Claire Shoemake
A DEFINITION
y Structure-activity relationship (SAR) is the relationship between the
chemical or three-dimensional structure of a molecule and its
biological activity.
y The analysis of SAR enables the determination of the chemical groups
responsible for evoking a target biological effect in the organism.
y This allows modification of the effect or the potency of a bioactive
compound (typically a drug) by changing its chemical structure.
y Medicinal chemists use the techniques of chemical synthesis &
computational drug design to insert new chemical groups into the
biomedical compound and test the modifications for their biological
effects.
y This method was refined to build mathematical relationships between
the chemical structure and the biological activity, known as
quantitative structure-activity relationships (QSAR).
THE SAR PARADOX
y The basic assumption for all molecule based hypotheses is that
similar molecules have similar activities.
y This principle is the basis of Structure-Activity Relationship
(SAR). The underlying problem is therefore how to define a small
difference on a molecular level, since each kind of activity, e.g.
reaction ability, biotransformation ability, solubility, target
activity, and so on, might depend on another difference.
y In general, one is more interested in finding strong trends.
y Created hypotheses usually rely on a finite number of chemical
data.
y The SAR paradox refers to the fact that it is not the case that all
similar molecules have similar activities.
HISTAMINE & ITS RECEPTOR SUBTYPES
y The biogenic amine histamine plays an important role in a
variety of pathophysiological conditions.
y In peripheral tissues, histamine is mainly stored in mast cells
and basophils.
y In allergic conditions, histamine is released from these cells and
is responsible for several of the well known symptoms of allergic
conditions of the skin and airways.
y In the gastric mucosa, gastrin induced histamine release fulfills
an important physiological role by stimulating parietal cells to
secrete gastric acid.
HISTAMINE & ITS RECEPTOR SUBTYPES
y In the CNS, histamine is synthesized in specific neurons that are
localized in the tuberomammillary nucleus of the posterior
hypothalamus.
y These neurons project to all major brain areas and are involved
in a variety of important physiological functions, including the
regulation of the sleep-wake cycle, cardiovascular control,
regulation of the hypothalamic pituitary adrenal-axis (HPA-axis),
learning and memory.
y Histamine exerts its action via at least four distinct receptor
subtypes. Molecular biological approaches have shown that all
y histamine receptors belong to the large family of G proteincoupled receptors.
HISTAMINE & ITS RECEPTOR SUBTYPES
y The gene encoding the H3 receptor has only recently been
cloned. In contrast to the H1 and H2 receptor gene, the H3
receptor gene contains intronic sequences, leading to the
identification of various H3 receptor isoforms following
alternative splicing ofthe introns.
y The isoforms show distinct expression patterns and signal
transduction mechanisms.
y Using the H3 receptor sequence, a new histamine (H4)
receptor was identified ‘in silico’.
y This receptor shows the strongest homology to the H3
receptor and also recognizes histamine with high affinity.
HISTAMINE & ITS RECEPTOR SUBTYPES
y For the H4 receptor, no pharmacological correlates are currently
y known.
y The H1 receptor couples mainly to Gq/11 thereby stimulating
phospholipase C,
y The H2 receptor interacts with Gs to activate adenylyl cyclase.
y The histamine H3 and H4 receptors couple to Gi proteins to
inhibit adenylyl cyclase, and to stimulate MAPK in the case of
the H3 receptor.
y In view of the important role of H1 and H2 receptors in allergic
responses and gastric acid secretion respectively, many potent
and selective antagonists have been developed as successful
drugs.
HISTAMINE & ITS RECEPTOR SUBTYPES
y Selective agonists are currently also available as
pharmacological tools.
y The H3 receptor was originally described as an autoreceptor,
inhibiting the release of histamine from histaminergic neurons in
brain.
y Recently, it was shown that this inhibitory effect is due to
constitutive activity of the H3 receptor.
y Recent evidence suggests that the H3 receptor regulates the
release of several important neurotransmitters (e.g.
acetylcholine, dopamine, GABA, norepinephrine, serotonin),
both in the peripheral and central nervous systems.
HISTAMINE & ITS RECEPTOR SUBTYPES
y Highly potent and selective agonists and antagonists have
recently been developed for the H3 receptor. These ligands are
useful pharmacological tools and are currently being assessed
for their clinical potential in allergy, inflammatory disorders,
attention deficit hyperactivity disorder, Alzheimer’s disease and
obesity.
y The H4 receptor is highly expressed in peripheral blood
leukocytes and intestinal tissue, making this receptor a
potentially interesting target in allergic and inflammatory
diseases.
y The receptor shows high affinity for several H3 receptor ligands
y (both agonists and antagonists), but shows a clearly different
pharmacological profile.
HISTAMINE & ITS RECEPTOR SUBTYPES
y These data strongly suggest that the discovery of selective H4
histamine receptor ligands can be expected.
y Because of the availability of many potent and subtype selective
ligands for histamine receptor subtypes, good radioligands are
y available.
y For the H1 receptor, the antagonist [3H]-mepyramine has been
successfully used in many preparations. The radioligand has
nanomolar affinity and shows high specificity, although in liver
y membranes, for example, binding to cytochrome P450
isoenzymes masks H1 receptor binding.
y For in vivo Positron Emission Tomography (PET) studies, [13C]y doxepin can be used to label H1 receptors. This ligand has been
used to label central H1 receptors in human brain.
HISTAMINE & ITS RECEPTOR SUBTYPES
y For the H2 receptor, the antagonist [125I]-iodoaminopotentidine
y has recently been developed as a high affinity radioligand.
y Because of its high sensitivity and subnanomolar affinity, this
radioligand has been a very useful tool for characterizing
y the H2 receptor.
y As for the H1 receptor, an agonist radioligand is lacking.
y In contrast, potent agonists and antagonists are
y available for the H3 receptor, some in radiolabeled form. Initially,
the agonists Nα-methylhistamine and (R)-α-methylhistamine
y were developed as tritiated radioligands.
HISTAMINE & ITS RECEPTOR SUBTYPES
y Both ligands show selective, high affinity labeling of the H3
receptor with almost no non-specific binding.
y The iodonated ligand [125I]-iodoproxyfan can also be used
as an agonist radioligand. Originally described as an
antagonist, iodoproxyfan acts as a partial agonist insome
H3 receptor models.
y [125I]-Iodophenpropit, [3H]-GR168320 and [3H]-clobenpropit
can be used as H3 receptor antagonist radioligands.
y For the H4 receptor, [3H]-histamine can be used to label the
receptor protein.
HISTAMINE & ITS RECEPTOR SUBTYPES
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y The term antihistamine historically has referred to drugs that
antagonize the actions of histamine at H1-receptors rather than
H2-receptors.
y The development of antihistamine drugs began more than 5
decades ago with the discovery that piperoxan was able to
protect animals from the bronchial spasm induced by histamine.
y This finding was followed by the synthesis of a number of Nphenylethylenediamines with antihistaminic activities superior to
piperoxan.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y Further traditional structure-activity studies in this series based
largely on the principles of isosterism and functional group
modification led to the introduction in the 1940s to 1970s of a
variety of H1-antagonists containing the diarylalkylamine
framework.
y These H1-antagonists, referred to now as the firstmgeneration or
classical antihistamines, are related structurally and include a
number of aminoalkyl ethers, ethylenediamines, piperazines,
propylamines, phenothiazines and dibenzocycloheptenes.
y In addition to H1-receptor antagonists, these compounds display
an array of other pharmacological activities which contribute
toward therapeutic applications and adverse reactions.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y More recently, a number of second generation or “non-sedating”
antihistamines have been developed and introduced.
y The second generation agents bear some structural resemblance
to the first generation agents, but have been modified to be more
specific in action and limited in their distribution profiles.
y H1-antagonists may be defined as those drugs that competitively
inhibit the action of histamine on tissues containing H1receptors.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y The structural features required for effective interaction with
these receptors will be discussed later on in this module.
y It should be noted that some H1-antagonists also block
histamine release. However the concentrations required to do so
are considerably greater than those required to produce
significant histamine receptor blockade.
y The H1-antagonists do not block antibody production or antigenantibody interactions.
y The H1-antagonists are now commonly subdivided into two
broad groups - the first generation or classical antihistamines
and the second generation or “non-sedating” antihistamines –
based primarily on their general pharmacological profiles.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y The differences between these two series are discussed
in more detail in the sections that follow.
y It is important to note, however, that most detailed
structure-activity analyses for H1-antagonists that have
been published focus on the structural requirements for
the first generation agents.
y From these studies the basic structural requirements
for H1-receptor antagonism have been identified as
shown on the following slide:
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y Ar is aryl (including phenyl, substituted phenyl, and heteroaryl
groups such as 2-pyridyl),
y Ar' is a second aryl or arylmethyl group.
y This diaryl substitution pattern is present in both the first and
second generation antihistamines and is essential for significant
H1-receptor affinity.
y Furthermore several SAR studies suggest that the two aryl
moieties must be capable of adopting a non coplanar
conformation relative to each other for optimal interaction with
the H1-receptor.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y The two aromatic systems may be linked as in the
tricyclic antihistamines (phenothiazines,
dibenzocycloheptanes and heptenes, etc.), but again
they must be non-coplanar for effective receptor
interaction.
y Most H1-antagonists contain substituents in one of the
aryl rings (usually benzene), and these influence
antihistamine potency, as well as biodisposition as is
discussed forindividual classes of compounds in the
sections that follow.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y A basic, terminal amine function which in many of the first
generation or classical antihistamines the terminal nitrogen atom
is a simple dimethylamino moiety.
y However, the amine may also be part of a heterocyclic structure,
as illustrated by the piperazine, some propylamines (pyrrolidines
y and piperdines), some phenothiazines, the
dibenzocycloheptenes and the second generation
antihistamines.
y In all cases the amino moiety is basic with pKas ranging from 8.5
to 10 and thus presumed to be protonated when bound on the
receptor.
y The moiety is also important in the development of stable, solid
dosage forms through salt formation.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y X is a connecting atom of O, C or N. The X connecting moiety of
typical H1-antagonists may be a saturated carbon-oxygen moiety
or simply a carbon or nitrogen atom.
y This group, along with the carbon chain appear (see below) to
serve primarily as a spacer group for the key pharmacophoric
moieties.
y Many of the anthistamines containing a carbon atom in the
connecting moiety are chiral, and exhibit stereoselective
receptor binding.
y For example, in the pheniramine series and carbinoxamine, this
atom is chiral and in vitro analyses indicate that those
enantiomers with the S-configuration have higher H1-receptor
affinity.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y The (CH2)n group represents a carbon chain which in typical H1antagonists consists of two or three atoms.
y The (CH2)n group and connecting atom results in a distance
between the central point of the diaryl ring system and the
terminal nitrogen atom in the extended conformation of the
antihistamines ranging from 5 to 6 angstroms (a "spacer" group).
y A similar distance between these key moieties is observed for
those antihistamines with less conformational freedom.
y In some series branching of the carbon chain results in a
reduction of antihistaminic activity. However, there are
exceptions as evidence by promethazine which has a greater
activity than its non branched counterpart.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y When the carbon adjacent to the terminal nitrogen atom
is branched, the possibility of asymmetry exists.
y However, stereoselective H1-receptor antagonism
typically is not observed when chirality exists at this
site.
y Also, in those compounds which possess an
asymmetrically substituted unsaturated carbon chain
(pyrrobutamine and triprolidine), one geometric isomer
typically displays higher receptor affinity than the other.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y Generally, the first and second generation anthistamines are
substantially more lipophilic than the endogenous agonist
histamine (or the H2-antagonists).
y This lipophilicity difference results primarily from the presence
of the two aryl rings, and the substituted amino moieties, and
thus may simply reflect the different structural requirements for
antagonist versus agonist action at H1-receptors.
y The nature of the connecting moiety and the structural nature of
the aryl moieties have been used to classify the anithistamines
as indicated in the sections that follow.
