The Potential of Inhibitors of Endocannabinoid Metabolism for Drug Development: A Critical Review

Out of the dark: the emerging roles of lncRNAs in pain

The dark genome, the nonprotein-coding part of the genome, is replete with long noncoding RNAs (lncRNAs). These functionally versatile transcripts, with specific temporal and spatial expression patterns, are critical gene regulators that play essential roles in health and disease. In recent years, FAAH-OUT was identified as the first lncRNA associated with an inherited human pain insensitivity disorder. Several other lncRNAs have also been studied for their contribution to chronic pain and genome-wide association studies are frequently identifying single nucleotide polymorphisms that map to lncRNAs. For a long time overlooked, lncRNAs are coming out of the dark and into the light as major players in human pain pathways and as potential targets for new RNA-based analgesic medicines.

Disrupted stress-induced analgesia in a neuropathic pain state is rescued by the endocannabinoid degradation inhibitor JZL195

Acute stress normally engages descending brain pathways to produce an antinociceptive response, known as stress-induced analgesia. Paradoxically, these descending pain modulatory pathways are also involved in the maintenance of the abnormal pain associated with chronic neuropathic pain. It remains unclear how stress-induced analgesia is affected by neuropathic pain states. We therefore examined the impact of a chronic constriction nerve-injury (CCI) model of neuropathic pain on restraint stress-induced analgesia in C57BL/6 mice. Thirty minutes of restraint stress produced analgesia in the hotplate thermal nociceptive assay that was less in CCI compared to control mice who underwent a sham-surgery. In sham but not CCI mice, stress-induced analgesia was reduced by the opioid receptor antagonist naltrexone. The cannabinoid CB1 receptor antagonist AM281 did not affect stress-induced analgesia in either sham or CCI mice. Low-dose pre-treatment with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195 increased stress-induced analgesia in CCI but not sham mice. The JZL195 enhancement of stress-induced analgesia in CCI mice was abolished by AM281 but was unaffected by naltrexone. These findings indicate that the acute opioid-mediated analgesic response to a psychological stressor is disrupted in a nerve-injury model of neuropathic pain. Importantly, this impairment of stress-induced analgesia was rescued by blockade of endocannabinoid breakdown via a cannabinoid CB1 receptor dependent mechanism. These findings suggest that subthreshold treatment with endocannabinoid degradation blockers could be used to alleviate the disruption of endogenous pain control systems in a neuropathic pain state.

Cannabis constituents for chronic neuropathic pain; reconciling the clinical and animal evidence

Chronic neuropathic pain is a debilitating pain syndrome caused by damage to the nervous system that is poorly served by current medications. Given these problems, clinical studies have pursued extracts of the plant Cannabis sativa as alternative treatments for this condition. The vast majority of these studies have examined cannabinoids which contain the psychoactive constituent delta-9-tetrahydrocannabinol (THC). While there have been some positive findings, meta-analyses of this clinical work indicates that this effectiveness is limited and hampered by side-effects. This review focuses on how recent preclinical studies have predicted the clinical limitations of THC-containing cannabis extracts, and importantly, point to how they might be improved. This work highlights the importance of targeting channels and receptors other than cannabinoid CB1 receptors which mediate many of the side-effects of cannabis.

Molecular basis of FAAH-OUT-associated human pain insensitivity

Chronic pain affects millions of people worldwide and new treatments are needed urgently. One way to identify novel analgesic strategies is to understand the biological dysfunctions that lead to human inherited pain insensitivity disorders. Here we report how the recently discovered brain and dorsal root ganglia-expressed FAAH-OUT long non-coding RNA (lncRNA) gene, which was found from studying a pain-insensitive patient with reduced anxiety and fast wound healing, regulates the adjacent key endocannabinoid system gene FAAH, which encodes the anandamide-degrading fatty acid amide hydrolase enzyme. We demonstrate that the disruption in FAAH-OUT lncRNA transcription leads to DNMT1-dependent DNA methylation within the FAAH promoter. In addition, FAAH-OUT contains a conserved regulatory element, FAAH-AMP, that acts as an enhancer for FAAH expression. Furthermore, using transcriptomic analyses in patient-derived cells we have uncovered a network of genes that are dysregulated from disruption of the FAAH-FAAH-OUT axis, thus providing a coherent mechanistic basis to understand the human phenotype observed. Given that FAAH is a potential target for the treatment of pain, anxiety, depression and other neurological disorders, this new understanding of the regulatory role of the FAAH-OUT gene provides a platform for the development of future gene and small molecule therapies.

