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Abstract

The term 'endocannabinoid' - originally coined in the mid-1990s after the discovery of membrane receptors for the psychoactive principle in Cannabis, Delta9-tetrahydrocannabinol and their endogenous ligands - now indicates a whole signalling system that comprises cannabinoid receptors, endogenous ligands and enzymes for ligand biosynthesis and inactivation. This system seems to be involved in an ever-increasing number of pathological conditions. With novel products already being aimed at the pharmaceutical market little more than a decade since the discovery of cannabinoid receptors, the endocannabinoid system seems to hold even more promise for the future development of therapeutic drugs. We explore the conditions under which the potential of targeting the endocannabinoid system might be realized in the years to come.
REVIEWS
The recreational use of Cannabis sativa preparations is
known to most people, largely as a result of the explosion
in its use in the late 1960s; indeed, marijuana is still one
of the most widespread illicit drugs of abuse in the
world
1
.However, the medicinal use of Cannabis also has
a millenarian history
2
, although this history has been
re-examined only very recently
3
.As early as 2600 BC, the
Chinese emperor Huang Ti advised taking Cannabis for
the relief of cramps, and rheumatic and menstrual pain;
however, the great therapeutic potential of Cannabis was
not scientifically assessed and publicized in the Western
world until the British physician O’Shaugnessy wrote on
the topic in the nineteenth century
2
.
This long history of Cannabis use has resulted in
the development of pharmaceutical drugs, such as
dronabinol (Marinol; Unimed). This drug is based on
(–)-9-TETRAHYDROCANNABINOL (THC; FIG. 1), which, in 1964
— and after decades of attempts to isolate and deter-
mine its chemical structure — was identified as the
major psychoactive component of Cannabis
4
.This
preparation — together with Cesamet, which is based
on the synthetic THC analogue nabilone
(FIG. 1) — was
being prescribed in the United States as an anti-emetic
and appetite-stimulant to patients with cancer and
AIDS
5
even before its molecular mode of action was
revealed. It took the design of more potent, and enan-
tiomerically pure, THC analogues, such as HU-210
(FIG. 1),to reveal that THC acts via specific sites of action
to produce its typical psychotropic effects. Labelling of
HU-210 to generate [
3
H]HU-245, and the development
of the non-classical (that is, bi-cyclic) cannabinoid
CP-55,245 by Pfizer
(FIG. 1),led to the identification of
plasma membrane
CANNABINOID RECEPTORS in 1988 (REF. 6).
The serendipitous cloning in 1990 of the first of such
proteins, the CB
1
receptor, came from the screening of
an orphan G-protein-coupled receptor (GPCR) with
several possible ligands
7
.Meanwhile, several other plant
cannabinoids that have little or no psychoactive action
had been identified; their biosynthetic relationships
have been established, and the possible contribution
that they make to some of the purported therapeutic
actions of Cannabis has been suggested. In particular,
cannabidiol
8
and the cannabinoic acids (FIG. 1) seemed
to be promising therapeutic tools, even though their
sites of action are still not well understood. Another
receptor for THC, the CB
2
receptor (located on blood
cells and immune tissues), was cloned in 1993
(REF. 9),
and the first endogenous ligands for cannabinoid CB
1
and CB
2
receptors — the ENDOCANNABINOIDS, as they were
termed in 1995
(REF. 10) — were isolated in the early
THE ENDOCANNABINOID
SYSTEM AND ITS THERAPEUTIC
EXPLOITATION
Vincenzo Di Marzo*, Maurizio Bifulco
and Luciano De Petrocellis
§
Abstract | The term ‘endocannabinoid’ — originally coined in the mid-1990s after the discovery of
membrane receptors for the psychoactive principle in Cannabis,
9
-tetrahydrocannabinol and
their endogenous ligands — now indicates a whole signalling system that comprises cannabinoid
receptors, endogenous ligands and enzymes for ligand biosynthesis and inactivation. This system
seems to be involved in an ever-increasing number of pathological conditions. With novel products
already being aimed at the pharmaceutical market little more than a decade since the discovery of
cannabinoid receptors, the endocannabinoid system seems to hold even more promise for the
future development of therapeutic drugs. We explore the conditions under which the potential of
targeting the endocannabinoid system might be realized in the years to come.
NATURE REVIEWS | DRUG DISCOVERY VOLUME 3 | SEPTEMBER 2004 | 771
Endocannabinoid Research
Group, Institutes of
*Biomolecular Chemistry
and
§
Cybernetics, National
Research Council,
Via Campi Flegrei 34,
Comprensorio Olivetti,
80078 Pozzuoli,
Napoli, Italy.
Dipartimento di Scienze
Farmaceutiche, Università
degli Studi di Salerno,
via Ponte Don Melillo,
84084 Fisciano, Salerno,
Italy.
*Correspondence to V.D.M.
e-mail:
vdimarzo@icmib.na.cnr.it
doi:10.1038/nrd1495
9-TETRAHYDROCANNABINOL
(THC). The major
psychotropic component of
Cannabis sativa, and one of
about 66 ‘cannabinoids’ found
in the flowers of this plant.
CANNABINOIDS
Natural lipophilic products from
the flower of Cannabis sativa,
most of which have a typical
bi-cyclic or tri-cyclic structure
and a common biogenetic origin
from olivetol.
CANNABINOID RECEPTORS
G-protein-coupled receptors for
THC, so far identified in most
vertebrate phyla. Two subtypes
are known: CB
1
and CB
2
.
ENDOCANNABINOIDS
Endogenous agonists of
cannabinoid receptors in
animal organisms.
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REVIEWS
cloned to date are mostly coupled to G
i/o
proteins,
through which they modulate the activity of adenylate
cyclases (which they mostly inhibit), mitogen-activated
protein kinases (which they stimulate), and, in the case
of CB
1
receptors, voltage-activated Ca
2+
channels (which
they inhibit) and inwardly rectifying K
+
channels
(which they stimulate), to transduce the binding of ago-
nists into biological responses
(FIG. 3)
14
.The chemical
prerequisites for the activation by synthetic and endo-
genous agonists of one intracellular signalling pathway
rather than another are being revealed, and might open
the way to new signalling-specific drugs.
The tissue distribution of CB
1
and CB
2
receptors
accounts for the well-known psychotropic and peripheral
effects of THC. CB
1
is one of the most abundant GPCRs
found so far in the central nervous system (CNS), and
reaches highest density in the basal ganglia, cerebellum,
hippocampus and cortex, but is also present in the
peripheral nervous system (PNS) and several peripheral
organs. CB
2
receptors, by contrast, are mostly restricted
to immune tissues and cells
18
.The previous knowledge
of THC pharmacology
19
— and, most importantly,
recent studies carried out by using pharmacological,
biochemical, analytical and genetic (for example, the use
of ‘knock-out’ mice) approaches
16
— are revealing several
possible functions of endocannabinoid signalling under
both physiological and pathological conditions. In the
CNS and PNS, the preferential (although not exclusive)
distribution of CB
1
receptors at presynaptic neurons,
their coupling to the inhibition of voltage-activated Ca
2+
channels, and the stimulation of endocannabinoid for-
mation by increased intracellular Ca
2+
and activation of
other GPCRs makes the endocannabinoid system an
ideal natural tool for modulating neurotransmitter
release
20,21
.In particular, endocannabinoids in the CNS
intervene in both short-term and long-term forms of
synaptic plasticity, including depolarization-induced
suppression of both excitatory and inhibitory neuro-
transmission, long-term potentiation and depression,
and long-term depression of inhibition
22,23
.The impli-
cations of these actions in the regulation of cognitive
functions and emotions in neuronal circuits of the
cortex, hippocampus and amygdala, and in the rein-
forcement of substances of abuse in the mesolimbic
system
24,25
,have been discussed elsewhere. The abun-
dance of both CB
1
receptors and endocannabinoids in
the basal ganglia and cerebellum makes targeting this
signalling system an ideal way to modulate movement
and posture.