HISTAMINE H1 RECEPTOR ANTAGONISTS: ANTIHISTAMINES
y Furthermore variations in the diaryl groups, X connecting
moieties and the nature of substitution in the alkyl side
chain or terminal nitrogen among the various drugs
accounts for differences observed in antagonist potency as
well as pharmacologic, biodisposition and adverse reaction
profiles.
y The ability of these drugs to display an array of
pharmacologic activities is due largely to the fact that they
contain the basic pharmacophore required for binding to
muscarinic as well as adrenergic, serotonergic receptors.
y The relationships of antihistamine structure to these
overlapping actions (H1-antagonist, anticholinergic, and
local anesthetic) are described later on.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y Antihistaminic Action:
y The classical antihistamines have been used extensively for
thesymptomatic treatment (sneezing, rhinorrhea, and itching of
eyes, nose, and throat) of allergic rhinitis (hay fever, pollinosis),
chronic idiopathic urticaria and a number of other histamine
related,diseases.
y These uses are clearly attributable to their antagonism of the
action of histamine at peripheral H1 receptors.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y Although the symptoms of the common cold might be
modified by antihistamines, these agents do not
prevent or cure colds nor do they shorten the course of
the disease.
y The antihistamines also are of little or no value in
diseases such as systemic anaphylaxis and bronchial
asthma, in which autacoids other than histamine are
important
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y Other Therapeutic Actions:
y A number of the antihistamines, particularly the phenothiazines
and aminoalkyl ethers, have antiemetic actions and thus may be
useful in the treatment of nausea, vomiting and motion sickness.
y Those agents which produce pronounced sedation have
applications as nonprescription sleeping aids.
y Several of the phenothiazines have limited utility in Parkinsonlike syndromes as a result of their ability to block central
muscarinic receptors.
y A number of antihistamines including promethazine, pyrilamine,
tripelennamine and diphenhydramine display local anaesthetic
activity that may be therapeutically useful.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y As the general pharmacologic profiles above suggest, the
majority of antihistamines are capable of interaction with a
variety of neurotransmitter receptors and other
biomacromolecular targets.
y This is most evident among the first generation agents many of
which function as antagonists at muscarinic receptors and, to a
lesser extent, adrenergic, serotonergic and dopamine receptors.
y While some of these non-target receptor interactions may be of
some therapeutic value (as discussed above), more frequently
they are manifested as adverse reactions that limit drug use.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y This is particularly true of the peripheral anticholingeric
effects produced by these drugs, and interactions with
a number of neurotransmitter systems in the CNS that
result in sedation, fatigue and dizziness.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y The primary objective of antihistamine research over the past 20
years has centered on development of new drugs with higher
selectivity for H1-receptors and lacking undesirable CNS actions.
y The pronounced sedative effects of some of the first generation
agents were thought to result from the ability of these drugs to
penetrate the blood-brain barrier, due to their lipophilic nature,
and then block cerebral H1-receptors and possibly other
receptors.
y Thus research efforts were initiated to design novel
antihistamines with reduced ability to penetrate the CNS and
decreased affinity for central histamine receptors.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y These efforts led to the introduction the second
generation antihistamines which are non-sedating and
have little antagonist activity at other neurotransmitter
receptors at therapeutic concentrations.
y The pharmacologic properties of these agents are
discussed in more detail later in this module.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y Surprisingly little information is available concerning the
pharmacokinetic and biodisposition profiles of the first
generation antihistamines.
y Generally the compounds are orally active and well absorbed,
but oral bioavailability may be limited by first pass metabolism.
y The metabolites formed depend on drug structure to a large
extent, but commonly involve the tertiary amino moiety.
y This functionality may be subject to succesive oxidative Ndealkylation, deamination, and amino acid conjugation of the
resultant acid.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y The amine group may also undergo N-oxidation, which
may be reversible, or direct glucuronide conjugation.
y Those first generation agents with unsubstituted and
activated aromatic rings (phenothiazines) may undergo
aromatic hydroxylation to yield phenols, which may be
eliminated as conjugates.
HISTAMINE H1 RECEPTOR ANTAGONISTS: Pharmacology
y The H1-antagonists display a variety of significant drug
interactions when co-administered with other therapeutic agents.
y For example, monoamine oxidase inhibitors prolong and
intensify the anticholinergic actions of the antihistamines.
y Also, the sedative effects of these agents may potentiate the
depressant activity of barbiturates, alcohol, narcotic analgesics
and other depressants.
y In recent years it has been discovered that several of the second
generation antihistamines may produce life-threatening
arrhythmias when co-administered with drugs that inhibit their
metabolism.
y These interactions are discussed in more detail in the sections
that follow.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethanolamines)
y The aminoalkyl ether antihistamines are characterized by
the presence of a CHO connecting moiety (X) and a two or
three carbon atom chain as the linking moiety between the
key diaryl and tertiary amino groups.
y Clemastine and diphenylpyraline differ from this basic
structural pattern in that the basic nitrogen moiety and at
least part of the carbon chain is part of a heterocyclic ring
system, and that there are three carbon atoms between the
oxygen and nitrogen atoms.
1st GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethanolamines)
y The simple diphenyl derivative diphenhydramine was the first
clinically useful member of the ethanolamine series and serves
as the protoype.
y In addition to antihistaminic action, diphenhydramine exhibits
anticholinergic, antidyskinetic, antiemetic, antitussive, and
sedative properties.
y Diphenhydramine is not a highly active H1-antagonist.
y Conversion to a quaternary ammonium salt does not alter the
antihistaminic action greatly, but does increase the
anticholinergic action.
X = H: Diphenhydramine
X = Br : Bromodiphenhydramine
-
Dimenhydrinate
Carbinoxamine Maleate
Doxylamine Succinate
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethanolamines)
y Diphenylpyraline is structurally related to diphenhydramine with
the aminoalkyl side chain incorporated in a piperidine ring. It is a
potent antihistaminic, and the usual dose is 2 mg three or four
times daily.
y Clemastine Fumarate is structurally related to
chlorodiphenhydramine with the aminoalkyl side chain
incorporated in a pyrrolidine ring, and it has an additional
benzylic methyl group.
y This compound has two chiral centers, each of which is of the
(R) absolute configuration in the dextrorotatory product.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethanolamines)
y A comparison of the activities of the antipodes indicates that the
asymmetric center close to the side chain nitrogen is of lesser
importance to antihistaminic activity.
y This member of the ethanolamine series is characterized by a
long duration of action, with an activity that reaches a maximum
in five to seven hours and persists for 10 to 12 hours.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethanolamines)
y Drowsiness is a side effect common to the tertiary aminoalkyl
ethers, presumably as a result of the ability of these compounds
to penetrate and BBB and occupy central H1-receptors.
y Although this side effect is exploited in over-the-counter (OTC)
sleeping aids, it may interfere with the patient's performance of
tasks requiring mental alertness.
y The diaryl tertiary aminoalkyl ether structure that characterizes
these compounds also serves as a pharmacophore for
muscarinic receptors.
y As a result the drugs in this group possess significant
y anticholinergic activity, which may enhance the H1-blocking
action on exocrine secretions.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethanolamines)
y The frequency of gastrointestinal side effects in this
series of antihistamines is relatively low compared to
the ethylenediamine antihistamines covered later.
y In spite of their extensive use, pharmacokinetic data on
this series of compounds is relatively limited.
y Most members of this series appear to be extensively
metabolized by pathways including N-oxidation, and
successive oxidative N-dealkylation followed by amino
acid conjugation of the resultant acid metabolites
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethylenediamines)
y The ethylenediamines were among the first useful antihistamines
and are characterized by the presence of a nitrogen connecting
atom (X) and a two carbon atom chain as the linking moiety
between the key diaryl and tertiary amino moieties.
y All compounds in this series are simple diarylethylenediamines
except for antazoline in which the terminal amine and a portion
of the carbon chain are included as part of an imidazoline ring
system.
y Because it differs significantly in its pharmacologic profile,
antazoline is not always classified as an ethylenediamine
derivative.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethylenediamines)
y Phenbenzamine was the first clinically useful member of this
class and served as the prototype for the development of more
effective derivatives.
y Replacement of the phenyl moiety of phenbenzamine with a 2pyridyl system yielded tripelennamine, a significantly more
effective histamine receptor blocker.
y Substitution of a para methoxy (pyrilamine or mepyramine),
chloro (chloropyramine) or bromo (bromtripelennamine) results
in a further enhancement in activity.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethylenediamines)
y Replacement of the benzyl group of tripelennamine with
a 2-thienylmethyl group provided methapyrilene, and
replacement of tripelennamine’s 2-pyridyl group with a
pyrimidinyl moiety (along with p-methoxy substitution)
yielded thonzylamine, both which function as potent
H1-receptor antagonists
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethylenediamines)
y In all of these compounds the aliphatic or terminal amino group
is significantly more basic than the nitrogen atom bonded to the
diaryl moiety; the non-bonded electrons on the diaryl nitrogen
are delocalized by the aromatic ring and the resultant reduction
in electron density on nitrogen decreases basicity.
y Thus the aliphatic amino group in the ethylenediamines is
sufficiently basic for the formation of pharmaceutically useful
salts.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethylenediamines)
y The ethylenediamines also display a relatively high
frequency of central nervous system depressant (sedation)
and gastrointestinal side effects.
y The anticholinergic and antiemetic actions of these
compounds is relatively low compared to most other
classical antihistamines.
y The piperazine and phenothiazine-type antihistamines also
contain the ethylenediamine moiety, but these agents are
discussed separately because they exhibit significantly
different pharmacologic properties.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Ethylenediamines)
y Relatively little information is available concerning the
pharmacokinetics of this series of compounds.
y Tripelennamine is known to metabolized in man by Nglucuronidation, N-oxidation and pyridyl oxidation followed by
phenol glucuronidation.
y It is anticipated that other members of this series are similarly
metabolized.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Piperazines/Cyclizines)
y The piperazines or cyclizines can also be considered to be
ethylenediamine derivatives or cyclic ethylenediamines
(cyclizines), however in this series the connecting moiety (X) is a
CHN group and the carbon chain, terminal amine functionality as
well as the nitrogen atom of the connecting group are all part of
a piperazine moiety.
y Both nitrogen atoms in these compounds are aliphatic and thus
display comparable basicities.
y The primary structural differences within this series involves the
nature of the para aromatic ring substituent (H or Cl) and, more
importantly, the nature of the terminal piperazine nitrogen
substituent.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Piperazines/Cyclizines)
y Cyclizine and chlorcyclizine are simple Nmethylpiperazines.
y Cyclizine HCl is used primarily in the prophylaxis and
treatment of motion sickness. The lactate salt (Cyclizine
Lactate Injection is used for intramuscular injection
because of the limited water solubility of the hydrochloride.
y Chlorcyclizine HCl has an additional ring Cl substituent
which reduces activity. Chlorcyclizine is indicated in the
symptomatic relief of urticaria, hay fever, and certain other
allergic conditions.
Cyclizine
Chlorcyclizine
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Piperazines/Cyclizines)
y Meclizine HCl and Buclizine HCl are N-benzyl substituted
piperazines. Although it is a moderately potent
antihistaminic, meclizine is used primarily as an
antinauseant in the prevention and treatment of motion
sickness and in the treatment of nausea and vomiting
associated with vertigo and radiation sickness.
y Buclizine Hydrochloride, is highly lipid-soluble and has
central nervous system depressant, antiemetic, and
antihistaminic properties.
Meclizine
Hydroxyzine
Buclizine
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Piperazines/Cyclizines)
y The piperazines are moderately potent antihistaminics with a
lower incidence of drowsiness.
y The activity of the piperazine-type antihistaminics is
characterized by a slow onset and long duration of action.
y These agents exhibit peripheral and central antimuscarinic
activity and this may be responsible for the antiemetic
(medullary chemoreceptor trigger zone) and antivertigo (diminish
y vestibular stimulation) effects.
y Thus as a group, these agents are probably more useful as
antiemetics and antinauseants and in the treatment of motion
sickness.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Piperazines/Cyclizines)
y Some members of this series have exhibited a strong teratogenic
potential, inducing a number of malformations in rats.
y Norchlorcyclizine, a metabolite of these piperazines, was
proposed to be responsible for the teratogenic effects of the
parent drugs.
y Metabolic studies in this series of compounds have focused
primarily on cyclizine and chlorcyclizine, and these compounds
undergo similar biotransformation.
y The primary pathways involve N-oxidation and N-demethylation,
and both of these metabolites are devoid of antihistaminic
activity.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Propylamines)
y The propylamine antihistamines are characterized structurally by
an sp3 or sp2 carbon connecting atom with a carbon chain of
two additional carbons linking the key tertiary amino and diaryl
pharmacophore moieties.
y Those propylamines with a saturated carbon connecting moiety
are commonly referred to as the pheniramines.
y All of the pheniramines consist of a phenyl and 2-pyridyl aryl
groups, and a terminal dimethylamino moiety.
y These compounds differ only in the phenyl substituent at the
para-position; H (pheniramine), Cl (chlorpheniramine) and Br
(brompheniramine).
