Analgesics: drug discovery
Jenny Laird
AstraZeneca Research & Development
Montréal
Choosing a target for drug
discovery
• should cover an “unmet medical need”
• has to show “demonstrable benefit” over
current treatments
• likely to make money!
Pain is an Unmet Medical Need
•1 in 8 of you will experience poorly relieved
persistent pain at some point in your life
•Only 50% of patients with post-operative pain are
satisfied with the pain therapy received
•Chronic unrelieved pain produces a disease state
with progressive physical & social dysfunction
•Reduction in Quality of Life similar to depression
•Heavy Socio-economic burden
•Poor choice of effective & safe analgesic drugs
•The two main classes of analgesics (opiates and
anti-inflammatories) were discovered two centuries
ago
PAIN
An unpleasant sensory
and emotional experience
associated with actual
or potential tissue
damage, or described in
terms of such damage
IASP, Subcommittee on Taxonomy, 1979
Nociceptive pathways:
peripheral sensory nerves
Dorsal horn of
spinal cord
Spinothalamic
tract
Nociceptive
Dorsal Root
Ganglion
Peripheral
nerve
Sympathetic ganglion
Viscera
Blood vessels
Skeletal
muscle
Tendon
bundle
Muscle and skin Nociceptive
terminals
receptors
C and Ad
fibres
sensory fibres are
C-fibres and Ad
fibres
C-fibres
umyelinated
Ad myelinated
Slow conduction
velocity
Signal variety of
noxious stimuli polymodal
Ascending Pain Pathways
Cortex
Thalamus
Mesencephalon
Pons
Trigeminal
ganglion
Medulla
oblongata
Spinal
cord
Topographic
representation
maintained
Sites for pain
modulation are spinal
cord and thalamus
Trigeminal system has
special significance for
migraine
One pain or many pains?
Cervero & Laird 1991 N. Physiol. Sci. 6 268-273
Pain, Hyperalgesia and Allodynia
100
pain sensation
hyperalgesia
75
injury
50
normal
pain
allodynia
25
pain
threshold
pain
threshold
0
innocuous
noxious
stimulus intensity
Cervero & Laird (1996)
INJURY
Spinal Cord
Ad / C
N
Primary
Hyperalgesia
LT
N
Nociceptive pathways:
peripheral sensory nerves
Dorsal Root
Ganglion
Peripheral
nerve
Sympathetic ganglion
Viscera
Blood vessels
Skeletal
muscle
Tendon
bundle
Muscle and skin
receptors
Nociceptors are
sensitised after
damage/inflammation
Peripheral Sensitisation
Peripheral injury or inflammation initiates
cascades of pro-inflammatory mediators
released from many tissues
These agents act on Nociceptors
decrease in threshold to stimulation
develop spontaneous activity
Sensory nerve terminals not only ‘passive’ but
contribute actively to the inflammatory
process
– neurogenic inflammation
Efferent functions of Nociceptors
Cervero & Laird, 1996
Pro-inflammatory mediators and nociceptors
TISSUE DAMAGE
inflammation
macrophages
polymorphs
platelets
H+
mast cells
NGF
TNF-1a
Cytokines
IL-6
IL-8
IL-1
Kinin B1
receptor
induction
5-HT
Kinins
PGs
Blood
vessels
histamine
COX-2
NGF
fibroblasts
substance P
gene transcription
EXCITE
SENSITISE
PGs
CGRP
H+
sympathetic nerve
substance P
KININS
Prostaglandins and Arachidonic
cyclo-oxygenase (COX)
Two isoforms of COX
– Both produce prostaglandins (PGE2, PGF2a, PGI)
COX-1 is constitutive, expressed in most tissues
– physiological and homeostatic role, cell signalling
COX-2 is inducible following inflammation, trauma
etc
– found in immunocompetent cells (e.g. leukocytes)
– pathophysiological role, initiates, maintains inflammation
Prostaglandins alone (particularly PGE2) do not
directly excite nociceptors but sensitise them to
other stimuli
Responses of a C-fibre nociceptor to a mixture of
Inflammatory Mediators (10-5 M histamine, bradykinin
and serotonin)
10
500 mV
Frequency (Hz)
5 ms
5
0
baseline
30 sec
I.M.
