NMDA showed signs of counteracting THC’s effect, and surprisingly this appeared to be CB1R-independent but CB2R-dependent. As sulpiride reduced the repetitive swimming, the THC-elicited behavior may indicate a psychosis-like state.Anandamide, the naturally occurring amide of arachidonic acid with ethanolamine, meets all key criteria of an endogenous cannabinoid substance: it is released on demand by stimulated neurons; it activates cannabinoid receptors with high affinity; and it is rapidly eliminated through a two-step process consisting of carrier-mediated transport followed by intracellular hydrolysis. Anandamide hydrolysis is catalyzed by the enzyme fatty acid amide hydrolase , a membrane-bound serine hydrolase that also cleaves other bio-active fatty acid ethanolamides such as oleoylethanolamide and palmitoylethanolamide. Mutant mice lacking the gene encoding FAAH cannot metabolize anandamide9 and, although fertile and generally normal, show signs of enhanced anandamide activity at cannabinoid receptors such as reduced pain sensation. This is suggestive that drugs targeting FAAH may heighten the tonic actions of anandamide, while possibly avoiding the multiple and often unwanted effects produced by ∆9 -tetrahydrocannabinol and other direct-acting cannabinoid agonists. To test this hypothesis, potent, selective and systemically active inhibitors of intracellular FAAH activity are needed. However, most current inhibitors of this enzyme lack the target selectivity and biological availability required for in vivo studies, whereas newer compounds, though promising, have not yet been characterized. Thus, the therapeutic potential of FAAH inhibition remains essentially unexplored.Kinetic analyses and dialysis experiments indicate that compounds 4 and 6 may inhibit FAAH activity through an irreversible interaction with the enzyme , possibly due to a nucleophilic attack of an active serine residue on the carbamate group. This mechanism sets the present compounds apart from the α-keto heterocycle derivatives described previously, which act as competitive FAAH inhibitors. A further indication of such a distinction is that in the α-keto heterocycle series potency is strongly dependent on the hydrophobicity of the flexible acyl chain,greenhouse benches whereas in the carbamate series potency is modulated by the shape of the rigid aromatic moiety. Accordingly, when we replaced the biphenyl of compound 5 with a 5-phenylpentyl group, representing the most effective acyl chain in the α-keto heterocycle series, the inhibitory activity was lost . Compounds 4 and 6 blocked the FAAHcatalyzed hydrolysis of exogenous [3 H]anandamide in primary cultures of intact cortical neurons, with IC50 values that paralleled those obtained in membrane preparations.
By contrast, compound 8, an analog of URB532 that does not inhibit FAAH in membranes , had no such effect . Moreover, URB532 and URB597 selectively impaired the breakdown of [ 3 H]anandamide without reducing its carrier-mediated uptake, causing non-metabolized [3 H]anandamide to accumulate in, and eventually exit from, the neurons. Thus, after a 4-minute incubation with [3 H]anandamide, the intracellular content of non-metabolized [3 H]anandamide was higher in inhibitortreated neurons than in control neurons . As expected, the anandamide transport blocker N-arachidonamide had an opposite effect, substantially reducing [3 H]anandamide internalization4 . When neurons treated with URB597 were exposed for 4 minutes to [3 H]anandamide and then incubated for 15 minutes in an [ 3 H]anandamide-free solution, 42.6 ± 8.7% of the accumulated [ 3 H]anandamide was released back into the medium . This process was linear with time and was not inhibited by AM404 , indicating that it occurred through passive diffusion rather than reverse transport. No such time-dependent release was observed in control neurons, the medium of which contained only residual levels of [3 H]anandamide carried over from the pre-incubation period. These studies identify a new class of carbamate inhibitors of FAAH activity, which potently block anandamide breakdown in intact brain neurons. We have developed a new class of agents that prevents anandamide inactivation by targeting the intracellular enzymatic activity of FAAH. URB597, the most potent member of this class, inhibited FAAH activity with an IC50 value of 4 nM in brain membranes and 0.5 nM in intact neurons, and an ID50 value of 0.15 mg kg–1 after systemic administration in the rat. This compound had much greater selectivity for FAAH than other cannabinoid related targets, including cannabinoid receptors and MGL, an enzyme involved in the deactivation of the endogenous cannabinoid ester 2-AG . Such target discrimination was matched by a lack of overt cannabimimetic effects in vivo. Thus, at doses that almost abolished FAAH activity and substantially raised brain anandamide levels, URB597 and its analog URB532 did not evoke catalepsy, reduce body temperature or stimulate feeding, three key symptoms of cannabinoid intoxication in the rodent. Nevertheless, the compounds did elicit marked anxiolytic-like responses, which paralleled their ability to inactivate FAAH and were prevented by the CB1 receptor antagonist rimonabant. We interpret these findings to indicate that URB597 and URB532 may selectively modulate anxiety-like behaviors by enhancing the tonic actions of anandamide on a subset of CB1 receptors which may normally be engaged in controlling emotions.