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Propylamines)
y The halogenated pheniramines are significantly more potent (2050 times) and have a longer duration of action than the parent
pheniramine.
y All of pheniramines are chiral molecules and are marketed as
racemates or the individual active dextro-enantiomers as
indicated below.
y The halogen-substituted derivatives have been resolved by
crystallization of salts formed with d-tartaric acid and
antihistaminic activity resides almost exclusively in the Sstereoisomers
y In addition to being an histamine H1 receptor antagonist,
chlorphenamine has been shown to work as a serotoninnorepinephrine reuptake inhibitor or SNRI.
y Brompheniramine led to the discovery of the SSRI zimelidine.
Cl
H
3
H
CH3
N
3
N
Br
H
Pheniramine
H
CH
3
N
N
H
H
CH
Brompheniramine
3
H
H
CH3
Chlorpheniramine
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Propylamines)
y Those propylamines with an unsaturated connecting moiety
include the open derivatives pyrrobutamine and triprolidine, and
the cyclic analogues dimethindene and phenindamine.
y The conformational rigidity of the unsaturated propylamines has
provided a useful model to determine distances between the key
diaryl and tertiary pharmacophoric moieties in H1-receptor
antagonists, a distance of 5-6.Å
y For pyrrobutamine and triprolidine the E-geometric isomers are
active.
y The relative potency of triprolidine is of the same order as that of
dexchlorpheniramine.
Triprolidine
Pyrrobutamine
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Propylamines)
y The antihistamines of the propylamine group are
among the most active H1-antagonists.
y The agents of this class also produce less
sedation than the other classical antihistamines
(yet a significant proportion of patients do
experience this effect), and have little antiemetic
action.
y They do, however, exhibit a signficant degree of
anticholinergic activity, albeit less than the
aminoalkyl ethers and phenothiazines.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Propylamines)
y In the propylamine series the pharmacokinetics of
chlorpheniramine have been studied most extensively in
humans.
y Oral bioavailability is relatively low (30-50%) and may be limited
by first pass metabolism.
y The primary metabolites for this compound, and other members
of this series, are the mono- and di-N-dealkylation products.
y Complete oxidation of the terminal amino moiety followed by
glycine conjugation has also been reported for
brompheniramine.
y Chlorpheniramine plasma half-lives range from about 12 hours to
28 hours, depending on the route of administration (oral versus
IV).
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Phenothiazines)
y Beginning in the mid-1940s, several antihistaminic drugs have
been discovered as a result of bridging the aryl units of agents
related to the ethylenediamines.
y The search for effective antimalarials led to the investigation of
phenothiazine derivatives in which the bridging entity is sulfur.
In subsequent testing, the phenothiazine class of drugs was
discovered to have not only antihistaminic activity, but also a
pharmacologic profile of its own, considerably different from that
of the ethylenediamines.
y Thus began the era of the useful psychotherapeutic agent.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Phenothiazines)
y The phenothiazine derivatives that display therapeutically
useful antihistaminic actions contain a two or three carbon
atom, branched alkyl chain between the ring system and
terminal nitrogen atom.
y This differs significantly from the phenothiazine
antipsychotic series in which an unbranched propyl chain is
required.
y The phenothiazines with a three carbon bridge between
nitrogen atoms are more potent in vitro.
y Also, unlike the phenothiazine antipsychotics, the
heterocyclic ring of the antihistamines is unsubstituted.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Phenothiazines)
y Promethazine, the parent member of this series, is moderately
potent by present-day standards with prolonged action and
pronounced sedative side effects.
y In addition to its antihistaminic action, it is a potent antiemetic,
anticholingeric and sedating agent, and significantly potentiates
the action of analgesic and sedative drugs.
y The other members of this series display a similar
pharmacologic profile and thus may cause drowsiness and so
may impair the ability to perform tasks requiring alertness.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Phenothiazines)
y Also, concurrent administration of alcoholic beverages
and other central nervous system depressants with the
phenothiazines should be avoided.
y In general, lengthening of the side chain and
substitution of lipophilic groups in the 2-position of the
aromatic ring results in compounds with decreased
antihistaminic activity and increased psychotherapeutic
properties
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Phenothiazines)
y The enantiomers of promethazine have been resolved and have
similar antihistaminic and other pharmacologic properties.
y This is in contrast with studies of the pheniramines and
carbinoxamine compounds in which the chiral center is closer to
the aromatic feature of the molecule.
y Asymmetry appears to be of less influence on antihistaminic
activity when the chiral center lies near the positively charged
side chain nitrogen.
y While little pharmacokinetic data is available for the
phenothiazine antihistamines, the metabolism of the close
structural analogue promethazine has been studied in detail.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐ Aminoalkyl Ethers (Phenothiazines)
y This compound undergoes mono and di-N-dealkylation,
sulfur oxidation, aromatic oxidation at the 3-position to
yield the phenol and N-oxidation. A number of these
metabolites, particularly the phenol, may yield
glucuronide conjugates.
y It is expected that the phenothiazine antihistamines
would display similar metabolic profiles.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐
Dibenzocycloheptenes/heptanes
y The dibenzocycloheptene and heptane antihistamines may be
regarded as phenothiazine analogues in which the sulfur atom
has been replaced by an isosteric vinyl group (cyproheptadine)
or a saturated ethyl bridge (azatadine), and the ring nitrogen
replaced by an sp2 carbon atom.
y The two members of this series are closely related in structure;
azatadine is an aza (pyridyl) isostere of cyproheptadine in which
the 10,11-double bond is reduced.
y Cyproheptadine HCl possesses both an antihistamine and an
antiserotonin activity and is used as an antipruritic agent.
st
1
GENERATION H1‐ANTAGONIST DRUG CLASSES‐
Dibenzocycloheptenes/heptanes
y Sedation is the most prominent side effect, and this is
usually brief, disappearing after three or four days of
treatment.
y Azatadine maleate: A potent, long-acting antihistaminic
with antiserotonin activity.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES
y The second generation antihistamines are more similar
pharmacologically than structurally.
y Structurally they are all diaryl substituted piperazines (cetirizine)
or piperidines (all others).
y As discussed earlier in this module, these compounds were
developed as selective H1-receptor antagonists with relatively
high potency.
y Most of these compounds also produce prolonged antihistaminic
effects as a result of slow dissociation from H1-receptors, and
the formation of active metabolites with similar receptor binding
profiles.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES
y The second generation agents have little affinity for
muscarinic, adrenergic or serotonergic receptors and
therefore display a lower degree of side effects
associated with antagonism at these receptors, but
their affinities for these receptors is somewhat variable.
y Generally, the large aralkyl groups or polar groups
linked to the piperidine/piperazine rings of these
compounds reduces their affinity for muscarinic or
adrenergic receptors.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES
y Perhaps most importantly, all of these compounds are devoid of
sedating effects at therapeutic concentrations due to poor CNS
penetration, and possibly lowered affinities for central
histaminic, cholinergic and adrenergic receptors.
y While these compounds offer several advantages over the
classical antihistamines, widespread use has revealed a number
of therapeutic limitations.
y This is probably most true for terfenadine and astemizole (since
withdrawn) which have been found to produce life-threatening
arrhythmias when used concurrently with drugs that inhibit their
y metabolism.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES
y These drug interactions have been most evident with
the imidazole antifungals ketoconazole, itraconazole
and fluconazole, and the macrolides erythromycin,
clarithromycin and troleandomycin which inhibit the
metabolism of terfenadine and astemizole, resulting in
elevated levels of the parent drugs which are
proarrhythmic.
y This adverse effect is evident by prolongation of QTc
intervals on ECG.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Terfenadine. Alpha-[4-(1,1-Dimethylethyl)phenyl] -4(hydroxydiphenylmethyl)-1-piperidinebutanol (Seldane®) is a
reduced butyrophenone derivative of an aminoalcohol-type
antihistaminic.
y Terfenadine was developed during a search for new
butyrophenone antipsychotic drugs as evident by the presence
of the N-phenylbutanol substituent. It also contains a
diphenylmethylpiperidine moiety analogous to that found in the
piperazine antihistamines.
y Terfenadine is a selective, longacting (>12 hours) H1-antagonist
with little affinity for muscarinic, serotonergic or adrenergic
receptors.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y The histamine receptor affinity of these compounds are
believed to be related primarily to the presence of the
diphenylmethylpiperidine moiety.
y The prolonged action results from very slow
dissociation from these receptors.
y The lack of anticholinergic, adrenergic or serotonergic
actions appears to be linked to the presence of the
Nphenylbutanol substituent.
y This substituent also limits distribution of terfenadine
to the CNS.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Terfenadine is rapidly absorbed producing peak plasma levels in
1-2 hours.
y The drug undergoes significant first pass metabolism with the
predominant metabolite being fexofenadine, an active metabolite
resulting from methyl group oxidation.
y When drugs that inhibit this transformation, such as the
imidazole antifungals and marolides, are used concurrently,
terfenadine levels may rise to toxic levels, resulting in potentially
fatal heart rhythm problems.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y This resulted in withdrawal of this drug product.
y Terfenadine is highly plasma protein bound (97%) and
has a half-life of about 20 hours.
y Terfenadine is widely distributed in peripheral tissues,
with highest concentrations in the liver.
y The major route of elimination of terfenadine and its
metabolites is in the faeces and elimination is biphasic.
y The mean elimination half-life is 16-23 hours.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Fexofenadine is a selective peripheral H1-receptor blocker that,
like terfenadine, produces no clinically significant
anticholinergic effects or alpha1-adrenergic receptor blockade at
therapeutic doses.
y The lack of anticholinergic, adrenergic or serotoninergic actions
appears to be linked to the presence of the N-phenylbutanol
substituent which limits binding to these receptors.
y No sedative or other CNS effects have been reported for this
drug and animal studies indicate that fexofenadine does not
cross the blood-brain barrier.
y In vitro studies also suggest that, unlike terfenadine,
fexofenadine does not block potassium channels in
cardiocytes.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Furthermore in drug interaction studies, no prolongation of the
QTc interval or related heart rhythm abnormalities were detected
when administered concurrently with erythromycin or
ketoconazole.
y Fexofenadine is rapidly absorbed after oral administration
producing peak serum concentrations in about 2.5 hours.
y Fexofenadine is 60-70% plasma protein bound.
y Unlike its parent drug, only 5% of the total dose of fexofenadine
is metabolized.
y The remainder is excreted in the bile and urine and the mean
elimination half-life is about 14 hours.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Astemizole, USP. 1-(4-Fluorobenzyl)-2-((1-(4methoxyphenyl)-4piperidyl)amino)benzimidazole (Hismanal®).
y Astemizole was developed from a series of
diphenylbutylpiperidine antihistamines in an effort to extend the
duration of action.
y During development it was discovered that this compound
produced little sedation or autonomic side effect.
y Astemizole is a selective and long acting H1-antagonist with little
affinity for muscarinic, serotoninergic, adrenergic receptors or
H2-receptors.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Generally, both the diaryl system and large aralkyl group linked
to the piperidine nitrogen appears to reduce its affinity for
muscarinic or adrenergic receptors.
y The piperidinoaminobenzimidazole moiety appears to be
required for H1-receptor affinity, and contributes significantly to
the persistent receptor binding that results in prolonged action.
y potent and longer acting than terfenadine.
y It does not penetrate the CNS readily, thus sedation and other
CNS side effects (dizziness, drowsiness, fatigue) are minimal.
y Astemizole also has no local anaesthetic actions.
y It is used for seasonal allergic rhinitis and chronic urticaria. It
has a slow onset of action (2 to 3 days).
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Astemizole is rapidly and completely absorbed orally and should
be administered 1 hour before meals.
y Peak plasma levels are observed within 1-4 hours.
y Astemizole is widely distributed in peripheral tissues, with
highest concentrations attained in the liver, pancreas and
adrenal glands.
y It undergoes extensive first pass metabolism by processes
including aromatic hydroxylation, oxidative
y dealkylation and glucuronidation.
y The main metabolites are desmethylastemizole, 6-hydroxy
y desmethylastemizole and norastemizole.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y The desmethyl metabolite has antihistaminic activity
comparable to the parent drug, and thus contributes to
the prolonged duration of action.
y Astemizole is highly protein bound (96%) and has a
plasma half-life of 1.6 days.
y The apparent half-life of the desmethyl metabolite
ranges from 10-20 days, depending on frequency of
dosing of the parent drug.
y The primary route of elimination is in the faeces.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Astemizole was discovered by Janssen in 1977
y It was withdrawn from the market because it produced life
threatening arrhythmia in a minority of subjects
y Recently there has been renewed interest in Astemizole owing
to the fact that it was found to be a potent treatment for malaria.
y It has a mechanism of action similar to chloroquine but has
activity even in chloroquine-resistant parasites.
