Latest advances in cannabinoid receptor antagonists and inverse agonists

Radiolabeling and in vitro /in vivo evaluation of N-(1-adamantyl)-8-methoxy-4-oxo-1-phenyl-1,4-dihydroquinoline-3-carboxamide as a PET probe for imaging cannabinoid type 2 receptor

The cannabinoid type 2 (CB2) receptor plays an important role in neuroinflammatory and neurodegenerative diseases such as multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer’s disease and is therefore a very promising target for therapeutic approaches as well as for imaging. Based on the literature, we identified one 4-oxoquinoline derivative(designated KD2) as the lead structure. It was synthesized, radiolabeled and evaluated as a potential imaging tracer for CB2. [11C]KD2 was obtained in 99% radiochemical purity.Moderate blood–brain barrier (BBB) passage was predicted for KD2 from an in vitro transport assay with P-glycoprotein-transfected Madin Darby canine kidney cells. No efflux of KD2 by P-glycoprotein was detected. In vitro autoradiography of rat and mouse spleen slices demonstrated that [11C]KD2 exhibits high specific binding towards CB2. High spleen uptake of [11C]KD2 was observed in dynamic positron emission tomography(PET) studies with Wistar rats and its specificity was confirmed by displacement study with a selective CB2 agonist, GW405833. A pilot autoradiography study with post-mortem spinal cord slices from amyotrophic lateral sclerosis (ALS)patients with [11C]KD2 suggested the presence of CB2 receptors under disease conditions. Specificity of [11C]KD2 binding could also be demonstrated on these human tissues. In conclusion, [11C]KD2 shows good in vitro and in vivo properties as a potential PET tracer for CB2.

Selective cannabinoid receptor 2 modulators: a patent review 2009--present

INTRODUCTION

The activation of the cannabinoid receptor 2 (CB2) affects a myriad of immune responses from inflammation to neuroprotection, demonstrates analgesic effects and suppresses responses in many animal models of pain. Questions around the involvement of CB1 activation in these effects remain, but efforts have been directed toward the discovery of highly selective CB2 modulators lacking the psychotropic effects of cannabinoids, which are mediated by the CB1 receptor.

AREAS COVERED

This review covers the patent literature which was published since April 2009 on CB2 selective modulators. It provides a general summary of the CB2 biology supporting the interest in CB2 as a drug target, new potential therapeutic indications and the development status of selective CB2 agonists.

EXPERT OPINION

There is a continuous interest in the CB2 receptor as a drug target. Many highly selective compounds of various chemotypes have been identified and their analgesic effects in animal models further support the potential of this mechanism in pain therapy. Several companies have initiated clinical trials. While some of these have been terminated for various reasons, one can anticipate the emergence of new drugs from CB2 modulation once a better understanding around the cannabinoid receptors is gained.

Discovery of indole alkaloids with cannabinoid CB1 receptor antagonistic activity

Three indole alkaloids, voacamine (1), 3,6-oxidovoacangine (2), and a new alkaloid, 5-hydroxy-3,6-oxidovoacangine (3), isolated from Voacanga africana were found to exhibit potent cannabinoid CB1 receptor antagonistic activity. This is the first example of CB1 antagonists derived from natural alkaloids.

Hair growth stimulator property of thienyl substituted pyrazole carboxamide derivatives as a CB1 receptor antagonist with in vivo antiobesity effect

A few thienyl substituted pyrazole derivatives were synthesized to aid in the characterization of the cannabinoid receptor antagonist and also to serve as potentially useful antiobesity agent. Structural requirements for selective CB1 receptor antagonistic activity of 5-thienyl pyrazole derivatives included the structural similarity with potent, specific antagonist rimonabant 1. Compound 3 has been identified as a hair growth stimulator and an antiobesity agent in animal models.

New 1,2,3,4-tetrahydropyrrolo[3,4-b]indole derivatives as selective CB2 receptor agonists

The preparation and evaluation of a novel class of CB2 agonists based on a 1,2,3,4-tetrahydropyrrolo[3,4-b]indole moiety are reported. They showed binding affinities up to 4.2 nM toward CB2 with sub-nanomolar EC(50) values. They also showed moderate to good (>350-fold) selectivity over the CB1 receptor.

Endocannabinoids and related N-acylethanolamines in the control of appetite and energy metabolism: emergence of new molecular players

PURPOSE OF REVIEW

Endocannabinoids (anandamide and 2-arachidonoylgycerol) and related N-acylethanolamines (N-oleoylethanolamine) exhibit opposite effects in the control of appetite. The purpose of this review is to highlight the similarities and differences of three major lipid-signaling molecules by focusing on their mode of action and the proteins involved in the control of food intake and energy metabolism.

