enannabinoid.degradation.inhibitors, biotransformation, Dokumenty
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Highlights
Cannabinoid-Receptor Agonists
Inhibitors of Endocannabinoid Degradation: Potential
Therapeutics for Neurological Disorders
Michaela Wendeler and Thomas Kolter*
Keywords:
cannabinoids · drug design · lipids · medical
chemistry · neurochemistry
Introduction
ing, stimulate the appetite, alleviate
cramps, and relax vascular muscles. At
the same time, however, they impair
cognitive and motor functions and in-
terfere with sensory perception and
short-term memory.
[2]
The underlying
causes of the effects of cannabinoids
have only been explored partially. In
1964, after about 20 years of research,
the structure of the main psychoactive
ingredientofmarijuana,(
)-D
9
-tetrahy-
drocannabinol (1) was finally deter-
one of the most abundant neuromodu-
latoryreceptorsinthebrain,CB2occurs
mainly on immune cells where it con-
tributes to the mediation of the immu-
nosuppressive effect of cannabinoids.
[5]
For a long time,
Cannabis sativa
preparations have been used as drugs
formedicinalandrecreationalpurposes.
Its dried blossom tips are known as
marijuana, and the dried resin as hash-
ish. For 60 years, chemical and physio-
logical effects of cannabinoids have
been investigated. Recently, this re-
search has focused on cannabinoid re-
ceptors in the human body and their
endogenous lipid ligands. Evidence is
accumulating that interference with en-
docannabinoid metabolism offers novel
prospects for the treatment of a multi-
tude of disorders such as pain, cancer,
epilepsy, and multiple sclerosis. Recent-
ly, a series of highly potent inhibitors of
endocannabinoid degradation has been
described, which promises the develop-
mentofnew strategiesforthe treatment
of anxiety and neurological disorders.
[1]
Ligands for Cannabinoid
Receptors
Cannabinoid-receptor agonists can
be assigned to different classes of com-
pounds. Tetrahydrocannabinol (1) be-
longs to a family of over 60 bi- and
tricyclic secondary metabolites that are
biosynthetically derived from geranyl
pyrophosphate and the polyketide oli-
vetol. Known under the name of mar-
inol, 1 is employed for the alleviation of
nausea in cancer patients undergoing
chemotherapy and for the treatment of
anorexia in patients with AIDS. Struc-
turalvariationsofthecannabinoidback-
bone led to the synthesis and pharma-
cological characterization of over 300
compounds.
[6]
Many of them bind to
both receptors, CB1 and CB2, but some
ligands with high selectivity for one or
the other receptor type have been
described.
[6]
Synthetic analogues are, for exam-
ple,CP-55,940(2),afullagonistforboth
receptors that is 4-50 times as potent as
1. The radio-labeled form of 2 allowed
the identification of the first cannabi-
noid receptor.
[7]
Another derivative,
HU-210 (3), is currently one of the most
potent cannabinoids.
[8]
Agonists with
aminoalkylindole structure, such as
WIN-55,212–2 (4),
[9]
as well as selective
antagonists of the CB1 receptor, for
example, SR141716A (5),
[10]
have pro-
ven to be valuable pharmacological
tools.
mined.
[3]
This laid the foundation for
the chemical synthesis of high-affinity
cannabinoid derivatives, which in turn
led to the identification of the first
cannabinoid receptor, CB1, in the cen-
tral nervous system.
[4]
Another receptor, CB2, was discov-
ered in 1993.
[5]
Both receptors are gly-
coproteins with seven transmembrane
helices. Their stimulation leads to the
activationofG proteinsbelongingtothe
G
i/o
family, thereby inhibiting the pro-
duction of adenylyl cyclase and cAMP
(cyclic adenosine-3’,5’-monophosphate)
and thus regulating numerous signal-
transduction pathways.
[6]
In addition,
CB1 receptors are known to inhibit
voltage-gated calcium channels and to
activate potassium channels. Whereas
CB1 is expressed predominantly in the
central nervous system and represents
Cannabinoids and Their
Receptors
The consumption of cannabinoids
leads to a highly complex spectrum of
pharmacological effects, which could be
of therapeutic benefit for a variety of
disorders. They exhibit sedative and
mood-altering properties, reduce pain
sensation, alleviate nausea and vomit-
[*] Priv.-Doz. Dr. T. Kolter,
Dipl.-Chem. M. Wendeler
Kekul-Institut fr Organische Chemie und
Biochemie
Universitt Bonn
Gerhard-Domagk Strasse 1
53121 Bonn (Germany)
Fax: (
+
49)228-73-7778
E-mail: tkolter@uni-bonn.de
2938
DOI: 10.1002/anie.200301641
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2003, 42, 2938 – 2941
 Angewandte
Chemie
ly expressed in brain and liver. Genet-
ically engineered mice that lack FAAH
show significantly elevated levels of
anandamide and related fatty acid
amides in the brain.
[24]
If these animals
are intraperitoneally treated with anan-
damide, they show an array of CB1-
dependent responses such as hypomo-
tility, catalepsy, and hypothermia. Re-
markably, these animals display a lower
pain perception, an effect that can be
eliminated by the CB1 antagonist 5.
These results conclusively indicate that
FAAHplaysakeyroleintheregulation
ofanandamideactionandthat thelatter
modulates pain sensation. Therefore,
together with FAAH and with the CB1
receptor, two protein members of the
endocannabinoid system are targets for
the development of novel analgesics.
[25]
FAAH is an integral membrane
protein, a homodimer composed of two
63-kD subunits and was previously crys-
tallized in the presence of the irrever-
sible active-site-directed inhibitor me-
thoxy arachidonyl fluorophosphonate
9.
