Therapeutic Opportunities Through Modulation of the Endocannabinoid System

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Neuropharmacology 48 (2005) 1068e1071 www.elsevier.com/locate/neuropharm

Therapeutic opportunities through modulation of the endocannabinoid system

Alexandros Makriyannis, Raphael Mechoulam, Daniele Piomelli *

University of California, Department of Pharmacology, Irvine, CA 92697-4260, United States

The discovery of the cannabinoid receptors and chemically from the polyunsaturated fatty acid, arach- endocannabinoid ligands has generated a great deal of idonic acid, which is used in nature as the starting interest in identifying opportunities for the development material for other important signaling compounds, such of novel cannabinergic therapeutic drugs. Such an effort as the eicosanoids. Additional endocannabinoid-related was first undertaken three decades ago by a number of compounds present in the body include virodhamine, pharmaceutical industries, but was rewarded with only which may act as an endogenous antagonist of CB1 modest success. However, the newly acquired knowl- receptors, and arachidonoylserine, which may engage an edge on the physiological roles of the endocannabinoid as-yet-uncharacterized cannabinoid-like receptor ex- system has significantly enhanced these prospects. pressed in the vasculature. At the June 27, 2004 workshop ‘‘Future Directions in As is well-known, the Cannabis plant contains more Cannabinoid Therapeutics II: From the Bench to the than 60 cannabinoids, which include (ÿ)-D9-tetrahydro- Clinic’’, sponsored by the University of California Center cannabinol (D9-THC), cannabigerol, cannabidiol, can- for Medicinal Cannabis Research, we on the Scientific nabinol, cannabichromene and cannabicyclol. Attention Planning Committee were asked to identify the areas has been mostly focused on D9-THC, because of its of research with the most immediate promise for the multiple biological properties. Nevertheless, less studied development of novel therapeutic agents. The Committee compounds such as cannabidiol may also be important, identified four broad areas involving modulation of the although we do not yet know at which receptors they endocannabinoid system as particularly promising in this may act to achieve their effects. D9-THC is the only regard: agonists for central CB1 cannabinoid receptors natural cannabinoid presently used in the clinic. and peripheral CB2 receptors, antagonists of CB1 In addition to these plant-derived cannabinoids, an receptors, inhibitors of endocannabinoid deactivation, extensive set of synthetic cannabinergic agonists has been and endocannabinoid-like compounds that act through developed over the last 30 years. Products of these efforts mechanisms distinct from CB1 and CB2 receptors include CP-55940 (Pfizer), created by opening one of the activation. Below, we summarize the data presented at rings of the tricyclic D9-THC structure and introducing the Workshop and the consensus of its participants on other small changes in its structure; HU-210 (Hebrew the most exciting opportunities for drug discovery. University), a very potent cannabinoid agonist resembling some D9-THC metabolites; and WIN55212-2 (Winthrop), which belongs to an altogether different class of chemicals, 1. Cannabinoid agonists the aminoalkylindoles. Additionally, the metabolically stable synthetic analog of anandamide R-methanandamide Two endogenous agonists of cannabinoid receptors (AM356) is routinely used as a pharmacological probe have been well characterized and are now widely used in to circumvent the short half life of the natural substance. research: anandamide (arachidonoylethanolamide), and Two important new additions to this armamentarium 2-arachidonoylglycerol (2-AG). Both molecules derive under discussion at the workshop include a peripherally acting cannabinoid agonist in preclinical development * Corresponding author. Tel.: C1 9498246180; fax: C1 9498246305. by Novartis for the treatment of neuropathic and E-mail address: [email protected] (D. Piomelli). inflammatory pain (structure as yet undisclosed), and

