Development of morphine analogue

Development of morphine
• The Opium Analgesics
•Variation of subtituen
• Drug extension
• Simplification
• Rigdification
History of opium
we are now going look in detail at one of
the oldest fields in medicinal chemistry. It is
important to appreciate that the opiates
are not the only compounds which are of
use in the relief of pain and that there are
several other classes of compounds
including aspirin, which combat pain. The
opiates have proved ideal for the treatment
of 'deep' chronic pain and work in the central
nervous system (CNS).
• To be precise, we should really only use the term for
those natural compounds which have been
extracted from opium- the sticky exudates obtained
from the poppy (papaver somniferum).
• The term alkaloids refers to a natural product which
contain a nitrogen atom and is therefore basic in
• These compounds provide a vast ‘library’ of
biologically active compounds which can be used as
lead compounds is many possible fields of
medicinal chemistry. However, we are only
interested at present in the alkaloids derived from
• The use of opium was recorded in China over 2000
years go and was known in Mesopotamia before
• Because of opium’s properties, the Greeks
dedicated the opium poppy to Thanatos ( the God
of death), Hypnos ( The God of Sleep) and
Morpheus ( The God of dreams). Later physicians
prescribed opium for a whole range of afflictions,
including chronic headache, vertigo, epilepsy,
asthma, colic, fevers, dropsies, melancholy, and’
troubles to which women subject’.
• opium use in medicine is quoted in a 12th century
prescription: “take opium, mandragora and
henbane in equal parts and mix with water. When
you want to saw or cut a man, dip a rag in this and
put it to his nostrils. He will sleep so deep that you
may do what you wish.
• Opium was first marketed in Britanian by Thomas
Dover, a one time irate who had taken up medicine
• Another popular remedy of the day was ‘Godfrey’s
cordial’ which contained opium, molasses, and
• Doctors started that stopping the drug after longterm used led to ‘great and intolerable distresses,
anxieties and depression of the spirit…..These were
the first reports of addiction and withdrawal
• Its has to be appreciated that in this days opium
and the opium trade were considered to be as
legitimated as tobacco or tea, and that this view
continued right up to the twentieth century.
Indeed, during the nineteenth century the opium
trade led directly to a war between the United
Kingdom and China
• During 19th century, China was ruled by an elite
class who considered all foreigners as nothing
better than barbarians and wanted nothing to do
with them
• Up until the early 17th century, China had grown its
own opium for use as an ingredient in cke and as a
medicine but strangely enough, it was introduction
of tobacco which changed all this. Tobacco was
discovered in the 15th century and sailor introduced
the habit in to far east
• However, because of china embargo most of this
had to be smuggled in via the port of canton by
British and American merchants.
• Eventually, the Chinese authorities decided to act.
They seized and burnt a shipload of opium, then
closed the port f Canton to the British . The British
traders were outraged and appealed to lord
Palmerston, the British foreign secretary. Relation
between the two counties steadily deteriorated and
led to the opium Wars of 1839-42. China was
quickly defeated and was forced to lease Hong Kong
to Britain as a trading port. They were also forced
to accept the principle of free trade and to pay
reparations of 21 million pounds
• In the mid 19th century, opium was smoked in much
the same way as cigarettes are today, and opium
dens were as much a part of London society as
coffee shop. These dens were used by many of te
romantic authors of the day, including Thomas de
Quincy, Edgar Allan Poe, and Samuel Taylor
Coleridge. De Quincy even wrote a book recording
his opium experiences around 4 pints of laudanum
a week when he wrote The Rime of The Ancient
Mariner. A later poem called Dejection, may have
been inspired by his experience of withdrawal
• Towards the end of the 19th century, doubts were
beginning to grow about the long term effects of
opium and its addictive properties
• Indeed, in 1882 a parliamentary report started, ‘if
Indian opium was stopped at once it would be a
very frightful calamity indeed. I should say that one
third of the adult population of China would die for
want of opium. Never theless, doubts persisted and
a motion was put forward in parliament in1893
stating that the ‘opium trade was morally
indefensible’. However, the motion was heavily
• It wasn’t until Chinese immigrants introduced
opium on a large scale into the USA, Australia, and
South America that governments really cracked
down on the trade. In 1909, the International
Opium Commission was set up and, by 1914, 34
nations had agreed to curb opium production and
trade. By 1924, 62 countries had signed up and the
league of Nations took over the role of control,
requiring countries to limit the use of narcotic drugs
to medicine alone. Unfortunately , many farmer in
India, Pakistan, Afghanistan, Turkey, Iran, and the
golden triangle (the border of Burma, Thailand, and
Laos) depended on the opium trade for survival, an
as a result the trade went underground and has
continued to his day.
