still inhibit some CYP isoforms including 3A4, 2D6 and 3A3. The inhibition is not as extensive
as with cimetidine, and very few drug-drug interactions are clinically significant.
Ranitidine is excreted largely unchanged, although some N-dealkylation and S-oxidation can
occur. None of the metabolites are active.
Famotidine
The guanidine group on the aromatic thiazole ring provides the basic center and makes the
moiety isosteric with imidazole. The thiazole ring binds to the H2 receptor via Van der Waals
forces, and the thiazole C2-guanidine group, in cationic conjugate acid form, anchors to the
receptor ASP98. The pKa of famotidine is 6.6. Therefore, the i:u ratio would be
approximately 1.3x105:1 at gastric pH 1.5, and approximately 1:6.3 at pH 7.4
Note that the ethylthiomethyl group is still with us! As with famotidine, it is needed to keep
the aromatic thiazole ring and the modified guanidine group separated by the proper
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distance and, possibly, to properly orient these groups for optimal binding to the H2
receptor. The thiazole ring cannot tautomerize, therefore the electron withdrawing
properties of the sulfur are not believed to be important to us here.
The sidechain guanidine group (where cations fear to tread) has been modified with an
electron withdrawing sulfonamide. Like the cyano and nitro groups, this group destroys the
basicity of the guanidine, and prevents the formation of cations in this area of the structure.
Potent and pure H2 antagonism results.
The potency of famotidine is approximately 40-60 times that of cimetidine and 9-15 times
that of ranitidine. Note that there is no terminal CH3 group on the modified terminal
guanidine moiety (which would probably have increased activity even further).
This H2 antagonist has a safety index along the lines of ranitidine. It does not inhibit CYP450
so few drug-drug interactions are observed. Like other H2 antagonists that increase gastric
pH, famotidine may influence the bioavailability of co-administered drugs that require strict
pH control for effective and controlled absorption (cephalosporins, iron, ketoconazole, etc.).
We’ll see the same drug-drug interaction potential with the proton pump inhibitors.
Nizatidine
Nizatidine is a hybrid of famotidine and ranitidine. It has the aromatic thiazole ring of
famotidine, and the diaminonitroethene and dimethylaminomethylgroups found on
ranitidine. Even though it has the same basic dimethylaminomethyl group as ranitidine, its
pKa is lower (6.8 vs. 8.2) since two thiazole heteroatoms (N and S) will pull electron density
away from the lone pair of electrons of the sidechain amino nitrogen, rather that the one
aromatic heteroatom (O) of the furan ring of ranitidine. The electron pull of these
heteroatoms is through  bonds.
Not surprisingly, nizatidine has an H2 antagonist activity similar to ranitidine, but it is much
more bioavailable after po administration. However, an identical dosing schedule of 300
mg/day is used. The antihistaminic potency of nizatidine is 5-18 times that of cimetidine.
Like famotidine, nizatidine does not inhibit CYP450. Unlike ranitidine, the N-desmethyl
metabolite retains some H2 antagonist action.
Attendance question
decreased white blood cell count
H1 vs H2 antagonists
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The imidazole ring, which is found on the agonist (histamine) and in one antagonist
(cimetidine), is not essential in and of itself. However, the critical chemical features of this
ring (aromaticity and a basic nitrogen atom) are.
We used the imidazole ring in the pharmacophore drawn above because it was initially
thought to be essential. However, we will soon see that other aromatic systems can provide
good antihistaminic action at H2 receptors as long as they have a basic functional group
either incorporated into the ring or attached to it as a substituent.
Again, the basic group is essential because it is believed to protonate at the acidic pH of the
parietal cell and anchor the antagonist to the anionic ASP98 residue on the H2 receptor. Any
aromatic system used in an H2 antagonist must be isosteric with the basic imidazole ring in
order to be effective in inhibiting histamine’s effects at H2 receptors.
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If the imidazole ring is present, its structure must promote and maintain the Nt-H tautomer
for receptor recognition. The drug must bind to the H2 receptor before it can antagonize it.
However, we do not want to promote tautomerization because we do not want to stimulate
the receptor. Remember, it’s the NH that has the right chemistry for receptor activation.
We don’t want this property in our H2 receptor antagonists.
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Two of the three “R” groups of the H2 antagonist pharmacophore serve to stabilize imidazole
ring-containing H2 antagonist structures in this most important NH tautomeric form. They
are:
Histamine binding site
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191ASP 98 should say 198
Revision of histamine binding at the histamine receptor
So what you end up with is an ion-dipole bond between the Asparagine 198 which now has a
positive charge and a H-bond between the proton on the pi nitrogen and the amine of Lysine
Proposed binding of an antagonist
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ASP 98 should say 198
From our SAR discussions thus far, we can see that, compared to the agonist histamine, H2
antagonists do a “flip-flop” at the receptor. The basic functional group of the aromatic ring
of the H2 antagonists (in cationic acid form) binds to the ASP198 that originally bound the
sidechain of histamine. In a similar fashion, the sidechain of the antagonists binds to the
receptor area [ASP186 and ARG257 (or TYR182)] that originally bound the nitrogen atoms of
the aromatic imidazole ring of histamine. As an analogy, both agonist (histamine) and
antagonist lie in the same bed (but not at the same time!) but one likes to sleep with its
head on the pillow while the other places its head at the foot of the bed.
Proposed binding of an antagonist (2)
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Similar to histamine, there are Van der Waals interactions between the pi electrons of the
imidazole ring or other aromatice ring in this case furan and aromatic amino acid
H2 – antagonists bind at the same site as histamine, - just the other way around – they
directly prevent histamine from binding – they are competitive antagonists.
Compare and contrast
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If you compare H1 antagonists with H2 antagonists, - remember that H1 antagonists are not
competitive antagonists they bind at a different site on the receptor and cause a
conformational change that prevents histamine from binding.