RECENT FINDINGS

Anandamide and 2-arachidonoylglycerol promote food intake and are the main endogenous ligands of the cannabinoid receptors. One of them, the cannabinoid receptor 1, is responsible for the control of food intake and energy expenditure both at a central and a peripheral level, affecting numerous anorexigenic and orexigenic mediators (leptin, neuropeptide Y, ghrelin, orexin, endogenous opioids, corticotropin-releasing hormone, alpha-melanocyte stimulating hormone, cocaine and amphetamine-related transcript). In the gut, N-oleoylethanolamine plays an opposite role in food regulation, by interacting with two molecular targets different from the cannabinoid receptors: the nuclear receptor peroxisome proliferator-activated receptor alpha and a G-protein coupled receptor GPR119.

SUMMARY

Recent findings on the molecular mechanisms underlying the promotion of food intake or, in contrast, the suppression of food intake by anandamide and N-oleoylethanolamine, are summarized. Potential strategies for treating overweight, metabolic syndrome, and type II diabetes are briefly outlined.

Acute anorectic response to cannabinoid CB1 receptor antagonist/inverse agonist AM 251 in rats: indirect behavioural mediation

Despite a large and consistent literature on the suppressant effects of cannabinoid CB1 receptor antagonists/inverse agonists (e.g. rimonabant, AM 251) on food intake and weight gain in rodents, surprisingly little is known about the behavioural selectivity of such effects. In this study, ethological scoring was used to characterize the acute behavioural effects of the rimonabant analogue AM 251 (1.5 and 3.0 mg/kg, intraperitoneally) in nondeprived male rats during a 1-h test with palatable mash. Data were also collected on daily weight gain and on retest food intake 7 days after dosing. Results showed that the higher dose of AM 251 significantly inhibited mash consumption (32% decrease relative to vehicle control), reduced time spent feeding during the test and suppressed body weight gain over the 48-h period that followed acute dosing. No effects on mash consumption were observed when the animals were retested drug-free 1 week after drug treatment. Detailed video analysis of the test sessions showed that, over the dose range tested, AM 251 did not significantly interfere with the vast majority of noningestive behaviours. Both doses of the compound, however, significantly increased the incidence of and the time spent on scratching, whereas the higher dose additionally increased both the number and duration of grooming episodes. The latter effect in particular disrupted the normal structure of behaviour (behavioural satiety sequence) with atypically high levels of grooming displacing feeding during the middle part of the test session. Overall, the behavioural profile of AM 251 in a free-feeding context is very similar to (but approximately two-fold less potent than) that recently reported for the parent molecule, rimonabant. Together, these data strongly suggest that the acute anorectic response to CB1 receptor antagonists/inverse agonists is indirectly mediated via major alterations to other components of the behavioural repertoire.

Blocking the Cannabinoid Receptors: Drug Candidates and Therapeutic Promises

The CB1 and CB2 cannabinoid receptors have been described as two prime sites of action for endocannabinoids. Both the localization and pharmacology of these two G-protein-coupled receptors are well-described, and numerous selective ligands have been characterized. The physiological effects of Cannabis sativa (cannabis) and a throughout study of the endocannabinoid system allowed for the identification of several pathophysiological conditions--including obesity, dyslipidemia, addictions, inflammation, and allergies--in which blocking the cannabinoid receptors might be beneficial. Many CB1 receptor antagonists are now in clinical trials, and the results of several studies involving the CB1 antagonist lead compound rimonabant (SR141716A) are now available. This review describes the pharmacological tools that are currently available and the animal studies supporting the therapeutic use of cannabinoid receptor antagonists and inverse agonists. The data available from the clinical trials are also discussed.

Discovery of pyrazine carboxamide CB1 antagonists: The introduction of a hydroxyl group improves the pharmaceutical properties and in vivo efficacy of the series

Structure-activity relationships for a series of pyrazine carboxamide CB1 antagonists are reported. Pharmaceutical properties of the series are improved via inclusion of hydroxyl-containing sidechains. This structural modification sufficiently improved ADME properties of an orally inactive series such that food intake reduction was achieved in rat feeding models. Compound 35 elicits a 46% reduction in food intake in ad libidum fed rats 4-h post-dose.

Optimization of imidazole amide derivatives as cannabinoid-1 receptor antagonists for the treatment of obesity

Several imidazole-based cyclohexyl amides were identified as potent CB-1 antagonists, but they exhibited poor oral exposure in rodents. Incorporation of a hydroxyl moiety on the cyclohexyl ring provided a dramatic improvement in oral exposure, together with a ca. 10-fold decrease in potency. Further optimization provided the imidazole 2-hydroxy-cyclohexyl amide 45, which exhibited hCB-1 K(i)=3.7nM, and caused significant appetite suppression and robust, dose-dependent reduction of body weight gain in industry-standard rat models.

Constrained analogs of CB-1 antagonists: 1,5,6,7-Tetrahydro-4H-pyrrolo[3,2-c]pyridine-4-one derivatives

A series of pyrrolopyridinones was designed and synthesized as constrained analogs of the pyrazole CB-1 antagonist rimonabant. Certain examples exhibited very potent hCB-1 receptor binding affinity and functional antagonism with Ki and Kb values below 10 nM, and with high selectivity for CB-1 over CB-2 (>100-fold). A representative analog was established to cause significant appetite suppression and reduction in body weight gain in industry-standard rat models used to develop new therapeutics for obesity.