30 sec
recovery
30 sec
Migraine: chronic, episodic pain
of ‘neurovascular’ origin
 Exact mechanism underlying migraine not
clearly understood
– assumed to be strong genetic linkage
 Neurogenic inflammatory mechanisms a
major component
 Major role for 5-Hydroxytryptamine (5-HT)
– principal current therapy is based on agonists at
5-HT1D receptor sub-type
– so far 11 5-HT receptor sub-types and still
counting……
Trigeminal neuronal ‘system’ main
‘pathway’ for initiation and pain perception
“Migraine” Ed
Pashke 1939
Spinal Cord
Ad / C
N
Primary
Hyperalgesia
A
LT
Secondary
Hyperalgesia
Ad / C
Allodynia
N
Central Sensitisation
Sensitisation occurs at the level of the spinal cord
and supra-spinally
Persistent and/or powerful nociceptive activation
elicits changes in the transmission of nociceptive
information within the CNS
Changes may last from hours to years
In certain cases these changes can become
pathological leading to unresolved persistent pain
Central Sensitisation in Thalamus of
Spinal Cord in Rats
Before Paw Inflammation
Nociceptive
neuronal recording
from Ventrobasal
thalamus
After Paw Inflammation
Appearance of responses to
stimulation of forepaw
Increase in
response to heat
Paw inflamed
with carrageenan
10 Hz
Pinch
Pinch
30 sec
50oC
50oC
50oC
50oC
Mechanisms of Central Sensitisation
Brief noxious
stimulus
milli
secs
secs
C-/ Ad fibres
transmitter release
brief activation in sc
transmission to brain
Transient Pain
min
Short term
inflammation
hours
days
Pathological
inflammation
Neuropathy
weeks
months
years
Peripheral sensitisation
Increase in synaptic efficacy
Central sensitisation
Induction of early genes, c-fos
Upregulation of neuropeptides
Recruitment of A fibres
Phenotypic changes
Sprouting of terminals
Inappropriate innervation
Expression of new receptors
Cell loss
Hyperalgesia
Sustained pain
Hyperalgesia
Allodynia
Chronic pain
Pathological
Mechanisms of Acute and
Persistent Pain
Some of the Main Players
Excitatory Neurotransmitters in Dorsal
Horn of Spinal Cord
 Excitatory Amino Acids (EAAs)
– glutamate, aspartate, (homocysteate)
– vast body of literature supporting major role in transmission in
spinal cord
– primary afferent transmitters
 EAAs act on 4 main receptor types
– ligand-gated ionotropic receptors
– kainate receptor
– AMPA receptor
– NMDA receptor
– G-protein coupled receptor
– metabotropic glutamate receptor
– Bewildering number of receptor sub-types
– individual function not clear
NMDA receptors contribute to spinal
cord sensitisation
EAA receptors:
Na+
AMPA
Na+
Glut
mGluR
NMDA
Glut
Na+
Ca2+
+
Mg2+
Brief
Depolarisation
iCa2+
Sustained
Depolarisation
EXCITATION
EXCITATION
PKC, NOS
Neuropeptides in Dorsal Horn
 Tachykinins
– excitatory neuropeptides localised in nociceptive
afferents
– Substance P, Neurokinin A,
– receptors NK1 and NK2
– ? transmitters or neuromodulators
 Calcitonin Gene-Related Peptide (CGRP)
– localised in greater % of nociceptive afferents than SP
– possibly two receptor sub-types
– excitatory centrally, powerful vasodilator peripherally,
– role unclear
Opioid receptors
3 subtypes : m, d, k
About 60% homology between
subtypes
G protein-coupled receptors
The ‘Grandfather’ of all
analgesics - Morphine - acts
here
Many synthetic opiates
available
Endogenous Opioid peptides
 Localised within several areas of CNS including dorsal
horn of spinal cord
– not exclusive to ‘nociceptive’ areas
 Relatively non-selective for opioid receptor sub-types
 Also produced by non-neuronal cells
Endogenous opioids and receptor selectivity
Endogenous peptide
-endorphin
Leu-enkephalin
Met-enkephalin
Dynorphin
m
+++
+
++
++
d
+++
+++
+++
+
k
+++
+++
Other opioid peptides e.g. nociceptin present in CNS in pain
pathways but significance to pain transmission unclear
Current Therapies for Pain
NSAIDs (Non-Steroidal Antiinflammatory Drugs,
COX-1 & COX-2)
Opiates (mu agonists)
Anticonvulsants (phenytoin), antidepressant
(amitriptyline), antiarrhythmics (mexylitine)
