Cannabinoid-1 Receptor Inverse Agonists: Current Understanding of Mechanism of Action and Unanswered Questions

Home , Inverse agonist, Taranabant

International Journal of Obesity (2009) 33, 947–955 & 2009 Macmillan Publishers Limited All rights reserved 0307-0565/09 $32.00 www.nature.com/ijo REVIEW Cannabinoid-1 receptor inverse agonists: current understanding of mechanism of action and unanswered questions

TM Fong1 and SB Heymsfield2

1Merck Research Laboratories, Department of Metabolic Disorders, Rahway, NJ, USA and 2Merck Research Laboratories, Global Center of Scientific Affairs, Rahway, NJ, USA

Rimonabant and taranabant are two extensively studied cannabinoid-1 receptor (CB1R) inverse agonists. Their effects on in vivo peripheral tissue metabolism are generally well replicated. The central nervous system site of action of taranabant or rimonabant is firmly established based on brain receptor occupancy studies. At the whole-body level, the mechanism of action of CB1R inverse agonists includes a reduction in food intake and an increase in energy expenditure. At the tissue level, fat mass reduction, liver lipid reduction and improved insulin sensitivity have been shown. These effects on tissue metabolism are readily explained by CB1R inverse agonist acting on brain CB1R and indirectly influencing the tissue metabolism through the autonomic nervous system. It has also been hypothesized that rimonabant acts directly on adipocytes, hepatocytes, pancreatic islets or skeletal muscle in addition to acting on brain CB1R, although strong support for the contribution of peripherally located CB1R to in vivo efficacy is still lacking. This review will carefully examine the published literature and provide a perspective on what new tools and studies are required to address the peripheral site of action hypothesis. International Journal of Obesity (2009) 33, 947–955; doi:10.1038/ijo.2009.132; published online 14 July 2009

Keywords: cannabinoid; CB1 receptor; inverse agonist; body weight; metabolism

Introduction psychiatric adverse effects exhibited by rimonabant and taranabant.3 Many reviews have been published summarizing Cannabinoid-1 receptors (CB1Rs) are predominantly ex- the observed in vivo efficacy of CB1R inverse agonists in pressed in the central nervous system (CNS) and PNS animals and humans.5–8 However, a critical assessment for (peripheral nervous system).1,2 Based on sensitive detection the peripheral site of action hypothesis has not been techniques, such as PCR, CB1R mRNA has also been detected conducted. We will examine the available data to outline a in some peripheral organs or isolated non-neuronal cells, consensus understanding for a MOA with widespread experi- although the detected mRNA in peripheral tissues or organs mental support, and highlight unanswered questions. This may represent mRNA from neurons within the peripheral review will focus on the physiological MOA of inverse tissues. Most of the literature in the CB1R research field agonists and whether peripheral CB1R plays a significant supports the prominent role of brain CB1R in mediating role in the metabolic effects of inverse agonists. Properly various pharmacological effects of drugs. Some investigators demonstrating the peripheral CB1R contribution should also proposed a hypothesis that includes peripheral non- provide a basis to evaluate whether a new generation of neuronal CB1R as part of the mechanism of action (MOA) of CB1R inverse agonists can be used in the treatment of CB1R inverse agonists.3,4 The peripheral MOA hypothesis metabolic disorders. received renewed interests when it became apparent that a peripherally restricted compound may be devoid of the Molecular and cellular MOA