BAY-387271 (Bayer), a centrally acting cannabinoid 1.2. Potential problems agonist in Phase II clinical studies for the treatment of stroke. The interest of the pharmaceutical industry in the Cannabinoid analogs are currently in development application of cannabinoid agonists to the treatment of or being tested by Astra, Bayer, Endo, GSK, GW pain conditions is not recent. Indeed, most of the Pharmaceuticals, Indevus, Kadmus Pharmaceuticals, compounds now in experimental use derive from such MAKScientific, and Pharmos. Some of the obvious an interest. Historically however cannabinoid agonist issues in drug development are efficacy, side effects, and development has not proved clinically fruitful, largely regulatory scheduling. With regard to efficacy, the because of the profound psychotropic side effects of primary clinical targets are pain, stroke, nausea, centrally active cannabinoid agonists, hence the attention appetite, and cough. Potential side effects in addition given to peripherally acting cannabinoids, which exhibit to the well-known central psychotropic actions include significant analgesic efficacy and low central activity in cardiovascular and immune perturbations, as well as animal models. fertility problems. Finally, a major question from the Neuroprotection is a relatively new area for canna- perspective of drug industry is one of regulatory binoid agonists, but one that appears to be already well scheduling: that is, whether any drug acting either advanced. Preclinical studies have made a convincing directly or indirectly through the endocannabinoid case for the efficacy of cannabinoid agents not only in system will be subject to the restrictive scheduling of experimental brain ischemia, but also in models of D9-THC, or whether scheduling will be determined by Parkinson’s disease and other forms of degenerative side effects, as is the case with most drugs. brain disorders. The results of a Phase II clinical trial with BAY-387271 are awaited with great excitement. Also highlighted during the conference were various 2. CB1 antagonists derivatives of (ÿ)cannabidiol. Particularly interesting in this regard was the compound (ÿ)-7-hydroxy-4#- A large number of pharmaceutical companies have dimethylheptyl-cannabidiol (7-OH-DMH-CBD) a hy- started active CB1 antagonist programs, mostly as droxylated, dimethylheptylated cannabidiol, structurally a result of the clinical success of SR141716A (rimona- 9 related to HU-210. Like D -THC, 7-OH-DMH-CBD is bant), the first CB1 antagonist to be developed. This a potent inhibitor of electrically evoked contractions in molecule has successfully completed Phase III studies the mouse vas deferens. However, 7-OH-DMH-CBD and is anticipated to become available within a year for the treatment of obesity and tobacco addiction. does not significantly bind to either CB1 or CB2 receptors and its inhibitory effects on muscle contractility are not Rimonabant is an inverse CB1 agonist with a Ki of 11 nM at the CB receptors and 1640 nM at CB . blocked by CB1 or CB2 receptor antagonists, suggesting 1 2 that the compound may target an as-yet-uncharacterized Additional agents currently in development include cannabinoid-like receptor. This hypothesis is reinforced SLV-326 (Solvay) and LY320135 (Lilly). However, all by pharmacological experiments, which suggest that of these compounds are inverse agonists. A series of 7-OH-DMH-CBD displays anti-inflammatory and in- neutral antagonists has been generated, but remains not testinal-relaxing properties, but does not exert overt as well characterized in the literature. Examples of this psychoactive effects in mice. However, the nature of this class are the compounds O-2654 (Organix) and AM5171 hypothetical receptor and its relationship to other (University of Connecticut). As noted above, therapeu- cannabinoid-like sites in the vasculature and in the brain tic areas for cannabinoid antagonists include obesity, hippocampus remains to be determined. drug addiction and perhaps CNS disorders.