Sumatriptan, Zomig (5HT agonists) etc for
migraine
Gabapentin (off label)
Tramadol (mu opioid plus ‘your guess as good as mine’)
Combinations (opioids plus)
Non Steroidal Anti-inflammatory
Drugs (NSAIDs)
Most widely used of all therapeutic agents
– Over 50 NSAIDs on the market
Three main effects
– anti-inflammatory
– antipyretic
– analgesic
Primary mechanism of action is inhibition of
arachidonic cyclo-oxygenase (COX) and therefore
reduction of prostaglandin levels
– most NSAIDs block both COX-1 and -2 e.g. naproxen,
indomethacin, ibuprofen, aspirin etc
Two recent selective COX-2 inhibitors - Vioxx and
Celecoxib
Indomethacin reduces the frequency of
spontaneous discharges of sensitised
nociceptors
50
Frequency (Hz)
500 mV
5 ms
40
30
20
10
0
baseline
5 min
vehicle
5 min
indomethacin
5 min
The Opiates
 Powerful analgesics all descended
from Morphine
 All activate m receptors and varying
degrees of d and k activation
 Pure agonist opiates
– morphine, codeine, oxymorphine,
methadone, pethidine, fentanyl,
sulfentanil, etc
 Partial/mixed agonists
– agonist on m, antagonist on d and/or k
– pentazocine, ketocyclazocine,
buprenorphine etc
 Antagonists
– e.g. naloxone, naltrexone
Morphine acts in several sites to produce
analgesia
+
Morphine
Activates
descending
inhibition
Descending inhibitory
control
Primary nociceptive afferent
Inhibits discharge of
1ry. afferents?
+
-
Inhibits transmitter
release
Hyperpolarises neurones
So, what’s wrong with current therapy?
Lack of efficacy
– in chronic pain 40% efficacy in Visual Analogue Scores
typical
– Nothing works well in neuropathic pain
Dose limiting adverse effects
– not only unpleasant but life-threatening as well
NSAIDs
– gastric haemorrhage, renal/kidney toxicity
Opiates
– respiratory depression, nausea & vomiting,
constipation, dependency
Sites of drug action
• non-specific
• physicochemical props.
• receptors
• neurotransmitters
• hormones
• enzymes
• transport systems
• ion channels
• active transport, eg. uptake blockers
Ion channels
• ligand dependent*
• voltage dependent
• voltage & ligand (eg. cardiac
Ca2+ channels)
Receptors
• ionotropic*
• metabotropic
• G-protein coupled (majority)
• tyrosine kinase (eg. insulin receptor)
Identifying novel targets from
the genome
• Isolating a target related to an existing target
• e.g. cloning COX-2 isoform
• Isolating a known target of unknown sequence
• e.g. cloning capsaicin receptor
• Identifying completely unknown targets
• DNA “chips”
• Human genome project
Once we have a target...
What are our goals?
Goals depend on mechanism of
action
• Similar to current compounds:
• reduce adverse effects
• increase benefits
• Novel mechanism:
• proof of concept
• mechanism-based adverse effects
Proof of concept
• animal models of human disease
• face validity
• model of mechanism
• Phase 1 models or trials
• surrogate end-points
• “Humanised” animals
• (transgenic technology)
What do we need to know
before going into man?
• effect at target site
• pharmacokinetics, including
bioavailability
• metabolism
• safety
• dose
• effectiveness in vivo (?)
Assays
• Binding assays
• Bioassays – Functional measure
• In vivo
• In vitro
Pharmacokinetics & Drug
metabolism (DMPK)
• Penetration/concentration and time course in
different compartments
• blood brain barrier (BBB)
• synovial capsule
• Metabolism
• Active metabolites
• Drug accumulation
Bioavailability
• Decide route of administration
• parenteral (i.v., i.m. )
• formulation
• oral
• first pass metabolism
• topical
• sensitivity reactions
Adverse effects
• related to dose
• mechanism-based
• structure-related
• not related to dose
• eg. hypersensitivity reactions
Summary and Conclusions
Pain is complex, particularly chronic pain
Transient pain is relatively well treated
Not all pain is the same
Present therapies are old, inadequate and
sometimes dangerous
There is a real need for novel, powerful, safe
analgesics in chronic pain
Drug discovery needs the skills of many
different disiplines