Despite a long history of medicinal use lasting back morethan 4,000 years [1, 2], the introduction of cannabinoids intomodern medicine is only beginning. Unlike opiates andmany other plant constituents used for therapeutic purposes,the most active ingredient
9-tetrahydrocannabinol (
9-THC,
dronabinol) was been identified only 40 years ago (see Fig.1). In addition, less than 20 years have passed since thedetection of an endogenous system of specific receptors andtheir endogenous ligands, the "endocannabinoid system" or"cannabinoid system".
In the 1930s and 1940s, the chemical structure of the firstphytocannabinoids had been successfully characterized [3],and the first synthetic derivatives of THC (parahexyl,DMHP) were successfully tested in clinical studies forepilepsy [4], depression [5] and dependency to alcohol andopiates [6]. However, it was not until 1964 that
9-THC was
stereochemically defined and synthesized [7].
Both discoveries, the identification of THC, whichallowed basic and clinical studies with defined andreproducible doses, and the detection of the endocannabinoidsystem in mammals, which allowed the investigation ofmechanisms of cannabinoid actions and the exploration ofthe therapeutic potential of inhibitors of endocannabinoid
*Address correspondence to the author at the Nova-Institut,
degradation and cannabinoid receptor antagonists, resulted ina considerable boost in research activities. The number ofpublications listed in the database PubMed(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi) rose fromvery few to about 400 per year in 1972, then declined to 250in 1982, remained at this level until 1989, and then steadilyrose to about 1100 per year in 2004.
OH
O
7
6
5
10
96'
5'
4'
3'2'1'3
21
5"3"1"8
OH
O
11
8
7
13
125
4
3
21
10b10d
109
5'3'1'6
Monoterpenoid numbering
Dibenzopyran numbering
Fig. (1). Chemical structure of THC, the main cannabinoid in the
cannabis plant, according to the monoterpenoid system (1-THC)
and dibenzopyran system (9-THC).
508 Current Drug Targets - CNS & Neurological Disorders, 2005, Vol. 4, No. 5 Franjo Grotenhermen
2. CANNABINOID RECEPTORS
To date, two cannabinoid receptors have been identified,the CB1 (cloned in 1990) [8], and the CB2 receptor (cloned in1993) [9], exhibiting 48% amino acid sequence identity.Besides their difference in amino acid sequence, they differin signaling mechanisms, tissue distribution, and sensitivityto certain agonists and antagonists that may show markedselectivity for one or the other receptor type [10].
Activation of the CB1 receptor produces effects oncirculation and psychotropic effects common to cannabisingestion, while activation of the CB2 receptor does not.Hence, selective CB2 receptor agonists have become anincreasingly investigated target for therapeutic uses ofcannabinoids, among them being analgesic, anti-inflammatory and anti-neoplastic actions [11, 12].
2.1. Genetics
The endocannabinoid system is teleologically, millions ofyears old and has been found in mammals, birds,amphibians, fish, sea urchins, molluscs, leeches and evenprimitive Hydra vulgaris [13, 14]. The nucleotide sequencesof genes encoding the cannabinoid receptors vary fromspecies to species, their identity with the human CBsequence being proportional to the evolutionary distancesbetween the organisms. The CB1 gene (CNR1) of the rhesusmonkey (Macaca mulata) is 100% identical to the sequenceof the human CNR1, whereas the CB1 gene of the leech(Hirudo medicinalis) shares only 58% of the human gene[14]. Comparisons between human, rat and mouse CB1
receptor sequences showed extensive homology both at thenucleotide and protein levels [15]. The CB1-receptornucleotide sequences of humans and rats are 90% and thoseof humans and mice are 91% identical. CB2 receptors showgreater interspecies differences with a similarity of 82%between the mouse and human receptor protein, the humanCB2 being 13 amino acids longer at the carboxyl terminus[16] and 81% amino acid homology between the rat andhuman CB2 receptor [17]. Pronounced species selectivity atthe rat cannabinoid CB2 receptor compared to the humanCB2 receptor was observed for two synthetic cannabinoids(AM-1710 and AM-1714), whereas JWH-015 andendocannabinoids were more human receptor selective [17].These findings of pharmacological species differences arecritical for characterizing cannabinoid receptor ligands in invivo rodent models for drug discovery purpose.
A first spliced amino-truncated variant (isoform) of theCB1 receptor cDNA, CB1a, has been isolated by Rinaldi-Carmona et al. (1996) [19]. Recently, another group reportedthe detection of another spliced variant (CB1b) and noted thatboth variants have a unique pharmacological profile and thattheir RNA's are expressed at low levels in several tissues[19].
Attempts have been made to investigate a possibleassociation between the predisposition to certain diseasesand mutations or variants in the CB1 receptor gene.Schizophrenia [20] and depression in Parkinson's disease[21] may be related to a genetic polymorphism of the CNR1,while Tourette's syndrome and delirium tremens inalcoholism [22] do not seem to be associated with a mutation
of the CB1 receptor gene [23]. Individuals with a 9-repeatallele of an AAT-repeat polymorphism of the gene may havea 2.3-fold higher susceptibility to schizophrenia [20].
Both CB1 and CB2 receptors belong to the class A(rhodopsin like) G-protein-coupled receptors (GPCR). G-proteins coupled to cannabinoid receptors are sensitive topertussis toxin, an inactivator of inhibiting Gi and Go
proteins. Among the class A GPCRs are receptors formelatonin, thyrotropin-releasing hormone, prostanoid andthe leukotriene B4 receptor. GPCRs are the most commonreceptors, containing 1000-2000 members in vertebrates(>1% of the genome) [24]. GPCRs are involved in therecognition and transduction of messages as diverse as light,Calcium ions, odorants, nucleotides and peptides, as well asproteins, controlling the activity of enzymes, ion channelsand transport of vesicles [24].
2.2. Distribution
CB1 receptors are mainly found on neurons in the brain,spinal cord and peripheral nervous system, but are alsopresent in certain peripheral organs and tissues, among thembeing immune cells, spleen, adrenal and pituitary glands,sympathetic ganglia, heart, lung and parts of thereproductive, urinary and gastrointestinal tracts [25]. In thecentral nervous system, the CB1 receptor is the mostabundant G-protein coupled receptor.
CB1 receptors are highly expressed in the cerebral cortex,basal ganglia (substantia nigra pars reticulata, globuspallidus, nucleus caudatus and putamen) cerebellum,hippocampus, periaqueductal grey, rostral ventromedialmedulla, certain nuclei of the thalamus and amygdala, anddorsal primary afferent spinal cord regions, which reflect theimportance of the cannabinoid system in motor control,memory processing and pain modulation, while theirexpression in the brainstem is low [10], which may accountfor the lack of cannabis-related acute fatalities, e.g. due todepression of respiration. Many CB1 receptors are expressedat the terminals of central and peripheral nerves and inhibitthe release of neurotransmitters.
CB1 receptors have also been found at the central andperipheral terminals of small diameter C-fibers and largerdiameter A /A -fibers of primary afferent neurons. Thishelps to explain the efficacy of CB1 receptor agonists inneuropathic pain, since this kind of pain is thought to beelicited in part by abnormal spontaneous discharges of Aand A fibers [26].
CB2 receptors occur principally in immune cells, amongthem being leukocytes, spleen and tonsils [27]. Immune cellsalso express CB1 receptors, but there is markedly moremRNA for CB2 than CB1 receptors in the immune system.Levels of CB1 and CB2 mRNA in human leukocytes havebeen shown to vary with cell type (B cells > natural killercells > monocytes > polymorphonuclear neutrophils, T4 andT8 cells) [28]. One of the functions of CB receptors in theimmune system is modulation of cytokine release. Activationof CB2 receptors has also been reported to produceantinociception, by stimulating peripheral release ofendogenous opioids [29].
Cannabinoids Current Drug Targets - CNS & Neurological Disorders, 2005, Vol. 4, No. 5 509
2.3. Mechanisms of Action
Agonistic stimulation of both types of cannabinoidreceptors activates a number of signal transduction pathways[25, 27]. Both are coupled through inhibiting G-proteins (Gi/o
proteins), negatively to adenylate cyclase and positively tomitogen-activated protein kinase. Inhibition of adenylatecyclase results in the inhibition of the conversion of ATP tocyclic AMP (cAMP). CB1, but not CB2 receptors are alsocoupled to several ion channels through Gi/o proteins,negatively to N-type and P/Q-type calcium channels and D-type potassium channels, positively to A-type and inwardlyrectifying potassium channels. CB1 receptors may alsomobilize arachidonic acid, close 5-HT3 receptors ionchannels, modulate nitric oxide production and mobilizearachidonic acid and intracellular calcium stores [26]. CB1
receptor activation can also initiate ceramide productionthrough a non-G protein mediated mechanism, and undercertain conditions, CB1 receptors may also activate adenylatecyclase and/or reduce outward potassium K current throughstimulating G proteins (Gs proteins) [26].
In vitro experiments have demonstrated that CB1
receptors can mediate inhibition of the neuronal release of amultitude of neurotransmitters and neuromodulators,including acetylcholine, dopamine, -aminobutyric acid(GABA), histamine, serotonin (5-hydroxytryptamine),glutamate, cholecystokinin, D-aspartate, glycine andnoradrenaline (norepinephrine) in several brain regions andoutside the brain (see Table 1 ). Inhibition ofneurotransmitters by CB1 receptor activation in the centralnervous system is caused by presynaptic inhibition ofneurotransmitter release from axon terminals [30]. In theCNS, this inhibition is caused by both endocannabinoids andexogenous cannabinoids. Presynaptic inhibition ofneurotransmitters by exogenous cannabinoids that bind tothe peripheral CB1 receptor has also been described in thesympathetic nervous system, but endocannabinoid-mediatedpresynaptic inhibition was not observed in all assaysinvestigating their action in the sympathetic nervous system[30].
In some experiments, CB1 receptor agonists have beenreported not to inhibit but to enhance the release of certainneurotransmitters. However, it is possible that these effectsalso result from a CB receptor-mediated inhibitory effect onneurotransmitter release, resulting in a stimulatory effect onneurotransmitter release at some point downstream of theside of the initial inhibitory effect [26].
Interaction of THC effects with other neurotransmitters issupported by the fact that antagonists of theseneurotransmitters blocked specific THC effects. The memorydisruptive effects of THC were completely reversed by theGABA antagonist bicuculline, while other THC effects wereunaffected [31]. Opioid receptor antagonists blocked severalbehavioral effects of CB1 agonists [32, 33]. A number ofpharmacological effects can be explained (at least in part) onthe basis of interactions with other neurotransmitters. Forexample, tachycardia and hyposalivation with dry mouth[34, 35] are mediated by effects of THC on release and turn-over of acetylcholine [34]. In a rat model, cannabinoidagonists inhibited activation of 5-HT3 receptors, explainingantiemetic properties of cannabinoids to be based oninteractions with serotonin [36]. Therapeutic effects inmovement and spastic disorders could be ascribed in part tointeractions with GABAergic, glutamergic and dopaminergictransmitters systems [37]. The effects on GABA, glutamateand glycine release in the periaqueductal grey, rostralventromedial medulla and substantia nigra may contribute tothe modulation of pain perception.