Mechanistic Modeling of Monoglyceride Lipase Covalent Modification Elucidates the Role of Leaving Group Expulsion and Discriminates Inhibitors with High and Low Potency

Inhibition of monoglyceride lipase (MGL), also known as monoacylglycerol lipase (MAGL), has emerged as a promising approach for treating neurological diseases. To gain useful insights in the design of agents with balanced potency and reactivity, we investigated the mechanism of MGL carbamoylation by the reference triazole urea SAR629 (IC₅₀ = 0.2 nM) and two recently described inhibitors featuring a pyrazole (IC₅₀ = 1800 nM) or a 4-cyanopyrazole (IC₅₀ = 8 nM) leaving group (LG), using a hybrid quantum mechanics/molecular mechanics (QM/MM) approach. Opposite to what was found for substrate 2-arachidonoyl-sn-glycerol (2-AG), covalent modification of MGL by azole ureas is controlled by LG expulsion. Simulations indicated that changes in the electronic structure of the LG greatly affect reaction energetics with triazole and 4-cyanopyrazole inhibitors following a more accessible carbamoylation path compared to the unsubstituted pyrazole derivative. The computational protocol provided reaction barriers able to discriminate between MGL inhibitors with different potencies. These results highlight how QM/MM simulations can contribute to elucidating structure–activity relationships and provide insights for the design of covalent inhibitors.

The endocannabinoid system and drug-associated contextual memories

Drug abuse and addiction can be initiated and reinstated by contextual stimuli previously paired with the drug use. The influence exerted by the context on drug-seeking behaviour can be modelled in experimental animals with place-conditioning protocols. Here, we review the effects of cannabinoids in place conditioning and the therapeutic potential of the endocannabinoid system for interfering with drug-related memories. The phytocannabinoid Δ9-tetrahydrocannabinol (THC) tends to induce conditioned place preference (CPP) at low doses and conditioned place aversion at high doses; cannabidiol is devoid of any effect, yet it inhibits CPP induced by some drugs. Synthetic CB1 receptor agonists tend to recapitulate the biphasic profile observed with THC, whereas selective antagonists/inverse agonists inhibit CPP induced by cocaine, nicotine, alcohol and opioids. However, their therapeutic use is limited by potential psychiatric side effects. The CB2 receptor has also attracted attention, because selective CB2 receptor agonists inhibit cocaine-induced CPP. Inhibitors of endocannabinoid membrane transport and hydrolysis yield mixed results. In targeting the endocannabinoid system for developing new treatments for drug addiction, future research should focus on 'neutral' CB1 receptor antagonists and CB2 receptor agonists. Such compounds may offer a well-tolerated pharmacological profile and curb addiction by preventing drug-seeking triggered by conditioned contextual cues.

Mechanisms of endocannabinoid control of synaptic plasticity

The endogenous cannabinoid transmitter system regulates synaptic transmission throughout the nervous system. Unlike conventional transmitters, specific stimuli induce synthesis of endocannabinoids (eCBs) in the postsynaptic neuron, and these travel backwards to modulate presynaptic inputs. In doing so, eCBs can induce short-term changes in synaptic strength and longer-term plasticity. While this eCB regulation is near ubiquitous, it displays major regional and synapse specific variations with different synapse specific forms of short-versus long-term plasticity throughout the brain. These differences are due to the plethora of pre- and postsynaptic mechanisms which have been implicated in eCB signalling, the intricacies of which are only just being realised. In this review, we shall describe the current understanding and highlight new advances in this area, with a focus on the retrograde action of eCBs at CB1 receptors (CB₁Rs).

The endocannabinoid system - current implications for drug development

In this review, the state of the art for compounds affecting the endocannabinoid (eCB) system is described with a focus on the treatment of pain. Amongst directly acting CB receptor ligands, clinical experience with ∆⁹ -tetrahydracannabinol and medical cannabis in chronic non-cancer pain indicates that there are differences between the benefits perceived by patients and the at best modest effect seen in meta-analyses of randomized controlled trials. The reason for this difference is not known but may involve differences in the type of patients that are recruited, the study conditions that are chosen and the degree to which biases such as reporting bias are operative. Other directly acting CB receptor ligands such as biased agonists and allosteric receptor modulators have not yet reached the clinic. Amongst indirectly acting compounds targeting the enzymes responsible for the synthesis and catabolism of the eCBs anandamide and 2-arachidonoylglycerol, fatty acid amide hydrolase (FAAH) inhibitors have been investigated clinically but were per se not useful for the treatment of pain, although they may be useful for the treatment of post-traumatic stress disorder and cannabis use disorder. Dual-acting compounds targeting this enzyme and other targets such as cyclooxygenase-2 or transient potential vanilloid receptor 1 may be a way forward for the treatment of pain.