• Opium contain a complex mixture of almost 25
alkaloids. the principle alkaloid in the mixture and
the one responsible for analgesic activity, is
morphine, named after the ancient god of sleepMorpheus Although pure morphine was isolated
in 1803, it was not until 1833 that chemist at
Macfarlane & Co. (now Macfarlane-Smith) in
Edinburgh were able to isolated and purify it on a
commercial scale. However , since morphine was
poorly absorbed orally, it was little used in
medicine until the directly into the blood supply.
• Morphine was then found to be a particularly good
analgesic and sedative, and was far more effective
than crude opium. But there was also the price to
be paid. Morphine was used during the American
Civil war (1861-65) and the Franco-Prussian war.
• At this stage, it is worth pointing out that all drugs
have side-effects of one sort or another
• The development of narcotic analgesics is good
example of the traditional approach to medicinal
chemistry and provides good examples of the
various strategies which can be employed in dug
development We can identify several stage:
• Stage 1
Recognition that natural plant or help (opium from
the poppy) has a pharmacological action.
• Stage 2
Extraction and identification of the active principle
• Stage 3
Synthetic studies (full and partial synthetics)
• Stage 4
Structure activity relationship-the synthetics of
analogues to see which part of the molecule are
important to biological activity.
• Stage 5
Drug development – the synthetic of analogues to try
and improve activity or reduce side-effects.
• Stage 6
Theories on the analgesic receptors. Synthesis of
analogues to test theories.
• Stage 5 and 6 are the most challenging and
rewarding part of the procedure as far as the
medicinal chemist is concerned, since the possibility
exists of improving on what nature has provided. In
this way, the chemist hope to again a better
understanding of the biological process involved,
which in turn suggests further possibilities for new
By 19th century standards,
morphine was an extremely
complex molecule and
provided a huge challenge
to chemists
By 1881, the functional groups on morphine
had been identified, but it took many more
years to establish the full structure.
In those day the only way to find the
structure of a complicated molecule was to
break it down into simpler fragments which
were already known and could be identified
The way to find the
structure of a complicated
break it down into
simpler fragments which
were already known and
could be identified.
A full synthesis of morphine
was achieved in 1952
synthesize the
What is morphine????
Morphine is the active
principle of opium and is
still one of the most
effective painkillers
available to medicine.
It is especially good for treating
dull, constant pain rather than
sharp, periodic pain
It acts in the brain and
appears to work by elevating
the pain threshold, thus
decreasing the brain’s
awareness of pain.
Unfortunately, it has a large number of
side-effects which include the following:
Depression of the respiratory centre,
constipation, excitation, euphoria, nausea,
pupil constriction, tolerance,dependence.
The dangerous side-effects of morphine
are those of tolerance and dependence,
allied with the effects morphine can have
on breathing. In fact, the most common
cause of deathfrom a morphine overdose
is by suffocation.
The molecule contains five rings, labeled A-E,
and has a pronounced T shape
It is basic because of the tertiary amino
group, but it also contains a phenolic
group, an alcohol group, an aromatic
ring, an ether bridge,and a double bond.
The fenolic Oh
• Codeine is the methyl ether of morphine and is
also present in opium. It is used for treating
moderate pain,coughs,and diarrhoea.
• If codeine is administeres to patients, its analgesic
effect is 20% that of morphine- much better than
expected. Why is this so?
• The answer lies in the fact that codeine can be
metabolized in the liver . The methyl ether is
removed to give the free phenolic group.
Thus,codeine can be viewed as a prodrug for
The-6 alcohol
 The result in fig 17.4 show that masking or the
complete loss of the alcohol group does not
decrease analgesic activity .
 In this case, the morphine analogues shown are
able to reach the analgesic receptor far more
efficiently than morphine itself.
 This is because the analgesic receptors are
located in the brain and, to reach the brain, the
drugs have to cross a barrier called the bloodbrain barrier.
 Since the barrier is fatty, highly polar compounds
are prevented from crossing. Thus, the more
polar groups a molecule has, the more difficulty it
has in reaching the brain.