The
NEUROMODULATORY actions of endocannabinoids
in the sensory and autonomic nervous systems also
result, mostly through CB
1
receptors, in the regulation
of pain perception
26
and of cardiovascular
27
,gastro-
intestinal
28
and respiratory
29
functions; their effects on
the release of hypothalamic hormones and peptides,
and the regulation of their levels by steroid hormones,
lead to modulation of food intake and of the pituitary–
hypothalamus–adrenal axis
30
,as well as of both female
and male reproduction
31
.The physiological importance
of CB
2
receptors in cellular and, particularly, humoral
immune responses is only now starting to be revealed
32
,
1990s (FIG. 2)
11–13
.All endocannabinoids identified so far
are derivatives (amides, esters and even ethers) of long-
chain polyunsaturated fatty acids, specifically arachi-
donic acid, and exhibit varying selectivity for the two
cannabinoid receptors
14
as well as for other molecular
targets
(FIG. 2)
15
.The two best-studied endocannabi-
noids are anandamide (N-arachidonoylethanolamine)
and 2-arachidonoylglycerol (2-AG)
11–13
.
Functions of the endocannabinoid system
The components (FIGS 3,4) and possible physiological
functions of the endocannabinoid system have been
extensively reviewed in recent articles
16,17
and will be
outlined here only briefly (although it should be noted
that the accounts given of the endocannabinoid system
need continuous updating). Both cannabinoid receptors
Synthetic 'cannabinoids'
O
H
OH
H
HO
OH
H
O
H
OH
H
COOH
HO
OH
H
COOH
O
OH
OH
HO
O
CH OH
2
OH
O
O
OH
OH
OH
OH
O
N
N
O
O
THC Cannabidiol
THC-acid Cannabidiolic acid
Cannabigerol Cannabichromene
HU-210 Nabilone
CP-55,940 WIN-55,212-2
Plant cannabinoids
Figure 1 | Chemical structures of some plant and synthetic cannabinoids. Of the plant
CANNABINOIDS shown, only
9
-tetrahydrocannabinol (THC)
4
binds to cannabinoid receptors with high
affinity. Of the synthetic ones, none is selective for one type of cannabinoid receptor over the other
18
.
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first studies on possible pathological alterations in
endocannabinoid signalling. There is now increasing
evidence that endocannabinoid levels undergo signifi-
cant changes in several animal models of both acute and
chronic disorders.
Neurological, psychiatric and eating disorders. Endo-
cannabinoids are selectively and transiently elevated in
specific brain areas during several pathological condi-
tions of the CNS. Endocannabinoid levels are elevated
in the hippocampus following glutamate excitotoxicity
35
,
and after a number of stressful stimuli: in the hypo-
thalamus and limbic forebrain after food deprivation
36
;
in the basolateral amygdala after retrieval of an
unpleasant memory
37
; and in the periaqueductal grey
matter after the administration of a painful stimulus
38
.
Endocannabinoid signalling is enhanced to protect
neurons from damage through feedback inhibition of
glutamatergic neuron activity
35
,or to minimize the
impact of the stressful stimulus by reinforcing appetite
through inhibition of anorectic signals
39,40
;by suppress-
ing aversive memories through inhibition of sig-
nalling by GABA (γ-aminobutyric acid)
37
; and by
producing central analgesia via suppression of activity
of nociceptive neurons
38
.
Anandamide levels were also increased in a clinical
case of hemispheric stroke
41
,which,taken together with
the findings that CB
1
receptors seem to contribute sig-
nificantly to protection from stroke in animals
42
and
that 2-AG is protective in a model of head trauma
43
,
support the notion that endocannabinoids are neuro-
protective agents. Indeed, endocannabinoid signalling is
also elevated in several animal models of neurodegener-
ative diseases: in the basal ganglia of reserpine- or
6-hydroxy-dopamine-treated rats (two models of
Parkinsons disease)
44,45
;in the hippocampi of β-amyloid-
treated rats (a model of Alzheimer’s disease) (V.D.M.,
unpublished observations); and in the brains and
spinal cords of mice with chronic relapsing experimen-
tal allergic encephalomyelitis (CREAE), a model of
multiple sclerosis
46
.
The function of this upregulated signalling, as
inferred from studies with CB
1
agonists/antagonists
and knockout mice, is presumably to counteract neu-
ronal hyperactivity, local inflammation and therefore
damage — or, in the case of multiple sclerosis, to
inhibit tremors and spasticity
47
.However,the progres-
sive nature of disorders such as Parkinsons and
Alzheimer’s diseases and multiple sclerosis could
result in a permanent, as opposed to transient, hyper-
activation of the endocannabinoid system. Such
hyperactivation could even contribute to the develop-
ment of the symptoms of Parkinsons and Alzheimer’s
diseases (such as inhibition of motor activity and loss
of memory, respectively — two typical effects of CB
1
agonists)
44,48
.Hyperactivation also results, in some cases,
in a compensatory downregulation of CB
1
-receptor
expression
49,50
.Interestingly, post-mortem analysis of the
brains of patients with Alzheimer’s disease revealed an
overexpression of normally unexpressed CB
2
receptors,
which indicates that endocannabinoids might confer
and has possible implications for inflammation and
chronic pain. In general — and in view of their chemical
nature as lipophilic compounds and their peculiar
biosynthetic mechanisms — endocannabinoids seem to
act as local mediators in an autocrine and paracrine
manner, and recent evidence also points to their involve-
ment in the control of cell metabolism, differentiation,
proliferation and death
33
.As cannabinoid receptors are
more ubiquitous in mammalian tissues than originally
thought, the function of endocannabinoid signalling is
likely to extend beyond what could be initially inferred
from the knowledge of THC pharmacology.
Pathologically altered endocannabinoid signalling
The development of sensitive and specific techniques
for the quantification of endocannabinoid and cannabi-
noid receptor levels in tissues and biological fluids
34
has
enabled us to answer the following questions: is endo-
cannabinoid signalling impaired or overactive during
certain disorders, and can this explain the symptoms or
the onset and progress of these disorders? Could it be
that alleviation of these symptoms with Cannabis results
from rectifying impaired levels of endocannabinoids in
some disorders? Although we now know that the effects
of endogenous cannabinoids and exogenously adminis-
tered THC can differ both qualitatively and quantita-
tively, this second rather simplistic hypothesis was not
too far from the truth, and provided impetus for the
ANANDAMIDE
One of the most studied
endocannabinoids, named
from the Sanskrit word
‘ananda’ for ‘bliss’.