Chong CR, Chen X, Shi L, Liu JO, Sullivan DJ (2006). "A clinical drug library
screen identifies astemizole as an antimalarial agent". Nat Chem Biol 2 (8):
415–16
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Loratadine, USP. 4-(8-chloro-5,6-dihydro-11H-benzo[5,6]cyclohepta[1,2-b]pyridin-11-ylidene-1-carboxylic acid ethyl ester.
y Loratadine is structurally related to the antihistamines azatadine
and cyproheptadine.
y It differs from azatadine in that a neutral carbamate group has
replaced the basic tertiary amino moiety, and the phenyl ring has
been substituted with a chlorine atom.
y The replacement of the basic group with a neutral functionality is
believed to preserve antihistaminic action while reducing CNS
effects.
y Loratadine is also structurally related to a number of tricyclic
antidepressants.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Loratadine is a selective peripheral H1-antagonist with a receptor
binding profile like the other members of this series, except that
it has more antiserotoninergic activity.
y Thus it produces no substantial CNS or autonomic side effects.
y Loratadine displays potency comparable to astemizole
y and greater than terfenadine.
y Loratadine is rapidly absorbed after oral administration
producing peak plasma levels in about 1.5 hours.
y This drug is extensively metabolized, primarily to the
descarboethoxy metabolite which retains some antihistaminic
activity.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Both the parent drug and metabolite have elimination
half-lives ranging from 8-15 hours.
y The metabolite is excreted renally as a conjugate.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperazines
y Cetirizine: [2-[4-[(4-chlorophenyl)phenylmethyl]-1piperazinyl]ethoxy]acetic acid (Zyrtec®).
y This compound is a racemic compound.
y Cetirizine is the primary acid metabolite of hydroxyzine resulting
y from complete oxidation of the primary alcohol moiety.
y This compound is zwitterionic and relatively polar and thus does
not penetrate the blood-brain barrier readily.
y Prior to its introduction in the US cetirizine was one of the most
widely prescribed H1-antihistamines in Europe.
y It is highly selective in its interaction with various hormonal
binding sites and highly potent (» terfenadine) as
y well.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperazines
y The advantages of this compound appear to be once-daily
dosing, a rapid onset of activity, minimized CNS effects and a
lack of clinically significant effects on cardiac rhythm when
administered with imidazole antifungals and macrolide
antibiotics.
y The onset of action is within 20 to 60 minutes in most patients.
y Cetirizine produces qualitatively different effects on
psychomotor/psychophysical functions compared to the first
generation antihistamines.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperazines
y However the most common adverse reaction associated with
cetirizine is dose-related somnolence and thus patients should
be advised that cetirizine may interfer with the performance of
certain psychomotor/psychophysical activities.
y Other effects of this drug include fatigue, dry mouth, pharyngitis
and dizziness.
y Because the drug is primarily eliminated by a renal route, its
adverse reactions may be more pronounced in individuals
suffering from renal insufficiency.
y No cardiotoxic effects, such as QT prolongation, are observed
with the new drug when used at its recommended or higher
doses or when coadministered with imidazole antifungals and
macrolide antibiotics.
y However, other typical drug interactions of H1-antihistamines
apply to cetirizine.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperazines
y Dose proportional Cmax values are achieved within 1 hour of oral
administration of cetirizine.
y Food slows the rate of cetirizine absorption but does not affect the
overall extent.
y Consistent with the polar nature of this carboxylic acid drug, less than
10% of peak plasma levels have been measured in the brain.
y Cetirizine is not extensively metabolized and »70% of a 10 mg oral dose
is excreted in the urine (>80% as unchanged drug) and 10% recovered
in the feces.
y The drug is highly protein bound and has a terminal half-life of 8.3
hours.
y The clearance of cetirizine is reduced in elderly subjects as well as in
renally and hepatically impaired patients
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Pyrrolidines
y Acrivastine, USP. (E,E)-isomer. It is an analogue of triprolidine
containing a carboxyethenyl moiety at the 6-position of the
pyridyl ring.
y Acrivastine shows antihistaminic potency and duration of action
comparable to triprolidine.
y Unlike triprolidine, acrivastine does not display significant
anticholinergic activity at therapeutic concentrations.
y Also, the enhanced polarity of this compound resulting from
carboxyethenyl substitution limits BBB penetration and thus this
compound produces less sedation than triprolidine.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Pyrrolidines y Limited pharmacokinetic data is available for this
compound.
y Orally administered drug has a half life of about 1.7
hours and a total body clearance of 4.4 mL/min/Kg.
y The mean peak plasma concentrations are reported to
vary widely, and the drug appears to penetrate the CNS
poorly.
y The metabolic fate of acrivastine has not been reported.
THIRD GENERATION ANTIHISTAMINES
y Third-generation H1-antihistamines are the active enantiomer
(levocetirizine) or metabolite (desloratadine & fexofenadine)
derivatives of second-generation drugs intended to have
increased efficacy with fewer adverse drug reactions.
y Indeed, fexofenadine is associated with a decreased risk of
cardiac arrhythmia compared to terfenadine.
y Second generation antihistamines (terfenadine, astemizole,
loratadine and cetirizine), which block peripheral H1 receptors
without penetrating the blood-brain barrier, were developed and
introduced from 1981 onwards to provide comparable
therapeutic benefit without the CNS side-effects.
THIRD GENERATION ANTIHISTAMINES
y Although largely successful in this goal, terfenadine and
astemizole were found to cause potentially serious
arrhythmias when plasma concentrations became elevated
subsequent to impaired metabolism.
y It was established that the cardiac toxicity was mainly due
to the parent drugs.
THIRD GENERATION ANTIHISTAMINES
y As active metabolites could account for most of the clinical
benefit, the goal for the third generation of antihistamines
became to develop therapeutically active metabolites that were
devoid of cardiac toxicity.
y The first of these drugs, fexofenadine (the active metabolite of
terfenadine), was approved in July 1996, after an unusually rapid
development programme. Its introduction set a new standard of
safety that led the FDA to request the withdrawal of terfenadine
in 1997 on the grounds that a safer version of an equivalent drug
was now available.
y Norastemizole and descarboethoxy loratadine, the metabolites of
astemizole and loratadine, respectively, are also in clinical
development. These offer comparable or superior clinical
benefits
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Fexofenadine Hydrochloride. (+/-)-4-[1-hydroxy-4-[4(hydroxydiphenylmethyl)-1-piperinyl]y butyl-α,α-dimethylbenzeneacetic acid (Allegra®).
y This compound is marketed as a racemate and exists as a
zwitterion in aqueous media at physiological pH.
y Fexofenadine is a primary metabolite of terfenadine.
y It was developed based on studies that revealed when
terfenadine’s hepatic conversion to the fexofenadine was
blocked by other drugs or disease, levels of the parent drug
(terfenadine) rise resulting in heart rhythm problems.
2nd GENERATION H1‐ANTAGONIST DRUG CLASSES‐ The Piperidines
y Subsequent clinical trials demonstrated that
fexofenadine was not only active and effective in
allergic disorders, but less cardiotoxic than terfenadine.
y This led to the approval of fexofenadine as an
alternative to relieve the symptoms of seasonal
allergies.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y IN THE PAST:
y Success in drug design largely due to serendipity (natural
sources)
y Analogues of naturally occurring molecules synthesised to
improve activity and/or reduce side effects
y Variations on a trial and error basis
y Wasteful with respect to time and effort involved
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y IN THE RECENT PAST (Last 20 Years):
y Emphasis on rational drug design
y Drugs designed to interact with a known biological system
y The cimetidine story is an excellent example of this
approach.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y THE REMARKABLE ASPECT OF THE CIMETIDINE STORY
y At the onset of the project there were no lead compounds,
y AND.......
y It was not even known whether or not the necessary
receptor protein existed.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y IN THE PAST:
y Success in drug design largely due to serendipity (natural
sources)
y Analogues of naturally occurring molecules synthesised to
improve activity and/or reduce side effects
y Variations on a trial and error basis
y Wasteful with respect to time and effort involved
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y ULCER THERAPY IN 1964
y Methods available few and unsatisfactory
y Ulcers localised erosions in mucous membranes of
stomach or duodenum
y Presence of gastric acid causes problem/aggravates
problem/delays recovery
y Untreated ulcer causes severe pain; internal bleeding;
mortality
y 1926: actor Rudolph Valentino dies at age 31 of perforated
ulcer
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y ULCER THERAPY IN 1964
y Conventional treatment neutralisation of gastric acid by
administration of bases such as Sodium Bicarbonate and
Calcium Carbonate
y High doses required to achieve neutralisation; relief
temporary; side effects great
y Surgery (removal of part of stomach) sometimes resorted
to.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y PHYSIOLOGY:
y Gastric acid (HCl) released by parietal cells which are
autonomically inervated
y Autonomic stimulation results in acetylcholine secretion from
the nerve endings adjacent to the parietal cells
y Acetylcholine crosses gap between nerve endings and parietal
cells
y Parietal cells activated- release of gastric acid into stomach.
y Triggers: sight, smell or thought of food implying that gastric
acid is released even before food has entered the stomach
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y PHYSIOLOGY:
y Nerve signals also stimulate the antral region of the
stomach which contains hormone producing cells known as
G cells.
y G cells release a hormone (a peptide called Gastrin)
y Gastrin is also released as food passes through the
stomach
y Gastrin moves into blood supply and travels to the parietal
cells further stimulating the release of gastric acid
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y IMPLICATION FROM A RATIONAL DRUG DESIGN POINT OF
VIEW:
y Release of gastric acid should be inhibited by:
y Antagonists of the acetylcholine receptor (anticholinergic
drugs)
y Gastrin Receptor Antagonists
y This thought process demonstrates a fundamental tenet in
rational drug design, namely the understanding of the
biological processes involved in the condition being
targeted, and the identification of receptors which require
modulation
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y RECEPTORS WITH THE POTENTIAL FOR SUCCESSFUL
MODULATION:
y ANTICHOLINERGICS:
y Would block the cholinergic receptor in the parietal cells
and inhibit the release of gastric acid
y BUT...
y They would also inhibit acetylcholine receptors in other
parts of the body and cause unwanted side effects
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y MORE RATIONAL DRUG DESIGN THOUGHT PROCESSES:
y WHAT IS THE DISTRIBUTION OF THE TARGET RECEPTOR IN THE
HUMAN ORGANISM?
y Are the receptors present exclusively in the target locus or are they
widespread?
y If present exclusively at target unwanted side effect probability is low.
y If widespread, do sub-types exist? And if subtypes do exist, is
distribution at different loci subtype determined?
y IN SHORT, MAY THE IDENTIFIED TARGET BE FEASABLY USED? (Ref.
Statement in Red Font on previous slide)
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y RECEPTORS WITH THE POTENTIAL FOR SUCCESSFUL
MODULATION:
y IDENTIFICATION OF A DRUG CAPABLE OF BLOCKING THE
HORMONE GASTRIN
y In the rational drug design process therefore, all efforts
were then concentrated on the complete understanding of
the gastrin release process
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y MORE RATIONAL DRUG DESIGN THOUGHT PROCESSES:
y WHAT ARE THE BIOLOGICAL PATHWAYS INVOLVED IN THE
RELEASE OF THE MEDIATOR THAT MUST BE CHANGED?
y IS THERE A COMPLETE UNDERSTANDING OF THESE
PATHWAYS?
y CONTRIBUTIONS BY BIOCHEMISTS AND PHARMACOLOGISTS
ARE ESSENTIAL AT THIS STAGE OF THE RATIONAL DRUG
DESIGN STUDY
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y RECEPTORS WITH THE POTENTIAL FOR SUCCESSFUL
MODULATION:
y Current (1962) knowledge also associated histamine with gastric
acid release stimulation.
y Could therefore an antihistamine be effective in the treatment of
gastric acid?
y Leap of faith in that although the association between histamine
and gastric acid release was known, the exact dynamics of the in
vivo association were not understood.
y Furthermore, conventional antihistamines failed to inhibit gastric
acid release.