Cannabinoids may cause contradictory effects withsuppression or induction/intensification of somatic andpsychic effects, including convulsion, emesis, pain, tremorand anxiety, depending on subject and condition. Cannabisand THC are used against nausea and vomiting caused byanti-neoplastic drugs but may rarely cause vomiting. Theyare used as analgesics but sometimes may increase pain [38];they may cause anxiety but may also be anxiolytic [39], etc.These observations are probably based on the control ofthese effects by several neuronal circuits influenced bycannabinoids. Recently, it has been demonstrated thatsignaling of the CB1 receptor is profoundly altered by a
Table 1. Neurotransmitter Functions Under Cannabinoid Control
Neurotransmitter Associated disorder
Excitatory amino acids
Glutamate Epilepsy, nerve-cell death in ischemia and hypoxia (stroke, head trauma, nerve gas toxicity)
Inhibitory amino acids
GABA Spinal cord motor disorders, epilepsy, anxiety
510 Current Drug Targets - CNS & Neurological Disorders, 2005, Vol. 4, No. 5 Franjo Grotenhermen
regulated association of CB1 and dopamine-2 receptors [40].The highest level of CB1/D2 receptor complexes wasdetected when both receptors were stimulated with sub-saturating concentrations of agonists. Concurrent receptorstimulation promoted a shift of CB1 signaling from apertussis toxin sensitive inhibition to a partly pertussis toxininsensitive stimulation of adenylate cyclase andphosphorylation by extracellular signal-regulated kinases 1and 2 (ERK1/2) [40]. A cross-talk between the CB1 receptorand other receptors in the brain has also been reported for thecorticotropin releasing hormone receptor type 1 (CRHR1) inolfactory regions, in several cortical and limbic structures,and in some hypothalamic and thalamic nuclei [41] and forthe -opioid receptor in the dorsal horn of the spinal cord[42].
Interactions of cannabinoids with other neurotransmittersystems may cause unexpected effects. While studies inanimals have demonstrated that opioid receptor antagonistsprecipitated a cannabinoid-like withdrawal syndrome incannabinoid-dependent rats [43], opioid receptor antagonistsdid not block the subjective effects of THC in humans in onestudy [44] or even increased the subjective effects THC inanother study [45].
Not all compounds that activate cannabinoid receptorsare cannabinoids. e.g. alkylamides of the Echinacea plantupregulated TNF-alpha mRNA, which was mediated by CB2
receptors [46].
2.4. Additional Cannabinoid Receptors
Not all cannabinoid effects are mediated by CBreceptors. The endocannabinoid 2-arachidonoyl glycerolsuppressed interferon-gamma expression in splenocytes andthere was no difference in magnitude of effect between cellsderived from C B1(-/-)/CB2(-/-) knockout mice and fromcontrols [47]. The ability of cannabidiol to impair themigration of tumor cells was independent of CB1 and CB2
receptor activation [48]. The mechanisms of these effects areunknown. There is increasing evidence for the existence ofadditional cannabinoid receptor subtypes in the brain andperiphery [49-51]. These receptors are more likely to befunctionally related to the known cannabinoid receptors thanhave a similar structure, as there is no evidence for additionalcannabinoid receptors in the human genome.
3. ENDOCANNABINOIDS
The identification of cannabinoid receptors was followedby the detection of endogenous ligands for these receptors,endogenous cannabinoids or endocannabinoids [52-54]. Allendocannabinoids are derivatives of arachidonic acid, thusdiffering in chemical structure from phytocannabinoids ofthe cannabis plant. To date, five endocannabinoids have beenidentified. These are N-arachidonoyl ethanolamide (AEA,anandamide) [52], 2-arachidonoyl glycerol (2-AG) [54, 55],2-arachidonoyl glyceryl ether (noladin ether) [56], O-arachidonoyl ethanolamine (virodhamine) [57], and N-arachidonoyl-dopamine (NADA) [58]. The most importantof these eicosanoid molecules are anandamide and 2-arachidonoyl glycerol (see Figs. 2 and 3). Noladin ether wasinitially synthesized as a stable analog of 2-AG. Whether itis present in mammalian brain is controversial, since it wasclaimed to be an endocannabinoid by Hanus et al. (2001)
[56], but could not be detected by another group inmammalian brains of several species, suggesting that it doesnot play a role in the central endocannabinoid system [59].
3.1. Binding to Cannabinoid and Vanilloid Receptors
When protected from enzymatic hydrolysis, anandamidehas a similar affinity to the CB1 receptor as
9-THC [26].
Affinity to the CB1 receptor is greater than CB2 affinity.Anandamide behaves as a full agonist at the CB1 receptor[60] and as a partial agonist at the CB2 receptor. This meansthat anandamide elicits a lower maximal response than thatof higher efficacy cannabinoid receptor agonists at the CB2
receptor, possessing the mixed agonist-antagonist propertiestypical of partial agonists. In one experiment, anandamidewas found to attenuate CB2 receptor mediated responses to2-AG, which was 3-fold more potent than anandamide [61].
O
O
OH
CH2OH
Fig. (3). 2-Arachidonoyl glycerol (2-AG).
Noladin ether and 2-AG are both cannabinoid receptoragonists, noladin ether having a much higher affinity for theCB1 receptor than for the CB2 receptor [26]. Virodhamine isa partial agonist with in vivo antagonist activity at the CB1
receptor and full agonist activity at the CB2 receptor [57].NADA is an agonist at the CB1 receptor [62].
Anandamide and NADA do not only bind to cannabinoidreceptors but also share the ability of capsaicin, a constituentof hot chilli peppers, to stimulate vanilloid receptors (VR1)[58, 62, 63]. The VR1 is associated with hyperalgesia andmay play a part in nociception. Since VR1 is also widelydistributed in the skin, it was recently proposed that thisreceptor does also play a central role in maturation andfunction of epithelial cells [64].
The historical designation of anandamide as an“endocannabinoid” seems to be only one part of thephysiological reality, and cannabinoid receptors seem toamount only to some of the “anandamide receptors”. Thepotency and efficacy of anandamide at the vanilloidreceptors is rather low. However, metabolites of anandamidemay serve as endogenous ligands for these receptors [65].NADA is a potent vasorelaxant, an effect mediated throughVR1 and CB receptors [66].
3.2. Production and Metabolism
The first two discovered endocannabinoids, anandamideand 2-AG, are best studied. They are synthesized in neuronalcells, including cortical and striatal neurons, but notastrocytes, and their synthesis is increased in response tomembrane depolarization. Specific membranedepolarization-induced release is characteristic of classicalneurotransmitters. However, in contrast to classical
Cannabinoids Current Drug Targets - CNS & Neurological Disorders, 2005, Vol. 4, No. 5 511
neurotransmitters that are being synthesized and stored inintraneural vesicles, endocannabinoids are produced “ondemand” by cleavage of membrane lipid precursors andreleased immediately from cells into the synapse in astimulus-dependent manner [53]. The precursor ofanandamide is N-arachidonoyl-phosphatidylethanolamine,which is hydrolyzed to the endocannabinoid by aphospholipase D-catalyzed process [67]. After release,endocannabinoids are rapidly deactivated by uptake intocells and metabolized. Metabolism of anandamide and 2-AGoccurs by enzymatic hydrolysis by fatty acid amidehydrolase (FAAH) and monoacylglycerol lipase [53, 68].FAAH degrades anandamide to arachidonic acid andethanolamide. In mice, lack of FAAH resulted insupersensitivity to anandamide and enhanced endogenouscannabinoid signaling [69]. Anandamide may also beconverted to other metabolites by lipoxygenase orcyclooxygenase [26]. Other metabolic processes includeacylation of noladin ether [70], oxidation of 2-AG andmethylation of the aromatic moiety of NADA.
In all cases, cellular uptake must proceed metabolismsince metabolism occurs only in the cells. Endocannabinoiduptake by cells seems to happen by “enhanced diffusion”through the cell membrane [57, 58, 71], even though anactive carrier system has not been detected so far. Simplepassive diffusion following a concentration gradient into thecells where they are quickly metabolized by FAAH, isregarded as unlikely, since several substances have beendeveloped that are thought to inhibit anandamide cellularuptake without inhibiting FAAH, among them being Arvanil[72] and VDM11 [73], and noladin ether and NADA arerapidly taken up into cells even though they are rather stableor refractory to enzymatic hydrolysis [58, 70]. However, thediscussion on the existence of a transport system is notfinished, and one group demonstrated that Arvanil and othersubstances regarded as anandamide transport inhibitors(olvanil, AM404) were actually inhibitors of FAAH [74].Intracellular uptake of endocannabinoids is a temperaturedependent and rapid process with a half time of a fewminutes, compared to hours in the case of exogenous plantcannabinoids.
As with the gene of the CB1 receptor, there is interest in apossible contribution of variations of the gene encodingFAAH to the etiology of diseases. The sparse data on apossible link between drug abuse and dependence and amutation of the FAAH gene and subsequent functionalabnormalities in the endocannabinoid system are conflicting[75, 76]. A naturally occurring missense gene polymorphism(FAAH 385 A/A genotype) was reported to be associatedwith overweight and obesity [77].
3.3. Tonic Activity of the Cannabinoid System
When administered alone, cannabinoid receptorantagonists may behave as inverse agonists in severalbioassay systems. In addition to blocking the effects ofexogenous cannabinoidsm, they may also produce effectsthat are opposite in direction from those produced bycannabinoid receptor agonists, e.g. resulting in hyperalgesia[78], suggesting that the endogenous cannabinoid system istonically active. Tonic activity may be due a constant release
of endocannabinoids or from a portion of cannabinoidreceptors that exist in a constitutively active state [27].
Tonic activity of the endocannabinoid system has beendemonstrated in several conditions. Endocannabinoids havebeen shown to be tonically active in the dorsal horn neuronsof the spinal cord, thus attenuating acute nociceptivetransmission at the level of the spinal cord [79].Endocannabinoid levels were increased in a pain circuit ofthe brain (periaqueductal gray) following painful stimuli[80]. Tonic control of spasticity by the endocannabinoidsystem has been observed in chronic relapsing experimentalautoimmune encephalomyelitis (CREAE) in mice, an animalmodel of multiple sclerosis [73]. An increase of cannabinoidreceptors following nerve damage was found in a rat modelof chronic neuropathic pain [81] and in a mice model ofintestinal inflammation [82]. This may increase the potencyof cannabinoid agonists used for the treatment of theseconditions. Tonic activity has also been demonstrated withregard to appetite control [83] and with regard to vomiting inemetic circuits of the brain [84]. Elevated endocannabinoidlevels have been detected in cerebrospinal fluid ofschizophrenic patients [85]. In other models, tonic orenhanced activity could not been demonstrated, e.g. in a ratmodel of inflammatory hyperalgesia [86].
4. MODULATORS OF THE ENDOCANNABINOIDSYSTEM
Exogenous modulators of the endocannabinoid systemcomprise cannabinoid receptor agonists (cannabinoids), CBreceptor antagonists and inhibitors of degradation and re-uptake of endocannabinoids that promote accumulation ofendocannabinoids in situ. Antagonists have allowed adetailed investigation of mechanisms of cannabinoid actionsand proved that not all cannabinoid effects were mediated bycannabinoid receptors, but that other mechanisms of actionwere involved. Both antagonists and agonist cannabinoidsare under clinical investigation for a broad number ofindications.