NEUROMODULATORY
A physiological action
consisting of the capability of
modulating neurotransmitter
release and/or action.
Stable endocannabinoid analogues
Anandamide (CB
1
>CB
2
)2-Arachidonoylglycerol (CB
1
= CB
2
)
Virodhamine (CB
2
>CB
1
) Noladin (CB
1
>>CB
2
)
N-Arachidonoyldopamine (CB
1
>>CB
2
)
Met-fluoro-anandamide (CB
1
>>CB
2
)(R)-Met-anandamide (CB
1
>>CB
2
)
Endocannabinoids and putative endocannabinoids
N
H
OH
O
O
NH
2
O
N
H
O
OH
OH
N
H
F
O
N
H
OH
O
O
O
OH
OH
OH
OH
O
Figure 2 | Chemical structures of endocannabinoids. Chemical structures of the two best-
studied endocannabinoids,
ANANDAMIDE and 2-arachidonoylglycerol
11–13
; of three recently
proposed endogenous ligands of cannabinoid receptors
159–161
; and of more metabolically stable
synthetic endocannabinoid analogues
162
. The rank of affinity of each compound for cannabinoid
receptor subtypes 1 or 2 is shown.
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lower susceptibility of CB
1
knockout mice to develop
obesity following a high-fat diet
57
.
Cardiovascular disorders. Elevated levels of macrophage
and/or platelet endocannabinoids are found in the
blood of rats during haemorrhagic and septic shock, or
following liver cirrhosis and experimental myocardial
infarction; these endocannabinoids produce the
hypotension typical of these pathological states
58–61
.
The levels of CB
1
receptors and endocannabinoids are
also elevated in the liver and blood, respectively, of
cirrhotic patients
60
.
Gastrointestinal disorders. The concentration of anan-
damide, and/or the expression of cannabinoid CB
1
receptors, is elevated in three mouse models of intestinal
disorders: small intestine inflammation
62
;cholera-toxin-
induced intestinal hyper-secretion and diarrhoea
63
; and
peritonitis-induced paralytic ileus
64
.Interestingly,
although enhanced signalling at CB
1
receptors affords
tonic protection against the symptoms in the first two
disorders, it contributes to a reduction of intestinal
motility during paralytic ileus. A recent study showed
that genetically engineered mice lacking either the CB
1
receptor or the major enzyme that catalyses anandamide
inactivation are more and less susceptible, respectively,
to developing colonic inflammation when treated with
di-nitro-benzene-sulphonic acid
65
.The importance of
the role of the endocannabinoid system in the control of
intestinal functions in men is indicated by the occurrence
of occasional diarrhoea after the repeated administration
of rimonabant (see below).
Reproductive disorders. There are a number of similarities
between mice and humans in the endocannabinoid
control of reproductive functions. Anandamide, by acting
preferentially at cannabinoid CB
1
receptors, has a dual
function in mouse embryo implantation, which it
stimulates at low concentrations and inhibits at higher
concentrations
66
.Accordingly, defective anandamide
hydrolysis leads to high levels of this compound in the
blood of pregnant women, which correlates with pre-
mature abortion or failure of implanted oocytes fertilized
in vitro
67,68
.
Cancer. Finally, increased endocannabinoid signalling is
found in some human malignancies compared with the
corresponding healthy tissues
69,70
, as well as in human
cancer cells with a high degree of invasiveness
69,71
.These
observations — together with the finding that stimula-
tion of either CB
1
or CB
2
receptors causes blockage of
the proliferation of cancer cells or induction of their
apoptosis in vitro
69,71,72
, and inhibition of cancer growth,
angiogenesis and metastasis in vivo
73–76
— indicate that
endocannabinoids might represent one of the many
adaptive responses aimed at counteracting tumour-cell
growth
69
.This possibility is supported by the recent
finding that inhibitors of endocannabinoid inactivation
can retard tumour growth both in vitro and in vivo
69,77
.
By contrast, aberrant overexpression of CB
2
receptors on
haematopoietic precursor cells has been suggested to be
protection in this disorder by activating CB
2
receptors, and
possibly by interfering with inflammatory reactions
51
.
In animal models of Huntingtons chorea,by contrast,
CB
1
-expressing fibres in the basal ganglia are progres-
sively lost from the early stages of the disorder
onwards, which results in impaired levels of both
endocannabinoids and CB
1
receptors — this subse-
quently contributes to the hyperkinesia typical of the
first phase of the disease
52,53
.Importantly, lower levels
of CB
1
receptors have also been found in post-mortem
brains from patients with Huntingtons chorea
54
.
It is interesting to note, with respect to the role of
endocannabinoids in food intake and energy balance,
how the endocannabinoid system seems to become
overactive in both the hypothalamus and adipocytes
of animal models of genetic obesity
39,55
.This observa-
tion might explain the transient inhibition of food
intake and the more persistent reduction in fat mass
that is observed following treatment of mice and rats
with the CB
1
-receptor antagonist/inverse agonist
56
SR141716 (Rimonabant; Sanofi-Synthelabo), and the
ATP
K
+
Ca
2+
cAMP
Gene
expression
Cannabinoid receptor agonists
AC
MAPK
PKA
CB
1
/CB
2
Heterotrimeric
G-proteins
Figure 3 | Major signalling pathways associated with cannabinoid receptor activation
by agonists. Activation of both cannabinoid CB
1
and CB
2
receptors, and the subsequent
stimulation of G
i/o
heterotrimeric proteins, is well known to be coupled to inhibition of
adenylate cyclase (AC) with corresponding inactivation of the protein kinase A (PKA)
phosphorylation pathway, or to stimulation of mitogen-activated protein kinase (MAPK).
These intracellular events lead to, among other effects, the regulation of expression of several
genes. However, more complex protein phosphorylation cascades — specifically, those
involving phosphoinositide-3-kinase and protein kinase B — are also proposed to be
triggered by CB
1
receptors
14–18
. Furthermore, stimulation, rather than inhibition, of AC by
CB
1
, but not CB
2
, receptors, via G
s
proteins, has also been described occasionally. CB
1
-,
but not CB
2
-, receptor stimulation of G
i/o
proteins is also directly coupled to inhibition of
voltage-activated Ca
2+
channels and stimulation of inwardly rectifying K
+
channels in neurons,
with subsequent inhibition of neurotransmitter release. The choice between which of these
pathways is modulated by cannabinoid receptor activation also depends on the type of
agonist under study
14–18
. cAMP, cyclic AMP.
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cases, endocannabinoids seem to have a protective role,
which, in certain diseases, can become ‘too much of a
good thing’. In view of the parallels that exist between
many experimental models and the corresponding con-
ditions seen in clinical studies, this conclusion opens the
way to the therapeutic use of substances that, depending
on the type of disorder, either prolong the half-life of
endocannabinoids or prevent their formation or action
(see below).