Forebrain sites that might be implicated in such actions include the basolateral amygdala, the anterior cingulate cortex and the prefrontal cortex, all key elements of an “emotion circuit” that contains high densities of CB1 receptors. CB1 receptors in these structures are localized to the axon terminals of a sub-population of GABAergic interneurons, which also express the peptide cholecystokinin. The anxiogenic properties of CCK35 and the ability of CB1 agonists to inhibit K+ -evoked CCK release from hippocampal slices36 indicate that interactions between this peptide and anandamide may participate in the control of anxiety. In addition to their anxiolytic-like actions, URB597 and URB532 exerted modest anti-nociception in a model of acute pain, which also was sensitive to CB1 receptor blockade. These findings are similar to those reported for Faah–/– mice9 and support the proposed roles of anandamide in the intrinsic modulation of pain. However, as emotional states may strongly influence pain sensation, it is possible that anxiolysis might have contributed to the mild anti-nociceptive effects of the FAAH inhibitors. URB597 and URB532 increased brain anandamide levels without modifying those of the second endogenous cannabinoid, 2-AG. It is therefore likely that the pharmacological actions of these compounds, which are sensitive to the CB1 antagonist rimonabant, are primarily due to anandamide accumulation. But the FAAH inhibitors also produced large elevations in the levels of two anandamide analogs, palmitoylethanolamide and oleoylethanolamide, whose biological effects are independent of CB1 receptors. Thus, we cannot exclude the possibility that additional properties of URB597 and URB532, mediated by these fatty ethanolamides, remain to be discovered. Our results define a novel class of inhibitors of FAAH activity, which enhance endogenous anandamide signaling without directly interacting with cannabinoid receptors. The behavioral profile of these agents—characterized by anxiolysis and mild analgesia—reveals a key role for anandamide in the regulation of emotional states and indicates a new mechanistic approach to anti-anxiety therapy.Many meta-analyses and reviews have been published in recent years. Studies of cannabinoid efficacy vary greatly: some have tested whole-leaf marijuana ; others assess isolated compounds such as THC, or known/controlled combinations of plant-derived products , and some study only synthesized products like nabilone. These differences make comparisons difficult. A recent Cochrane-style review7 looked at the quality of evidence supporting the use of cannabinoids in CINV, as appetite stimulant in HIV/AIDS, in chronic pain, spasticity from MS or paraplegia, depression, anxiety, sleep problems, psychosis, glaucoma, or Tourette’s syndrome.