The-6 alcohol
Morphine has three polar groups (phenol,alcohol,and an amine),
whereas the analogues above have either lost the polar alcohol
group or have it masked by an alkyl or acyl group. They therefore
enter the brain more easily and accumulate at the receptor sites in
greater concentrations; hence, the better analgesic activity.
The comparison of morphine, 6
acetylmorphine, and diamorphine
• The most active (and the most dangerous)
compound of the three is 6-acetylmorphine,
which is four times more active than morphine.
Heroin is also more active than morphine by a
factor of two, but it less active than 6acetylmorphine.
• 6-Acetylmorphine, as we have seen already, is
less polar than morphine and will enter the
brain more quickly and in greater
• Heroin and 6-acetylmorphine are both more
potent analgesics than morphine. Unfortunately,
they also have greater side-effects and have
severe tolerance and dependence characteristics.
 The double bond at 7-8
Several analogues, including dihydromorphine have shown that the
double bond is not necessary for analgesic activity.
 The N-methyl group
The N-oxide and N-methyl quaternary salts of morphine are both
inactive, no analgesic is observed, since a charged molecule has very
little chance of crossing the blood-brain barier.
If these same compound are injected directly into the brain, a totally
different result is obtained and both these compounds are found to have
similar analgesic activity to morphine.
The replacement of the NMe group with NH reduces activity but does
not eliminate it.
The fact that significant activity is retained shows that the methyl
substituent is not essensial to activity.
However, the nitrogen itself is crucial. If it is removed completely, all
analgesic activity is lost. To conclude, the nitrogen atom is essential to
analgesic activity and interacts with the analgesic receptor in the
ionized form.
The aromatic ring
The aromatic ring is essential. Compounds lacking it show no analgesic activity.
The ether bridge
As we shall see later, the ether bridge is not required for analgesic activity.
Morphine is asymmetric molecule containing several symmetric centres, and
exist naturally as a single enantiomer.
We have identified that there are at least three important interactions
involving the phenol, the aromatic ring, and the amine on morphine.
The receptor has complementary binding group placed in such a way that
they can interact with all three group.
To summarize, the important functional groups for analgesic activity in
morphine are shown.
Variation of subtituen
• A series of alkyl chain on the phenolic group
give compounds which are inactive or poorly
• Phenol group must be free for analgesic
• The removal of N-methyl group to give
normorphine allows a series of alkyl chain to
be added to the basic centre
Drug Extension
• Strategy by which the molecule to extended
by the addition of extra binding group
• The aim is to probe for further binding region
which might be available in the receptor’s
binding site and improve the interaction
between drug and receptor (fig 17.11)
• Fig 17.11
There are four important binding regions in
the binding site and morphine only uses three
of them
Search for further binding region for that
fourth binding interaction would be
productive because morphine can act as
analgesic and morphine able to interact with
painkilling receptor in the body
The result from the alkylation of
• As the alkyl group is increased in size a methyl
to butyl group, the activity drops to zero
• With a large group such as a pentyl a hexyl
group, activity recover slightly
• When a phenethyl group is attached, the
activity increases 14-fold, a strong indication
that a hydrophobic binding region has been
located which interacts favourably with the
new aromatic ring
Varying subtituen on the nitrogen
Naloxone and naltrexone have no analgesic
activity but these molecule can act as
antagonists to morphine. They have bound to
the receptor and they block morphine from
binding, morphine can no longer act as an
analgesic. The fact that morphine is blocked
from all its receptor means that none of its
side-effects are produced either and it is the
bocking of these effect which make antagonist
extremely useful.
Naltrexone is eight times more active then
naloxone as an antagonist and is given to
drung addicts who have been weaned off
morphine or heroin.
Nalorphine is the antagonist displaced morphine
from the receptor and binds more strongly, this
can prevent from an overdose of morphine.
There are no analgesic activity should be
observed. However, a very weak analgesic activity
is observed and this analgesia appears to be free
of the undesired side effect. This was the first
sign that a non-addictive, safe analgesic might be
possible. Unfortunately, nalorphine has
hallucinogenic side-effect resulting from the
activation of a non analgesic receptor and is
therefore unsuitable as an analgesic
• Trere are five ring present in the structureof
• The presence of those ring can be altered
• Now, we will learn how necessary the
complete carbon sceleton
Removing Ring E
• Removing Ring E leads to a complete loss of
• This result emphasizes the importance of the
basic N to analgesic activity
Removing Ring D
• Removing the oxygen bridge give a series of
compounds called the morphinas which have useful
analgesic activity
• N-Methyl morphinas has only 20% as active as
morphine because the phenolic is missing
• Levorphanol five times more active than morphine
although the side-effects are increase to.