Correspondence: Dr TM Fong Merck & Co., Department of Metabolic The MOA of a drug can be considered at the molecular, Disorders, PO Box 2000, Rahway, NJ 7065, USA cellular and physiological level. At the molecular level, a or Dr SB Heymsfield, Merck & Co., Global Center of Scientific Affairs, Rahway, drug targeting a receptor can be acting as an agonist, a NJ, USA. neutral antagonist or an inverse agonist.9 This definition is E-mail: [email protected] Received 13 March 2009; revised 20 May 2009; accepted 25 May 2009; on the basis of data from in vitro assay rather than in vivo published online 14 July 2009 assay. Some receptors exhibit constitutive activity, which is CB1R mechanism of action TM Fong and SB Heymsfield 948 defined as receptor signaling activity in the absence of As we showed, inverse agonists also competitively antag- agonist. When a receptor does not exhibit constitutive onize the agonist effect.11 Hence, qualitative terms such as activity, neutral antagonist and inverse agonist collapse into inverse agonist/antagonist at the CB1R do not explain the the same class of ‘‘antagonist’’. However, when a receptor difference between these two molecules. They differ in areas does exhibit constitutive activity as in the case of CB1R, of potency, selectivity, off-target activity, pharmacokinetic neutral antagonist and inverse agonist are distinct pharma- properties and toxicological profiles. cological entities and can be experimentally differentiated At the cellular level, it has been well documented that under conditions when agonist is not present.10 In the case CB1R agonists inhibit neurotransmitter release, whereas of CB1R ligands, all three classes of compounds (agonists, inverse agonists increase neurotransmitter release.18–22 This neutral antagonists and inverse agonists) have been dis- cellular MOA is also consistent with the molecular distinc- covered. Taranabant and rimonabant have been shown in tion between agonist and inverse agonist. Although most numerous studies to exhibit inverse agonist properties comparisons of agonists to inverse agonists are on the basis (Figure 1).11–17 In the literature, some authors have con- of recombinant expression systems, ex vivo assays of brain tinued to use the term antagonist, and some authors have slice electrophysiology have indicated that rimonabant’s used the term antagonist/inverse agonist to describe rimo- effect is consistent with an inverse agonist effect under these nabant and taranabant. It should be pointed out here that conditions, in which the likelihood of endocannabinoid the term antagonist emphasizes its ability to antagonize the release is low.20–22 agonist effect, and the term inverse agonist emphasizes its ability to inhibit CB1R activity in the absence of agonist. Physiological MOA 12 The physiological effects after CB1R inverse agonist treat- 10 ment in animals and humans include inhibition of food MK-0364 11, 23–25 8 intake and increase of energy expenditure. Although Inv. Ag. the molecular basis and cellular basis for the action of inverse AM251 agonists have been well documented and the physiological 6 Inv. Ag. endpoints are well defined, the detailed physiological mechanisms through which modulation of CB1R translate

cAMP, pmole 4 to changes in peripheral tissue metabolism can be difficult to 2 dissect. The following sections will address the physiological CP55940 (agonist) MOA related to the site of action for inverse agonists and the 0 physiological pathways involved. 1E-12 1E-11 1E-10 1E-09 1E-08 1E-07 When evaluating the MOA of drugs in whole animals, the [ compound ] , M effects of a drug need to be separated from the site of action of the drug. A drug can act on a brain target and produce peripheral tissue effects mediated through the autonomic nervous system. Alternately, a drug can act on a molecular target within the peripheral organs to produce an effect (Figure 2). Many bodily functions, such as body temperature, metabolism and endocrine functions, are controlled by the brain. Hence, the demonstration of a peripheral tissue effect

Site of drug action Central CB1R Peripheral CB1R (in peripheral organs) Via PNS

Peripheral organs

Physiological Energy Intake Energy Expenditure effects and behavior Figure 1 Inverse agonist property of taranabant/MK-0364 (a) and rimonabant/SR141716A (b). The data were derived from cell-based assay measuring Figure 2 Schematic framework for evaluating the MOA of CB1R agents. The intracellular cAMP level utilizing recombinant expression system. Panel a is top portion represents the site of drug action. The middle represents how the reproduced, with permission, from Fong et al.11 Panel b is reproduced, with drug exerts its effect. The bottom portion represents what physiological effects permission, from Sink et al.15 and behaviors are elicited by the drug.