The endocannabinoid system in modulating fear, anxiety, and stress

The endocannabinoid system is widely expressed in the limbic system, prefrontal cortical areas, and brain structures regulating neuroendocrine stress responses, which explains the key role of this system in the control of emotions. In this review, we update recent advances on the function of the endocannabinoid system in determining the value of fear-evoking stimuli and promoting appropriate behavioral responses for stress resilience. We also review the alterations in the activity of the endocannabinoid system during fear, stress, and anxiety, and the pathophysiological role of each component of this system in the control of these protective emotional responses that also trigger pathological emotional disorders. In spite of all the evidence, we have not yet taken advantage of the therapeutic implications of this important role of the endocannabinoid system, and possible future strategies to improve the treatment of these emotional disorders are discussed.


Advances in the discovery of fatty acid amide hydrolase inhibitors: what does the future hold?

INTRODUCTION

Fatty acid amide hydrolase (FAAH) is a membrane-bound enzyme, that inactivates endogenous signaling lipids of the fatty acid amide family, including the endocannabinoid anandamide (N-arachidonoylethanolamine, AEA). The latter compound has been shown to regulate a number of important pathophysiological conditions in humans, like feeding, obesity, immune response, reproductive events, motor coordination, and neurological disorders. Hence, direct manipulation of the endocannabinoid tone is thought to have therapeutic potential. A new opportunity to develop effective drugs may arise from multi-target directed ligand (MTDL) strategies, which brings the concept that a single compound can recognize different targets involved in the cascade of pathophysiological events.

AREAS COVERED

This review reports the latest advances in the development of new single targeted and dual-targeted FAAH inhibitors over the past 5 years.

EXPERT OPINION

In recent years, several FAAH inhibitors have been synthesized and investigated, yet to date none of them has reached the market as a systemic drug. Due to the diligence of inherent redundancy and robustness in many biological networks and pathways, multitarget inhibitors present a new prospect in the pharmaceutical industry for treatment of complex diseases.

Endogenous cannabinoid modulation of restraint stress-induced analgesia in thermal nociception

It is thought that endogenous cannabinoids have a role in the analgesia induced by specific forms of stress. We examined if the role of endogenous cannabinoids is also dependent upon the mode of nociception, and whether this could be altered by drugs which block their enzymatic degradation. In C57BL/6 mice, restraint stress produced analgesia in the hot-plate and plantar tests, two thermal pain assays that engage distinct supraspinal and spinal nociceptive pathways. Stress-induced analgesia in the hot-plate test was abolished by pre-treatment with the opioid receptor antagonist naltrexone but was unaffected by the cannabinoid receptor antagonist 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281). By contrast, stress-induced analgesia in the plantar test was abolished by pre-treatment with naltrexone plus AM281, but not by either antagonist individually. Remarkably, inhibiting the breakdown of endocannabinoids, with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195, rescued stress-induced analgesia in the hotplate test when endogenous opioid signalling was blocked by naltrexone. Furthermore, JZL195 recruited analgesia induced by sub-threshold restraint stress in both thermal pain assays. These findings indicate the role of endocannabinoids in stress-induced analgesia differs with the type of thermal pain behaviour. However, by inhibiting their breakdown, endocannabinoids can be recruited to substitute for endogenous opioid signalling when their activity is blocked, indicating a degree of redundancy between opioid and cannabinoid systems. Together these data suggest targeting endocannabinoid breakdown could provide an alternative, or adjuvant to mainstream analgesics such as opioids.