Cannabinoids were originally regarded as any of a classof "typical C21 groups of compounds present in Cannabissativa L." [87]. The modern definition is termed with moreemphasis on synthetic chemistry and on pharmacology, andencompasses kindred structures, or any other compound thataffects cannabinoid receptors. This has created severalchemical sub-categories that take into consideration thevarious forms of natural and synthetic compounds.
It has been proposed to use the term phytocannabinoidfor the natural plant compounds [88] and endocannabinoidsfor the natural animal compounds [89], the endogenousligands of the cannabinoid receptors. Synthetic agonists ofthese receptors have been classified according to their degreeof kinship ("classical" vs. "non-classical") withphytocannabinoids.
4.1. Phytocannabinoids and their Metabolites
Natural plant cannabinoids are oxygen-containingaromatic hydrocarbons. In contrast to most other drugs,including opiates, cocaine, nicotine and caffeine, they do notcontain nitrogen, and hence are not alkaloids.
512 Current Drug Targets - CNS & Neurological Disorders, 2005, Vol. 4, No. 5 Franjo Grotenhermen
Phytocannabinoids were originally thought to be onlypresent in the cannabis plant (Cannabis sativa L.), butrecently, some cannabinoid type bibenzyls have also beenfound in liverwort (Radula perrottetii and Radula marginata)[90], with the chemical structure of perrottetinenic acid inliverwort being similar to that of
9-THC in cannabis.
O
H
OH
Fig. (4). Cannabidiol.
More than 60 cannabinoids have been detected incannabis, mainly belonging to one of 10 subclasses or types[91], of which the cannabigerol type (CBG), thecannabichromene type (CBC), the cannabidiol type (CBD),the
9-THC type, and the cannabinol type (CBN) are the
most relevant in quantity. Cannabinoid distribution variesbetween different cannabis strains and usually only three orfour cannabinoids are found in one plant in relevantconcentrations.
9-THC is largely responsible for the
pharmacological effects of cannabis including itspsychotropic properties, but other compounds of thecannabis plant are involved in these effects [92].Concentrations of
9-THC are below 0.2% in fiber-type
cannabis varieties (hemp) used for the production of fiberand seeds, and may vary between 2 and 30% in the floweringtops and upper leaves in the female drug-type cannabis plantused for recreational and medicinal purposes.
CH2OH
O
OH
Fig. (5). 11-OH-THC (11-hydroxy-THC).
11-OH-9-tetrahydrocannabinol (11-OH-THC) is the
most important psychotropic metabolite of 9-THC with a
similar spectrum of actions and similar kinetic profiles as theparent molecule [93, 94]. 11-nor-9-carboxy-THC (THC-COOH) is the most important non-psychotropic metaboliteof
9-THC.
COOH
O
OH
Fig. (6). THC-COOH (11-nor-9-carboxy-THC).
4.2. Synthetic Cannabinoids
According to their degree of kinship with thephytocannabinoids, synthetic cannabinoids may be describedas classical or non-classical derivatives. Among the classical
synthetic cannabinoids that retain the phytocannabinoid ringstructures and their oxygen atoms are nabilone, nantradol,HU-210, and HU-211. Nabilone is available on prescriptionin several countries with a similar pharmacological profile asTHC [95]. HU-210, an analog of
8-THC with a
dimethylheptyl side chain, is between 80 and 800 times morepotent than THC [96, 97], while its enantiomer (mirrorimage) HU-211, is completely devoid of psychoactivity [98].The latter, also called dexanabinol, is an NMDA antagonistwith neuroprotective properties in hypoxia and ischemia[99]. CT3 or ajulemic acid, a derivative of the
8-THC
metabolite THC-COOH, is under clinical investigation forinflammation and pain [100].
O
O
OH
Fig. (7). Nabilone.
Levonantradol (Pfizer), a non-classical cannabinoid witha more radical deviation of the typical structure, was underclinical investigation for the treatment of pain [101] and theside effects of chemotherapy [102] and radiotherapy [103].Other non-classical cannabinoids are the aminoalkylindolWIN-55,212-2, which has a 6.75-fold affinity towards theCB2 receptor [104] and the bicyclic cannabinoid analog CP-55,940, a widely-used agonist for the testing of cannabinoidreceptor affinity with potency 4-25 times greater than THCdepending on assay [105].
CH2OH
O
OH
Fig. (8). Dexanabinol (HU211).
4.3. Endocannabinoid Analogs
Several anandamide congeners have been synthesized,among them is (R)-(+)- -methanandamide that possessesboth a four-fold higher affinity for the CB1 receptor and agreater catabolic resistance than anandamide. Furtheranandamide analogs are arachidonoyl-2'-chloroethylamide(ACEA) and arachidonoyl cyclopropylamide (ACPA).
4.4. Inhibitors of Endocannabinoid Degradation
Fatty acid-based compounds have been synthesized thatmimic the structure of anandamide, but act as inhibitors ofmembrane transport or of the catabolic enzyme FAAH [68].The first of these compounds was N-(4-hydroxyphenyl)arachidonoylamide, usually designated as AM404. AM404increases the plasma levels of anandamide in rats and causesa time-dependent decrease of motor activity, which isreversed by a cannabinoid CB1 receptor antagonist [106]. Itis unclear whether AM404 is a selective inhibitor of
Cannabinoids Current Drug Targets - CNS & Neurological Disorders, 2005, Vol. 4, No. 5 513
anandamide transport or an inhibitor of FAAH [74]. It alsoactivates vanilloid receptors and binds to CB1 receptors, butit does not seem to be a CB1 receptor agonist or antagonist[26]. Another inhibitor of membrane transport or FAAH isVDM-11, which also binds to CB1 receptors.
COOH
O
OH
Fig. (9). CT3 (ajulemic acid, IP751).
A frequently used inhibitor of FAAH isphenylmethylsulfonyl fluoride (PMSF). The irreversibleFAAH inhibitors palmitylsulphonoyl fluoride (AM374) andstearylsulphonoyl fluoride (AM381) are approximately 20times more potent than PMSF in preventing the hydrolysis ofanandamide in brain homogenates [107]. They both are onlyweak CB1 receptor ligands [107]. Even more potent FAAHinhibitors have recently been developed [108].
4.5. Cannabinoid Receptor Antagonists
The first reported antagonist of a cannabinoid receptorbinding site was announced by Rinaldi-Carmona et al.(1998) as the potent and orally-active CB1-selectivecompound, SR141716A, that is also called rimonabant(Acomplia
®) in clinical studies [109]. Rimonabant is a
diarylpyrazole and the majority of CB1 receptor antagonistscan be regarded as a structural modification of this molecule[110]. A CB2 selective antagonist synthesized by the samegroup at Sanofi-Synthelabo is SR144528. Both also bind tothe other receptor type in higher concentrations. Further CBreceptor antagonists with CB1 selectivity are thediarylpyrazoles AM251 and AM281, and the substitutedbenzofuran LY320135. A CB2 selective antagonist is theaminoalkylindole 6-iodopravadoline (AM630). Severalpharmaceutical companies besides Sanofi-Synthelabo havepatented CB receptor antagonists, among them being Pfizer,Bayer, Merck, Solvay Pharmaceuticals, Hoffmann-La Rocheand AstraZeneca [110].
4.6. Affinity to the Cannabinoid Receptor
Both antagonists and CB receptor agonists show differentaffinity to CB1 and CB2 receptors. Synthetic cannabinoidshave been developed that act as highly selective agonists atone of these receptor types [111, 27]. Several syntheticcannabinoid receptor agonists with significant selectivity forCB2 receptors have a classical structure (L-759633, JWH-133, HU-308). The hydroxy group of THC has been replacedby a methoxy group or removed in these molecules. Apotentially important class of CB1 receptor selective agonistsare anandamide analogs, including (R ) - ( + ) - -methanandamide, ACEA and ACPA.
9-THC has approximately equal affinity for the CB1 and
CB2 receptor, while anandamide has marginal selectivity forCB1 receptors [111]. However, the efficacy of THC andanandamide is less at CB2 than at CB1 receptors.
The affinity to CB receptors and the pharmacologicalactivity of 9-THC is stereoselective, with the natural (-)-trans isomer (dronabinol) being 6-100 times more potentthan the (+)-trans isomer depending on the assay [113]. Notall phytocannabinoids are agonists to the cannabinoidreceptor. The mechanism of action and the pharmacology ofCBD considerably differ from that of THC. As with THCand other phytocannabinoids, the natural CBD is a (-)-enantiomer. However, in contrast to (-)- 9-THC, (-)-cannabidiol does not have a significant affinity tocannabinoid receptors, while its synthetic (+)-enantiomerdoes [114].
Cl
Cl
NN
Cl
N
O
HNA
NN
Cl
N
O
HB
Fig. (10). Cannabinoid receptor antagonists, SR 141716A (A), a
selective CB1 receptor antagonist,and SR 144528 (B), a selective
CB2 receptor antagonist.
5. PHARMACOLOGY
The best-studied modulator of the endocannabinoidsystem is the phytocannabinoid and CB receptor agonist
9-
THC. While THC is still favored in clinical studies; basicresearch is often conducted with more potent CB receptoragonists, such as WIN55,212-2, HU-210 and CP-55,940.
5.1.9-THC and Other CB Receptor Agonists
The majority of THC effects are mediated throughagonistic actions at cannabinoid receptors. Some non-CBmediated effects of THC and synthetic derivatives have alsobeen described, e.g. some effects on the immune system[115], some neuroprotective effects [116], and anti-emeticeffects. The anti-emetic effects of THC are reported to be inpart mediated by CB1 receptors [117] and in part by non-CBmechanisms; the rationale for the clinical use of THC as ananti-emetic in children receiving cancer chemotherapy [118].Due to the lower CB1 receptor density in the brain ofchildren compared with adults, they tolerated relatively high
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doses of 8-THC in a clinical study, without significant CB1
receptor mediated adverse effects [118]. In a study with cellsstably transfected with the human 5-HT3A receptor, several(endo)cannabinoids (THC, WIN55,212-2, anandamide, etc.)directly inhibited currents induced by 5-hydroxytryptamine[119]. Since 5-HT3 antagonists are potent anti-emetic drugs,this may be one mechanisms by which cannabinoids act asanti-emetics.
It is possible that several effects previously thought to benon-receptor mediated are mediated by cannabinoid receptorsubtypes that have not yet been identified.
The activation of the cannabinoid system viaphytocannabinoids such as THC, and synthetic andendogenous cannabinoids causes numerous actions that havebeen extensively reviewed [113, 120-128]. Some effects ofcannabinoid receptor agonists show a biphasic behavior independency of dose, e.g. low doses of anandamidestimulated phagocytosis and stimulated behavioral activitiesin mice, while high doses decreased activities and causedinhibitory effects on immune functions [129].
Psyche, Cognition and Behavior
In many species, the behavioral actions of low doses ofTHC are characterized by a unique mixture of depressant andstimulant effects in the CNS [113]. In humans, THC orcannabis consumption, respectively, is usually described as apleasant and relaxing experience. Use in a social contextmay result in laughter and talkativeness. Occasionally, thereare unpleasant feelings such as anxiety that may escalate topanic. A sense of enhanced well-being may alternate withdysphoric phases. THC improves taste responsiveness andenhances the sensory appeal of foods [35]. It may inducesleep [130, 131].