τ
2
3
2
α
2
3
2
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y A STUDY OF HISTAMINE:
y Histamine is made up of an
imidazole ring whic can exist in two
tautomeric forms.
y Attached to the imidazole ring is a
two carbon chain with a terminal αamino group.
y The pKa of this amino group is 9.80
which means that at a plasma pH of
7.4, the side chain of histamine is
99.6% ionised.
y The pKa of the imidazole ring is
5.74, implying that at pH 7.4, the
ring exists in the unionised form.
τ
α
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y A STUDY OF HISTAMINE:
y Whenever cell damage occurs, histamine is released
stimulating the dilatation and increased permeability of
small blood vessels.
y This allows defensive cells, eg white blood cells, to be
released from the blood supply to an area of tissue damage
to combat any potential infection.
y Unfortunately, allergy and irritation also cause histamine
release and are responsible for clinical conditions including
hay fever, rash and asthma.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y THEORY: TWO HISTAMINE RECEPTORS?
y From where did this histamine approach therefore arise?
y Scenario: Conventional antihistamines fail to have an effect
on gastric acid release.
y BUT....
y They also failed to inhibit other actions of histamine eg
failed to fully inhibit the dilatation of blood vessels induced
by histamine.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y THEORY: TWO HISTAMINE RECEPTORS?
y Perhaps 2 types of histamine receptor exist?
y Could it be that histamine, the natural messenger, is
capable of acting as an agonist equally effectively at both,
and does not distinguish between the two subtypes.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y MORE RATIONAL DRUG DESIGN THOUGHT PROCESSES:
y IN RATIONAL DRUG DESIGN, THE DETERMINATION OF
WHETHER OR NOT TARGET RECEPTOR SUB-TYPES EXIST IS
PARAMOUNT IMPORTANCE
y IF RECEPTOR SUBTYPES ARE DETERMINED TO EXIST, AND
THE SUBTLE DIFFERENCES BETWEEN THEM ARE
DETERMINED, THEN HIGHLY SELECTIVE DRUG MOLECULES
MAY BE DESIGNED, CAPABLE OF ELICITING THE DESIRED
PHARMACOLOGICAL EFFECTS FOR A REDUCED SIDE EFFECT
PROFILE.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y THEORY: TWO HISTAMINE RECEPTORS?
y Perhaps 2 types of histamine receptor exist?
y If 2 types of histamine receptor exist, then theoretically (as
highlighted in principle on the previous slide), it should be
possible to design antagonist molecules capable of
distinguishing between the receptor subtypes.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y THEORY: TWO HISTAMINE RECEPTORS?
y Perhaps 2 types of histamine receptor exist?
y And if this theory was correct, then further extrapolation
would continue to suggest that the conventional
antihistamines known in the early 60s were already
selective, in that they were capable of inhibiting histamine
receptors involved in inflammation (arbitrarily called H1
Receptors), and were unable to inhibit the proposed
histamine receptors involved in gastric acid secretion (the
proposed H2 Receptors)
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y THEORY: TWO HISTAMINE RECEPTORS?
y PROBLEM:
y To date no known antagonist for the proposed H2 receptors
y Until such a compound found, it could not be certain that
the H2 receptors even existed
y No receptor was available to study
y The aim was to design a selective antagonist for this
hypothetical receptor
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y SEARCHING FOR A LEAD
y With a theory and no lead molecule, histamine itself was an
obvious starting point, because if histamine was stimulating
the release of gastric acid by binding to a hypothetical H2
receptor, then clearly, histamine was being recognised by
the receptor.
y The task then was to vary the structure of histamine in such
a way that it would still be recognised by the receptor, but
bind in such a way that it acted as an antagonist rather than
an agonist.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Determine how histamine itself was binding to its receptors.
y Structure-activity studies on histamine and histamine
analogues revealed that the binding requirements for
histamine to the H1- and the proposed H2- receptors were
slightly different:
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y H1- RECEPTOR REQUIREMENTS:
y The side-chain had to have a
positively charged nitrogen atom
with at least one attached proton.
(Quaternary ammonium salts which
lacked such a proton were
extremely weak in activity)
y There had to be a flexible side chain
N
between the above cation and the
heteroaromatic ring.
..
y The heteroaromatic ring did not
have to be imidazole in nature, but
it did have to have a nitrogen atom SAR FOR AGONIST AT H1 RECEPTOR
with a lone pair or electrons orthoto the side chain.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FOR THE PROPOSED H2- RECEPTOR, SAR STUDIES WERE
CARRIED OUT TO DETERMINE WHETHER HISTAMINE
ANALOGUES COULD BRING ABOUT THE PHYSIOLOGICAL
EFFECTS PROPOSED FOR THIS RECEPTOR ie
STIMULATING GASTRIC ACID RELEASE.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y H2- RECEPTOR REQUIREMENTS:
y Essential SAR requirements were the same as for the H1receptor except that the heteroaromatic ring had to contain
an amidine unit (HN-CH-N:)
..
SAR FOR AGONIST AT PROPOSED H2 RECEPTOR
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Having gained a knowledge of structure activity
relationships for histamine, the task was now a molecule
which would be recognised by the H2 Receptor, but which
would not activate it.
y More clearly, an agonist had to be converted into an
antagonist
y It was consequently necessary to alter the way in which the
molecule was bound to the receptor.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y Pictorially one can imagine
histamine fitting into its
receptor site and inducing a
change in shape which
switches on, or activates the
receptor.
y An antagonist might be found
by adding a functional group
which would bind to another
binding region on the
receptor and prevent the
conformational change
required for activation.
y Pg 557 fig 18.8
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y To begin with, a study of known agonists and antagonists in
other fields of medicinal chemistry was carried out.
y The structural differences between agonists and antagonists for
a particular receptor were identified and then similar alterations
were attempted on histamine.
y For example, fusing an aromatic ring onto noradrenaline had
been a successful tactic used in the design of antagonists for
the noradrenaline receptor.
y This same tactic was attempted unsuccessfully with histamine.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Another approach which had been used unsuccessfully in
the development of anticholinergic agents was the addition
of non-polar, hydrophobic substituents.
y This approach was also attempted unsuccessfully with
histamine by attaching various alkyl and arylalkyl groups to
different locations on the histamine skeleton.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y This latter strategy, did however,
yield one interesting result which
was relevant to later studies.
y It was shown that 4methylhistamine was a highly
selective H2-agonist, showing far
greater activity for the H2- than for
the H1- receptor.
y This fact obviously led researchers
to question why such a small
simple alteration should result in
such a drastic change in selectivity
..
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y The selectivity observed suggested that 4-methylhistamine,
and by inference histamine, must adopt 2 conformationsone to fit the H1- and the other to fit the H2- receptor.
y Since 4-methylhistamine was more active at the H2receptor, then the implication was that the conformation
required for the H2- receptor was a stable one for 4methylhistamine, whereas the conformation required for the
H1- receptor is unstable for 4-methylhistamine
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
..
Stable conformation of 4-methylhistamine
Selective for H2-Receptor
..
Unstable conformation of 4-methylhistamine.
Selective for H1- Receptor
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Up to this point thus, research had concentrated on searching
for an additional hydrophobic binding region on the receptor
y Then the focus of research switched to determine the effect of
replacing the terminal α-NH3+ group with a variety of different
polar functional groups.
y The reasoning was that different polar groups could bond to the
same region on the receptor as the NH3+ group, but that the
geometry of bonding might be altered sufficiently to produce an
antagonist
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y It was from this study that the first crucial breakthrough was
achieved with the discovery that Nα-guanylhistamine was acting
very weakly as an antagonist (partial agonist).
y This means that Nα-guanylhistamine activates the H2- Receptor,
but not to the same extent as histamine.
y As a result, the amount of gastric acid released is lower.
y Most significantly, as long as Nα-guanylhistamine is bound to
the receptor, it prevents histamine from binding and thus
prevents complete receptor activation.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y Nα- guanylhistamine:
HN
N
C
H2
H2
C NH
NH2 +
HN
N
NH2
HN
N
C
H2
H2 +
C N
NH2
H
NH2
C
H2
H2
C NH
NH2
NH2+
+
HN
N
C
H2
H2
C N
NH2
H
NH2
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y QUESTIONS:
y Which part or parts of the Nα-guanylhistamine skeleton
were responsible for the observed effects?
y Was the guanidine group itself acting as an antagonist?
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Various guanidine structures were synthesised which lacked the
imidazole ring, but none had the desired antagonist activity.
y This means that both the imidazole ring and the guanidine group
were required.
y The structures of Nα-guanylhistamine and histamine were then
compared.
y Both structures contain an imidazole ring and a positively
charged group linked by a 2 carbon bridge.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y The guanidine group is basic and protonated at pH 7.4 so
that the analogue has a positive charge similar to histamine.
y However, the charge on the guanidine group can be spread
around a planar arrangement of 3 nitrogens, and can
potentially be further away from the imidazole ring.
y This leads to the possibility that the analogue could be
interacting with a binding region on the receptor which is
out of reach of histamine.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
p.560 Figs 18.13 and 18.14
Fig 18.14 histamine capable of reaching only the agonist
region
Fig 18.13 the analogue with extended functionality is capable
of reaching either region.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y It was postulated that 2 alternative binding regions might be
available for the cationic group- an agonist region where binding
leads to activation of the receptor, and an antagonist binding
region where binding does not activate the receptor.
y If most of the analogue molecules bind to the agonist region, and
the remainder bind to the antagonist region, then this could
explain the partial agonist activity.
y Regardless of the mode of binding, histamine would be
prevented from binding, and an antagonism would be observed
due to the percentage of Nα-guanylhistamine bound to the
antagonist region.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPING THE LEAD- A CHELATION BONDING THEORY:
y Variations were attempted in order to evaluate if an analogue
could be made which binds only to the antagonist region.
y An isothiourea was synthesised. In this structure the nitrogen
nearest to the imidazole ring was replaced with a sulfur atom.
y The positive charge in this molecule was restricted to the
terminal portion of the chain .
y The scope was that this latter should interact more strongly with
the proposed antagonist binding region if this is indeed further
away.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Antagonist activity did increase, but the compound was still
a partial agonist, showing that binding was still possible to
the agonist region.
y Two other analogues were synthesised, where one of the
terminal amino acids in the guanidine group was replaced
with either a methylthio group or a methyl group.
y Both the resulting structures were partial agonists, but with
poorer antagonist activity.
X= SMe; Me
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y From these results, it was concluded that both terminal amino
groups were required for binding to the antagonist binding
region.
y It was proposed that the charged guanidine group was
interacting with a charged carboxylate residue on the receptor
via 2 hydrogen bonds.
y If either of these terminal amino groups were absent, then
binding would be weaker, resulting in a lower level of
antagonism.
H
H2
X
C C
H2
HN
N H
+
O
O
N H
H
N
Strong
Interaction
H
H
+ N
H2
NH
C C
H2
HN
N
X
RECEPTOR
O
-
X = NH, S
RECEPTOR
O
Weak
Interaction
X = Me, SMe
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y The chain was then extended from a 2-carbon to a 3-carbon unit
in order to see what would happen if the guanidine group was
moved further away from the imidazole ring.
y The antagonist activity increased for the guanidine structure,
but, strangely enough decreased for the isothiourea structure.
y It was therefore proposed that with a chain length of 2 carbon
units, hydrogen bonding to the receptor involved the terminal
NH2 groups, but with a chain length of 3 carbonunits, hydrogen
bonding involved one terminal NH2 group along with the NH
group within the chain.
GUANIDINE & ISOTHIOUREA STRUCTURES
GUANIDINE‐ Increased Antagonist Activity
ISOTHIOUREA‐ Decreased Antagonist Activity
2
2
2
2
2
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Support for this theory was provided by the fact that
replacing one of the terminal NH2 groups in the guanidine
analogue with SMe or Me did not adversely affect the
antagonist activity. (Ref Graphic Next Slide)
y This was completely different from the results obtained
when similar changes were carried out on the 2-carbon
bridged compound. (Ref Graphic Two Slides Over)
PROPOSED BINDING INTERACTIONS FOR ANALOGUES OF DIFFERENT CHAIN LENGTH
C
H
N
NH
H
2
C X
HN
2
+
NH
2
2
O
_
‐
RECEPTOR
O
X = S, NH
HN
N
C
H
2
H
2
C C
H
2
+ X
N
NH
H
2
O _‐
O
RECEPTOR
X = NH2 SMe Me
EFFECT OF VARYING THE GUANIDINE GROUP ON BINDING TO THE ANTAGONIST REGION
y Fig 18.22 & 18.23 on pg 563
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y The problem now was to completely remove the agonist
activity in order to obtain compounds with pure antagonist
activity.
y This meant designing a structure which would differentiate
between the agonist and the antagonist binding regions.
y At first sight this seemed impossible because both regions
appear to involve the same type of bonding
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y Histamine’s activity as an agonist depends on the imidazole
ring and the charged amino function, with the 2 groups
taking part in hydrogen and ionic bonding respectively.
y However, the antagonist activity of the partial agonists
described so far also appears to depend on a hydrogen
bonding imidazole ring and an ionic bonding guanidine
group.
y HOWEVER.................