‘Endocannabinoid enzymes’ as drug targets
If the endocannabinoid system is involved in pathological
states, then cannabinoid CB
1
and CB
2
receptors
which, however, recent evidence suggests are not the
sole molecular targets for the endocannabinoids
15
can certainly be considered as new targets for drug
development. Furthermore, in view of the findings dis-
cussed in the previous section, attempts to pharmaco-
logically manipulate endocannabinoid levels might also
result in novel pharmaceuticals. Hence, an understand-
ing of how these molecules are made, and how cells
regulate their levels under physiological and pathological
conditions, has been recognized as a high priority in
cannabinoid research.
Most of the pathways and enzymes for the biosynthe-
sis and degradation of endocannabinoids have now been
identified
(BOX 1; TABLE 1) and have been extensively
reviewed
16,17,79
.Both anandamide and 2-AG are pro-
duced by the hydrolysis of precursors that derive from
phospholipid remodelling
(FIG. 4)
80–83
.The hydrolysis of
the phosphodiester bond of N-arachidonoylphos-
phatidylethanolamine, a minor component of animal
membranes, yields anandamide in one step. The enzyme
that catalyses this reaction, which was identified in the
1980s
84
but remained uncharacterized until only a few
months ago, is N-acylphosphatidylethanolamine-
selective phospholipase D (NAPE-PLD)
(BOX 1)
85
.
NAPE-PLD, a member of the zinc-metallo-hydrolase
family of enzymes of the β-lactamase fold, is chemically
and enzymatically distinct from other PLD enzymes, and
is almost equally efficacious with most NAPEs as sub-
strates. It is therefore responsible for the formation of
other biologically active N-acylethanolamines (NAEs),
such as the C16:0, C18:0 and C18:1 congeners. This
basic information on the structural and enzymatic
properties of NAPE-PLD should soon result in the
development of selective inhibitors of the enzyme. The
NAPE precursors for anandamide and NAEs are, in
turn, produced from the action of an as yet uncharac-
terized trans-acylase enzyme, which catalyses the
transfer of an acyl group from the sn-1 position of
phospholipids to the nitrogen atom of phosphatidyl-
ethanolamine in a Ca
2+
-sensitive manner
86,87
.
2-AG is obtained from the hydrolysis of sn-1-acyl-2-
arachidonoyl-glycerols (DAGs) through the action of
two sn-1-selective-diacylglycerol lipases (DAGL-α and
DAGL-β), which have been recently cloned and charac-
terized
(FIG. 4; BOX 1)
88
.These two isoforms seem to be
members of the serine lipase (Ser-lipase) family,
because they contain the typical lipase-3 and Ser-lipase
signature sequences. Within this latter domain, two
associated with, and possibly a causative factor of,
human acute myeloid leukaemias
78
.
In summary, it can be concluded from these
ANALYTI-
CAL STUDIES ON ENDOCANNABINOIDS
and cannabinoid recep-
tors that altered endocannabinoid signalling accompa-
nies several disorders. Such changes in signalling
sometimes represent an attempt to counteract a patho-
logical process, and in other instances are one of the
causative factors underlying the disease or its symptoms.
Although it is premature to view endocannabinoids as
markers of pathological states, a general conclusion from
the studies carried out in the past decade is that, in most
MAGL
FAAH
DAGL
PLC
NAT
NAPE-PLD
CB
1
Presynaptic neuron
2-AG inactivation
2-AG
Glycerol +
arachidonic
acid
Glycerophospholipid
Glycerophospholipid +
phosphatidylethanolamine
NArPE
Anandamide
Postsynaptic neuron
Ethanolamine +
arachidonic acid
sn-1-Acyl-2-
arachidonyl glycerol
2-AG
2-AG biosynthesis
Anandamide
biosynthesis
Anandamide
inactivation
EMT
EMT
EC
EC
Figure 4 | Anabolic and catabolic pathways of endocannabinoids and their most likely
subcellular localization. Hydrolytic enzymes are involved in both the biosynthesis of
endocannabinoids (ECs) and in their inactivation
(BOX 1). The enzymes for 2-arachidonoylglycerol
(2-AG) biosynthesis, the phospholipases C (PLC)
82,83
and the sn-1-selective diacylglycerol
lipases (DAGLs)
88
seem to be mostly localized on the plasma membrane. The DAGLs, in
particular, are located on postsynaptic neurons in the adult nervous system
88
, whereas the
monoacylglycerol lipase (MAGL) for 2-AG inactivation is localized in presynaptic neurons
110
, which
supports a possible role as retrograde messenger at presynaptic CB
1
receptors for this
compound
157
. The anandamide biosynthetic enzymes N-acyltransferase (NAT)
84
and
N-acylphosphatidyl-ethanolamine-specific phospholipase D (NAPE-PLD)
85
and the inactivating
enzyme fatty acid amide hydrolase (FAAH)
104
are all located on intracellular membranes. FAAH
seems to be most abundant on neurons postsynaptic to CB
1
receptors
158
, indicating that
anandamide acts principally on these neurons. However, whether NAT and NAPE-PLD are pre- or
postsynaptic is not known. Finally, an as yet uncharacterised
ENDOCANNABINOID MEMBRANE
TRANSPORTER
(EMT) seems to facilitate both endocannabinoid release and re-uptake
80,94–97
, and
might be localized on both pre- and postsynaptic neurons. NArPE, N-arachidonoyl-
phosphatidyl-ethanolamine.
ENDOCANNABINOID
MEMBRANE TRANSPORTER(S)
Putative and elusive membrane
protein(s) that has (have) been
postulated to be capable of
binding selectively to the
endocannabinoids and to
facilitate their transport across
the plasma membrane according
to concentration gradients .
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REVIEWS
these compounds are not stored in secretory vesicles,
but are instead biosynthesized and released from cells
only ‘when and where needed’. This
ON DEMAND’charac-
ter of endocannabinoid production fits very well with
the local modulatory role proposed for the endo-
cannabinoid system under pathological conditions,
which are normally accompanied by increases in intra-
cellular Ca
2+
to high (mM) concentrations.
Once released from cells, and having activated their
molecular targets, endocannabinoids need to be rapidly
inactivated
(FIG. 4).The importance of endocannabinoid
inactivation mechanisms is underlined by a number of
observations: low pain sensitivity and low susceptibility
to developing colon inflammation
65,91
on the one hand,
and pathological states such Parkinsons disease
45
,pre-
mature abortion
67,68
,stronger epileptic seizures
92
and,
possibly, higher predisposition to drug abuse
93
on the
other hand seem to be associated with impaired endo-
cannabinoid (mostly anandamide) hydrolysis. To be
hydrolysed, however, endocannabinoids need first to be
rapidly cleared away from the receptor active site and,
therefore, to be taken up by the cell.
This process occurs via rapid diffusion through the
cell membrane; this is facilitated by intracellular degra-
dation and — as suggested by indirect but robust data
— at least one more selective mechanism
94
.This possibly
relies on the presence of a membrane transporter
80,95–97
,
which would mediate a more rapid uptake of endo-
cannabinoids according to their gradient of concentra-
tion across the plasma membrane
(BOX 1).This putative
endocannabinoid membrane transporter (EMT) also
seems to be responsible for endocannabinoid release
95
,
because immediately after their biosynthesis endo-
cannabinoids are more abundant inside compared with
outside the cell. It must be emphasized, however, that
there is no molecular evidence for the existence of the
EMT, and that some authors have suggested that this
process might uniquely depend on endocannabinoid
intracellular hydrolysis
98,99
.