Adult Cellular Neuroadaptations Induced by Adolescent THC Exposure in Female Rats Are Rescued by Enhancing Anandamide Signaling

BACKGROUND

In rodent models, chronic exposure to cannabis' psychoactive ingredient, Δ9-tetrahydrocannabinol, during adolescence leads to abnormal behavior in adulthood. In female rats, this maladaptive behavior is characterized by endophenotypes for depressive-like and psychotic-like disorders as well as cognitive deficits. We recently reported that most depressive-like behaviors triggered by adolescent Δ9-tetrahydrocannabinol exposure can be rescued by manipulating endocannabinoid signaling in adulthood with the anandamide-inactivating enzyme FAAH inhibitor, URB597. However, the molecular mechanisms underlying URB597's antidepressant-like properties remain to be established.

METHODS

Here we examined the impact of adult URB597 treatment on the cellular and functional neuroadaptations that occurred in the prefrontal cortex and dentate gyrus of the hippocampus upon Δ9-tetrahydrocannabinol during adolescence through biochemical, morphofunctional, and electrophysiological studies.

RESULTS

We found that the positive action of URB597 is associated with the rescue of Δ9-tetrahydrocannabinol-induced deficits in endocannabinoid-mediated signaling and synaptic plasticity in the prefrontal cortex and the recovery of functional neurogenesis in the dentate gyrus of the hippocampus. Moreover, the rescue property of URB597 on depressive-like behavior requires the activity of the CB1 cannabinoid receptor.

CONCLUSIONS

By providing novel insights into the cellular and molecular mechanisms of URB597 at defined cortical and hippocampal circuits, our results highlight that positive modulation of endocannabinoid-signaling could be a strategy for treating mood alterations secondary to adolescent cannabis use.

Enhanced endocannabinoid tone as a potential target of pharmacotherapy

The endocannabinoid system is up-regulated in numerous pathophysiological states such as inflammatory, neurodegenerative, gastrointestinal, metabolic and cardiovascular diseases, pain, and cancer. It has been suggested that this phenomenon primarily serves an autoprotective role in inhibiting disease progression and/or diminishing signs and symptoms. Accordingly, enhancement of endogenous endocannabinoid tone by inhibition of endocannabinoid degradation represents a promising therapeutic approach for the treatment of many diseases. Importantly, this allows for the avoidance of unwanted psychotropic side effects that accompany exogenously administered cannabinoids. The effects of endocannabinoid metabolic pathway modulation are complex, as endocannabinoids can exert their actions directly or via numerous metabolites. The two main strategies for blocking endocannabinoid degradation are inhibition of endocannabinoid-degrading enzymes and inhibition of endocannabinoid cellular uptake. To date, the most investigated compounds are inhibitors of fatty acid amide hydrolase (FAAH), an enzyme that degrades the endocannabinoid anandamide. However, application of FAAH inhibitors (and consequently other endocannabinoid degradation inhibitors) in medicine became questionable due to a lack of therapeutic efficacy in clinical trials and serious adverse effects evoked by one specific compound. In this paper, we discuss multiple pathways of endocannabinoid metabolism, changes in endocannabinoid levels across numerous human diseases and corresponding experimental models, pharmacological strategies for enhancing endocannabinoid tone and potential therapeutic applications including multi-target drugs with additional targets outside of the endocannabinoid system (cyclooxygenase-2, cholinesterase, TRPV1, and PGF_2α-EA receptors), and currently used medicines or medicinal herbs that additionally enhance endocannabinoid levels. Ultimately, further clinical and preclinical studies are warranted to develop medicines for enhancing endocannabinoid tone.

In Vitro and in Vivo Evaluation of 11C-Labeled Azetidinecarboxylates for Imaging Monoacylglycerol Lipase by PET Imaging Studies

Monoacylglycerol lipase (MAGL) is the principle enzyme for metabolizing endogenous cannabinoid ligand 2-arachidonoyglycerol (2-AG). Blockade of MAGL increases 2-AG levels, resulting in subsequent activation of the endocannabinoid system, and has emerged as a novel therapeutic strategy to treat drug addiction, inflammation, and neurodegenerative diseases. Herein we report a new series of MAGL inhibitors, which were radiolabeled by site-specific labeling technologies, including 11C-carbonylation and spirocyclic iodonium ylide (SCIDY) radiofluorination. The lead compound [11C]10 (MAGL-0519) demonstrated high specific binding and selectivity in vitro and in vivo. We also observed unexpected washout kinetics with these irreversible radiotracers, in which in vivo evidence for turnover of the covalent residue was unveiled between MAGL and azetidine carboxylates. This work may lead to new directions for drug discovery and PET tracer development based on azetidine carboxylate inhibitor scaffold.