Acute THC intoxication impairs learning and memory[132-134], and adversely affects psychomotor and cognitiveperformance [135], reducing the ability to drive a car and tooperate machinery. Reduced reaction time also affects theiris constriction response of the eye. A brief light flashshows decreased amplitude of constriction and a deceleratedvelocity of constriction and dilation [136]. Tolerance maydevelop to the impairment of psychomotor and cognitiveperformance with long-term use. In a study that comparedinformation processing in heavy cannabis users with non-users there was no difference when users were under theinfluence of the drug, while there were significantly slowedinformation-processing speeds in the subacute phase in thecannabis users, which may be attributed to withdrawal [137].
The most conspicuous psychological effects of THC inhumans have been divided into four groups: affective(euphoria and easy laughter), sensory (increased perceptionof external stimuli and of the person’s own body), somatic(feeling of the body floating or sinking in the bed), andcognitive (distortion of time perception, memory lapses,difficulty in concentration) [138].
These effects only appear if an individually variablethreshold of dose is exceeded. During a study on the efficacyof dronabinol (THC) in 24 patients with Tourette's syndromewho received up to 10 mg THC daily for 6 weeks nodetrimental effects were seen on neuropsychological
performance (learning, recall of word lists, visual memory,divided attention) [139].
Stress, Anxiety and Fear
The endocannabinoid system may serve as a novelapproach to the treatment of anxiety-related disorders.Endocannabinoid signaling negatively modulates thefunction of the hypothalamic-pituitary-adrenal axis in acontext-dependent manner [140]. Upon exposure of mice toacute stress, hypothalamic 2-arachidonoyl glycerol contentwas reduced compared with the control value; however, after5 days of stress, which resulted in an attenuatedcorticosterone response, the hypothalamic 2-AG content wasincreased compared with the control value. The CB1 receptoragonist CP55940 reduced blood corticosterone levels instressed mice, while a CB1 receptor antagonist increasedcorticosterone concentrations [140]. A similar effect wasachieved by the administration of the putativeendocannabinoid transport inhibitor AM404, or the FAAHinhibitor URB597. Another group observed reducedhippocampal 2-AG levels following chronic stress [141].Chronic stress impaired reversal learning and inducedperseveratory behavior in the Morris water maze, animpairment that was reversed by exogenous cannabinoidadministration, suggesting deficient endocannabinoidsignaling. Data on the effects of cannabinoids on stress areconflicting. In humans, cannabis may cause anxiety andpanic, reactions that may be potentiated by stress. Animalresearch demonstrated that there may be a synergisticinteraction between stress and CB1 receptor agonists (THC,CP55940) in the effects on amygdalar activity [142].
CB1-deficient mice showed strongly impaired short-termand long-term extinction of aversive memories [143], andblockade of the CB1 receptor in rats led to a dose-dependentdecrease in extinction of conditioned fear [144]. Theseeffects may explain the anxiety reducing effects inposttraumatic stress disorder and similar conditions.However, in contrast to AM404, which enhanced theextinction of conditioned fear, the administration of the CB1
agonist WIN 55,212-2 did not appear to affect extinction[144].
Neuroprotection
One important physiological role of endocannabinoidsseems to be neuroprotection [145]. Ischemia and hypoxia inthe CNS induce abnormal glutamate hyperactivity and otherprocesses that cause neuronal damage. These processes alsoplay a role in chronic neurodegenerative diseases such asParkinson’s and Alzheimer’s disease and multiple sclerosis.Neuroprotective mediators are also released in ischemia andhypoxia, including anandamide and 2-AG. When these twoendocannabinoids were administered after traumatic braininjury in animals, they reduced brain damage [145].Neuroprotective cannabinoid mechanisms observed inanimal studies include reduction of glutamate toxicity byinhibition of excessive glutamate production, inhibition ofcalcium influx into cells, anti-oxidant properties whichreduce damage caused by oxygen radicals and modulation ofvascular tone [99, 116, 146]. Cannabinoids reduce braininflammation. The CB agonist WIN55,212-2 reduced theproduction of several key inflammatory mediators byactivated human astrocytes, including NO (nitric oxide),
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TNF-alpha (tumor necrosis factor alpha), and severalchemokines [147]. CB1 receptors seem to play a major rolein neuroprotection by endocannabinoids, since CB1(-/-) miceshowed little spontaneous recovery after closed head injurycompared to controls [148]. Receptor-stimulated inhibitionof protein kinase A was reported to be required for theneuroprotective effects of CB1 receptor activation [149].
THC was neuroprotective in rats given the toxic agentouabain. THC treated animals showed reduced volume ofedema by 22% in the acute phase and 36% less nervedamage after 7 days [150]. CB1 and CB2 receptor agonistsreduce amyloid-beta toxicity in vitro and in vivo [151, 152].Enhanced amyloid-beta peptide deposition along with gliacell activation in senile plaques plays a major role in thepathology of Alzheimer's disease (AD). CB1 positiveneurons are greatly reduced in areas of microglial activation,and CB1 receptor protein expression is markedly decreasedin AD brains [152]. Amyloid-beta induced activation ofmicroglial cells, cognitive impairment and loss of neuronalmarkers was prevented by cannabinoids in rats [152].
Circulatory System
THC can induce tachycardia [138] and increase cardiacoutput with increased cardiac labor and oxygen demand[153]. It can also produce peripheral vasodilation, orthostatichypotension [125, 154] and reduced platelet aggregation[155]. Data on cerebral blood flow effects are contradictory.Regional increases and decreases of blood flow with nomean change of flow were reported by one group [156],while a recent report suggests that systolic velocity and thepulsatility index, a measure of cerebrovascular resistance,were significantly increased in cannabis users [157]. Theseeffects persisted in heavy users for more than one month ofmonitored abstinence and were regarded as a partialexplanation for cognitive deficits in heavy cannabis users.
In young healthy subjects, the heart is under control ofthe vagus that mediates bradycardia. Tachycardia by THCmay easily be explained by vagal inhibition (inhibitedrelease of acetylcholine) through presynaptic CB1 receptors[158], which can be attenuated by beta-blockers [138] andblocked by the selective CB1 antagonist SR141716A [159].Regular use can lead to bradycardia [154]. Theendocannabinoid system plays an important role in thecontrol of blood pressure. Hypotension is mediated bycentral inhibition of the sympathetic nervous system,apparently by activation of CB1 receptors, since this effectcan also be prevented by a CB1 antagonist [160].Endocannabinoids are produced by the vascularendothelium, circulating macrophages and platelets [161].Vascular resistance in the coronaries and the brain is loweredprimarily by direct activation of vascular cannabinoid CB1
receptors [162].
Appetite, Eating and Digestion
The endogenous cannabinoid system plays a critical rolein milk ingestion of new-born mice [50]. Blockade of theCB1 receptor results in death of new-borns [163].Anandamide induces over-eating in rats through a CB1
receptor mediated mechanism [164]. Endocannabinoids inthe hypothalamus are part of the brain's complex system for
controlling appetite including a significant component that isregulated by leptin [83]. Leptin is a major signal throughwhich the hypothalamus senses nutritional state andmodulates food intake and energy balance. Leptin reducesfood intake by upregulating appetite-reducing neuropeptides,such as alpha-melanocyte-stimulating hormone, anddownregulating appetite-stimulating factors, primarilyneuropeptide Y. In animal research, reduced levels of leptinwere associated with elevated levels of endocannabinoids inthe hypothalamus, and application of leptin reducedendocannabinoid levels [83]. Cannabinoid-induced eating isascribed to an increase of the incentive value of food [165].
Cannabinoid agonists inhibit gastrointestinal motility andgastric emptying in rats [166]. In a study with humans, THCcaused a significant delay in gastric emptying [167]. Inaddition, CB agonists inhibited pentagastrin-induced gastricacid secretion in the rat [168], mediated by suppression ofvagal drive to the stomach through activation of peripheralCB1 receptors [169, 170].
Immune System
Animal and cell experiments have demonstrated thatTHC exerts complex effects on cellular and humoralimmunity [171, 172]. It is not clear; to what extent theseeffects are of clinical relevance in humans with respect tobeneficial (inflammation [173, 174], allergies [175],autoimmune processes [172]) and undesirable effects(decreased resistance towards pathogens and carcinogens)[171]. THC was shown to modulate the immune response ofT lymphocytes [176]. It suppressed the proliferation of Tcells and changed the balance of T helper 1 (Th1) and Thelper 2 (Th2) cytokines. It decreased the pro-inflammatoryTh1 reaction (e.g. the production of interferon-gamma) andincreased the Th2 reaction. This may explain why THC iseffective against inflammation with a strong Th1 reaction,e.g. in multiple sclerosis, Crohn's disease and arthritis. Theregulation of the activation and balance of human Th1/Th2
cells seems to be mediated by a CB2 receptor-dependentpathway [176].
Additional Organ Systems and Effects
Antiviral actions. Incubation with THC reduced theinfectious potency of herpes simplex viruses [177].Micromolar concentrations of THC inhibit Kaposi's SarcomaAssociated Herpes virus and Epstein-Barr virus reactivationin virus infected B cells [178].
THC also strongly inhibited lytic replication of severaloncogenic viruses in vitro [178].
Bones and cartilage. Preliminary observations presentedby Mechoulam et al. in 2003 at the First EuropeanWorkshop on Cannabinoid Research in Madrid, Spain, showthat endocannabinoids seem to stimulate bone formation.During differentiation, osteoblast precursor cells haveprogressively increased levels of CB2 but not of CB1 asmeasured by reverse transcription polymerase chain reaction.In addition, normal mice treated systematically with 2-AGshowed a dose dependent increase in trabecular boneformation. The peptide leptin is not only known tonegatively regulate endocannabinoid activity in appetite
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control, but also to influence osteoblastic activity. Results ofexperimental research by another group suggest that somecannabinoids may prevent cartilage resorption, in part, byinhibiting proteoglycan degradation and also by inhibitingcytokine production of chondrocytes induced by the freeradical nitric oxide (NO) [179].
Eye. The evidence of cannabinoid receptors at differentsites (anterior eye, retina, corneal epithelium) suggests thatcannabinoids influence different physiological functions inthe human eye [180]. Vasodilation in the eye is observed asconjunctival reddening after THC exposure [113]. THC andsome other cannabinoids decrease intraocular pressure [180,181]. CB1 receptors in the eye are involved in this effect,while CB2 receptor agonists do not reduce intraocularpressure [182].
Genetic and cell metabolism. THC can inhibit DNA,RNA, and protein synthesis, and can influence the cell cycle.However, very high doses are required to produce this effectin vitro [183]. Cannabinoid agonists inhibited human breastcancer cell proliferation in vitro [184, 185], and, directlyapplied at the tumor site, showed antineoplastic activityagainst malignant gliomas in rats [186].
Hormonal system and fertility. THC interacts with thehypothalamic-pituitary adrenal axis influencing numeroushormonal processes [187]. Minor changes in humanhormone levels due to acute cannabis or THC ingestionusually remain in the normal range [125]. Tolerancedevelops to these effects, however, and even regularcannabis users demonstrate normal hormone levels.
Pregnancy. Anandamide levels during pregnancy show acharacteristic pattern [188]. Mean plasma levels were 0.9 nmin the first trimester and 0.4 nm in the second and thirdtrimester. During labor, anandamide plasma levels rose to2.5 nm. Postmenopausal and luteal-phase levels were similarto those in the first trimester. It is currently unclear whetherimplantation of the embryo can be disrupted by THC.