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y A distinction could be made between the charged groups.
y Structures showing antagonist activity are all capable of forming
chelated bonding structures (Slide 176)
y This interaction involves two hydrogen bonds between two
charged species.
y The question arose as to whether it was really necessary for the
chelating group to be charged, or more clearly.....
y Could a neutral group also chelate to the antagonist region by
hydrogen bonding alone?
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y If yes, then it could be possible to distinguish between the
agonist and the antagonist region, especially since ionic
bonding appeared necessary for agonist binding.
y The decision was therefore, to evaluate the consequence of
replacing the strongly basic guanidine group with a neutral
group capable of interacting with the receptor by 2 hydrogen
bonds.
y These groups were selected also on the basis of not causing any
other significant changes to the other properties of the molecule.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y Thus, in order to study the effect of changing the basic
guanidine group with a neutral group, it was necessary to ensure
that the new group was as similar as possible to guanidine in
terms of size, shape and hydrophobicity.
y Several functional groups were tried, but success was ultimately
achieved by using a thiourea group.
y In fact, the thiourea derivative SK&F91581 proved to be a weak
antagonist with no agonist activity.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y Apart from basicity, the properties of the thiourea group
were very similar to those of the guanidine group.
y Both groups were planar, similar in size, and capable of
participating in hydrogen bonding.
y Thus the alteration in biological activity could be reasonably
attributed to the differences in basicity between the two
groups.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y Unlike guanidine, the thiourea group is neutral.
y This is due to the C=S group which has an electron withdrawing
effect on neighbouring nitrogens making them non-basic and
more like amide nitrogens.
y The fact that a neutral group was capable of binding to the
antagonist region and not to the agonist site was taken to imply
that the agonist binding region required ionic bonding, and that
the antagonist binding region required hydrogen bonding.
BURIMAMIDE
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y FROM PARTIAL AGONIST TO ANTAGONIST- BURIMAMIDE
DEVELOPMENT:
y Further chain extension and the addition of an N-methyl group
led to burimamide which was found to have enhanced activity.
y These results suggested that chain extension served to move
the thiourea group closer to the antagonist binding region, and
that addition of the N-methyl group resulted in a beneficial
increase in hydrophobicity.
y Burimamide is a highly specific competitive antagonist of
histamine at H2-receptors and is 100x more potent than Nαguanylhistamine.
y Its discovery finally proved the existence of the H2-receptor
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y Despite apparent success, burimamide was not suitable for
progression to clinical trials because its antagonist activity was
still too low for oral administration.
y Attention was next turned to the imidazole ring of burimamide
and to the possible tautomeric forms of this ring.
y It was argued that if one particular tautomer was preferred for
binding to the H2 receptor, then activity could be enhanced by
modifying the burimamide structure to favour that tautomer.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y At pH 7.4 it is possible for the imidazole ring to equilibrate
between the 2 tautomeric forms I and II via the protonated
intermediate III shown on the next slide.
y The necessary proton for this process is supplied by water
or by an exchangeable proton on a suitable amino acid
residue in the binding region.
y If the exchange is slow, it is possible that the drug will enter
and leave the receptor at a faster rate than the equilibration
between the 3 tautomeric forms.
IMIDAZOLE RING CAN EQUILIBRATE BETWEEN VARIOUS TAUTOMERIC FORMS
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y And if this latter hypothesis is correct, then the preferred
tautomer in a strong agonist such as histamine should also
be the preferred tautomer for a strong antagonist.
y The graphic (2 slides previous) indicated that the imidazole
ring can exist as one ionised and 2 unionised tautomers.
y It was thus necessary to consider the likelihood of the
preferred tautomer being ionised or otherwise.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y And if this latter hypothesis is correct, then the preferred
tautomer in a strong agonist such as histamine should also
be the preferred tautomer for a strong antagonist.
y The graphic (2 slides previous) indicated that the imidazole
ring can exist as one ionised and 2 unionised tautomers.
y It was thus necessary to consider the likelihood of the
preferred tautomer being ionised or otherwise.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y It has already been seen that the pKa for the imidazole ring
in histamine is 5.74, meaning that the ring is a weak base,
and mostly unionised.
y The pKa value for imidazole itself is 6.80, and for
burimamide 7.25.
y These values show that these imidazole rings are more
basic than histamine, and more likely to be ionised.
y The question is why this is so..................
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y This may be explained through consideration of the side
chain, which must have an electronic effect on the imidazole
ring.
y If the side chain is electron withdrawing, or electron
donating, then it will affect the basicity of the ring.
y A measure of the side chain’s electronic effect can be
worked out by the Hammett equation......
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y
pKa(R) = pKa(H) + ρσR
y Where pKa(R) is the pKa of the imidazole ring bearing the side
chain R,
y pKa(H) is the pKa of the unsubtituted imidazole ring
y ρ is a constant
y And ρ(R) is the Hammett substituent constant for the side chain
R.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y From the pKa values, the value of the Hammett substituent
constant can be calculated to show whether the side chain
R is electron withdrawing or donating.
y In burimamide, the side chain was calculated to be slightly
electron donating- of the same order of a methyl group.
y Therefore, the imidazole ring in burimamide is more likely to
be ionised than that in histamine in which the side chain is
electron withdrawing.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y At pH 7.4, 40% of burimamide is ionised in the imidazole ring,
compared to 3% of histamine.
y This represents quite a difference between the 2 structures, and
since the binding of the imidazole ring is important both for
antagonist and agonist activity, the implication is that a pKa
value closer to that of histamine might lead to better binding and
to better antagonist activity.
y It was necessary therefore, in this drug design project to make
the side chain electron withdrawing rather than donating.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y This may be done by inserting an electronegative atom into
the side chain- which also has a minimal effect on the rest
of the molecule.
y In other words, an isostere for a methylene group was
sought,one that had the desired electronic effect, but which
also had the same size and properties as the methylene
group.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y The first isostere to be tried was a sulfur atom.
y Sulfur is quite a good isostere for a methylene unit, in that
both groups have similar van der Waals radii and similar
bond angles.
y However, a C-S bond is slightly longer than a C-C bond,
leading to a light extension (15%) of the structure.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y The methylene group replaced was next but one to the imidazole
ring.
y This site was chosen, not for any strategic reasons, but because
a synthetic route was readily available to cary out this
transformation.
y As hoped, the resulting compound, thiaburimamide, had a
significantly lower pKa of 6.25, and was found to have enhanced
antagonistic activity.
y This result supported the theory that a reduction in the
proportion of ionised tautomer was beneficial to receptor binding
and antagonist activity.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y Thiaburimamide had been synthesised in order to favour
the unonised imidazole ring over the ionised ring.
y But as previously demonstrated, there are 2 possible
unionised tautomers.
y The question consequently arose as to whether either of
these was preferred for receptor binding.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y In order to answer this question, histamine was once again
considered.
y If one of the unionised tautomers was found to be preferred
over the other in histamine, then the reasonable assumption
would be that this is favoured tautomer for receptor binding.
y The preferred tautomer for histamine is tautomer 1
y Why is tautomer 1 favoured?.................
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y The answer lies in the fact that the side chain on histamine is
electron withdrawing.
y This electron withdrawing effect on the imidazole ring is
inductive, and therefore the strength of the effect decreases the
distance around the ring.
y The implication is that the nitrogen atom on the imidazole ring
closest to the side chain (Nπ) experiences a greater electron
withdrawing effect than the one further away (Nτ).
y As a result, the closer nitrogen is less basic, which in turn
means that it is less likely to bind to hydrogen.
y Since the side chain in thiaburimamide is electron withdrawing,
then it too will favour tautomer 1.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y It was now argued that this tautomer could be further enhanced
if an electron donating group was placed at position 4 in the ring.
y At this position, the inductive effect would be felt most at the
neighbouring nitrogen (Nτ), further enhancing its basic character
and increasing the population of tautomer 1.
y It was also important to choose a group which would not
interfere with the normal receptor binding interaction.
y For example, a large substituent would be too bulky and prevent
the analogue fitting the receptor.
y A methyl group was chosen since it was known that 4mthylhistamine was an agonist, and was highly selective for the
H2 receptor.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y METIAMIDE DEVELOPMENT:
y The compound obtained was metiamide which was found to
have enhanced activity as an antagonist, supporting the
previous theory.
y Compared to burimamide, the percentage of ionised imidazole
ring was lowered in metiamide, and the ratio of the two possible
unionised imidazole tautomers reversed.
y The fact that activity is increased with respect to
thiaburimiamide suggests that the increase in the population of
tautomer 1 outweighs the increase in population of the ionised
tautomer 111
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y 4-METHYLBURIMAMIDE DEVELOPMENT:
y 4-methylburimamide was also synthesised for comparison.
y Here, the introduction of the 4-methyl group did not lead to
an increase in activity.
y The pKa of 4-methylburimamide is high- 7.80, resulting in
the population of the ionised imidazole ring to rise to 72%
y This demonstrates the importance of rationalising structural
changes- adding a 4-methyl group to thiaburimamide is
advantageous, but adding it to burimamide is not.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y The design and synthesis of metiamide followed a rational
approach aimed at favouring one specific tautomer in an
approach known as dynamic structure activity analysis.
y But strangely enough...........
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y It has since transpired that the improvement in antagonism
may have also resulted from conformational effects.
y X-ray crystallography studies have indicated that the longer
thioester linkage in the chain increases the flexibility of the
side chain and that the 4- methyl substituent in the
imidazole ring may help to orientate the imidazole ring
correctly for receptor binding.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y It is significant that the oxygen analogue oxaburimamide
was less potent than burimamide despite the fact that the
electron withdrawing effect of the oxygen containing chain
on the ring is similar to the sulfur containing chain.
y The bond lengths and angles of the ether link are similar to
the methylene unit, and in thisrespect is a better isostere
than sulfur.
y However, the oxygen atom is substantially smaller.
y It is also significantly more basic and more hydrophilic than
either sulfur or methylene.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Oxaburimamide’s lower activity might be due to a variety of
reasons.
y For example, the oxygen may not allow the same flexibility
permitted by the sulfur atom.
y Alternatively, the oxygen may be involved in a hydrogen
bonding interaction either with the receptor or with its own
imidazole ring resulting in a change in receptor binding
interaction.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRATEGY:
y Metiamide is 10x more active than burimamide, and showed
promise as an anti-ulcer agent.
y Unfortunately, a number of patients suffered kidney damage
and granulocytopaenia- a condition which results in the
reduction of circulating white blood cells, and makes
patients susceptible to infection.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPMENT OF CIMETIDINE:
y It was proposed that metiamide’s side effects were
associated with the thiourea group- a group which is not
particularly common in the body’s biochemistry.
y Therefore consideration was given to replacing this group
with a group which was similar in property, but would be
more acceptable in a biochemical context.