Anti-Inflammatory Effects of Omega-3 Fatty Acids in the Brain: Physiological Mechanisms and Relevance to Pharmacology

Classically, polyunsaturated fatty acids (PUFA) were largely thought to be relatively inert structural components of brain, largely important for the formation of cellular membranes. Over the past 10 years, a host of bioactive lipid mediators that are enzymatically derived from arachidonic acid, the main n-6 PUFA, and docosahexaenoic acid, the main n-3 PUFA in the brain, known to regulate peripheral immune function, have been detected in the brain and shown to regulate microglia activation. Recent advances have focused on how PUFA regulate the molecular signaling of microglia, especially in the context of neuroinflammation and behavior. Several active drugs regulate brain lipid signaling and provide proof of concept for targeting the brain. Because brain lipid metabolism relies on a complex integration of diet, peripheral metabolism, including the liver and blood, which supply the brain with PUFAs that can be altered by genetics, sex, and aging, there are many pathways that can be disrupted, leading to altered brain lipid homeostasis. Brain lipid signaling pathways are altered in neurologic disorders and may be viable targets for the development of novel therapeutics. In this study, we discuss in particular how n-3 PUFAs and their metabolites regulate microglia phenotype and function to exert their anti-inflammatory and proresolving activities in the brain.

Enantiospecific Allosteric Modulation of Cannabinoid 1 Receptor

The cannabinoid 1 receptor (CB1R) is one of the most widely expressed metabotropic G protein-coupled receptors in brain, and its participation in various (patho)physiological processes has made CB1R activation a viable therapeutic modality. Adverse psychotropic effects limit the clinical utility of CB1R orthosteric agonists and have promoted the search for CB1R positive allosteric modulators (PAMs) with the promise of improved drug-like pharmacology and enhanced safety over typical CB1R agonists. In this study, we describe the synthesis and in vitro and ex vivo pharmacology of the novel allosteric CB1R modulator GAT211 (racemic) and its resolved enantiomers, GAT228 (R) and GAT229 (S). GAT211 engages CB1R allosteric site(s), enhances the binding of the orthosteric full agonist [³H]CP55,490, and reduces the binding of the orthosteric antagonist/inverse agonist [³H]SR141716A. GAT211 displayed both PAM and agonist activity in HEK293A and Neuro2a cells expressing human recombinant CB1R (hCB1R) and in mouse-brain membranes rich in native CB1R. GAT211 also exhibited a strong PAM effect in isolated vas deferens endogenously expressing CB1R. Each resolved and crystallized GAT211 enantiomer showed a markedly distinctive pharmacology as a CB1R allosteric modulator. In all biological systems examined, GAT211's allosteric agonist activity resided with the R-(+)-enantiomer (GAT228), whereas its PAM activity resided with the S-(-)-enantiomer (GAT229), which lacked intrinsic activity. These results constitute the first demonstration of enantiomer-selective CB1R positive allosteric modulation and set a precedent whereby enantiomeric resolution can decisively define the molecular pharmacology of a CB1R allosteric ligand.

Chronic inhibition of fatty acid amide hydrolase by URB597 produces differential effects on cardiac performance in normotensive and hypertensive rats

BACKGROUND AND PURPOSE

Fatty acid amide hydrolase (FAAH) inhibitors are postulated to possess anti-hypertensive potential, because their acute injection decreases BP in spontaneously hypertensive rats (SHR), partly through normalization of cardiac contractile function. Here, we examined whether the potential hypotensive effect of chronic FAAH inhibition by URB597 in hypertensive rats correlated with changes in cardiac performance.

EXPERIMENTAL APPROACH

Experiments were performed using perfused hearts and left atria isolated from 8- to 10-week-old SHR, age-matched deoxycorticosterone acetate (DOCA)-salt rats and normotensive controls chronically treated with URB597 (1 mg·kg^-1 ) or vehicle.

KEY RESULTS

URB597 decreased BP only in the DOCA-salt rats, along with a reduction of ventricular hypertrophy and diastolic stiffness, determined in hypertension. We also observed normalization of the negative inotropic atrial response to the cannabinoid receptor agonist CP55940. In the SHR model, URB597 normalized (atria) and enhanced (hearts) the positive ino- and chronotropic effects of the β-adrenoceptor agonist isoprenaline respectively. Ventricular CB₁ and CB₂ receptor expression was decreased only in the DOCA-salt model, whereas FAAH expression was reduced in both models. URB597 caused translocation of CB₁ receptor immunoreactivity to the intercalated discs in the hearts of SHR. URB597 increased cardiac diastolic stiffness and modified the ino- and lusitropic effects of isoprenaline in normotensive rats.