• Levorphanol can be taken orally and lasts more longer
in the body because it not metabolized in the liver
• The miror image og levorphanol (dextrophan) has
insignificant analgesic activity
• Adding an allyl substituent on the nitrogen gives antagonist
• Adding a phenethyl group to the nitrogen greatly increase
• Adding 14-OH group also increase activity
• Morhinas are more potent and longer acting than their
morphine counterparts, but they also have higher toxity
and comparable dependence characteristics
• The modifications carried out on morphine, when carried
out on the morphinans, lead to the same biological result.
This implies that both type of molecule are binding to the
same receptors in the same way
• The morpinans are easier to synthesize since they are
simple molecules
Removing Rings C and D
• Opening both rings C and D gives an interesting
group of compound called the bonzomorphans
which are found to retain analgesic activity.
• Rings C and D are not essential to analgesic
• Analgesia and addiction are not necessarily coexistent
• 6,7-benzomorphans are clinically useful
compound whice resonable analgesic activity,
less addictive liabbility and less tolerance
• Benzhomorphans are simple to synthesize
Removing Rings B, C, and D
• Removing rings B, C, and D gives an series of compound known as
• Their structural relationship to morphine was only identified when
they werw found to be analgesics
• Rings C, D, and E are not essential for analgesic activity
• Piperidines retain side-effect such as addition and depression of the
respiratory centre
• Piperidine analgesics are faster acting and have shorter duration
• The quartenary centre present in the piperidines is usually
• The aromatic ring and basic nitrogen are essensial to activity but
the phenol group is not
• Piperidine analgesics appear to bind with analgesic receptors in a
different manner to previous groups
Removing Rings B, C, D, and E
• Methadone retains morphine-like side-effect,
hoever it is orally active and has less severe
emetic and constipation effects.
• Side-effect such as sedation, euphoria, and
withdrawal are also less severe and therefore the
compound has been given to drug addicts as a
substitute for morphine or heroin in order to
wean them off these drugs.
• This is not complete cure since it merely swaps an
addiction to heroin/morphines for an addiction to
• The molecule has a single asymmetric centre
and when the molecule is drown in the same
manner as morphine, R-enantiomer being
twice as ppowerful as morphine whereas the
S-enantiomer is inactive
A completely different strategy is to make the molecule
more complicated or more rigid. This strategy to remove the
side-effects of a drug or to increase activity.
The side-effects of a drug are due to interactions with
additional receptors. This interactions are probably because
of the molecule taking up different conformations or shapes.
If we make the molecule more rigid, we might eliminate the
conformations which are recognized by undesireable
receptors and restrict the molecule to the specific
conformation which fits the desired receptor. In this way, we
would hope to eliminate such side-effects as dependence
and respiratory depression. We might also expect increased
activity since the molecules is more likely to be in the
correct conformation to interact with the receptor.
The example of this tactic in the analgesic field is
provided by a group of compounds known as the
oripavines. These structures often show remarkably
high activity.
The oripavines are made from an alkaloid which we
have not described so far-thebaine (Fig. 17.26).
Thebaine, codeine, and morphine is similar in
structure. Unlike morphine and codeine, thebaine has
no analgesic activity.
There is a diene group present in ring C and
when thebaine reacts with methyl vinyl
ketone, a Diels Alder reaction takes place to
give an extra ring and increased rigidity to the
The Grignard reaction is stereospecific. By
varying the groups added by the Grignard
reaction, some remarkably powerful compounds
have been obtained.
For example :
Etorphine = 10 000x more potent than morphine.
A combination hydrophobic molecule and can
cross the blood-brain barrier 300x more easily
than morphine, has 20x more affinity for the
analgesic receptor site because of better binding
At slightly higher doses than those required for
analgesia can act as sedative. The compound has a
considerable margin safety and is used to
immobilize large animals such as elephants. Only
very small doses are required and these can be
dissolved in such small volumes (1 mL) that they
can be placed in crossbow darts.