Sperm. After several weeks of daily smoking 8-10cannabis cigarettes, a slight decrease in sperm count wasobserved in humans, without impairment of their function[189]. In animal studies high doses of cannabinoids inhibitedthe acrosome reaction [190].
5.2. Endocannabinoids and Inhibitors ofEndocannabinoid Degradation
Several similarities have been described betweenexogenous cannabinoid receptor agonists andendocannabinoids, with regard to their pharmacology andmedicinal effects [26]. Inhibitors of endocannabinoiddegradation may be promising candidates for therapeuticmodulation of the endocannabinoid system.
FAAH inhibitors produced analgesia in animal models[191] and AM404 reduced the expression of c-fos, a markerof activated neurons in an experimental model ofneuropathic pain [192]. In the latter study, both CB1 and CB2
receptors as well as vanilloid receptors were involved in theobserved effect. Another group demonstrated that blockersof anandamide hydrolysis were able to reduce anxiety inanimal tests [193]. These benzodiazepine-like propertieswere accompanied by augmented brain levels of anandamide
and were prevented by CB1 receptor blockade. Recently, ithas been shown that two selective inhibitors of the putativeendocannabinoid transporter and hence of endocannabinoidinactivation, provide an effective therapy for Theiler murineencephalomyelitis, a virus-induced demyelinating diseaseand an animal model of multiple sclerosis [194]. Treatmentof infected mice with the transport inhibitors OMDM1 andOMDM2 enhanced anandamide levels in the spinal cord,ameliorated motor symptoms and decreased inflammatoryresponses. This effect resembles that of exogenouscannabinoid receptor agonists [195].
However, there are several differences in thepharmacology of exogenous cannabinoids andendocannabinoids [196]. An animal study found cross-tolerance between THC and anandamide for antinociception,but not for suppression of spontaneous activity, catalepsyand hypothermia in mice tolerant to THC [196]. Theseresults suggest that the pharmacology of anandamide onlypartially overlaps with that of THC and other exogenouscannabinoids.
Some non CB effects of anandamide may be mediated byvanilloid receptors. For example, inhibition of cellproliferation of rat C6 glioma cells by endocannabinoids wasreported to involve combined activation of both vanilloidreceptors and to lesser extent, cannabinoid receptors [197].The vasodilation caused by anandamide in the splanchnicarteries was also reported to be mediated by both CB1 andvanilloid receptors [198].
While anandamide and 2-AG have similar affinities tothe CB1 and to the CB2 receptor, there seem to be differencesin their pharmacology. 2-arachidonoyl glycerol was shownto induce the migration of natural killer cells, which wasabolished by treatment with a CB2 receptor antagonist(SR144528) [199]. In contrast to 2-AG, anandamide andTHC did not induce the migration. In fact, the combinedapplication of THC and 2-AG abolished the migrationinduced by the latter [199]. Experiments with rathippocampal slices provide another example for differencesin the effects of anandamide and 2-AG [63]. While 2-AGreduced paired-pulse depression of population spikesmediated by CB1 receptors, anandamide increased paired-pulse depression by acting on hippocampal vanilloidreceptors.
5.3. Cannabidiol
The mode of action of cannabidiol is not fully understoodand several mechanisms have been proposed: (1) CBD actsas antagonist at the central CB1 receptor and was able toinhibit several CB1 mediated THC effects [200]. In a studyby Petitet et al. (1998) [201], CBD considerably reduced thereceptor activation by the potent classical CB1 receptoragonist CP55940. (2) CBD stimulates the vanilloid receptortype 1 with a maximum effect similar in efficacy to that ofcapsaicin [202]. (3) CBD inhibits the uptake and hydrolysisof the endocannabinoid anandamide, thus increasing itsconcentration [202, 203]. (4) Finally, CBD may also increasethe plasma THC level [204] by inhibiting hepaticmicrosomal THC metabolism through inactivation of thecytochrome P-450 oxidative system [205, 206]. However,there was no or minimal effect of CBD on plasma levels ofTHC in man [207, 208].
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In a study that analyzed the mode of action of the anti-inflammatory and anti-hyperalgesic effects of CBD,simultaneous administration of a VR1 receptor antagonistfully reversed the anti-hyperalgesic effects [209]. A CB2
receptor antagonist was partly effective and a CB1 receptorantagonist had no effect. The anti-inflammatory efficacy ofCBD was unrelated to cyclooxygenase (COX) activity, butCBD inhibited the endothelial isoform of nitric oxidesynthase (eNOS). In a rat model of arthritis, low doses ofCBD decreased prostaglandin E2, nitric oxide and lipidperoxide level, mediators that are all known to be involvedin the development and maintenance of arthritis [209].
CBD exerts sedating [210], anti-epileptic [211], anti-dystonic [212], anti-emetic [213], and anti-inflammatory[214, 215] effects. It reduced intraocular pressure [216], wasneuroprotective [116], and antagonized the psychotropic andseveral other effects of THC [200].
5.4. THC Metabolites and Derivatives
After intravenous administration in humans, 11-OH-THCwas equipotent to THC in causing psychic effects andreduction in intraocular pressure [94]. In somepharmacological animal tests, 11-OH-THC was 3 to 7 timesmore potent than THC [217].
THC-COOH possesses anti-inflammatory and analgesicproperties by mechanisms similar to non-steroidal anti-inflammatory drugs (NSAIDs) [218-220]. Typical propertiesof NSAIDs apparently not shared by THC-11-oic acid arethe major adverse effects of NSAIDs, namely,gastrointestinal and kidney toxicity [100]. The precise basisfor this difference is not well-understood; however, it may bepartly due to a selective inhibition by THC-11 oic acid forCOX-2 vs COX-1 (cyclooxygenase). THC-COOHantagonizes some effects of the parent drug through anunknown mechanism, e.g. the cataleptic effect in mice [221].Ajulemic acid (CT3), a synthetic derivative of THC-COOH,shows a similar pharmacological profile as the naturalsubstance. Recently, a possible mechanism of action wasproposed for this derivative [222]. Ajulemic acid bindsdirectly and specifically to the peroxisome proliferator-act ivated receptor gamma (PPARgamma), apharmacologically important member of the nuclear receptorsuperfamily. In addition, it was shown that ajulemic acidinhibited interleukin-8 promoter activity in a PPARgamma-dependent manner, suggesting a link between the anti-inflammatory action of the cannabinoid acid and theactivation of PPARgamma. Additionally, CT3 binds to CB1
and CB2 receptors, but has a more limited brain access and issomewhat better tolerated than THC [223].
5.5. Antagonists
SR141716A was able to block the psychological andphysiological effects of THC in humans in a dose-dependentmanner [159]. Three mechanisms are proposed to accountfor inverse cannabimimetic effects of antagonists, (1)antagonism of endocannabinoids, (2) modulation of CB1
receptors, possibly through an allosteric mechanism shiftingthem from a constitutively active state to an inactive state,(3) CB1 receptor-independent mechanisms, for exampleantagonism at A1 receptors of endogenous adenosine [224].
Experiments with mice lacking CB1 receptors (CB1-/- mice)or with tissues from CB1-/- mice suggest that rimonabant(SR141716A) produces at least some of its effects bybinding to CB1 receptors. CB1 receptor knockout mice eatless than their wild-type littermates [83]. SR141716A hasbeen found to reduce food intake in CB1+/+ mice but not inCB1-/- mice [224].
Reviews of the pharmacological effects of cannabinoidreceptor antagonists are presented by Pertwee (2005) andLange and Kruse (2004) [110, 224]. Among the effects ofCB1 antagonists reported are increased locomotor activity inrats and mice, improvement of memory in rats and mice,increased nociception (inflammatory and neuropathic pain orallodynia) in rats and mice, evocation of emesis in shrews,spasticity and tremor in a mouse model of multiple sclerosis,accelerated intestinal transit in rats and mice, increase ofseverity of induced colitis in mice, and decrease of milk-ingestion and growth in mouse pups. Schlicker andKathmann (2001) reviewed the inverse cannabimimeticeffects of SR141716A on neurotransmitter release in vitro,among them being the evocation of acetylcholine release,noradrenaline release, GABA release, dopamine release,glutamate release, and D-aspartate release in several braintissues in rats, mice and guinea-pigs [225].
The effects of physiological or increasedendocannabinoid levels may be reduced by the application ofCB antagonists as proven in several animal models.Rimonabant may produce hyperalgesia due to the blockadeof endocannabinoid effects [80]. Increased anandamide and2-arachidonoyl glycerol levels are observed in basolateralamygdala complex of auditory fear-conditioned mice.Endocannabinoids are thought to promote extinction ofaversive memories in these animals, a process that wasimpaired by the application of rimonabant [143].Endocannabinoid concentrations are also elevated in thebrain and spinal cord of spastic mice with chronic relapsingexperimental allergic encephalomyelitis (CREAE).Spasticity in these animals may be exacerbated by the CB1
receptor antagonist [73].
6. THERAPEUTIC POTENTIAL
Cannabis preparations and single cannabinoids have beenemployed in the treatment of numerous diseases [123, 126,127, 226]. Besides phytocannabinoids, several syntheticcannabinoid derivatives are under clinical investigation thatare devoid of psychotropic effects or have fewer side effects.Antagonists at the CB1 receptor have followed and inhibitorsof endocannabinoid degradation will presumably follow.
6.1. Cannabinoid Receptor Agonists
Clinical studies with single cannabinoids and whole plantpreparations (smoked cannabis, encapsulated cannabisextract, sublingual liquid preparations) have often beeninspired by anecdotal experiences of patients employingcrude cannabis products. The anti-emetic [227], and theappetite enhancing effects [228], muscle relaxation [229],analgesia [230], and therapeutic use in Tourette's syndrome[231] were all discovered or re-discovered in this manner.
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Incidental observations have also revealed therapeuticallyuseful effects. This occurred in a study of Volicer et al.(1997) in patients with Alzheimer's disease, wherein theprimary issue was an examination of the appetite-stimulatingeffects of
9-THC [232]. Not only appetite and body weight
increased, but disturbed behavior among the patients alsodecreased following the intake of the drug. The discovery ofdecreased intraocular pressure with THC administration inthe beginning of the 1970s was also serendipitous [233],when several research groups screened for effects ofcannabis on the human body. The investigation of anti-cancer effects of THC and other cannabinoids wasstimulated by a long-term animal study in rats and mice thatstudied possible cancer causing actions of THC [234].
Several surveys have shown that cannabis is self-administered to treat a broad range of chronic illnesses. Thelargest of these investigations with 2969 returnedquestionnaires conducted in the United Kingdom revealedthat cannabis was used by a considerable percentage ofpatients, in chronic pain (25%), multiple sclerosis (22%),depression (22%), arthritis (21%) and neuropathy (19%)[235]. Sixty-eight per cent said cannabis considerably easedtheir symptoms. Another survey among 252 HIV patients, ofwhom 175 (69%) were on antiretroviral therapy (ART)found that those who suffered from nausea were about threetimes more likely to adhere to ART if they used cannabiscompared to non-users [236].