Among the experimental data supporting the pres-
ence of a cellular uptake mechanism that is independent
of enzymatic hydrolysis, the finding of synthetic sub-
stances that are capable of selectively inhibiting anan-
damide re-uptake over anandamide hydrolysis, and with
extremely stringent chemical prerequisites, is certainly
one the most convincing
(FIG. 5)
94,100,101
.Moreover,
genetic deletion of the major enzyme that catalyses the
hydrolysis of anandamide does not seem to prevent its
rapid and saturable cellular uptake
94,102
.
It is possible that the process of endocannabinoid
uptake and release is not as simple as originally pro-
posed
80,103
,and instead involves various organized forms
of the plasma membrane as well as binding proteins.
Clearly, further efforts will have to be dedicated to
solving the controversial issue of the actual existence of
a selective mechanism for endocannabinoid membrane
transport; this could possibly be achieved by cloning,
expressing and molecularly characterizing the specific
protein(s) involved. This will facilitate the pharmaco-
logical targeting of endocannabinoid membrane trans-
port, which will subsequently prolong the duration of
highly conserved amino-acid residues — Ser443 and
Asp495, which normally participate in the catalytic
triad of these enzymes — were shown to be necessary
for DAGL activity. This opens up the possibilty of
developing specific DAGL inhibitors on the basis of
knowledge of Ser-hydrolase and lipase-3 inhibitors.
DAGLs also contain four unusual hydrophobic, and
possibly trans-membrane, domains that are probably
responsible for DAGL localization to the plasma mem-
brane
88
.Interestingly, although the α- and β-isoforms
are preferentially, although not exclusively, expressed
in the adult and developing brain, respectively, both
enzymes experience a shift in their cellular localization
during brain development. Although they exhibit
axonal co-localization with CB
1
in the pre- and post-
natal nervous system, they seem to be localized in
postsynaptic neurons in the adult brain
88
.This shift
reflects the proposed role for 2-AG as an autocrine mes-
senger in axonal guidance
89,90
, and as a retrograde
messenger in the adult brain
22,23
.In both cases, the
enzymes clearly use DAGs as substrates, and their
action depends on that of other enzymes capable of
producing these compounds — such as phospho-
lipase C, for which inhibitors suitable for use in vivo
have already been developed but are likely to also
affect other pathways.
Both the NAPE-PLD and the DAGLs are signifi-
cantly stimulated by high Ca
2+
concentration, which
explains, in part, why Ca
2+
influx or its mobilization
from intracellular stores triggers anandamide and 2-AG
biosynthesis in intact cells. This observation, and the fact
that endocannabinoid biosynthesis relies greatly on
phospholipid-derived precursors, strongly indicates that
ANALYTICAL TECHNIQUES FOR
ENDOCANNABINOID STUDIES
Methodologies for quantifying
the levels of the
endocannabinoids and of
cannabinoid receptors, consisting
mostly of isotope-dilution mass-
spectrometric techniques for
anandamide and 2-AG,
polymerase chain reaction and
in situ hybridization techniques
for receptor and enzyme mRNAs,
western immunoblotting and
immunohistochemistry for
receptor and enzyme proteins.
ON DEMAND
A typical property of the
production of
endocannabinoids, which are
made in the organism only
‘when and where needed’.
Box 1 | ‘Endocannabinoid enzymes’ — state of the art
The enzymes regulating endocannabinoid levels can be pharmacologically targeted to
manipulate the concentration of endocannabinoid in tissues (see also
TABLE 1). Five
potential therapeutic enzymes have been identified to date: N-acylphosphatidyl-
ethanolamine-selective phospholipase D (NAPE-PLD), involved in the conversion
of N-arachidonoyl-phosphatidyl-ethanolamine to anandamide
85
; fatty acid amide
hydrolase (FAAH), which catalyses anandamide hydrolysis and subsequent
inactivation at cannabinoid receptors
104
;the sn-1-selective diacylglycerol lipase
isozymes α and β (DAGL-α and DAGL-β), which hydrolyse diacylglycerols to
2-acylglycerols
88
; and monoacylglycerol lipase (MAGL), which catalyses the hydrolysis
of 2-AG
109,110,156
.All these enzymes lack selectivity for one particular member of the
families of their substrates, which means that an inhibitor of these enzymes will affect
the levels of both cannabinoid receptor-active and -inactive N-acylethanolamines
(NAEs) and MAGs. These enzymes are also located where their hydrophobic
substrates are most likely to be partitioned
(FIG. 4),which indicates different
subcellular compartmentalization of anandamide and 2-AG, and therefore different
functions for the two compounds — for example, in short- and long-term synaptic
plasticity
157
. This hypothesis is also supported by the complementary cellular and
subcellular localization of the two ‘inactivating’ enzymes in the adult central nervous
system
(FIG. 3)
88,110,158
. The endocannabinoid membrane transporter is an as yet
uncharacterized and still controversial
103
membrane protein thought to be crucial in
the regulation of the distribution of all endocannabinoids between the intracellular
and extracellular milieu, and subsequently their ability to interact with cannabinoid
receptors and degrading enzymes. Early and indirect evidence
94,102,103
indicates that
endocannabinoid transport across the cell membrane is an active process and is not
simply driven by intracellular hydrolysis, as suggested by some authors
99
.
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REVIEWS
Developing endocannabinoid-based drugs
The therapeutic applications of cannabinoid CB
1
-
receptor agonists stem from anecdotal accounts of the
medicinal use of Cannabis, as well as, most importantly,
from undergoing controlled clinical trials (see below).
However, the past decade of studies on plant cannabi-
noids and the endocannabinoid system have introduced
the design of more innovative therapeutic strategies that
are based on several approaches, each with its own
advantages and disadvantages
(TABLE 2).
CB
2
-receptor agonists. The cloning of CB
2
receptors,
and their virtual absence in the healthy brain, was
welcomed as the opening of a possible gateway to the
development of
NON-PSYCHOTROPIC CANNABINOID drugs.
However, research on the physiological and pathological
function of CB
2
receptors is somehow lagging behind
that of their cognate receptors, due to their apparent
selective localization in immune cells as opposed to the
seemingly more widespread distribution of CB
1
recep-
tors. Nevertheless, we now know that at least three
selective, non-psychotropic CB
2
agonists (HU-308,
AM-1241 and JWH-133)
(FIG. 6) hold promise in
inflammatory and neuropathic pain
112,113
and, possibly,
gliomas and malignant lymphomas
71,114
.
CB
1
-receptor antagonists (and inhibitors of biosynthesis).