CONCLUSION AND IMPLICATIONS

Hypotensive effect of chronic FAAH inhibition depend on the model of hypertension and partly correlate with improved cardiac performance. In normotensive rats, chronic FAAH inhibition produced several side-effects. Thus, the therapeutic potential of these agents should be interpreted cautiously.

Metabolism of the Endocannabinoid Anandamide: Open Questions after 25 Years

Cannabis extracts have been used for centuries, but its main active principle ∆⁹-tetrahydrocannabinol (THC) was identified about 50 years ago. Yet, it is only 25 years ago that the first endogenous ligand of the same receptors engaged by the cannabis agents was discovered. This “endocannabinoid (eCB)” was identified as N-arachidonoylethanolamine (or anandamide (AEA)), and was shown to have several receptors, metabolic enzymes and transporters that altogether drive its biological activity. Here I report on the latest advances about AEA metabolism, with the aim of focusing open questions still awaiting an answer for a deeper understanding of AEA activity, and for translating AEA-based drugs into novel therapeutics for human diseases.

Endocannabinoid Turnover

In this review, we consider the biosynthetic, hydrolytic, and oxidative metabolism of the endocannabinoids anandamide and 2-arachidonoylglycerol. We describe the enzymes associated with these events and their characterization. We identify the inhibitor profile for these enzymes and the status of therapeutic exploitation, which to date has been limited to clinical trials for fatty acid amide hydrolase inhibitors. To bring the review to a close, we consider whether point block of a single enzyme is likely to be the most successful approach for therapeutic exploitation of the endocannabinoid system.

Plasma levels of the endocannabinoid anandamide, related N-acylethanolamines and linoleic acid-derived oxylipins in patients with migraine

There is evidence that patients with migraine have deficient levels of the endogenous cannabinoid receptor ligand anandamide (AEA). It is not known, however, if this is a localised or generalised phenomenon. In the present study, levels of AEA, related N-acylethanolamines (NAEs) and linoleic acid-derived oxylipins have been measured in the blood of 26 healthy women and 38 women with migraine (26 with aura, 12 without aura) who were matched for age and body-mass index. Blood samples were taken on two occasions: the first sample near the start of the menstrual cycle (when present) and the second approximately fourteen days later. For a subset of migraine patients, two additional blood samples were taken, one during a migraine attack and one approximately 1 month later (to be at the same stage in the menstrual cycle, when present). NAEs and oxylipins were measured by liquid chromatography coupled to mass spectrometry. Twenty-nine lipids were quantified, of which 16 were found to have a high reproducibility of measurement. There were no significant differences in the levels of AEA, the related NAEs stearoylethanolamide and oleoylethanolamide or any of the nine linoleic acid-derived oxylipins measured either between migraine patients with vs. without aura, or between controls and migraine patients (after stratification to take into account whether or not the individuals had regular menstruation cycles) in either of the first two samples. Levels of linoleoylethanolamide were lower in the patients with vs. without aura on the second sample but not in the first sample, but the biological importance of this finding is unclear. Due to time-dependent increases in their concentrations ex vivo prior to centrifugation, AEA and oleoylethanolamide levels in the samples collected during migraine attacks were not analysed, but for the other fourteen lipids, there were no significant differences in plasma concentrations during migraine vs. one month later. It is concluded that migraine is not associated with a generalised (as opposed to localised) deficiency in these lipids.

Combination Chemistry: Structure-Activity Relationships of Novel Psychoactive Cannabinoids