International Journal of Obesity CB1R mechanism of action TM Fong and SB Heymsfield 949 does not necessarily imply the drug acts on the peripheral direct actions of rimonabant on adipocytes4 or hepato- tissue directly. Resolving this question can be challenging and cytes41,42 may contribute to the observed physiological requires specific tools and appropriately designed studies. effects. It is impossible to show by conventional in vivo studies whether the physiological effects of CB1R inverse agonists may include a component of the peripheral site of Peripheral physiological effects vs peripheral site of action, because of the intact communication between the action for a drug brain and peripheral tissues. To address the question of potential peripheral site of action, three experimental When assessing the effects of CB1R inverse agonists, it is approaches can be used as follows: (a) a pharmacological important to recognize that many groups have reproduced a approach using a non-brain-penetrating compound; (b) a wide variety of physiological effects in vivo, including combination of pharmacological approach and genetic or reduction of food intake, increase in energy expenditure, surgical approach in which CNS CB1R can be deleted or the reduction of fat mass and reduction of liver lipid content. CNS–PNS connection is disrupted; (c) in vitro cell culture Food intake reduction and energy expenditure increase are studies in which the influence of CNS is absent. two important components of the MOA of taranabant,11, 25 and both of these pharmacodynamic effects can lead to reduction of fat mass and liver lipid content. Food intake (a) Non-brain-penetrating compounds or other pharmacological reduction and energy expenditure increase have also been approaches shown in rodents for rimonabant or AVE1625.26–28 Although In vitro and in vivo behavioral studies using one non-brain- the in vivo effects of CB1R inverse agonists are well penetrating CB1R antagonist SR140098 have been pub- replicated, the site of action for some of these drugs has lished,43 although its structure and its effect on energy been debated, specifically regarding whether peripherally balance and metabolism have not been disclosed. A non- located CB1R mediates the in vivo effects. brain-penetrating compound clearly represents an extremely Taranabant clearly acts on brain CB1R as shown by the valuable tool in addressing whether inhibiting peripheral brain penetration of the compound, the brain CB1R CB1R can lead to metabolic efficacy. Toward that end, occupancy and the correlation between brain CB1R occu- Pavon44 hypothesized that a newly discovered molecule, LH- pancy and efficacy on weight loss and food intake.11, 29 21, might not cross the blood–brain barrier. However, LH-21 Other groups also showed that rimonabant occupies brain has since been shown to be brain penetrant and its weight CB1R at doses that produce significant physiological loss efficacy is not mediated by CB1R, because LH-21 is still effects.30 There is only one report suggesting that central effective in CB1R KO mice.45 More recently, another administration of rimonabant or endocannabinoids through molecule (URB447) was reported to be non-brain pene- intracerebroventricular (icv) injection did not affect food trant.46 Like LH-21, URB447 has a low affinity at CB1R intake.31 In contrast, it has been shown in independent (IC50 ¼ 313 nM). As URB447 has not been shown to lack studies that direct injection of agonists into brain can efficacy in CB1R KO mice, its mechanism-based efficacy in stimulate food intake,32,33 and icv administration of rimo- wild-type (WT) mice needs to be confirmed before this nabant does reduce food intake and body weight.34 Based on molecule can be used to test the peripheral CB1R hypothesis. the totality of these reports, it seems to be that the negative Without a non-brain-penetrating compound, one group result of Gomez et al.31 may represent a special limitation of has attempted to determine whether central administration the study. of rimonabant can produce different effects compared with One of the reasons for the difficulty of proving the systemic administration. In the studies by Nogueiras contribution of peripherally located CB1R by conventional et al.,34 it was reported that differential effects were observed in vivo studies is the communication between the CNS and with icv or i.p. administration of rimonabant. Rimonabant peripheral organs. Autonomic nerve innervation of adipose at 10 mpk (i.p.) but not 5 ug (icv) led to reduction of tri- tissue, liver and pancreas has been well established in the glyceride content in white adipose tissue (WAT) or improved literature.35–40 As the brain clearly controls whole body insulin sensitivity. However, these differences are also metabolism through the autonomic nervous system, it is consistent with other explanations. For example, with i.p. generally expected that centrally acting CB1R inverse administration of a brain-penetrating compound, the entire agonists can lead to various peripheral physiological effects. brain is exposed to the compound. In contrast, icv admin- Showing the direct contribution of peripherally located istration of a compound does not necessarily produce the CB1R will require specifically designed experiments. same brain exposure to the compound as i.p. administration. It has been well documented that certain small molecules Experimental approaches to show a peripheral site may not reach the entire brain after icv administration,47,48 of drug action whereas systemic administration of brain-penetrating molecules will expose the whole brain. Different brain substrates As low levels of CB1R mRNA can be detected by PCR in can also independently affect food intake and energy expen- adipocytes and liver, it has also been hypothesized that diture.49 In the absence of CB1R occupancy measurement or

International Journal of Obesity CB1R mechanism of action TM Fong and SB Heymsfield 950 compound diffusion measurement, it would be difficult to weight and whole-body nutrient utilization are mediated by ascertain that rimonabant through icv administration would CNS CB1R. Genetically, deleting liver CB1R leads to a smaller reach the same brain sites as through i.p. administration. These increase in liver triglyceride compared with the WT mice alternative interpretations clearly highlight the need to have a under a high-fat diet condition. When a non-brain-penetrat- non-brain-penetrating compound to address the specific role ing compound is available, it will be important to test of peripherally located non-neuronal CB1R. whether pharmacological inhibition of liver CB1R can lead to the improvement of metabolic parameters.42