1.1. CB2 agonists 2.1. Obesity

Another way to reduce central side effects is to target The mechanism by which cannabinoid antagonists peripheral CB2 receptors, which are expressed in the brain exert their anti-obesity effects is still not fully un- only during inflammatory states and even then are limited derstood. Data on rimonabant presented at the work- to microglia. Selective CB2 receptor agonists include the shop identified two possibilities. First, there is a loss of compounds AM1241 (University of Connecticut) and appetite. Mutant mice that are deficient in CB1 receptors HU-308 (Hebrew University). Compounds of this type eat less than wild-type controls. Second, there is an offer a great deal of promise in the treatment of pain and increase in metabolic rate and a loss of fat mass. These inflammation. Studies conducted on multiple animal effects may be linked, on the one hand, to the ability of models of pain have shown that the CB2-selective agonist rimonabant to affect corticotropin-releasing hormone AM1241 has robust analgesic effects and is very potent in (CRH), as suggested by the fact that CB1 receptors neuropathic pain models. These effects are maintained in colocalize with CRH receptors in the hypothalamus. CB1-deficient, but not in CB2-deficient mice. This may be significant for explaining the drug’s effects 1070 A. Makriyannis et al. / Neuropharmacology 48 (2005) 1068e1071 on appetite drive, as it is known that CRH is received a great deal of attention. First, the possibility of anorexigenic. On the other hand, mice that lack CB1 neuropsychiatric sequelae, such as anhedonia and anxi- receptors display a hyperactivity of the hypothalamic- ety: preclinical studies have consistently shown such pituitary-adrenal axis, with increases in both ACTH and effects in animals, though they have not yet been observed corticosterone. This phenotype may be important in in the clinic. Second, pain and hyperalgesia, because of regard to overall metabolic rate. Another possible the pervasive role played by the endocannabinoid system mediator of the long-lasting effect on body weight in the control of pain processing. Last, hypertension, as reduction unrelated to altered food intake is the indicated by the contribution of the endocannabinoids to adipocyte, because CB1 receptor activation causes blood pressure regulation and the pressor effects of lipogenesis, which is blocked by rimonabant. rimonabant in animal models of hypertension.