The urea analogue was tried but was found to be less active
CHEMICAL STRUCTURE OF THE GUANIDINE ANALOGUE
y The guanidine analogue (top)
was also less active, but it was
interesting to note that this
compound had no agonist
activity.
y This contrasts with the 3-carbon
bridged guanidine (below) which
has already been shown to be a
partial agonist.
y This made the guanidine
analogue (top) the first example
of a guanidine having pure
antagonist activity.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPMENT OF CIMETIDINE:
y One possible explanation for this is that the longer 4-unit
chain extends the guanidine binding group beyond the
reach of the agonist binding region, whereas the shorter 3unit chain still allows binding to both agonist and
antagonist regions.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
4‐Carbon Unit Chain
3‐Carbon Unit Chain
y Fig 18.33
y Fig 18.34
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPMENT OF CIMETIDINE:
y The antagonist activity for the guanidine analogue was
weak, but consideration was given to this compound since
it was thought that the guanidine unit would be less likely to
have toxic side effects than the thiourea.
y This was a reasonable assumption since the guanidine unit
is naturally present in the amino acid arginine.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPMENT OF CIMETIDINE:
y The problem now was to retain the guanidine unit but to
increase activity.
y It seemed likely that the low activity was due to the fact that
the basic guanidine group would be ionised at pH7.4.
y The problem was how to make this group neutral- no easy
task- considering that guanidine is one of the strongest
bases in organic chemistry.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPMENT OF CIMETIDINE:
y Nevertheless, a search of the literature revealed a useful
study on the ionisation of monosubstituted guanidines.
y A comparison of pKa values of these compounds with the
inductive substituent complexes σi for the substituents X
yielded a straight line indicating that pKa is inversely
proportional to the electron withdrawing power of the
substituents.
y Thus, strongly electron withdrawing substituents make the
guanidine group less basic and less ionised.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
Ionisation of monosubstituted
guanidines
y Fig 18.36
pKa vs Inductive Substituent
Constants (σi) for X on the LHS
y Fig 18.37
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y DEVELOPMENT OF CIMETIDINE:
y Both the nitroguanidine and cyanoguanidine analogues of
metiamide were synthesised and found to have comparable
antagonist activities to metiamide.
y The cyanoguanidine analogue was cimetidine, and was the
more potent analogue that was chosen for clinical studies
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y BIOLOGICAL ACTIVITY OF CIMETIDINE:
y Cimetidine inhibitsed H2 receptors and consequently also
inhibited gastric acid release.
y It did not have the toxicity of metiamide, and was also more
potent than metiamide.
y It also inhibited pentagastrin from stimilating the release of
gastric acid.
y Pentagastrin is an analogue of gastrin, and the fact that
cimetidine was capable of blocking its stimulatory activity
suggested a relationship between histamine and gastrin in the
release of gastric acid.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y CIMETIDINE:
y Cimetidine was first marketed in the UK in 1976 under the
trade name Tagamet (derived from anTAGonist and
ciMETidine)
y It was the first really effective ant-ulcer drug that
successfully did away with the need for surgery, and for
several years it was the world’s best selling prescription
drug until it was pushed into second place in 1988 by
ranitidine.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRUCTURE & ACTIVITY OF CIMETIDINE:
y The finding that metiamide and cimetidine were both good
H2-antagonists of similar activity shows that the
cyanoguanidine group is a good bioisostere for the thiourea
group.
y This is despite the fact that three tautomeric forms are
possible for the guanidine group compared to only one for
the isothiourea group.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRUCTURE & ACTIVITY OF CIMETIDINE:
y The 3 tautomeric forms of the guanidine unit were foundto
be more apparent than real, since the imino tautomer (II) is
the preferred tautomeric form for the guanidine unit.
y Tautomer II is favoured since the cyano group has a
stronger electron withdrawing effect on the neighbouring
nitrogen compared to the two nitrogens further away.
y This makes the neighbouring nitrogen less basic and less
likely to be protonated.
THE CIMETIDINE STORY‐ A Rational Approach to Drug Design
y STRUCTURE & ACTIVITY OF CIMETIDINE:
y Since tautomer II is favoured, the guanidine group does in fact
bear a close structural resemblance to the thiourea group.
y Both groups have a planar π electron system with similar
geometries (equal C-N distances and angles).
y They are polar and hydrophilic with high dipole moments and
low partition coefficients.
y They are weakly basic and weakly acidic such that they are
unionised at pH7.4
VARIATION OF THE IMIDAZOLE RING‐ Ranitidine
y Ranitidine:
y Further studies on cimetidine analogues showed that the
imidazole ring could be replaced with other nitrogen
containing heterocyclic rings.
y However, Glaxo moved one step further by showing that the
imidazole ring could be replaced by a furan ring bearing a
nitrogen containing substituent.
y This led to the introduction of ranitidine
CHEMICAL STRUCTURE OF RANTIDINE (ZANTAC®)
Ranitidine has
fewer side effects
than cimetidine,
has a longer half
life, and is 10x
more active.
VARIATION OF THE IMIDAZOLE RING‐ Ranitidine
y SAR RESULTS FOR RANITIDINE INCLUDE:
y The nitroketeneaminal group is optimum for activity, but
may be replaced by other planar π systems capable of
hydrogen bonding
y Replacing the sulfur atom with a methylene group leads to a
drop in activity
y Replacing the furan ring with more hydrophobic rings such
as phenyl or thiophene reduces activity
VARIATION OF THE IMIDAZOLE RING‐ Ranitidine
y SAR RESULTS FOR RANITIDINE INCLUDE:
y 2,5-disubstitution is the best substitution pattern for the
furan ring.
y Substitution on the dimethylamino group may be varied,
showing that the basicity and hydrophobicity of this group
are not crucial to activity.
VARIATION OF THE IMIDAZOLE RING‐ Ranitidine
y SAR RESULTS FOR RANITIDINE INCLUDE:
y Methyl substitution at carbon 3 of the furan ring eliminates
activity, whereas the equivalent substitution on the
imidazole series increases activity.
y Methyl substitution at carbon 4 of the furan ring eliminates
activity, whereas the equivalent substitution in the
imidazole series increases activity.
VARIATION OF THE IMIDAZOLE RING‐ Ranitidine
y SAR RESULTS FOR RANITIDINE INCLUDE:
y These latter 2 results imply that the heterocyclic rings for
cimetidine and ranitidine are not interacting in the same way
with the H2 receptor.
y This is suported by the fact that a corresponding
dimethylaminomethylene group attached to cimetidine
leads to a drop in activity.
y Ranitidine was introduced into the market in 1981, and by
1988 became the world’s best selling prescription drug.
FAMOTIDINE & NIZATIDINE
y During 1985 and 1987 two new antiulcer drugs were
introduced to the market- famotidine and nizatidine.
y Famotidine is 30x more active than cimetidine in vitro.
y The side chain contains a sulfonylamidine group while the
heterocyclic imidazole ring of cimetidine has been replaced
with a 2-guanidinothiazole ring
FAMOTIDINE & NIZATIDINE
y SAR STUDIES ON FAMOTIDINE INDICATE THAT:
y The sulfonylamidine binding group is not essential and may
be replaced by a variety of structures as long as they are
planar, have a dipole moment, and are capable of
interacting with the receptor by hydrogen bonding. A low
pKa is not essential, which allows a larger variety of planar
groups to be used than is possible for cimetidine.
y Activity is optimum for a chain length of 4 or 5 units.
y Replacement of sulfur with a CH2 group increases activity
FAMOTIDINE & NIZATIDINE
y SAR STUDIES ON FAMOTIDINE INDICATE THAT:
y Modification of the chain is possible with, for example, inclusion
of an aromatic ring
y A methyl substituent ortho to the chain leads to a drop in activity
(unlike the cimetidine series).
y 3 of the 4 hydrogens in the 2 NH2 groups are required for
activity.
y There are several results which are markedly different from
cimetidine, implying that famotidine and cimetidine are not
interacting in the same way with the H2-receptor
FAMOTIDINE & NIZATIDINE
y NIZATIDINE:
y Nizatidine was introduced into the UK in 1987 by the Lilly
Corporation.
y It is equipotent to ranitidine
y The furan ring in ranitidine is replaced with a thiazole ring
H2 ANTAGONISTS WITH PROLONGED ACTIVITY
y There is presently need for longer lasting antiulcer agents
which require once daily doses.
y GSK carried out further development on ranitidine by
placing the oxygen of the furan ring exocyclic to a phenyl
ring and replacing the dimethylamino group with a
piperidine ring to give a series of novel structures.
H2 ANTAGONISTS WITH PROLONGED ACTIVITY
y The most promising of these compounds were lamitidine
and loxitidine which were 5 – 10x more potent than
ranitidine and 3x longer lasting.
y Unfortunately, these compounds showed toxicity in long
term animal strudies, with the possibility of causing gastric
ulcer.
y They were subsequently withdrawn from clinical study.
COMPARISON OF H1 & H2 ANTAGONISTS
y The structures of the H2 antagonists are markedly different
to the classical H1 antagonists, so there is little surprise
that the original antihistamines failed to antagonise the H2
receptor.
y H1 antagonists like H1 agonists possess an ionic amino
group at the end of a flexible chain.
y Unlike the agonists they possess 2 aryl or heteroaryl rings
in place of the imidazole ring see 18.62
COMPARISON OF H1 & H2 ANTAGONISTS
y Because of the aryl rings, H1 antagonists are hydrophobic
molecules having high partition coefficients.
y In contrast, H2 antagonists are polar and hydrophilic,
having high dipole moments and low partition coefficients.
y At the end of the flexible chain they have a polar p electron
system which is weakly amphoteric and unionised at pH 7.4
y This binding group appears to be the key feature leading to
antagonism of H2 receptors.
COMPARISON OF H1 & H2 ANTAGONISTS
y The 5- membered heterocycle generally contains a nitrogen
atom, or, in the case of furan or phenyl, a nitrogen
containing side chain.
y The hydrophilic character of H2 antagonists helps to explain
why H2 antagonists are less likely to have CNS side effects
often associated with H1 antagonists.
THE H2 RECEPTOR & H2 ANTAGONISTS
y H2 receptors are present in a variety of organs and tissues,
but their main role is acid secretion.
y As a result, H2 antagonists are remarkably safe, and mostly
free of side effects.
ANTIBACTERIAL AGENTS
y The fight against bacterial infection is one of the great
success stories of medicinal chemistry
y Bacteria were first identified in the 1670s by van
Leeuwenhoek following his invention of the microscope.
y It was not until the 19th century that their link with disease
was appreciated following the experiments of Pasteur who
demonstrated that specific bacterial strains were crucial to
fermentation, and that these and other microrganisms were
far more widespread than previously thought.
ANTIBACTERIAL AGENTS
y An early advocate of a germ theory of disease was the
Edinburgh surgeon Lister.
y Despite the protests of several colleagues who took offence
at the suggestion that they might be infecting their own
patients, Lister introduced carbolic acid as an antiseptic
and sterilising agent for operating theatres and wards.
y The improvement in surgical survival rates was significant.
ANTIBACTERIAL AGENTS
y During the latter half of the 19th century, scientists such as
Koch were able to identify the micro-organisms responsible
for diseases such as tuberculosis, cholera and typhoid.
y Methods such as vaccination for fighting infections were
studied.
y Research was also carried out to try to find effective
antibacterial agents or antibiotics
ANTIBACTERIAL AGENTS
y Erlich’s Principle of Chemotherapy was that a chemical
could directly interfere with the proliferation of
microorganisms at concentrations tolerated by the host.
y Thus was the notion of selective toxicity, introduced to
therapeutics.
y This selectivity came to be represented by a
chemotherapeutic index which compared the minimum
effective dose of a drug with the maximum dose which
could be tolerated by the host.
y This principle was later extended to all drug classes- hence
the widely used term therapeutic index.
MECHANISMS OF ANTIBACTERIAL ACTION‐ 5 MAIN MECHANISMS
y INHIBITION OF CELL METABOLISM BY ANTIMETABOLITES
y They inhibit the metabolism of a microorganism but not of
the host
y They do this by inhibition of an enzyme catalysed reaction
present in the bacterial cell but not in animal cells
y The best known examples of antibacterial agents acting in
this way are the sulphonamides
ANTIBACTERIAL AGENTS
y INHIBITION OF CELL WALL SYNTHESIS
y This leads to bacterial cel lysis and death.
y Agents acting in this way include penicillins &
cephalosporins.
y Since animal cells do not have a cell wall, they are
unaffected by such agents.
ANTIBACTERIAL AGENTS
y INTERACTION WITH THE PLASMA MEMBRANE
y Some antibacterial agents interact with the plasma
membrane of bacterial cells to affect membrane
permeability.
y This has fatal results for the bacterial cell.
y Polymyxins and tyrothricin operate in this way
ANTIBACTERIAL AGENTS
y DISRUPTION OF PROTEIN SYNTHESIS.
y This means that essential enzymes required for the bcterial
cell’s survival can no longer be made.
y Agents which disrupt protein synthesis include the
rifamycins, aminoglycosides, tetracyclines &
chloramphenicol.