(b) Combination of surgical or genetic approach with pharmacological approach (c) In vitro cell culture approach In the absence of validated non-brain-penetrating CB1R To remove the influence of CNS, in vitro cell culture studies ligands, one can combine pharmacological approach with a can be conducted to evaluate the effect of CB1R inverse surgical or genetic approach to eliminate or reduce the agonists on adipocyte metabolism, hepatocyte metabolism contribution of CNS CB1R. In a recent study by Verty et al.,50 and pancreatic b-cell functions. Although cell culture rimonabant-induced BAT (brown adipose tissue) tempera- conditions do not necessarily represent in vivo cellular ture increase was eliminated when BAT was sympathetically conditions and positive outcomes from such in vitro studies denervated. These data indicate that the rimonabant- do not necessarily translate into in vivo beneficial effects, induced energy expenditure increase in BAT requires intact these studies could provide a theoretical basis for a direct communication between the brain and periphery, and BAT action of CB1R inverse agonists on non-neuronal cells. temperature increase is likely mediated by rimonabant acting on CNS CB1R. Similar combination of surgical and pharmacological approaches could potentially be applied to Adipocytes the study of metabolism at other peripheral tissues. Of all the peripheral organs (besides the GI (gastrointestinal) A genetic approach may also be useful when combined tract), adipose tissue is the tissue where CB1R mRNA has with a pharmacological approach. For instance, one can been consistently detected by PCR, which is a highly produce conditional CNS CB1R KO mice and test brain- sensitive method of detection. Although CB1R mRNA (or penetrating CB1R inverse agonists. In principle, the CNS at least the fragment as detected by PCR) is clearly detected, CB1R KO mice can be designed to retain all CB1R in whether the CB1R protein is functional in adipocytes is peripheral tissues, but lack CB1R in the CNS. Such a genetic controversial. model can be combined with pharmacological approaches to Using primary adipocytes from mice, it was reported that a evaluate the contribution of peripheral CB1R. Although CNS non-selective agonist WIN55212 can increase LPL (lipopro- 53 CB1R KO mice have not been reported, CB1R KO in a tein lipase) activity at 1 uM (Figure 3). However, rimona- subpopulation of neurons has been reported.51 The latter is bant alone at 1 uM did not reduce LPL activity, and not appropriate to address the site-of-action question WIN55212 alone at 100 nM did not have any effect on LPL because not all CB1R’s are removed from the CNS. activity. As WIN activates CB1R with an EC50 of 20 nM in 54 More recently, liver-specific CB1R KO mice have been cell-based cAMP assay, it is not clear whether the LPL effect evaluated.52 This model provides the potential to evaluate of WIN at 1 uM is mediated by CB1R. Furthermore, WIN has CB1R inverse agonist in the presence or in the absence of not been tested using adipocytes isolated from CB1R KO liver CB1R. With regard to diet-induced obesity (DIO), the liver CB1R KO mice show the same phenotype of DIO as the WT mice, whereas the global KO mice are resistant to DIO. When rimonabant was tested in the WT or liver CB1R KO mice or global KO mice, rimonabant reduces respiratory quotient (RQ) in both the WT and the liver CB1R KO mice but not in the global KO mice. These data indicate that body weight reduction and whole-body fatty acid oxidation increase are not mediated by liver CB1R. When analyzing the high-fat diet-induced steatosis and liver triglyceride content, the liver CB1R KO mice seem to have an intermediate phenotype. Specifically, the liver triglyceride increase and steatosis in the liver KO are still observed, albeit to a lesser extent compared with the WT mice. On the other hand, the global CB1R KO mice are completely resistant to high-fat diet-induced steatosis. Figure 3 In vitro effect of WIN55212 (WIN) on heparin-releasable LPL activity of primary mouse adipocytes. SR represents SR141716A. *Po0.05 w Based on these pharmacological and genetic studies, it compared to vehicle control, Po0.05 compared to 10À6 M WIN. Figure is seems that the effects of rimonabant treatment on body reproduced, with permission, from Cota et al.53