2.2. Drug addiction 3. Endocannabinoid deactivation inhibitors CB1 cannabinoid receptors are present on the cell surface of neurons within the brain reward circuitry. The endocannabinoid signaling system differs from Furthermore, endocannabinoids may be released from classical neurotransmitter systems, picking up where dopamine neurons in the ventral tegmental area (VTA), classical neurotransmitters leave off. That is, the activa- and from medium spiny neurons in the nucleus tion of receptors (D2, glutamate, NMDA) initiates accumbens of the brain reward circuit. Additionally, a series of chemical events that leads to the release of 9 endocannabinoids and D -THC activate CB1 receptors endocannabinoids from the postsynaptic spine e the final and by doing so regulate reward strength and drug step of which is the enzymatic production and subsequent craving. Though we do not know how this occurs, it is release of anandamide and/or 2-AG. Once released, the likely that these mechanisms extend to all drugs of endocannabinoids are then directed to the presynaptic abuse, because collectively these drugs show the pro- cell and the CB1 receptor responds by inhibiting further pensity to increase VTA dopamine neuron activity, release of that cell’s neurotransmitters. The termination which might be coupled to augmented endocannabinoid of this cascade is accomplished via a transporter that production from the dopamine neurons themselves. internalizes the endocannabinoids, after which intracel- Finally, cannabinoid receptor antagonists block the lular enzymes such as fatty-acid amide hydrolase effects of endocannabinoids in these reward circuits. (FAAH) break them down. Preclinical work shows that priming injections of There is a general consensus that endocannabinoids cannabinoid agonists (WIN55212-2 and CP-55940) are transported into cells via a facilitated diffusion reinstate heroin-seeking behavior after a prolonged mechanism. This process may differ both kinetically and period of abstinence in rats trained to self-administer pharmacologically from cell to cell. In brain neurons, heroin. The cannabinoid antagonist rimonabant fully endocannabinoid transport is blocked by certain agents, prevents heroin-induced reinstatement of heroin-seeking which include the compounds AM404, OMDM-8 behavior. Additionally, rimonabant significantly attenu- and AM1172 (University of Connecticut/University of ates cannabinoid-induced reinstatement of heroin- California). However, the pharmacological properties of seeking behavior. these drugs in vivo are only partially understood. All these findings clearly support the hypothesis of Once inside cells, endocannabinoids are hydrolyzed a functional interaction between opioid and cannabi- by three principal enzyme systems. FAAH is a key noid systems in the neurobiological mechanisms of enzyme of anandamide deactivation in the brain. Potent relapse and might suggest a potential clinical use of and selective FAAH inhibitors have been developed and cannabinoid antagonists for preventing relapse to shown to exert profound antianxiety and antihyperten- heroin abuse. It has also been shown that cannabinoid sive effects in animals. The latter effects were discussed at antagonists can prevent drug reinstatement with co- length at the workshop, highlighting the important role caine, alcohol, and nicotine. Thus, it seems that the of anandamide in two important examples of vascular future of cannabinoid antagonists in substance abuse allostasis e shock and hypertension. In addition to treatment is particularly promising, especially in the FAAH, another amide hydrolase has been recently clinical setting, where polydrug abuse is exceedingly characterized, which may participate in the degradation more common than isolated single-drug abuse. of anandamide and other fatty-acid ethanolamides such as oleoylethanolamine (OEA). This amidase prefers 2.3. Potential side effects acid pH values and has a different tissue distribution than FAAH, being notably high in lung, spleen and The available data suggest that CB1 antagonism inflammatory cells. Inhibitors of this enzyme are being produces relatively mild side effects in people. Yet several developed. Finally, 2-AG is hydrolyzed by an enzymatic potential risks were discussed and three in particular system separate from FAAH, which probably involves A. Makriyannis et al. / Neuropharmacology 48 (2005) 1068e1071 1071 a monoacylglycerol lipase recently cloned from the rat plish their effects is through binding to intracellular brain. Inhibitors of this enzyme are currently under receptors like the ligand-activated transcription factor development. peroxisome proliferators-activate receptor-a (PPAR-a). OEA exerts profound satiety-inducing, weight-reducing 3.1. Direct vs. indirect agonists and anti-inflammatory effects in rodents, which are absent in mice deficient in PPAR-a and are closely What are the therapeutic advantages and draw- mimicked by synthetic PPAR-a agonists. Furthermore, backs of using a direct agonist vs. an indirect agonist? OEA was found to bind to and activate PPAR-a with Several parallels can be drawn to the well-known SSRIs high affinity (KD of 37 nM) in vitro. These findings (selective serotonin reuptake inhibitors), which have suggest that OEA is an endogenous PPAR-a agonist shown such powerful and useful therapeutic applications involved in the regulation of energy balance. An in effecting indirect agonism of the serotonergic system. analogous role has been recently suggested for palmi- Indeed, there is ample evidence that pharmacological toylethanolamide, the antinflammatory properties of profiles for the indirectly-acting agonists can generally which have been known for decades and are now been be attributed to enhanced selectivity based on more exploited in veterinary medicine. localized action. A prime reason for favoring the indirect agonism approach is the possibility of obtaining new drugs devoid of the psychoactive effects and perceived 4. Conclusions abuse potential of directly acting CB1 agonists. If we accept the postulate of on-demand modulation of endo- The discovery of the endocannabinoid signaling cannabinoid signaling as contributing to some disease system e with its network of ligands, membrane states, we are likely to witness the development of more receptors, and regulatory proteins e has revealed a score specific medications acting indirectly such as inhibitors of potential new therapeutic targets in the fields of of cannabinoid uptake or breakdown. obesity, drug abuse, pain, stroke and hypertension. Most of these targets must still be fully validated through pharmacology and genetics, and better ligands 3.2. Endocannabinoid-like compounds for these targets are still needed. Yet, it is clear that endocannabinoid modulation has become one of the An avenue through which endocannabinoid-like exciting new areas of current drug discovery, and that fatty-acid ethanolamides have been shown to accom- efforts to fill these gaps are well worthwhile.