Antagonists are usually desirable compounds for drug
companies to develop as therapeutics. Indeed, the most
advanced clinical studies performed so far on a novel
compound based on the endocannabinoid system are
those on rimonabant (SR141716A;
FIG. 6), a selective
CB
1
antagonist/inverse agonist
115
,which is being tested
against obesity and tobacco dependence. Phase II clinical
trials have been completed, with a successful outcome
for both indications, although the dose used in the
smoking cessation trials (40 mg per day) did cause
some minor side effects. Most of the large-scale Phase III
clinical trials will be completed in 2004, although the
results of two of these have just been made public (‘The
endocannabinoid system: a new target for multi-risk
management’ presented at the Satellite Symposium at
the American College of Cardiology meeting, New
all endogenous cannabinoid signals identified so far.
This presents potential therapeutic advantages in many
disorders (see above): two examples include the inhibi-
tion of spasticity with few side effects in the CREAE
model of multiple sclerosis, and blockade of diarrhoea
after exposure to cholera toxin
46,63
.
As mentioned above, the intracellular metabolism
of endocannabinoids occurs mostly, although not
uniquely, through enzymatic hydrolysis. Indeed, the first
‘endocannabinoid enzyme’ to be cloned was the fatty
acid amide hydrolase (FAAH)
104
,which catalyses the
hydrolysis of anandamide and other long-chain fatty
acids and, under certain conditions, 2-AG
(BOX 1;see
REF. 79 for a review). Genetically engineered mice lacking
this enzyme have also been developed
105
and seem to
have 15-fold higher brain levels of anandamide (but not
2-AG), and a higher threshold sensitivity to nociceptive
stimuli
91,105
.Accordingly, pharmacological inhibition of
FAAH also leads to analgesic effects (although so far this
has been assessed only in animal models of acute
pain
106
), and to an antispasticity action in mice with
CREAE
46
.
Specific FAAH inhibitors have been developed that
are able to significantly enhance anandamide levels in
nervous tissues and to exhibit analgesic activity in
rodents
(FIG. 5).In one case, such compounds were
shown to also elicit anxiolytic effects in two experi-
mental models of anxiety
107
.An innovative proteomics
approach has been also used to develop new inhibitors
of FAAH, and led to the identification of nanomolar
reversible inhibitors, and enabled promiscuous
inhibitors to be discarded in favour of equally potent
compounds with at least 500-fold selectivity for their
targets
108
.With regard to 2-AG hydrolysis, there are,
unfortunately, no available selective inhibitors of the
enzyme principally responsible for this process — a
monoacylglycerol lipase (MAGL) cloned in the
1990s
109
and recently shown to control 2-AG levels in
isolated cells
(BOX 1)
110
.A cocktail of cannabinoid-
receptor-inactive monoacylglycerols is the only tool
developed so far to specifically inhibit 2-AG degradation
and enhance its pharmacological and therapeutically
beneficial actions
43,111
.
NON-PSYCHOTROPIC
CANNABINOID
Any plant or synthetic
cannabinoid-like compound
that does not induce, in animal
models and in humans, the
central cannabimimetic effects
typical of THC.
Table 1 | Major features of ‘endocannabinoid enzymes’*
Target Cloned Three- Catalytic/ Selectivity Tissue and Regulation Knockout Inhibitors
dimensional active site cellular mice promising in
structure localization animal models
of disorders
NAPE-PLD Yes Unknown Unknown All NAPEs tested Unknown Yes (+) No No
DAGLα Yes Unknown Yes All DAGs tested Yes Yes (+) No No
DAGLβ Yes Unknown Yes All DAGs tested Yes Yes (+) No No
FAAH Yes Yes Yes All long-chain NAEs and Yes Yes (+/–) Yes Yes
primary amides tested
MAGL Yes Unknown Yes All MAGs tested Yes Yes (–) No No
EMT No Unknown Unknown Only polyunsaturated long- Unknown Yes (+/–) No Yes
chain NAEs, 2-AG, and
other endocannabinoids
*See also BOX 1. 2-AG, 2-arachidonoylglycerol; DAGL, sn-1-selective diacylglycerol lipase; EMT, endocannabinoid membrane transporter; FAAH, fatty acid amide hydrolase;
MAGL, monoacylglycerol lipase; NAE. N-acylethanolamine; NAPE-PLD, N-acylphosphatidylethanolamine-selective phospholipase D.
778 | SEPTEMBER 2004 | VOLUME 3 www.nature.com/reviews/drugdisc
REVIEWS
carried out in animal models, might be in the pallia-
tive care of Parkinsons and Alzheimer’s diseases, and
of premature spontaneous abortion. Similarly, yet-to-
be-developed selective inhibitors of endocannabinoid
biosynthesis might, in the future, be used in these
same disorders.
Inhibitors of metabolism. As pointed out above, endo-
cannabinoids seem to be produced on demand’, and
in several cases fulfil a protective role ‘when and where
needed’. This protection, however, is rendered incom-
plete by the rapid degradation of endocannabinoids
in vivo.Therefore, one possible therapeutic approach
would be to retard the inactivation of endocannabi-
noids when they are being produced with a protective
function — for example, at the onset of some neuro-
logical, cardiovascular and intestinal disorders, or during
anxious states. Promising results in preclinical studies
have already been published with inhibitors of endo-
cannabinoid metabolism
(FIG. 5) in experimental models
of acute pain
106
,epilepsy
35
,multiple sclerosis
46
,Parkinsons
disease
45
, anxiety
107
and diarrhoea
63
.This approach, as
opposed to the direct stimulation of cannabinoid
Orleans, 9 March 2004). Rimonabant (20 mg per day)
was efficacious in reducing body weight and waist cir-
cumference in obese people after 1 year of treatment
(with more than 44% of the treated patients having
lost more than 10% of their weight), and in ameliorating
most of the symptoms of their metabolic syndrome. In
another study, the same dose of rimonabant almost
doubled the abstinence rate in smokers during 4 weeks
of treatment, with no change in body weight in normal
smokers and a significant loss of weight in overweight
smokers. Interestingly, the minor and occasional
adverse events observed with the 40-mg-per-day dose
of rimonabant (particularly diarrhoea and nausea) in
the Phase II trials were exactly what would have been
expected from previous knowledge of those functions
that are clearly tonically regulated by the endocannabi-
noid/CB
1
system in animals — specifically colon
motility
116
and emesis
117,118
.These findings in humans
indicate that the appropriate dose of rimonabant
specifically counteracts that part of endocannabinoid
signalling that becomes overactive and participates in
some pathological states. Other possible applications of
CB
1
antagonists/inverse agonists, based on observations
AM404 VDM11
UCM-707 OMDM-1 and OMDM-2
Endocannabinoid membrane transporter (EMT) inhibitors
Fatty acid amide hydrolase (FAAH) inhibitors
AM374 N-arachidonoyl-serotonin
URB-597 O-1624Compound 7
N
H
O
OH
N
H
O
O
N
H
O
OH
N
H
O
OH
OH
N
H
O
N
H
OH
P
F
OCH
3
O
O
N
O
N
O
NH
2
ON
H
O
SF
O
O
Figure 5 | Inhibitors of endocannabinoid inactivation. Inhibitors of both endocannabinoid cellular uptake and intracellular
degradation by fatty acid amide hydrolase (FAAH) that have been tested in vivo are shown. Of the uptake inhibitors, AM404 was the first
to be developed
96
, but was not particularly selective. VDM-11, UCM-707 and the two OMDM isomers are more selective, but the last
two compounds are more metabolically stable
100,101
. Of the FAAH inhibitors shown
106,107
,N-arachidonoyl-serotonin
163
is the least potent
in vitro, but is possibly more selective over cannabinoid receptors or phospholipase A
2
. Compound 7 was developed by Cravatt
and co-workers using a non-conventional proteomics approach
108
. No inhibitors have yet been developed for monoacylglycerol lipase.