Several states of the United States allow the medical useof cannabis and some of them have established anidentification card system. In Oregon, about 10,000 subjectsare registered card-holders with recommendations by about1,500 physicians (http://www.dhs.state.or.us/publichealth/mm/data.cfm). Since September 2003, cannabis is availablein Dutch pharmacies, distributed by the Office of MedicinalCannabis of the Health Ministry. In Canada, patients mayapply to the Health Ministry for a legal use of cannabis onthe basis of recommendations by their physicians.
Besides THC (Marinol , Solvay Pharmaceuticals),several whole cannabis plant preparations are currently underclinical investigation, including Cannador of the Institutefor Clinical Research in Berlin, Germany, a capsulatedcannabis extract, and Sativex of the British company GWPharmaceuticals, an under the tongue spray. Cannador andSativex are both standardized on THC and CBD content,Sativex containing equal amounts of the two cannabinoidsand Cannador containing more THC than CBD. At theCenter for Medicinal Cannabis Research at the University ofCalifornia, the therapeutic potential of smoked cannabis andinhaled cannabis by means of a vaporizer are investigated.
Hierarchy of Therapeutic Effects
Possible indications for cannabis preparations have beenextensively reviewed [123, 126, 127, 226, 237-240]. To dojustice to the scientific evidence with regard to differentindications, a hierarchy of therapeutic effects can be devised,with established effects, effects with clinical and preclinicalconfirmation and effects based on preclinical, mechanisticstudies. However the history of research into the therapeuticbenefits of cannabis and cannabinoids has demonstrated thatthe scientific evidence for a specific indication does not
necessarily reflect the actual therapeutic potential for a givendisease.
Established Effects
9-THC (dronabinol, Marinol) is approved in several
countries for medicinal use in refractory nausea andvomiting caused by antineoplastic drugs used for thetreatment of cancer (for review see: [112]) and for appetiteloss in anorexia and cachexia of HIV/AIDS patients [228,241, 242]. These effects can be regarded as establishedeffects for THC and cannabis. Nabilone (Cesamet ) is thesecond cannabinoid available on prescription, against nauseaand vomiting associated with cancer chemotherapy.
In more than 30 studies, THC and nabilone have beenshown to have a similar anti-emetic efficacy as thephenothiazines [112]. In the 1980s, several clinical studieswith smoked cannabis have been performed in the USA, inwhich smoked cannabis was effective similar as THC (forreview see: Musty & Rossi 2001). In the Lynn PiersonResearch Program of New Mexico, 256 subjects whounderwent chemotherapy received either THC or smokedcannabis, both drugs reduced nausea from 4.5 to 2.0 onaverage on a scale between 1 (no problem) and 5 (severe)and emesis from 4.3 to 1.7 on average [243]. There are noclinical studies comparing cannabinoids and 5-HT3
(serotonin) antagonists, but a study with healthy subjectsshowed that ondansetron was significantly more effectivethan smoked cannabis [244]. Nausea and emesis wereinduced by syrup of ipecac. Cannabis significantly reducedratings of nausea and slightly reduced the incidence ofvomiting compared to placebo, while ondansetroncompletely eliminated the emetic effects of ipecac. In aclinical setting, 5-HT3 antagonists are usually superior toTHC, but the cannabinoid has proven to be effective at leastin some cases of intractable nausea and vomiting [245].Animal research demonstrated that THC reinforces the anti-emetic effects of ondansetron in vomiting produced bycisplatin [246], suggesting that a combination of both drugsmay be meaningful in clinical practice.
THC is effective in AIDS wasting and cancer cachexia[247]. THC (2 x 2.5 mg) was less effective than megestrolacetate (800 mg daily). However, in these and some otherstudies, THC or cannabis may have been underdosed. Itappears that 10 to 20 mg of THC are necessary for thetreatment of weight loss [248]. In HIV-patients withsignificant loss of muscle mass, oral THC (10, 20 and 30mg) and smoked cannabis (1.8, 2.8 and 3.9% THC) caused asignificantly increased caloric intake compared to controls(HIV-patients without weight loss), but the highest THCdose was not well-tolerated.
Neuropathic pain seems to be the next indication, whichcan be regarded as established for a treatment with cannabis-based medicines. In 2005 Sativex received approval byHealth Canada for relief of neuropathic pain in multiplesclerosis. Several case reports and small clinical studiesindicate that THC and cannabis may be effective in treatingseveral conditions of chronic pain [249-252, 254, 255]. Areview of 2001 concluded that cannabinoids are no moreeffective than codeine in controlling chronic pain [256], anda commentary noted that cannabinoids may have potential intreating neuropathic pains, particularly those with spastic
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components [257]. A therapeutic potential in neuropathicpain is supported by experimental data on anti-hyperalgesicproperties of cannabinoid receptor agonists [258]. Later andstudies with a larger patient population confirmed thisassumption. Berman et al. (2004) observed a significantdecrease in pain and improved sleep by two differentcannabis extracts in 48 patients with neuropathic pain frombrachial plexus avulsion [259]. In addition, there are severalreports of pain reduction in multiple sclerosis by THC [260,261] and cannabis [261, 262]. One recent study observedsignificant analgesic effects of THC and Sativex in chronicpain of different origins (multiple sclerosis, spinal cordinjury, brachial plexus avulsion, stiff-man syndrome, etc.)[263]. In contrast to these studies, Attal et al. (2004) onlyfound a therapeutic effect of oral THC in one of sevenpatients with chronic refractory neuropathic pain [264].
Cannabinoid receptor agonists and opiates were shown toact synergistically in animal models of pain [265-267], arationale for the observation of additive effects of THC andmorphine in pain therapy.
Effects with Clinical and Preclinical Confirmation
Several small studies in the 1980s and 1990s investigatedthe therapeutic potential of THC, cannabis and nabilone inspasticity due to spinal cord injury [25, 255, 268] andmultiple sclerosis [255, 268- 272]. This indication is anothertarget of current interest for cannabis-based drugs [38, 261273-277]. Spasticity is difficult to treat with available drugs,as reviewed by Shakespeare et al. (2000) [278]. Onlyintrathecal application of baclofen resulted in a significantimprovement in spasticity scores according to the AshworthScale and activities of daily living (ADL). Tizanidineresulted in improved spasticity but did not improve ADL,and side effects (sleepiness, dry mouth).
Recent larger placebo controlled trials for theinvestigation of the efficacy of cannabis and THC inspasticity were restricted to multiple sclerosis patients [261,273, 275-277], and there is only one small study oncannabinoids in spasticity due to spinal cord injuryconducted in the past few years [38]. Killestein et al. (2002)were unable to find any benefits of THC and Cannador inmultiple sclerosis patients with severe spasticity, but dosesapplied (2 x 2.5 mg or 2 x 5 mg THC) were probably too lowto get the desired therapeutic effects [274]. Other studiesgenerally indicate significant effects in subjective ratings,and not in objective spasticity scores [261, 275, 276].Preliminary results of a first long term study of THC andCannador in multiple sclerosis suggest that THC may havelong-term beneficial effects on the course of the disease.Results of the short-term trial (15 weeks) with eligible 630patients are conflicting [408]. Eighty per cent of the originalstudy population participated in a 12 months follow-upstudy. In the 15-week study, 657 patients with stablemultiple sclerosis and muscle spasticity received a maximumdaily dose of 10-25 mg THC as single agent or in a cannabisextract. There was no significant effect of cannabinoids onobjective spasticity scores according to the Ashworth scale,but patients reported subjective improvements in pain andspasticity. In the long-term study, there was a significantimprovement of spasticity scores in the THC group.
Cannabinoids may also be effective in some othermovement disorders, including Tourette's syndrome,dystonia and levodopa-induced dyskinesia [17, 229, 279-283]. Results were conflicting in two small clinical trials oncannabinoids in levodopa-induced dyskinesia in Parkinson'sdisease. While a study with nabilone (n=7) significantlyimproved dyskinesia [284], a cannabis extract wasineffective in another investigation (n=19) [285].
In 1971, during a systematic investigation of its effects inhealthy cannabis users, cannabis reduced intraocularpressure, suggesting a therapeutic potential in glaucoma. Inthe following 12 years, a number of studies in healthyindividuals and glaucoma patients with cannabis and severalnatural and synthetic cannabinoids were conducted [233,286-289]. Cannabis decreased intraocular pressure by anaverage 25-30%, occasionally up to 50%, the effect lasting4-6 h. Neuroprotective benefits of cannabinoids may be ofadditional value in preventing damage to the optical nerve[180].
Experiments examining the anti-asthmatic effect of THCor cannabis in healthy and asthmatic subjects date mainlyfrom the 1970s, and are all acute studies [290-292]. Theeffects of a cannabis cigarette (2% THC) or oral THC (15mg), respectively, approximately correspond to thoseobtained with therapeutic doses of common bronchodilatordrugs (salbutamol, isoprenaline). Experimental researchsuggests that cannabinoid receptor agonists possess furtherproperties that may be of value in asthma. They inhibitedcapsaicin-induced bronchial smooth muscle contraction andreduced inflammation of the respiratory tract by blocking therelease of the inflammatory cytokine tachykinin [293].Several phytocannabinoids possess an anti-allergic potential.THC and cannabinol attenuated the increase of theinterleukins IL-2, IL-4, IL-5, and IL-13 in reaction tosensitization with ovalbumin in mice. In addition, theelevation of serum IgE and the mucus overproductioninduced by ovalbumin was markedly attenuated by the twocannabinoids [175].
A few studies investigated anxiolytic properties ofnabilone [39, 294, 295]. In one of these investigations, 25outpatients suffering from anxiety received either placebo ornabilone over a 28-day treatment period [39]. Those treatedwith the verum showed a dramatic improvement in anxiety.
THC was shown to reduce agitation in patients withAlzheimer's disease. Results of a small placebo controlledtrial [232] were confirmed by Ross and Shua-Haim in aphase II open label parallel-group study involving 54patients. The latter findings were presented at the AmericanSociety of Consultant Pharmacists' annual meeting inNovember 2003. Evaluation at nine weeks of treatment with2 x 2.5 or 2 x 5 mg THC found significant reductions ofagitation scores in both groups. There also was a trendtowards a decrease in the caregiver burden scores.
In addition to the indications, above, there are severalindications, in which benefits are generally implied in casereports. These include allergies [296], inflammation [127],epilepsy [297], intractable hiccups [298], depression [241],bipolar disorders [299], dependency to opiates and alcohol[296, 300,], withdrawal symptoms [300], and post-traumatic
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stress disorder [127]. A first clinical study on THC in post-traumatic stress disorder of Israeli soldiers is currently underway.
Preclinical, Mechanistic Studies
Basic research shows promising possible futuretherapeutic uses, among them being neuroprotection inhypoxia and ischemia [99, 116]. Some immunologicalmechanisms of THC hint to possible benefits in autoimmunediseases, such as multiple sclerosis, arthritis, and Crohn'sdisease [172]. In a murine model of multiple sclerosis,cannabinoids significantly improved the neurological deficitsin a long-lasting way. On a histological level, they reducedmicroglial activation and decreased the number of CD4+infiltrating T cells in the spinal cord [301]. Another groupfound that amelioration of clinical disease in the samemultiple sclerosis model was associated with downregulationof myelin epitope-specific Th1 effector functions (delayed-type hypersensitivity and IFN-gamma production) and theinhibition of the proinflammatory cytokines, TNF-alpha,interleukin 1-beta, and interleukin-6 [302]. Raman et al.(2004) reported that treatment with THC was effective in amouse model of amyotrophic lateral sclerosis if administeredeither before or after onset of disease signs, which theyattributed to the reduction of oxidative damage and anti-excitotoxic effects of the cannabinoid in vitro [303].