International Journal of Obesity CB1R mechanism of action TM Fong and SB Heymsfield 951 mice. For this WIN effect to be translated to meaningful pharmacological potency of WIN55212 or rimonabant, and endocannabinoid effect in vivo, the endocannabinoid levels these adipocyte results have not been replicated by other need to be much higher than their CB1R EC50 values. Thus groups. It remains a challenge to show a consistent direct far, such a high level of endocannabinoids has not been effect of CB1R inverse agonists on adipocyte functional reported. For example, plasma anandamide levels were readouts in vitro. reported to be B50-fold lower than its EC50 value at CB1R.55 Whether the local anandamide level within the adipose tissue is higher than circulating level is not yet Hepatocytes known. Osei-Hyiaman et al.41 have reported the presence of CB1R Using a preadipocyte 3T3 cell line, adipocytes can be mRNA in liver, although the level is clearly very low. Using differentiated from preadipocytes in the presence of insulin.4 primary hepatocytes from mice, it was found that CB1R Under such conditions, rimonabant was reported to increase agonist HU210 (100 nM) can increase fatty acid synthesis, adiponectin mRNA levels at 50 or 100 nM but not at 25 nM or whereas rimonabant (10 nM) can decrease fatty acid synthesis 200 nM (Figure 4). The reason for such a narrow concentra- (Figure 5). The HU210-induced increase was shown to be tion-dependence for the rimonabant effect is not clear. absent in hepatocytes from CB1R KO mice.41 Whether the Furthermore, it is not clear whether adiponectin protein rimonabant-induced fatty acid synthesis reduction is secretion is affected by rimonabant. In humans, the plasma mediated by CB1R has not been tested, presumably because Cmax of rimonabant at 20 mg after 21 days of dosing is of the fact that fatty acid synthesis in the hepatocytes of 56 B380 nM. the KO mice is already very low. The published data of The direct effects of rimonabant on adipocytes has not Osei-Hyiaman et al.41 are comprehensive, encompassing been widely replicated by other groups.57,58 Using a similar both genetic model and pharmacological studies. As dis- cell culture approach, Hamilton et al. 58 reported that cAMP cussed in section (b), the role of liver CB1R awaits further level and lipolysis were not affected by rimonabant, whereas pharmacological validation using a non-brain-penetrating a b-agonist did increase cAMP or lipolysis. Using either a cell inverse agonist. culture approach or in vivo studies, Hamilton et al.58 reported that adiponectin mRNA is not affected by rimonabant, whereas rosiglitazone did increase adiponectin mRNA. Pancreatic islets Similar studies conducted by us did not detect significant Matias et al.60 reported that a CB1R agonist stimulates changes in lipolysis or lipogenesis induced by CB1R agonists glucose-dependent insulin secretion (GDIS) in rat insulino- or inverse agonists in cell culture assays (Fong et al, ma RIN-m5F cells. These results would predict that inverse unpublished). In another study using human primary agonists should inhibit glucose-dependent insulin release. adipocytes, adiponectin mRNA was not affected by either Given that rimonabant has been shown to improve glycemic rimonabant or WIN55212.59 In addition, Lofgren et al.57 control in diabetic patients, the prediction of Matias et al.60 reported that human CB1R mRNA levels in adipose biopsy is counter-intuitive as many anti-diabetic agents such as samples were not related to fat-cell function or adiponectin GLP-1 are known to stimulate GDIS. level. Using isolated mouse islets, Juan-Pico61 reported that In summary, some of the positive findings in adipocyte CB1R agonists inhibit GDIS. Nakata et al.62 also reported studies are not easily reconciled with the known in vitro that CB1R agonists inhibit GDIS in mouse islets. Both

Figure 4 In vitro effect of rimonabant on adiponectin mRNA level in Figure 5 In vitro effects of CB1R agonist (HU210) and inverse agonist differentiated mouse 3T3 F442A adipocytes. *Po0.05; **Po0.01. Figure is (SR141716) on FA synthesis in isolated hepatocytes from mice. Figure is reproduced, with permission, from Bensaid et al.4 reproduced, with permission, from Osei-Hyiaman et al.41

International Journal of Obesity CB1R mechanism of action TM Fong and SB Heymsfield 952 Table 1 Reported effects of CB1R agonist or inverse agonist on glucose- In clinical studies, statistical modeling has suggested that a dependent insulin secretion (GDIS) in vitro portion of the lipid profile improvement and glucose profile Reference Agonists Inverse agonists improvement is not explained by a weight loss effect. However, post-hoc statistical modeling is not generally 60 Matias et al. increase GDIS block agonist effect accepted as proof of a weight loss-independent effect.66 Juan-Pico et al. 61 decrease GDIS not determined 63 Specifically designed studies will be needed to address this Getty-Kaushik et al. not determined decrease GDIS Nakata et al. 62 decrease GDIS not determined question.