Seventy-nine randomized trials involving 6462 patients were identified. Moderate quality evidence supports the use of cannabinoids for chronic pain and spasticity. Low quality evidence supports the use of cannabinoids for CINV, wasting, sleep problems, and Tourette’s syndrome. Many other uses of cannabinoids are rationalized based on cultural traditions, small case series, open label trials, anecdote, or opinion. The American Academy of Neurology completed its own recent review and concluded that cannabinoids, particularly nabiximols,grower equipment provide small benefits for patients with MSrelated spasticity, central pain, and urinary symptoms but shows little evidence of efficacy for other neurologic conditions.Orrin Devinsky, a leading U.S. investigator in the use of CBD for treatment-refractory pediatric epilepsy, has published an open-label trial showing some efficacy that warrants randomized controlled trials. The already-mentioned National Academies monograph draws nearly identical conclusions to those above. Readers are referred to it for a comprehensive review of the evidence. There is also interesting preclinical data suggesting that cannabinoids may have a role in reversing opioid-associated hyperalgesia,may reduce craving and relapse risk in opioid dependence,and could help prevent or treat chemotherapy induced peripheral neuropathic pain.There is emerging, equivocal data regarding whether cannabinoids improve quality-of-life and reduce symptom burden or disease activity in inflammatory bowel disease.In a recent review of the common indications for use listed in state cannabis regulations, Alzheimer’s disease, amyotrophic lateral sclerosis , cachexia, cancer, Crohn’s/ inflammatory bowel disease , epilepsy, severe/chronic pain, glaucoma, hepatitis C, HIV/AIDS, MS, and post-traumatic stress disorder appear regularly. This listing suggests the degree to which state guidelines depart from, and generally exceed, evidence-based uses, and in turn the degree to which lawmaking may be influenced by popular beliefs or political processes. For the palliative care practitioner, these efficacy data are complicated not just by the generally low volume of only moderate quality evidence, and by state regulations that regularly depart from that evidence, but also increasingly by the growing reach of our field beyond cancer and HIV into the realms of adult degenerative diseases of the central nervous system, heart, lung, liver, digestive tract, and to serious non-cancer childhood disorders.
As the National Academies monograph and other recent excellent reviews suggest,recreational use of cannabinoids is particularly concerning in young people with still-developing brains, persons with preexisting mental illness, and those with existing substance abuse problems. In these populations regular cannabis use can unmask or hasten the onset of psychotic illness, is associated with reduced IQ, addiction/dependence, and a withdrawal syndrome. Other widely recognized sequela of regular/chronic use include dropping out of school, decreased motivation, socialization, and life-satisfaction, and chronic bronchitis. Other than bronchitis, the data regarding respiratory consequences of cannabis consumption are equivocal.The data on marijuana use and cardiac disease have not shown compelling evidence for concern. One recent 20-year comparison of daily tobacco versus marijuana smokers showed only increased risk for periodontal disease with cannabis, whereas tobacco users had expected increases in lung, cardiac, and metabolic risk factors. Cannabis appears to have some anti-inflammatory properties; according to the National Academies report, there is insufficient evidence to support any conclusions about the impacts of cannabis on other immune functions. Interesting public health data suggest that there may be significant trends toward decreases in opioid overdose deaths in states with legalized cannabis,fewer overall traffic deaths after legalization,and reductions in pre-versus-post legalization Medicare expenditures on prescription analgesics, sedative-hypnotics, anxiolytics, and other agents.A separate but important consideration is whether a standard risk-benefit analysis makes sense for palliative care patients when contemplating cannabis. Risks of over-generalization aside, most palliative care patients are NOT young people with unlimited life prospects who are early in their school years or social developmental trajectories, are NOT climbing career ladders, are NOT parenting small dependent children, or operating heavy industrial machinery. Thus, I would argue that this brief summary of safety risks should, in the palliative care clinical setting, be balanced against the exigencies of attempting to help patients achieve symptom relief in the context of serious illness, particularly if they are facing difficult symptoms not responsive to conventional treatments.In addition to those scientific and public policy matters outlined above, there are many other uncertainties facing the palliative care clinician and his/her patient contemplating cannabis. Chief among them is what the patient actually receives when he/she purchases medical marijuana at a dispensary: a recent small study of marijuana edibles showed accurate labeling in only 17%.The majority of products were ‘‘over labeled’’ , while 23% were ‘‘underlabeled’’ . Geographic differences were noted as well, with Los Angeles dispensaries showing a significant inclination to underlabel. FDA has recently published a report of its analysis of CBD products purchased over the internet, which showed most of the products to contain little or no active ingredient.These findings undermine a fundamental element of physician practice, namely the ability to identify and recommend specific, reliable doses of compounds. It should also be noted that under most of the state laws, physicians are not prescribing medical marijuana at all. Instead, they are asked to endorse, attest, or certify that in their professional judgment the patient has a disorder for which medical marijuana may have efficacy. This, too, is unfamiliar territory for many of us.