Anti-neoplastic activity of THC came into the focus ofresearch after a long-term animal study, designed toinvestigate THC’s potential carcinogenicity, resulted inbetter survival of rats dosed with THC than controls due tolower incidence for several types of cancer [234]. Frequencyof testicular interstitial cell, pancreas and pituitary glandadenomas in male rats, mammary gland fibroadenoma anduterus stromal polyp in female rats was reduced in a dose-related manner. Later studies showed that cannabinoidsexerted antineoplastic activity in malignant gliomas [12,197], malignant skin tumors [304], colorectal cancer [305]and prostate cancer [306]. CB1 and CB2 receptor agonistswere both effective in some of the studies. Cannabinoidsseem to be able to control the cell survival/death decision[307]. Thus, cannabinoids may induce proliferation, growtharrest, or apoptosis in a number of cells depending on dose[307]. Cannabinoids were also shown to inhibit angiogenesisof malignant gliomas by at least two mechanisms, directinhibition of vascular endothelial cell migration and survivalas well as the decrease of the expression of proangiogenicfactors [308]. On the other hand it should be noted that theimmunosuppressive and proliferative action of THC andother cannabinoids may have unfavorable consequences oncancer growth, especially if tumours do not expresscannabinoid receptors [309, 310]. A first human phase I-IItrial to investigate the tolerability and efficacy ofintracranially applied THC in glioblastoma multiforme isunder way in Spain.
Other fields of research with CB agonists includedisorders of circulation and blood pressure [162, 311]. Inrats, daily application of a CB1 agonist after experimentalinfarction prevented signs of heart failure, endothelialdysfunction and hypotension, however, the cannabinoid alsoincreased left-ventricular end-diastolic pressure, which maybe negative in the long run [312].
Several effects observed in animal studies provide thebasis for further research, among them are effects againstdiarrhea in mice [313] and stabilization of respiration insleep-related breathing disorders (e.g. apnea) [314].Cannabinoids were effective in an animal model of attention-deficit hyperactivity disorder (ADHD), in the spontaneouslyhypertensive-rat (SHR) [315]. A very impulsive subgroup ofSHR rats presented a reduced density of CB1 cannabinoidreceptors in the prefrontal cortex of the brain. Theadministration of WIN55,212-2 normalized the impulsivebehavioral profile in this subgroup, but had no effect oncontrols.
6.2. Other Cannabinoids
Two other cannabinoids under clinical investigation areHU-211, which is also called dexanabinol, and CT3 orajulemic acid, which is now called IP751 by IndevusPharmaceuticals. The cannabis extracts Cannador andSativex contain considerable amounts of cannabidiol (CBD).
Cannabidiol
The rationale for the combination of THC and CBD isthe observation that CBD reduced the psychic side effects ofTHC, which may result in an improved tolerance of the drug.Simultaneous oral administration of CBD (1 mg/kg) with ahigh THC dose (0.5 mg/kg) in healthy volunteers attenuatedthe anxiogenic and psychotomimetic symptoms induced byTHC, but not the increase in pulse-rate [200]. Other THCeffects including anti-emesis and anti-inflammation may beincreased, since these therapeutic properties are shared byCBD, but it cannot be excluded that some therapeutic effectsof THC may be antagonized as well.
In small human studies, CBD was shown to exert potentanxiolytic and anti-psychotic effects [210]. Oral CBD (300mg) attenuated anxiety induced in healthy volunteers byhaving them prepare a 4-minute speech to a similar extent asipsapirone (5 mg) and diazepam (10 mg) [316]. In a singlecase study, CBD in increasing doses up to 1500 mg/day wasas effective as an antipsychotic as haloperidol in aschizophrenic patient, who had significant hormonal sideeffects during treatment with a typical antipsychotic [210].
In an open clinical study with four patients sufferingfrom Huntington's disease, CBD (2 x 300 mg) reducedchoreic symptoms in three participants (Sandyk et al. 1988,cited from [92]). However, these effects could not beconfirmed in a double-blind crossover study with 15 patients[317]. Cannabidiol caused a 20 to 50 per cent reduction invarious dystonias of five patients [212]. Results from threecontrolled clinical studies on CBD in epilepsy were mixed,but the cannabinoid seems to have some anti-epilepticpotential [318-320].
Dexanabinol
Dexanabinol or HU-211 is an antagonist of the NMDAsubtype (NMDA = N-methyl-D-aspartic acid) of theglutamate receptor [99]. Glutamate is the main excitatoryneurotransmitter in the brain, and excessive activation ofglutamate receptors may mediate neuronal injury or death ina variety of pathological conditions, including stroke,mechanical brain trauma, hypoxia and variousneurodegenerative disorders, including Parkinson's
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syndrome, amyotrophic lateral sclerosis, neuropathic painsyndromes and perhaps Alzheimer's disease [321]. HU-211proved to be neuroprotective in several animal models,including closed head injury in rats [99], a model of focalcerebral ischemia in spontaneously hypertensive rats [322],and optic nerve crush injury in rats [323]. The cannabinoidalso reduced the consequences of experimental autoimmuneencephalomyelitis (EAE), a model of multiple sclerosis[324].
So far, HU-211 has been studied in humans for twoindications, in severe head trauma and for the prevention ofcognitive deficits following coronary artery bypass surgery.In a phase II double blind clinical trial in patients with severehead trauma, dexanabinol attenuated elevations ofintracranial pressure and reductions of cerebral perfusionpressure [325]. A consistent trend towards better overalloutcome as determined by the Glasgow outcome scale wasobserved in the severe patient subgroup. However, clinicalphase III studies did not confirm efficacy (Press release ofPharmos Corporation of 20 December 2004). A phase IIstudy of dexanabinol for the prevention of cognitive deficitsfollowing heart surgery is under way [326].
Ajulemic Acid (CT3, IP751)
The anti-inflammatory activity of CT3 was demonstratedin animal models of acute and chronic inflammation [100].The animals did not exhibit evidence of tolerance to ajulemicacid during 30 days of treatment; a clear divergence in themechanism of action between the cannabinoid acids andother cannabinoids. CT3 also markedly reduced thebehavioral responses to painful stimuli [100]. In unrelatedstudies, CT3 appeared to have potent anti-cancer effects[11]. According to a press release by IndevusPharmaceuticals on 7 March 2005, the drug was effective inan animal model of interstitial cystitis. It significantlyreduced the bladder overactivity associated with the disease,without affecting the normal voiding mechanism of thebladder.
In a first clinical study with 20 subjects who sufferedfrom chronic pain, CT3 proved to have a significantanalgesic effect without psychic side effects [223].
6.3. Cannabinoid Receptor Antagonists
A possible therapeutic potential of cannabinoid receptorantagonists was proposed for obesity [159], schizophrenia[159], in conditions with lowered blood pressure, e.g. livercirrhosis [162] and septic shock [327], Parkinson's disease[328], Huntington's disease [329], alcohol and nicotinedependency [330], heroine addiction [331], problems insexual behavior and sexual performance [110], asthma [110]and to improve memory in Alzheimer's disease [159]. Theidea that CB receptor antagonists may be useful in livercirrhosis follows the observation that rats with carbontetrachloride-induced liver cirrhosis present with low bloodpressure, which is elevated by SR141716A [332]. Theantagonist also reduced the elevated mesenteric blood flowand portal pressure. Compared with non-cirrhotic controls, incirrhotic human livers, there was a three-fold increase in CB1
receptors on isolated vascular endothelial cells [332].Recently, it was reported that the density of CB1 receptors
and the levels of anandamide and 2-AG in the dorsolateralprefrontal cortex of alcoholic suicides was higher than in acontrol group of chronic alcoholics suggesting ahyperactivity of endocannabinoidergic signaling in alcoholicsuicides and a therapeutic potential for CB1 antagonists[333].
Many of the studies on the therapeutic potential ofcannabinoid receptor antagonists are found in the basicresearch literature, and only one paper on clinical researchhas been published in peer-reviewed journals [334]. Thecompound most clinically advanced is rimonabant,registered in 2005 for approval in the US and Europe asAcomplia. Phase IIb clinical data showed that rimonabant indaily doses of 5, 10 and 20 mg caused a significant weightloss in a 16-week study (cited according to [335]). Twophase III trials were initiated for the treatment of obesity inAugust 2001 [336] and by September 2002 for smokingcessation [336]. Results of these trials showed that the drugsignificantly reduced body weight within one year oftreatment (5 or 20 mg rimonabant once daily). The lowerdose resulted in a mean weight reduction of 3.4 kg, thehigher dose in a mean reduction of 6.6 kg, while the placebogroup lost 1.8 kg on average. The higher dose also had asignificant positive effect on blood lipids. Results of thesmoking cessation study were presented at the 53
rd annual
scientific meeting of the American College of Cardiology inMarch 2004. According to a press release by Sanofi-Synthelabo, 700 smokers were treated either with 5 or 20 mgof rimonabant, and about a quarter of the subjects whoreceived 20 mg stopped smoking in 10 weeks, which wasabout twice the quit rate in the placebo group.
7. SIDE EFFECTS
7.1. Agonists
Adverse effects of medical cannabis use are within therange of effects tolerated for other medications [126, 127].Long-term medical use of cannabis for more than 15 yearshas been reported to be well-tolerated without significantphysical or cognitive impairment [337].
The median lethal dose (LD50) of oral THC in rats was800-1900 mg/kg depending on sex and strain [338]. Therewere no cases of death due to toxicity following themaximum oral THC dose in dogs (up to 3000 mg/kg THC)and monkeys (up to 9000 mg/kg THC) [338]. Acute fatalcases in humans have not been substantiated. However,myocardial infarction may be triggered by THC due toeffects on circulation [339, 340]. This is unlikely to happenin healthy subjects but in persons with coronary heart diseasefor whom orthostatic hypotension or a moderately increasedheart rate may pose a risk. The THC derivative nabilonereduced choreatic movements in Huntington`s disease in asingle case study [329]. Thus, cannabinoid receptor agonistsmay be contraindicated in Huntington's disease.
It is controversial whether heavy regular consumptionmay have a long-term negative impact on cognition [341-343], but this impairment seems to be minimal if it exists[341, 344]. Early users who started their use before the ageof 17 presented with poorer cognitive performance,
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especially verbal IQ compared to users who started later ornon-users [345].
Possible reasons for this difference may be (1) innatedifferences between groups in cognitive ability, antedatingfirst cannabis use; (2) a neurotoxic effect of cannabis on thedeveloping brain; or (3) poorer learning of conventionalcognitive skills by young cannabis users who have eschewedschool and university [345]. In a longitudinal study withyoung adults current heavy cannabis use was associated withreduced overall IQ, processing speed, immediate, anddelayed memory, while former heavy users showed nodifference to non-users [346]. In a twin study cannabis-usingtwins significantly differed from their non-using co-twins ingeneral intelligence [347]. However, this difference wasminimal and authors concluded that these results indicate anabsence of marked long-term residual effects of cannabis useon cognitive abilities. In a meta-analysis of studies thatinvestigated residual effects of cannabis on theneurocognitive performance of adult human subjects, chronicuse was associated with decrements in the ability to learnand remember new information, whereas other cognitiveabilities were unaffected [348]. There is conflicting evidencethat infants exposed to THC in utero suffer developmentaland cognitive impairment [349].