Adding a cyclopropyl group gives a very powerful
antagonist called diprenorphine which is 100x
more potent than nalorphine (oripavine
equivalent) and can be used to reverse the
immobilizing effects of ethorphine. Diprenorphine
has no analgesic activity.
Replacing the methyl group derived from the
Grignard reagent with a t-butyl group gives
buprenorphine (Fig.17.31), which has the similar
properties to drug like nalorphine that it has
analgesic activity with a very low risk of addiction.
Buprenorphine is the most lipophilic compounds
and therefore enters the brain very easily, such a
drug would react quickly with its receptor.
Buprenorphine = 100x more active than morphine
as an agonist and 4x more active than nalorphine
as an antagonist.
 the risks of suffocation from a drug overdose <
 to treat patients suffering from cancer and also
following surgery.
 can’t be taken orally, so drawbacks include side
effect such as nausea and vomiting.
 weaning addicts off heroin.
Buprenorphine binds slowly to analgesic receptors
but, once it does bind, it binds very strongly.
Overall, buprenorphine’s stronger affinity for
analgesic receptors outweights its relatively weak
action, such that buprenorphine can produce
analgesia at lower doses than morphine.
Buprenorphine provides another example of an
opiate analogue where analgesia has been
separated from dangerous side-effects.
17.4. Receptor Theory
-There are the least four different receptors
with which morphine can interact, three of
which are analgesic receptors.
-The initial theory on receptor binding (the
beckett-casy hypothesis) assumed a single
analgesic receptor
17.4.1. Beckett-Casy hypothesis
• it was assumed that there was a rigid binding site and that
morphine and its analogues fitted into the site in a classic lock
and key analogy.
• the following features were proposed as being assential if an
analgesic was to interact with its receptors. (fig 17.32)
• There must be a basic centre (nitrogen) which can be
ionized at physiological pH to form a positively charged
group. This group then forms an ionic bond with a
comparable anionic group in the receptor.
• As a consequence of this, analgesics have to have a pKa
of 7.8-8.9 such that there is an approximately equal
chance of the amine being ionized or un-ionized at
physiological pH.
• This is necessary since the analgesic has to cross the
blood-brain barrier as the free base, but once across has
to be ionized to interact with the receptor. The pKa
values of useful analgesics all match this prediction.
• The aromatic ring in morphine has to be properly
oriented with respect to the nitrogen atom to allow a
van der Waals interaction with a suitable hydrophobic
location on the receptor
• The phenol group is probably hydrogen bonded to a
suitable residue at the receptor site
• There might be a "hollow" just large enough for the
ethylene bridge of carbons 15 and 16 to fit to align the
molecule and enhance the overall fit.
• This first theory fitted in well with the majority of results.
• the aromatic ring, phenol, and the nitrogen groups are all
important, but there is some doubt as to whether the ethylene
bridge is important, since there are several analgesics which lack it
(e.g. fentanyl)
• The theory also fails to include the extra binding region which was
discovered by drug extension.
• These results strongly suggested that a simple one receptor theory
was not applicable.
17.4.2. Multiple analgesic receptors
 The Beckett-Casy theory tried to explain analgesic results
based on a single analgesic receptor. It is now known that
there are three different analgesic receptors which are
associated with different types of side-effects.
 The important binding groups for each receptor are the
phenol, the aromatic ring, and the ionized nitrogen centre.
 Beyond that, there are subtle differences between each
receptor which can distinguish between the finer details of
different analgesic molecules. As a result, some analgesic
show preference for one analgesic receptor over another or
interact in different ways
There are three analgesic receptors which are
activated by morphine, and which have been
labeled with Greek letters:
• The mu receptor (µ)
• the kappa receptor (κ)
• the delta receptor(δ)
The mu receptor (µ)
• Morphine binds strongly to this receptor and produces
• Receptor binding also leads to the undesired side-effect of
respiratory depression, euphoria, and addiction
• It is difficult to remove the the side -effects of morphine
because the receptor with which they bind most strongly is
also inherently involved with these side-effect
the kappa receptor (κ)
 morphine binds less strongly to this receptor.
The biological response is analgesia with
sedation and none of the hazardous sideeffects. It is this receptor which provides the
best hope for the ultimate safe analgesic.