ANTIBACTERIAL AGENTS
y INHIBITION OF NUCLEIC ACID TRANSCRIPTION &
REPLICATION
y Inhibition of nucleic acid function prevents cell division
and/or the synthesis of essential enzymes.
y Agents acting in this way include nalidixic acid and
proflavine
THE ANTIMETABOLITES‐ the sulphonamides
y The sulphonamide story starts in 1935 when it was
discovered that a red dye, prontosil rubrum, had in vivo
antibacterial properties, when these were adiminstered to
laboratory animals.
y It was also significant, that prontosil rubrum, could not kill
bacteria in a test tube.
y This fact remained a mystery until it was discovered that
prontosil itself was not the antibacterial agent.
THE ANTIMETABOLITES‐ the sulphonamides
y Instead, it was found that the dye was metabolised by
bacteria present in the small intestine of the test animal, and
broken down to give a product called sulphanilamide.
y It was this compound which was the true antibacterial
agent.
y Prontosil thus became the first example of a pro-drug, and
sulphanilamide was synthesised in the laboratory and
became the first synthetic antibacterial agent, active
against a wide variety of infections.
Metabolised
Prontosil (Pro-Drug)
Sulphanilamide
METABOLISM OF PRONTOSIL
THE ANTIMETABOLITES‐ the sulphonamides
y Further developments led to a range of sulphonamides which
proved effective against Gram positive organisms specifically
meningococci and pneumococci.
y Despite their undoubted benefits, sulpha drugs were ineffective
against Salmonella- the organism responsible for typhoid.
y Other problems resulted from the way in which these drugs are
metabolised, since toxic products are frequently obtained.
y For this reason, sulphonamides were largely superseded by
penicillin
THE ANTIMETABOLITES‐ the sulphonamides ‐SAR
The synthesis of a large number of sulphonamide analogues
(see next slide) led to the following conclusions:
yThe para amino group is essential for activity and must be
unsubstituted (ie R=H). The only exception is when R is an
acyl group ie amides. The amides themselves are inactive, but
may be metabolised in vivo to regenerate the active
compound (see 2 slides over). This means that amides can be
used as sulphonamide prodrugs.
SULPHONAMIDE ANALOGUES
Metabolism of acyl group to regenerate active compound
THE ANTIMETABOLITES‐ the sulphonamides ‐SAR
y The aromatic ring and the sulphonamide functional group
are both required.
y The aromatic ring must be para substituted only
y The sulphonamide nitrogen must be secondary
y R’’ is the only possible site that can be varied in
sulphonamides
SULPHANILAMIDE ANALOGUES
y R’’ can be varied by incorporating a large range of
heterocyclic or aromatic structures which affect the extent
to which the drug binds to plasma proteins
y This in turn controls the blood levels of the drug such that it
can be short acting or long acting
y Thus a drug which binds strongly to plasma protein will be
released more slowly into the circulation and will be longer
lasting
SULPHANILAMIDE ANALOGUES
y Changing the nature of the R’’ has also helped to reduce the
toxicity of some sulphonamides.
y The primary amino group of sulphonamides is acetylated in
the body and the resulting amides have reduced solubility
which can lead to toxic effects.
y For example, the metabolites formed from an early
sulphonamide, sulphathiazole, (see slide overleaf), is poorly
soluble and may prove fatal if it blocks the renal tubule.
N-acetylation
INSOLUBLE
SULPHANILAMIDE ANALOGUES
y It was discovered that the solubility problem could be overcome
by replacing the thiazole ring in sulphathiazole with a pyrimidine
ring to give sulphadiazine.
y The reason for the improved solubility lies in the acidity of the
solubility lies in the acidity of the sulphonamide NH proton.
y In sulphathiazole, this proton is not very acidic (high pKa).
y Therefore, sulphathiazole and its metabolite are mostly
unionised at blood pH.
SULPHANILAMIDE ANALOGUES
y Replacing the thiazole ring with a more electron
withdrawing pyrimidine ring increases the acidity of the NH
proton by stabilising the anion which results.
y Therefore, sulphadiazine and its metabolite are significantly
ionised at blood pH.
y As a consequence, they are more soluble and less toxic.
pKa 6.48
86% Ionised
SULPHANILAMIDE ANALOGUES‐
Sulphadiazine
y Was found to be more active than sulphathiazole, and soon
replaced it in therapy.
y The corollary therefore is that varying R” can affect the
solubility of sulphonamides or the extent to which they bind
to plasma protein.
y These variations are therefore affecting the
pharmacokinetics of the drug rather than its mechanism of
action.
SULPHANILAMIDES‐The Future
y The penicillins largely superseded the sulphonamides in the
fight against bacterial infections, and for a long time,
sulphonamides were relegated backstage.
y There has of late, been a revival of interest with the
discovery of a new breed of longer lasting sulphonamides.
y One example of this new generation is sulphamethoxine,
(see next slide), which is so stable in the body it need only
be taken once a week.
SULPHA DRUGS‐Current Applications in Medicine
y Treatment of Urinary Tract Infections
y Ophthalmic Use.
y Treatment of Mucous Membrane Infection.
y Treatment of Gut Infections
EXAMPLES OF OTHER ANTI‐
METABOLITES
y There are other antimetabolites in medical use apart from
the sulphonamides.
y Two examples are trimethoprim and a group of compounds
known as sulfones.
SULFONE STRUCTURE
TRIMETHOPRIM
y Is a diaminopyrimidine structure which has proved to be a
highly selective, orally active, antibacterial and antimalarial
agent.
y Unlike the sulphonamides it inhibits dihydrofolate
reductase- the enzyme which carries out the conversion of
folic acid to tetrahydrofolate.
y The overall effect, however, is the same as with the
sulphonamides- the inhibition of DNA synthesis and cell
growth
TRIMETHOPRIM
y Dihydrofolate reductase is present in mammalian cells as
well as in bacterial cells.
y Trimethoprim distinguishes between enzymes in either cell
type, owing to the fact that mutations over millions of years
have resulted in a significant difference in structure
between the two enzymes such that trimethoprim
recognises and inhibits the bacterial enzyme, but does not
recognise the mammalian enzyme.
TRIMETHOPRIM
y Is often given in conjunction with the sulphonamide
sulphamethoxazole (co-trimoxazole Septrin®)
y Sulphamethoxazole inhibits the incorporation of PABA into folic acid;
trimethoprim inhibits dihydrofolate reductase. This results in the
inhibition of two enzymes in the same biosynthetic route; and is a very
effective way of inhibiting a biosynthetic route and has the advantage
of keeping the doses of both drugs to sub-toxic levels.
y The acquistion of the same level of inhibition using a single drug would
require much higher doses, leading to potential side effects.
y This approach has been described as SEQUENTIAL BLOCKING
H
N
CO2 H
TETRAHYDROFOLATE
FOLIC ACID
H
OMe
O
H
CH
3
N
H
H
S
H
N
N
CH
N
H
O
2
N
OMe
N O
SULFAMETHOXAZOLE
OMe
TRIMETHOPRIM
BACTERIAL CELL WALL SYNTHESIS INHIBITORS
y There are 2 major classes of drug which
act in this fashion
y These are the penicillins & the
cephalosporins
BRIEF HISTORY OF THE PENICILLINS
y In 1877, Pasteur & Joubert discovered that certain moulds could
produce toxic substances which killed bacteria. Unfortunately, these
substances were also toxic to humans and had no clinical value. They
demonstrated however, that moulds could be a source of antibacterial
agents.
y In 1928, Fleming noted that a bacterial culture which had been left open
to the air for several weeks had become infected by a fungal colony. Of
interest was the fact that there was an area surrounding the fungal
colony where the bacterial colonies were dying.
y He correctly concluded that the fungal colony was producing an
antibacterial agent which was spreading to the surrounding area.
BRIEF HISTORY OF THE PENICILLINS
y Recognising the significance of this, he set out to culture
and identify the fungus, and showed it to be a relatively rare
species of Penicillium.
y It has since been suggested that the Penicillium spore
responsible for the fungal colony originated from another
laboratory in the building, and that the spore was carried by
air currents, and was eventually blown through the window
of Fleming’s laboratory.
BRIEF HISTORY OF THE PENICILLINS
y Fleming spent several years investigating the novel
antobacterial substance, and showed it to have significant
anti-bacterial properties, and to be remarkably non-toxic to
humans.
y Unfortunately, the substance was also unstable, and
Fleming was unable to isolate & purify the compound.
y He therefore came to the conclusion that penicillin was too
unstable to be used clinically.
BRIEF HISTORY OF THE PENICILLINS
y The problem of isolating penicillin was eventually solved in 1938
by Florey & Chain by using a process known as freeze drying,
which allowed isolation of the antibiotic under much milder
conditions than had been previously available.
y By 1941, Florey & Chain were able to carry out the first clinical
trials on crude extracts of penicillin, and achieved spectacular
success.
y Further developments aimed at producing the new agent in large
quantities were developed in the US, such that by 1944 there was
enough penicillin for casualties arising from the D-Day landings.
BRIEF HISTORY OF THE PENICILLINS
y This led to a widespread use of penicillin- however, the
structure of the compound remained unresolved.
y The issue was finally settled in 1945 when Dorothy
Hodgkins established the exact structure (see overleaf)
through X-ray analysis.
R=
Benzyl Penicillin (PEN G)
6-Aminopenicillanic Acid
R=
Phenoxymethylpenicillin
(PEN V)
Acyl Side
Chain
β -Lactam Ring
Thiazolidine Ring
BRIEF HISTORY OF THE PENICILLINS
y The synthesis of such a highly strained molecule presented
a huge challenge. This was overcome by Sheenan who
completed a full synthesis of the molecule by 1957.
y This synthetic pathway was too involved to be of
commercial use, but the following year Beechams isolated a
biosynthetic intermediate of penicillin.
y This revolutionised the field of penicillins by providing the
starting material for a huge range of semisynthetic
penicillins
BRIEF HISTORY OF THE PENICILLINS
y Penicillins were widely & carelessly used so that the
evolution of penicillin resistant bacteria became more and
more of a problem.
y The fight against these penicillin-resistant bacteria was
promoted greatly when, in 1976 Beechams discovered a
natural product called clavulanic acid which has proved
highly effective in protecting penicillins from the bacterial
enzymes which protect them.
Sulphur replaced by Oxygen
No acylamino side chain
6
7
9
4
5
8
3
1
2
β -Lactam
Oxazolidine Ring
THE STRUCTURE OF PENICILLIN
y The structure of penicillin is unusual, such that many scientists
were not convinced of its veracity until this was proven through
X Ray crystallography.
y It is comprised of a highly unstable looking bicyclic system
consisting of a 4-membered β-lactam ring fused to a 5 membered
thiazolidine ring.
y The skeleton of this molecule suggests that it is derived from the
amino acids cysteine and valine.
y The overall shape of the molecule is like that of a half open book:
CYS
3
3
2
VAL
PENICILLIN APPEARS TO BE DERIVED FROM CYSTEINE & VALINE
THE STRUCTURE OF PENICILLIN
y The acyl side chain R varies depending on the make up of
the fermentation media used during the synthetic process.
y For example, corn steep liquor was used as a medium when
penicillin was first produced in the United States, and this
gave Penicillin G (R = benzyl)
y This was due to the high levels of phenylacetic acid
(PhCH2CO2H) present in the medium.
PENICILLIN ANALOGUES
y In 1957, Sheehan succeeded in synthesising penicillin and
obtained a 1% yield of penicillin V using a multistep
synthetic route.
y Clearly full synthesis was not an efficient way of making
penicillin analogues.
y In 1958-1960, Beechams managed to isolate a biosynthetic
intermediate of penicillin which was also one of Sheehan’s
synthetic intermediates.
PENICILLIN ANALOGUES
y The compound was 6-APA and it allowed the synthesis of a
huge number of analogues by a semi-synthetic method.
y This means that fermentation yielded 6-APA which could
then be treated synthetically to give semi-synthetic
analogues.
y This was achieved by acylating the 6-APA with a range of
acid chlorides
2
3
3
3
3
2
PENICILLIN ANALOGUES ACHIEVED BY ACYLATING 6-APA
PENICILLIN ANALOGUES
y 6-APA is now produced by hydrolysis of
penicillin G or V with an enzyme- penicillin
acylase or by other chemical methods which will
be discussed later on in this unit.
y These are more efficient procedures than
fermentation.
H
H
N
O
H
H
S
O
C H2
N
O
C H3
C H3
CO2 H
H
H N
C H2
PRODUCTION OF 6-APA
H
S
N
O
HYDROLYSIS
H
C H3
C H3
CO2 H