Cannabis can induce a schizophrenic psychosis invulnerable persons [124, 135], and there is increasingevidence that there is a distinct cannabis psychosis [350] andthat cannabis increases the risk to develop a psychosis bytwo-fold [351].
Cannabis use is associated with depression, suicidalthoughts and attempted suicide [352]. However, a study withdizygotic and monozygotic twins suggests that thisassociation may not be causal and that genetic vulnerabilitiesmake substantial contributions, since the association washigher in dizygotic than in identical twins [352]. Heavycannabis use has been reported to cause an "amotivationalsyndrome" [353]. A systematical examination of the mentalhealth of cannabis users suggests that symptoms associatedwith this syndrome are due to depression [353].
The consequences of the use of THC or cannabis bypatients with liver cirrhosis are unclear. In experimentalstudies, activation of the CB2 receptor was shown to causeantifibrogenic effects [354]. In liver biopsy specimens frompatients with active cirrhosis of various etiologies, CB2
receptors were expressed in non-parenchymal cells. Incontrast, CB2 receptors were not detected in normal humanliver. In cultured hepatic myofibroblasts and in activatedhepatic stellate cells activation of CB2 receptors triggeredpotent antifibrogenic effects, namely, growth inhibition andapoptosis. On the other, hand an epidemiological studypresented by Hezode et al. at the 39th Annual EuropeanAssociation for the Study of the Liver Conference in April2004 suggests that daily use of cannabis may promote thedevelopment of liver cirrhosis in persons with chronichepatitis C, while moderate use did not increase the risk. Theendocannabinoid system was proposed to play an importantrole in the vasodilated state in advanced cirrhosis, andanandamide is regarded as a selective splanchnic vasodilator[198]. Thus, the use of cannabinoids may have unfavorableeffects in advanced liver cirrhosis.
The harmful effects of combustion products produced bysmoking cannabis have to be distinguished from effects bycannabis or single cannabinoids [127]. The risks of smokingcannabis are probably similar to those from smoking otherdried plant material, including tobacco [355].
Tolerance develops to most of the THC effects [356],among them being the cardiovascular, psychological andskin hypothermic effects [357, 358], analgesia [359],immunosuppression [360], corticosteroid release [361], anddisruption of the hypothalamo-hypophyseal axis [362],causing alterations in endocannabinoid formation andcontents in the brain [363]. In a 30-day study, volunteersreceived daily doses of 210 mg oral THC and developedtolerance to cognitive and psychomotor impairment and tothe psychological high by the end of the study [357]. After afew days, an increased heart rate was replaced by a normalor a slowed heart rate. Tolerance develops also tocannabinoid-induced orthostatic hypotension [154].
Tolerance can mainly be attributed to pharmacodynamicchanges, presumably based on receptor downregulationand/or receptor desensitization [363, 364]. Rate and durationof tolerance varies with different effects. Rats receiving THCover a period of five days exhibited a decreased specificbinding ranging from 20 to 60% in different receptor sites ofthe brain compared to controls [356]. However, in anotherstudy, no significant alteration in receptor binding wasobserved after chronic administration of THC resulting intwenty-sevenfold behavioral tolerance [365]. Chronicadministration of anandamide as well, resulted in behavioraltolerance without receptor downregulation [366], and it wasproposed that desensitization of the CB1 receptor mightaccount for this observation [366]. Tolerance has beenobserved to occur together with modified biotransformationactivities with regard to mitochondrial oxygen consumption,monooxygenase activities, and the content of livermicrosomal cytochrome P450 [367]. However, only a smallproportion of tolerance can be attributed to changes inmetabolism [368].
After abrupt cessation of chronic dosing with THC,especially with high doses, withdrawal has been observed inhumans [357, 369]. Subjects complained of inner unrest,irritability, and insomnia and presented "hot flashes",sweating, rhinorrhea, loose stools, hiccups, and anorexia.Withdrawal symptoms in humans are usually mild and therisk for physical and psychic dependency is low compared toopiates, tobacco, alcohol, and benzodiazepines [370-372]. Areview of several indicators of the abuse potential of oraldronabinol in a therapeutic context found little evidence ofsuch a problem [373].
7.2. Antagonists
A 90-mg dose of rimonabant was well-tolerated inhealthy subjects with a history of cannabis use [159]. Incurrent clinical studies, doses of 5 to 20 mg are used.Adverse effects with rimonabant are reported to be transientand slightly greater than placebo, with the most commonbeing nausea [334, 374]. A high drop-out rate was reportedfor rimonabant's phase III studies in the press (New YorkTimes of 5 December 2004) which may be due to tasteaversion [375] and anxiety-like responses [376].
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Further possible side effects of CB1 receptor antagonistsmight be appetite loss [377] and other inverse agonisticactions at the CB1 receptor observed in vivo, includinghyperalgesia [78] and inflammation. There is a report of asubject who developed multiple sclerosis after startingtreatment with rimonabant for obesity [378], an effectconsistent with reported protective effects ofendocannabinoids in animal models of multiple sclerosis[73].
8. DRUG INTERACTIONS
The knowledge of drug interactions is restricted to THC,nabilone and cannabis, which are in use for a considerabletime. Not much knowledge is available for othercannabinoids and cannabinoid receptor antagonists.Interactions of THC and cannabis with other drugs maydepend on activity on similar effector systems or metabolicinteractions [380]. Since cannabinoids are strongly bound toproteins, interactions with other protein bound drugs mayalso occur. They might also interact with drugs that, such asTHC, are metabolized by enzymes of the cytochrome P-450complex. However, there was only a minor influence ofcannabis smoking and oral dronabinol on pharmacokineticparameters of antiretroviral medication used in HIV infectionand metabolized by cytochrome P-450 enzymes, and the useof cannabinoids was regarded as unlikely to impairantiretroviral efficacy [380]. Tobacco and cannabis smokingcessation was reported to result in elevated blood levels ofantipsychotic medication (clozapine or olanzapine), due tocessation of induction of cytochrome P4501A2 (CYP1A2) bysmoke constituents [381].
Other medicines may enhance or attenuate certain actionsof THC or certain actions of these medicines may beenhanced or attenuated by THC [382, 383]. Moreover, it ispossible that certain effects are enhanced and others reduced,as is the case with phenothiazines applied against side effectsof cancer chemotherapy. In a study by Lane et al. (1991), acombination of prochlorperazine and dronabinol was moreeffective in reducing unwanted effects of the antineoplasticmedication than the phenothiazine alone, and the incidenceof cannabinoid-induced adverse effects was decreased whendronabinol was combined with prochlorperazine, which alsohas antipsychotic properties [384]. Cannabis, caffeine andtobacco reduced the blood pressure reactivity protection ofascorbic acid, probably through their dopaminergic effects[385].
Of greatest clinical relevance is reinforcement of thesedating effects of other psychotropic substances (alcohol,benzodiazepines), and the interaction with substances thatact on heart and circulation (amphetamines, adrenaline,atropine, beta-blockers, diuretics, tricyclic antidepressants,etc.) [382, 383].
A number of additive effects may be desirable, such asthe enhancement of muscle relaxants, bronchodilators andanti-glaucoma medication [180] of analgesia by opiates [265,267], the antiemetic effect of phenothiazines [384] and 5-HT3 antagonists [246], and the antiepileptic action ofbenzodiazepines [386]. THC may antagonize theantipsychotic actions of neuroleptics [383] and may improve
their clinical responsiveness in motor disorders [387]. Acombination with other drugs may be desirable not only toreduce side effects of the single drugs, but also to prevent thedevelopment of tolerance. In animals studies, tolerance tomorphine was reduced by simultaneous administration ofTHC [388]. Chronic treatment with high doses of oralmorphine produced a threefold tolerance of pain-reducingeffects. Tolerance to morphine was prevented in groupsreceiving a daily co-treatment with low doses of THC [388].
Since the cannabinoid system is linked with hormonalcontrol, there may be interactions in this area. Theprogesterone receptor inhibitor mifepristone, which isapproved for the termination of early pregnancy, and theglucocorticoid synthesis inhibitor, metyrapone, were shownto potentiate the sedating effects of high THC doses in mice[389].
The cyclooxygenase inhibitors indomethacin,acetylsalicylic acid, and other non-steroidal anti-inflammatory drugs antagonize THC effects. Indomethacinsignificantly reduced subjective "high" [390], tachycardia[390], decrease of contractile performance in heart muscle[391] and decrease of intraocular pressure following topicalTHC (eye drops) [392], reflecting the involvement ofcyclooxygenase activity in several THC effects.
9. CONCLUSION
The cannabinoid system plays a major role in signaltransduction in neuronal cells, and anandamide seems to be acentral inhibitory compound in the central nervous system[393]. Modulators of the cannabinoid system withtherapeutic potential include agonists and antagonists at bothreceptors subtypes and inhibitors of endocannabinoiddegradation. Mechanisms of action of endogenous andexogenous cannabinoids are complex, not only involvingactivation of and interaction at the cannabinoid receptor, butalso activation of vanilloid receptors [197], influence ofendocannabinoid concentration [202], antioxidant activity[116], metabolic interaction with other compounds, andseveral others.
Due to the millennia-long use of cannabis forrecreational, religious and medicinal purposes, which inrecent decades was accompanied by scientists from severaldisciplines, medical CB receptor agonism is predicted toexhibit previously described side effects or adverse events.On the other hand, information on possible side effects ofantagonists is still sparse, albeit they usually seem to bewell-tolerated in clinical studies.
The psychotropic effects of CB1 receptor agonists and thestigma of cannabis as a recreational and addicting drug arestill major obstacles to the legal therapeutic utilization of thewhole range of potentially beneficial effects. In recent years,large and properly designed clinical studies have beenconducted and further trials are under way or planned toverify anecdotic experiences and the results from smalleruncontrolled studies, and to overcome uncertainties andskepticism.
Apart from CB1 receptor agonists and cannabispreparations that cause psychic side effects, cannabinoid
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analogs that do not bind to the CB1 receptor are attractivecompounds for clinical research. Additional ideas for theseparation of the desired therapeutic effects from thepsychotropic action comprise the concurrent administrationof THC and CBD, the design of CB1 receptor agonists thatdo not cross the blood brain barrier, and the development ofcompounds that influence endocannabinoid levels byinhibition of their membrane transport or hydrolysis.Inhibitors of endocannabinoid degradation may exert themaximum effect in brain areas where endocannabinoid levelsare already increased in reaction to a disease. It is remarkablethat FAAH inhibitors may already be in clinical use [394].The non-steroidal anti-inflammatory agent flurbiprofen,inhibits the metabolism of FAAH and intrathecallyadministrated flurbiprofen reduced inflammatory pain by amechanism that was blocked by a CB1 receptor antagonist.The anaesthetic agent propofol and the non-steroidal anti-inflammatory drugs indomethacin also activate cannabinoidreceptors as an important part of their actions [394].
The discovery of the cannabinoid system has accelerateda broad and exciting field of research, and we are in themiddle of discovering all its facets that may be of use for theprevention and treatment of illnesses.