Originally developed as research tools for use in structure-activity relationship studies, synthetic cannabinoids contributed to significant scientific advances in the cannabinoid field. Unfortunately, a subset of these compounds was diverted for recreational use beginning in the early 2000s. As these compounds were banned, they were replaced with additional synthetic cannabinoids with increasingly diverse chemical structures. This chapter focuses on integration of recent results with those covered in previous reviews. Whereas most of the early compounds were derived from the prototypic naphthoylindole JWH-018, currently popular synthetic cannabinoids include tetramethylcyclopropyl ketones and indazole-derived cannabinoids (e.g., AB-PINACA, AB-CHMINACA). Despite their structural differences, psychoactive synthetic cannabinoids bind with high affinity to CB1 receptors in the brain and, when tested, have been shown to activate these receptors and to produce a characteristic profile of effects, including suppression of locomotor activity, antinociception, hypothermia, and catalepsy, as well as Δ9-tetrahydrocannabinol (THC)-like discriminative stimulus effects in mice. When they have been tested, synthetic cannabinoids are often found to be more efficacious at activation of the CB1 receptor and more potent in vivo. Further, their chemical alteration by thermolysis during use and their uncertain stability and purity may result in exposure to degradants that differ from the parent compound contained in the original product. Consequently, while their intoxicant effects may be similar to those of THC, use of synthetic cannabinoids may be accompanied by unpredicted, and sometimes harmful, effects.

1-Heteroarylpropan-2-ones as inhibitors of fatty acid amide hydrolase: Studies on structure-activity relationships and metabolic stability

The serine hydrolase fatty acid amide hydrolase (FAAH) catalyzes the degradation of the endocannabinoid anandamide, which possesses analgesic and anti-inflammatory effects. A new series of 1-heteroarylpropan-2-ones was synthesized and evaluated for FAAH inhibition. Structure-activity relationship studies revealed that 1H-benzotriazol-1-yl, 1H-7-azabenzotriazol-1-yl, 1H-tetrazol-1-yl and 2H-tetrazol-2-yl substituents have the highest impact on inhibitory potency. Furthermore, attempts were made to increase the limited metabolic stability of the ketone functionality of these compounds towards metabolic reduction by introduction of shielding alkyl substituents in proximity of this serine reactive group.

Fatty Acid Amide Hydrolase Binding in Brain of Cannabis Users: Imaging With the Novel Radiotracer [11C]CURB

BACKGROUND

One of the major mechanisms for terminating the actions of the endocannabinoid anandamide is hydrolysis by fatty acid amide hydrolase (FAAH), and inhibitors of the enzyme were suggested as potential treatment for human cannabis dependence. However, the status of brain FAAH in cannabis use disorder is unknown.

METHODS

Brain FAAH binding was measured with positron emission tomography and [¹¹C]CURB in 22 healthy control subjects and ten chronic cannabis users during early abstinence. The FAAH genetic polymorphism (rs324420) and blood, urine, and hair levels of cannabinoids and metabolites were determined.

RESULTS

In cannabis users, FAAH binding was significantly lower by 14%-20% across the brain regions examined than in matched control subjects (overall Cohen's d = 0.96). Lower binding was negatively correlated with cannabinoid concentrations in blood and urine and was associated with higher trait impulsiveness.

CONCLUSIONS

Lower FAAH binding levels in the brain may be a consequence of chronic and recent cannabis exposure and could contribute to cannabis withdrawal. This effect should be considered in the development of novel treatment strategies for cannabis use disorder that target FAAH and endocannabinoids. Further studies are needed to examine possible changes in FAAH binding during prolonged cannabis abstinence and whether lower FAAH binding predates drug use.

A Personal Retrospective: Elevating Anandamide (AEA) by Targeting Fatty Acid Amide Hydrolase (FAAH) and the Fatty Acid Binding Proteins (FABPs)

This perspective was adapted from a Career Achievement Award talk given at the International Cannabinoid Research Society Symposium in Bukovina, Poland on June 27, 2016. As a biochemist working in the neurosciences, I was always fascinated with neurotransmitter inactivation. In 1993 we identified an enzyme activity that breaks down anandamide. We called the enzyme anandamide amidase, now called FAAH. We and other laboratories developed FAAH inhibitors that were useful reagents that also proved to have beneficial physiological effects and until recently, new generations of inhibitors were in clinical trials. Nearly all neurotransmitters are water soluble and as such, require a transmembrane protein transporter to pass through the lipid membrane for inactivation inside the cell. However, using model systems, we and others have shown that this is unnecessary for anandamide, an uncharged hydrophobic molecule that readily diffuses across the cellular membrane. Interestingly, its uptake is driven by the concentration gradient resulting from its breakdown mainly by FAAH localized in the endoplasmic reticulum. We identified the FABPs as intracellular carriers that "solubilize" anandamide, transporting anandamide to FAAH. Compounds that bind to FABPs block AEA breakdown, raising its level. The cannabinoids (THC and CBD) also were discovered to bind FABPs and this may be one of the mechanisms by which CBD works in childhood epilepsy, raising anandamide levels. Targeting FABPs may be advantageous since they have some tissue specificity and do not require reactive serine hydrolase inhibitors, as does FAAH, with potential for off-target reactions. At the International Cannabis Research Society Symposium in 1992, Raphe Mechoulam revealed that his laboratory isolated an endogenous lipid molecule that binds to the CB1 receptor (cannabinoid receptor type 1) and this became the milestone paper published in December of that year describing anandamide (AEA, Devane et al., 1992). As to be expected, this discovery raised the issues of AEA's synthesis and breakdown.