NATURE REVIEWS | DRUG DISCOVERY VOLUME 3 | SEPTEMBER 2004 | 779
REVIEWS
Soft drugs, pro-drugs, partial agonists and others.
These new strategies for the development of non-
psychotropic cannabinoids with therapeutic value
have been much discussed, but have, to date, made
little progress. More effort needs to be directed to sev-
eral areas. First, the development of compounds that
although unable to cross the blood–brain barrier are
still able to activate peripheral CB
1
receptors. Second,
the development of cannabinoid pro-drugs. And last, the
development of drugs with a short half-life and that
can activate only peripheral receptors after local
administration (soft-drugs). These strategies are rec-
ommended to solve the problem of the unwanted
psychotropic effects with cannabinoid-based therapies,
a problem that is perhaps principally of social, rather
than pharmaceutical, importance. On the other hand,
the possible use of
PARTIAL AGONISTS of CB
1
,which
should not cause tolerance and dependence phenomena,
particularly after dosage uptitration, seems to be an
easier option, as evidenced by a few examples of such
compounds that already exist
125–127
.
Cannabis extracts and non-psychotropic plant canna-
binoids. Cannabinoids of either plant or synthetic origin
that are non-psychotropic because they are only
weakly active on cannabinoid receptors have been
studied. The most promising of the plant compounds
receptors with systemically administered agonists, is
likely to influence endocannabinoid levels principally
in those tissues in which there is an ongoing production
of otherwise ‘silent’ endocannabinoids, and therefore
should produce fewer side effects.
‘Dual target’ strategies. Synergistic actions of cannabi-
noid CB
1
-receptor agonists with substances activating
other receptors have recently been explored. In particular,
two strategies could result in the potential development
of either new drugs or new therapeutic treatments. The
first approach is through the exploitation of synergistic
analgesic effects of CB
1
and µ- or δ- opioid-receptor
agonists, which, through the co-administration (for
example, of THC and morphine), produces analgesic
actions stronger and longer-lasting (through the avoid-
ance of the development of morphine tolerance) than
those obtained with each agonist alone
119,120
.The second
approach builds on the observation that CB
1
agonists
and substances that activate the receptor for capsaicin
(the vanilloid transient receptor potential cation
channel-1 (TRPV1) receptor) show partly overlapping
medicinal actions; this fact could subsequently guide the
development of ‘hybrid’ CB
1
/TRPV1 agonists, such as
the prototypical arvanil, which has promising analgesic/
anti-inflammatory
121–123
,antispastic
123
and antiprolif-
erative actions
124
.
PARTIAL AGONIST
Any receptor agonist that does
not induce a full functional
response in a given functional
assay of receptor activation.
Table 2 | Therapeutic strategies from the endocannabinoid system*
Strategy Available Routes of Advantages Disadvantages Clinical Indications tested clinically
administration trials or preclinically
tested complete?
CB
1
agonists Yes Oral, Wide range of Psychotropic Yes Nausea, Tourette’s, Parkinson’s
suppository applications effects, tolerance disease, pain
, cachexia, MS,
glaucoma, cancer, diarrhoea, stroke
CB
2
agonists Yes Oral No psychotropic Limited range of No Pain, gliomas, lymphomas,
effect applications inflammation
Partial Yes None Unlikely Limited No Pain
agonists development efficacy
of tolerance
‘Soft’ agonists and No None tested No psychotropic Applications N/A N/A
agonists unable to effect limited to ‘peripheral
cross the BBB disorders’
§
CB
1
antagonists Yes Oral No psychotropic Limited range of Yes Obesity, nicotine and alcohol
effect, very few applications dependence, ileus
side effects
Inhibitors of No None tested No psychotropic Limited range N/A N/A
biosynthesis effect, very few of applications
side effects
Inhibitors of Yes None Higher selectivity, Residual side No Pain, anxiety, diarrhoea,
inactivation wide range of effects Parkinson’s disease
applications
Multi-target Yes Oral, mixed Higher efficacy, Limited range Yes Pain, spasticity in MS
preparations and low tolerance of applications
‘hybrid’ agonists
Cannabinoid Yes Oral No psychotropic Unknown Yes Pain, head injury, rheumatoid
receptor-inactive effect; very mechanism of arthritis
cannabinoids few side effects action
Cannabis extracts Yes Sublingual spray Toxicology well Initial side Yes Pain, spasticity in MS
investigated effects
*See text for details and references.
‘Pain’ denotes chronic, neuropathic, inflammatory, MS-related and post-oprative pain.
§
‘Peripheral disorders’ denote those disorders
that occur in peripheral organs or tissues as opposed to those developing in the central nervous system. BBB, blood–brain barrier; MS, multiple sclerosis and its animal
model (allergic experimental encephalomyelitis); N/A, not applicable.
780 | SEPTEMBER 2004 | VOLUME 3 www.nature.com/reviews/drugdisc
REVIEWS
which are due soon
130
.These trials could open the way
to the marketing of Sativex, a sublingual spray devel-
oped by GW Pharm and licensed to Bayer for possible
future distribution in European countries. Several
synthetic cannabinoids have been developed from
plant cannabinoids: HU-211 (dexanabinol), a neuro-
protective compound developed by Pharmos, now
undergoing Phase III clinical trials for severe head
injury
131
;ajulemic acid (CT3), a THC-11-oic acid ana-
logue with potent analgesic and anti-inflammatory
is cannabidiol, for which interesting anti-inflammatory,
anti-emetic, neuroprotective and anticancer actions
have been reported
128
.The wide range of beneficial
effects of cannabidiol and of other cannabinoids from
Cannabis
(FIG. 1) prompted the therapeutic use of
cannabinoid-rich extracts of the dried flowers of this
plant
129
.The genetic selection of Cannabis strains with
exactly reproducible ratios of THC/cannabidiol
allowed the preparation of such extracts and their test-
ing in at least five Phase III clinical trials, the results of
HU-308
CB
2
-selective agonist
Arvanil
CB
1
/TRPV
1
'hybrid' agonist
M123,WO0128498
CB
1
-selective agonist
HU-211
CB-receptor-inactive
synthetic cannabinoid
AM-1241
CB
2
-selective agonist
Ajulemic acid
(CT-3)
Rimonabant
(SR141716A) CB
1
-selective
antagonist/reverse agonist
SR144228
CB
2
-selective
antagonist/reverse agonist
WO0170700
CB
1
-selective antagonist
JTE-907
CB
2
-selective
antagonist/reverse agonist
BAY 38-7271
CB-receptor agonist
M122,WO0128557
CB
1
-selective agonist
N
N
Cl
Cl
Cl
O
N
HN
Cl
N
NN
S
O
O
NH
H
3
C
Cl
Cl
N
N
HN O
N
O
I
F
N
H
O
H
3
C
O
O
N
H
O
O
O
HO
OO
SCF
3
OO
CH OH
2
OCH
3
H
3
CO
N
H
O
Cl
Cl
CH
3
CH
3
N
H
O
OH
O
O
CH OH
2
OH
O
COOH
OH
N
O
I
NO
2
CH
3
N
Figure 6 | Chemical structures of some therapeutically promising, patented drugs based on the endocannabinoid system.