[26]
The crystal structure reveals that
theactivesiteisaccessiblefromboththe
cytosolic leaflet of the membrane, and
from the aqueous phase of the cytosol.
The nucleophilic side chain of Ser241 is
part of an unusual Ser-Ser-Lys catalytic
triad. The structure of the enzyme
suggests that fatty acid amides destined
for degradation can directly reach the
activesite fromthemembraneandneed
not be transported through the aqueous
phase.
are not stored in synaptic vesicles, but
are rapidly synthesized in neurons in
response to membrane depolarization
and calcium influx.
[15]
Compounds 6
[16,17]
and 7
[18]
are biosynthetically derived
from different membrane lipids. In the
case of anandamide 6, a transacylase
transfers arachidonic acid from the
sn
1-
position of phosphatidylcholine to the
amino group of phosphatidylethanol-
amine. Subsequently, 6 is released by
the action of a phospholipase D.
Another peculiarity of endocanna-
binoids is that they act as retrograde
synaptic messengers: They are released
by postsynaptic neurons, diffuse across
the synaptic cleft, and activate the CB1
receptors on presynaptic nerve cells.
[19]
Endocannabinoid action is terminated
within several minutes
[20]
through intra-
cellular degradation: Endocannabinoids
are transported across the cell mem-
brane by a protein-mediated,
[21]
bidirec-
tionalprocess,
[22]
thenanandamide6 and
related substances are degraded by an
enzyme, fatty acid amide hydrolase
(FAAH).
[23]
Inhibitors of Fatty Acid Amide
Hydrolase
During the last few years, research
concentrated on FAAH inhibitors with
theaimofdevelopingdrugsthatamplify
the pharmacologically useful effects of
endogenous cannabinoids by inhibiting
their degradation. These substances
would have the advantage of avoiding
the unwanted psychotropic effects dis-
played by D
9
-THC and other direct-
acting exogenous cannabinoid ago-
nists.
[27]
Previously described FAAH in-
hibitors, however, did not show the
required target selectivity or bioavail-
ability,
[28–30]
or were not sufficiently
investigated with respect to their bio-
logical effects.
[31]
Endogenous Cannabinoids
Several endogenous lipidspresent in
mammalianbraintissuehavebeeniden-
tified as cannabinoid-receptor agonists.
They mimic the pharmacological effects
of synthetic or plant-derived cannabi-
noids, but are metabolically less stable.
The first endocannabinoid to be identi-
fied was
N
-arachidonoylethanolamine
(anandamide, 6).
[11]
Later, 2-arachido-
noylglycerol (7),
[12,13]
and 2-arachidonyl-
glycerylether (8) were found.
[14]
In con-
trast to classical neurotransmitters, they
Fatty Acid Amide Hydrolase
The concentration and signaling du-
ration of several endogenous cannabi-
noidsisregulatedprimarilybyFAAH,a
membrane protein that is predominant-
2939
Angew. Chem. Int. Ed. 2003, 42, 2938 – 2941
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Highlights
be administered in verylow, nonpsycho-
tropicconcentrations,orderivativesthat
cannot pass the blood–brain barrier, are
particularly important.
A better knowledge of the endocan-
nabinoid system will reveal the full
therapeutic potential of its modulation
and will facilitate the rational design of
potent inhibitors with improved biolog-
ical properties.
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of selective FAAH inhibitors with car-
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50
=18.6 m
m
), which
could be enhanced by replacing the
N
-
methyl group with an
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-cyclohexyl sub-
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lead structure resulted in URB597
(12),whichprovedtobethemostpotent
FAAH inhibitor in this series of com-
pounds with an IC
50
-value of 4 n
m
in
membrane preparations and 0.5 n
m
in
intact neurons.
Kinetic analyses suggest that a nu-
cleophilic attack of the serine residue of
the active site at the carbamate irrever-
sibly inactivates the enzyme. Most nota-
bly, other serine hydrolases are not
inhibited by this compound. In experi-
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dependent inhibition of FAAH activity
in the brain and a significant increase in
thelevelsofanandamideandotherfatty
acid ethanolamides in the brain. This
wasaccompaniedby anxiolyticandmild
analgesic effects, which could be re-
versedbytheCB1receptorantagonist 5.
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docannabinoid system plays a major
role in the modulation of emotional
states.
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ties,
[2]
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effects of cannabinoids prevent their
broad clinical application. The increase
in endocannabinoid concentration by
selective inhibition of their degradation
promises to circumvent these disadvan-
tages, but the complexity of the endo-
cannabinoid system must be considered
priortoclinicalapplicationofinhibitors:
Notonlyanandamide 6,butalsoaseries
of related fatty acid amides such as
palmitoylethanolamine, oleamide, and
the sleep-inducing
N
-oleoylethanol-
amine, are degraded by FAAH and
might compete together with oxygenat-
ed metabolites for the endocannabinoid
transporter. Receptors for these bioac-
tive lipids have not yet been identified,
but it is likely that inhibition of FAAH
might also affect other cellular signal-
transduction pathways. In addition to
CB1 and CB2 receptors, anandamide
also binds to the vanilloid receptor with
partly opposing cellular effects. More-
over, some data suggest the existence of
nonclassical cannabinoid receptors,
[6]
whose influence on signal transduction
and cell fate is largely unknown.
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of neurological disorders, endocannabi-
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exogenous analogues, endocannabi-
noids show significant antitumour ef-
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selective inhibition of growth-factor-de-
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[33]
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2940
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Angew. Chem. Int. Ed. 2003, 42, 2938 – 2941
Angewandte
Chemie
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