Challenges to develop novel anti-inflammatory and analgesic drugs

Chronic inflammatory diseases and persistent pain of different origin represent common medical, social, and economic burden, and their pharmacotherapy is still an unresolved issue. Therefore, there is a great and urgent need to develop anti-inflammatory and analgesic agents with novel mechanisms of action, but it is a very challenging task. The main problem is the relatively large translational gap between the preclinical experimental data and the clinical results due to characteristics of the models, difficulties with the investigational techniques particularly for pain, as well as species differences in the mechanisms. We summarize here the current state-of-the-art medication and related ongoing strategies, and the novel targets with lead molecules under clinical development. The first members of the gold-standard categories, such as nonsteroidal anti-inflammatory drugs, glucocorticoids, and opioids, were introduced decades ago, and since then very few drugs with novel mechanisms of action have been successfully taken to the clinics despite considerable development efforts. Several biologics targeting different key molecules have provided breakthrough in some autoimmune/inflammatory diseases, but they are expensive, only parenterally available, their long-term side effects often limit their administration, and they do not effectively reduce pain. Some kinase inhibitors and phosphodiesterase-4 blockers have recently been introduced as new directions. There are in fact some promising novel approaches at different clinical stages of drug development focusing on transient receptor potential vanilloid 1/ankyrin 1 channel antagonism, inhibition of voltage-gated sodium/calcium channels, several enzymes (kinases, semicarbazide-sensitive amine oxidases, and matrix metalloproteinases), cytokines/chemokines, transcription factors, nerve growth factor, and modulation of several G protein-coupled receptors (cannabinoids, purinoceptors, and neuropeptides). WIREs Nanomed Nanobiotechnol 2017, 9:e1427. doi: 10.1002/wnan.1427 For further resources related to this article, please visit the WIREs website.

Synthesis and Preclinical Evaluation of Sulfonamido-based [(11)C-Carbonyl]-Carbamates and Ureas for Imaging Monoacylglycerol Lipase

Monoacylglycerol lipase (MAGL) is a 33 kDa member of the serine hydrolase superfamily that preferentially degrades 2-arachidonoylglycerol (2-AG) to arachidonic acid in the endocannabinoid system. Inhibition of MAGL is not only of interest for probing the cannabinoid pathway but also as a therapeutic and diagnostic target for neuroinflammation. Limited attempts have been made to image MAGL in vivo and a suitable PET ligand for this target has yet to be identified and is urgently sought to guide small molecule drug development in this pathway. Herein we synthesized and evaluated the physiochemical properties of an array of eleven sulfonamido-based carbamates and ureas with a series of terminal aryl moieties, linkers and leaving groups. The most potent compounds were a novel MAGL inhibitor, N-((1-(1H-1,2,4-triazole-1-carbonyl)piperidin-4-yl) methyl)-4-chlorobenzenesulfonamide (TZPU; IC₅₀ = 35.9 nM), and the known inhibitor 1,1,1,3,3,3-hexafluoropropan-2-yl 4-(((4-chlorophenyl)sulfonamido) methyl)piperidine-1-carboxylate (SAR127303; IC₅₀ = 39.3 nM), which were also shown to be selective for MAGL over fatty acid amide hydrolase (FAAH), and cannabinoid receptors (CB1 & CB2). Both of these compounds were radiolabeled with carbon-11 via [¹¹C]COCl₂, followed by comprehensive ex vivo biodistribution and in vivo PET imaging studies in normal rats to determine their brain permeability, specificity, clearance and metabolism. Whereas TZPU did not show adequate specificity to warrant further evaluation, [¹¹C]SAR127303 was advanced for preliminary PET neuroimaging studies in nonhuman primate. The tracer showed good brain permeability (ca. 1 SUV) and heterogeneous regional brain distribution which is consistent with the distribution of MAGL.