The most important feature of each compound is shown (collated from
REFS 112–115,124,131,132,137–142). Arvanil activates both CB
1
and
vanilloid (TRPV
1
)receptors, and for this reason is defined as ‘hybrid’.
NATURE REVIEWS | DRUG DISCOVERY VOLUME 3 | SEPTEMBER 2004 | 781
REVIEWS
still no consensus
152,153
), other therapeutic uses might
arise in the future from the direct activation of
cannabinoid receptors by either THC, its synthetic
analogues or Cannabis extracts, particularly if higher
doses (that is, at the upper threshold for side effects),
or alternative routes to oral administration (which
leads to poor bioavailability of the active principle)
are used.
Clinical trials have also been carried out with CT-3
(40 mg per day), which in a randomized, double-
blind, placebo-controlled crossover trial was effective
against neuropathic pain
154
, and with dexanabinol
(48–150 mg), which in a similar, multi-centre study
led to more rapid recovery from serious head
injury
155
.These trials underline again, for substances
developed from plant cannabinoids, the reasonably
good agreement found so far between the results of
preclinical studies in animal models and the outcome
of clinical trials.
Conclusions
From the literature reviewed in this article it is possible
to conclude that there is high potential for the open-
ing of new therapeutic avenues from research on the
endocannabinoid system, particularly for some disor-
ders for which no satisfactory treatment exists to date.
Some strategies aimed at obtaining new pharmaceuticals
from THC and Cannabis extracts, although partly
hampered by the unavoidable social implications con-
cerning the use of psychotropic compounds and by
preconceived ideas supported by little scientific evi-
dence, are currently being explored in controlled trials.
If successful, these trials — which build on millennia
of anecdotal observations of the effects of Cannabis
on man — will soon be translated into medicinal
preparations readily available through medical
prescription.
Other strategies, such as the use of synthetic receptor
antagonists and agonists, now seem to be possible
thanks to the enterprising energy of pioneering drug
companies who have invested in this idea — in some
cases before cannabinoid receptors were even discov-
ered. More indications might be suggested for these
compounds as our knowledge of the pathological role
of the endocannabinoid system improves and as more
efficacious routes for their administration are devised.
Finally, the realization of the most innovative
approaches, such as the use in the clinic of inhibitors
of endocannabinoid biosynthesis or inactivation, will
require even further enthusiasm and investment, but
might prove in the end more gratifying than the
strategies currently explored, in terms of safety, efficacy,
selectivity and exact knowledge of the mechanism of
action of the new drugs developed. In conclusion,
only the overturning of old taboos, the understanding
that cannabinoids are not ‘just for fun, and further
basic and preclinical research involving more than a
handful of pharmaceutical companies and often
underfunded scientists will reveal whether all the
promises held by the endocannabinoid system can be
realized in the future.
activity
132
; and HU-320, a cannabidiol analogue recently
shown to have interesting anti-inflammatory activity
133
.
Cannabidiol weakly interacts with TRPV1 receptors
and some endocannabinoid-degrading enzymes
134
,
HU-211 inhibits NMDA (N-methyl-
D-aspartate) glu-
tamate receptors
135
, and CT3 activates peroxisome-
proliferative activated receptor-γ (PPAR-γ)
136
.However,
the molecular mode of action of these compounds is
still open to investigation.
Clinical trials: work in progress
The cloning of cannabinoid receptors and the identi-
fication of their endogenous ligands has stimulated
an ever-increasing effort by pharmaceutical compa-
nies to develop novel, potent, selective and possibly
water-soluble CB
1
- and CB
2
-receptor agonists and
antagonists, and several relevant patents have been
filed since the mid-1990s. Some compounds, shown
in
FIG. 6,are particularly noteworthy because they
either exhibit novel features (such as that of being
selective agonists at CB
1
or CB
2
receptors, or being
water-soluble), or they are being tested in humans, or
both
137–143
.Indeed, apart from the above-mentioned
great expectations raised from rimonabant and
Sativex, clinical trials have mostly been performed so
far with oral THC and non-psychotropic cannabi-
noids. The results of a recent, multi-centre, random-
ized placebo-controlled study, including more than
600 patients with multiple sclerosis, were recently
published
144
.Although no benefit on spasticity was
found with a low oral dose of THC (5 mg per day),
when this symptom was assessed using the Ashworth
scale, more than 60% of the treated patients exhibited
objective improvements in mobility and reported
subjective improvements in pain, with an overall very
low degree of adverse events
144
.
Other promising results were obtained in random-
ized, double-blind, placebo-controlled crossover trials
on the alleviation of tics in Tourettes syndrome
145
and
on levo-3,4(OH)
2
-phenylalanine (L-DOPA)-induced
dyskinesia in Parkinsons disease
146
,with THC and
nabilone, respectively. Nabilone, however, was ineffec-
tive in reducing generalized and segmental primary
dystonia in Parkinsons disease patients
147
.The topical
administration of the synthetic cannabinoid receptor
agonist WIN55212-2 was also very effective in reduc-
ing intraocular pressure in glaucoma patients
148
.
When co-administered orally with morphine (30 mg)
to humans, THC (20 mg) exhibited an analgesic effect
only slightly synergistic with that of the opiate
120
, and,
at the dose of 5 mg, it did not significantly reduce
post-operative pain in hysterectomized women
149
.The
emerging picture of the side effects of oral THC
administration, however, seems to be reassuring —
only minor events were reported in most cases, and
there were few, if any, of the serious immune-suppressive
actions and neurocognitive deficits feared in the
past
150,151
.Therefore, these current clinical trials raise
the hope that, in addition to the traditional indica-
tions against nausea/vomiting and weight loss in can-
cer and AIDS patients (for which, however, there is
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Acknowledgements
The work of the authors is currently supported by grants from the
Ministry of Italian University and Research (MIUR, Fondo Italiano per
la Ricerca di Base, to V.D.M.), the Volkswagen Stiftung (to V.D.M.),
GW Pharm Ltd (to V.D.M., M.B. and L.D.P.), the Associazione
Italiana per la Ricerca sul Cancro (AIRC, to M.B.) and the
Associazione ERMES (to M.B.).
Competing interests statement
The authors declare competing financial interests: see Web version
for details.
Online links
DATABASES
The following terms in this article are linked online to:
Entrez: http://www.ncbi.nlm.nih.gov/entrez
CB1 receptor | CB2 receptor | FAAH | NAPE-PLD | PPAR-γ |
TRPV1
Online Mendelian Inheritance in Man:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM
Alzheimer’s disease | Huntington’s chorea | multiple sclerosis |
Parkinson’s disease | Tourettes’s syndrome
Access to this interactive links box is free online.
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