 It earlier results obtained:
- nalorphine,
- pentazocine, and
- buprenorphine
 nalorphine acts as an antagonist at the mu receptor,
thus blocking morphine from acting there
 it acts as weak agonist at the kappa receptor (as does
morphine) and so the slight analgesia observed with
nalorphine is due to the partial activation of the kappa
 nalorphine has hallucinogenic side-effects. this is
caused by nalorphine also binding to a completely
different, non-analgesic receptor in the brain called the
sigma receptor (see section 17.7.4.) where it acts as an
• pentazocine interacts with the mu and k
receptors in the same way, but is able to
'switch on' the k receptor more strongly.
• it 'switches on' the sigma receptor.
 Buprenorphine is slightly different.
 it binds strongly to all three analgesic receptors and acts
as an antagonist at the delta receptor(see below) and
kappa receptor, but acts as a partial agonist at the mu
receptor to produce its analgesic effect.
 that buprenorphine has same side-effects as morphine.
 buprenorphine interacts strongly with the receptor. It is
slow to bind out, once it has bound, it is slow to leave.
the delta receptor(δ)
• the delta receptor(δ) is where the brain's natural painkillers interact.
Morphine can also bind quite strongly to this receptor.
• Table 17.1 shows the relative activities of morphine, nalorphine,
pentazocine, enkephalins, pethidine, and naloxone. A plus sign
indicates that the compound is acting as an agonist. A minus sign
means that it acts as an antagonist. A zero sign means that there is no
activity or minor activity.
• There is now a search going on for orally active opiate
structure which can act as antagonists at the mu receptor.
Some success has been obtained, especially with the
compounds shown in fig. 17.33, but even these compounds
still suffer from side-effects, or lack the desired oral activity
Agonist and Antagonist Properties of
Morphine Analogues
Molecule act at a receptor
Molecule will act as an agonist or antagonist depending
on which of the extra binding region is used.
• Example : phenazocine
• Example : nalorphine
• Example :
Enkephalins and Endorphins
• Morphine : alkaloid which relieves pain and
acts in the CNS.
• There must be an analgesic receptor in the
• There must be chemicals in the body which
interact with these receptors.
• Natural painkiller : produced by human
• Enkephalin (Greek) ; “in the head”
Inhibitors of Peptidases
An alternative approach is to enhance the
activity of natural enkephalins by inhibiting the
peptidase enzyme which metabolizes them.
The enzyme responsible for metabolism has a
zinc ion present in the active site, which
normally accepts the phenylalanin residue
present in enkephalins.
Receptor Mechanisms
The mu Receptor (µ)
This increase in potassium permeability also
decreases the influx of calcium ions into the
nerve terminal and this in turn reduces
neurotransmitter release.
Both effects, therefore, ‘shut down’ the nerve
and block the pain messages.
Unfortunately, this receptor is also associated
with the hazardous side-effects of narcotic
analgesics. There is still a search to see if there
are possibly two slightly different µ receptors,
one which is solely due to analgesia and one
responsible for the side-effects.
The kappa Receptor (κ)
The nerves affected by the κ
mechanism are those related to pain
induced by non-thermal stimuli.
This is not the case with the µ
receptor, where all pain messages are
inhibited. This suggest a different
distribution of κ receptors from µ receptors.
The delta Receptor (δ)
The sigma Receptor (σ)
This receptor is not an analgesic receptor,
but we have seen that it can be activated by
opiate molecules such as nalorphine. When
activated, it produces hallucinogenic effects.
The σ receptor may be the receptor
associated with the hallucinogenic and
psychotomimetic effects of phencyclidine
(PCP), known as ‘angel dust’.
The Future
κ Agonists
Such compounds should have much reducedside effects. However, a completely specific κ
agonist has not yet been found.
Selectivity between µ receptor subtypes
There might be two slightly different µ
receptors, one is purely for analgesia (µ1) and
the other solely responsible for unwanted sideeffects such as respiratory depression (µ2).
Peripheral Opiate Receptors
Peripheral opiate receptors have been
identified in the ileum and are responsible
for the antidiarrhoeal activity of opiates.
If peripheral sensory nerves also possess
opiate receptors, drugs might be designed
versus these sites and as a result would
not need to cross the blood-brain barrier.
Blocking Postsynaptic Receptors
Blocking the postsynaptic receptors which
are responsible for the transmission of pain with
selective antagonists may well be the best
approach to treating pain and the best way of
eliminating side-effects.
Agonists for the Cannabinoid Receptor
Cannabinoid agonists may have a role to play
in enhancing the effects of opiate analgesics
and may allow less opiate to be administered.

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