How does the CB1R affect feeding behaviorFrole Juan-Pico61 and Nakata62 did not evaluate CB1R inverse of CB1R on appetite, satiety/satiation and craving agonists. In a 2007 American Diabetes Association (ADA) presentation, Getty-Kaushik et al.63 reported that rimonabant The central control of feeding behavior can be manifested in at high concentration (1 uM) can inhibit GDIS in islets from several different ways. Appetite refers to the tendency to seek lean Zucker rats. The conclusions from these four published food and consume food.67,68 A drug can act on distinct studies vary greatly (Table 1), hence the direct effect of CB1R components of the appetite control mechanism, including inverse agonists on islets seems difficult to reconcile. psychological events of hunger perception or craving, or temporally distinct components of pre-meal hunger, satiation during a meal (which causes meal termination) or post- meal state of satiety. Limited experimental data exist to Skeletal muscle elucidate the details of how CB1R inverse agonists affect Liu et al.27 conducted in vivo experiments with rimonabant these different components. In one clinical trial of rimona- and showed that the treatment of ob/ob mice with bant, it was shown that rimonabant reduces food craving rimonabant for 7 days led to increased glucose uptake in and hunger at the start of a meal with no change in the muscle. This study is often misquoted as evidence that sensation of fullness after meals, implying an improved rimonabant acts on skeletal muscle directly. However, this is control of the pre-meal motivation to eat. In addition, the an in vivo study and does not address the site of drug action. reduction in food intake is independent of food type and A recent article reported that incubating isolated human pleasantness, and rimonabant does not affect the pleasant- myotubes with 5 uM of AM251 led to a small change in the ness of food.69 In another clinical trial of taranabant, Addy mRNA levels of several enzymes, such as AMPK or PDK4.64 As et al.25 showed that taranabant reduces the intake of all three the requirement of high concentration is in contrast to the major macronutrients (carbohydrate, fat and protein). These known in vitro binding affinity and intrinsic inverse agonist data show that CB1R inverse agonists inhibit pre-meal potency of AM251, it is unclear whether these effects are motivation to eat, craving and hunger and enhance satiation indeed mediated by CB1R. Further studies will be needed to (that is, meal termination). address these unanswered questions. Based on the co-localization of CCK1R and CB1R in vagal nerves and the observation that CCK agonist or antagonist can affect the expression level of CB1R in vagal afferent Do CB1R agents produce weight loss-independent neurons,70 it has been hypothesized that CB1R inverse effects? agonist may affect satiety.71 However, the co-localization per se does not necessarily support a satiety effect. To further Currently, there are no published experimental data to validate this hypothesis, it is necessary to show that CB1R support the hypothesis of weight loss-independent effects. inverse agonists affect vagal nerve activity in a similar It has been reported that AM251 lacks acute effects on manner as CCK or CB1R inverse agonists affect satiety in glucose homeostasis in DIO mice when AM251 acutely behavioral studies. affects body weight in DIO mice.65 Whether long-term In rodent lever-pressing studies in which food (either treatment with a low dose of a CB1R inverse agonist can palatable or non-palatable) was used as reinforcer, rimona- produce glycemic effect without body weight effect has not bant was found to suppress the lever-press response rate, been reported. It is also possible that a significant reduction consistent with the interpretation that CB1R inverse agonists of regional fat mass may not be reflected in a body weight reduce the rewarding efficiency of food with no selectivity change, but the reduction of regional fat mass may be on palatability.72,73 In addition, food intake studies showed sufficient to improve insulin sensitivity. If this is true, an that rimonabant inhibited the intake of both regular chow or apparent weight loss-independent effect can actually be a palatable food.24, 74,75 Tetrahydrocannabinol (THC) has also regional fat mass reduction-dependent effect. Further studies been shown to stimulate the intake of either chow or high- are needed to clarify the relationship among abdominal fat fat sweetened diet.76 These data generally support a model in mass, liver lipid content, body weight and other metabolic which CB1R inverse agonists reduce the intake of all parameters. macronutrients with no selectivity related to palatability.

International Journal of Obesity CB1R mechanism of action TM Fong and SB Heymsfield 953 Acknowledgements

We thank Drs Joyce Harp, Cai Li, Ravi Shankar and Ira Gantz for critically reading the paper and providing valuable CB1R ↓ suggestions.

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