Design, synthesis, and structure-activity relationships of alkylcarbamic acid aryl esters, a new class of fatty acid amide hydrolase inhibitors

Tetrahydro-β-carboline derivatives targeting fatty acid amide hydrolase (FAAH) and transient receptor potential (TRP) channels

A series of twenty-five derivatives of tetrahydro-β-carbolines 1-3 was synthesized and assayed on FAAH and TRPV1 and TRPA1 channels. Four carbamates, that is, 5a,c,e, and 9b inhibited FAAH with significant potency and interacted also effectively with TRPV1 and TRPA1 nociceptive receptors, while ureas 7b,d,f, and 8a,b were endowed with specific submicromolar TRPV1 modulating activities.

Sulfonyl fluoride inhibitors of fatty acid amide hydrolase

Sulfonyl fluorides are known to inhibit esterases. Early work from our laboratory has identified hexadecyl sulfonylfluoride (AM374) as a potent in vitro and in vivo inhibitor of fatty acid amide hydrolase (FAAH). We now report on later generation sulfonyl fluoride analogs that exhibit potent and selective inhibition of FAAH. Using recombinant rat and human FAAH, we show that 5-(4-hydroxyphenyl)pentanesulfonyl fluoride (AM3506) has similar inhibitory activity for both the rat and the human enzyme, while rapid dilution assays and mass spectrometry analysis suggest that the compound is a covalent modifier for FAAH and inhibits its action in an irreversible manner. Our SAR results are highlighted by molecular docking of key analogs.

Antinociceptive effects of the N-acylethanolamine acid amidase inhibitor ARN077 in rodent pain models

Fatty acid ethanolamides (FAEs), which include palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), are endogenous agonists of peroxisome proliferator-activated receptor-α (PPAR-α) and important regulators of the inflammatory response. They are degraded in macrophages by the lysosomal cysteine amidase, N-acylethanolamine acid amidase (NAAA). Previous studies have shown that pharmacological inhibition of NAAA activity suppresses macrophage activation in vitro and causes marked anti-inflammatory effects in vivo, which is suggestive of a role for NAAA in the control of inflammation. It is still unknown, however, whether NAAA-mediated FAE deactivation might regulate pain signaling. The present study examined the effects of ARN077, a potent and selective NAAA inhibitor recently disclosed by our group, in rodent models of hyperalgesia and allodynia caused by inflammation or nerve damage. Topical administration of ARN077 attenuated, in a dose-dependent manner, heat hyperalgesia and mechanical allodynia elicited in mice by carrageenan injection or sciatic nerve ligation. The antinociceptive effects of ARN077 were prevented by the selective PPAR-α antagonist GW6471 and did not occur in PPAR-α-deficient mice. Furthermore, topical ARN077 reversed the allodynia caused by ultraviolet B radiation in rats, and this effect was blocked by pretreatment with GW6471. Sciatic nerve ligation or application of the proinflammatory phorbol ester 12-O-tetradecanoylphorbol 13-acetate decreased FAE levels in sciatic nerve and skin tissue, respectively. ARN077 reversed these biochemical effects. The results identify ARN077 as a potent inhibitor of intracellular NAAA activity, which is active in vivo by topical administration. The findings further suggest that NAAA regulates peripheral pain initiation by interrupting endogenous FAE signaling at PPAR-α.

Cannabinoid receptor subtypes 1 and 2 mediate long-lasting neuroprotection and improve motor behavior deficits after transient focal cerebral ischemia

The endocannabinoid system is crucially involved in the regulation of brain activity and inflammation. We have investigated the localization of cannabinoid CB1 and CB2 receptors in adult rat brains before and after focal cerebral ischemia due to endothelin-induced transient occlusion of the middle cerebral artery (eMCAO). Using immunohistochemistry, both receptor subtypes were identified in cortical neurons. After eMCAO, neuronal cell death was accompanied by reduced neuronal CB1 and CB2 receptor-linked immunofluorescence. In parallel, CB1 receptor was found in activated microglia/macrophages 3 days post eMCAO and in astroglia cells at days 3 and 7. CB2 receptor labeling was identified in activated microglia/macrophages or astroglia 3 and 7d ays post ischemia, respectively. In addition, immune competent CD45-positive cells were characterized by pronounced CB2 receptor staining 3 and 7 days post eMCAO. KN38-72717, a potent and selective CB1 and CB2 receptor agonist, revealed a significant, dose-dependent and long-lasting reduction of cortical lesion sizes due to eMCAO, when applied consecutively before, during and after eMCAO. In addition, severe motor deficits of animals suffering from eMCAO were significantly improved by KN38-7271. KN38-7271 remained effective, even if its application was delayed up to 6h post eMCAO. Finally, we show that the endocannabinoid system assembles a comprehensive machinery to defend the brain against the devastating consequences of cerebral ischemia. In summary, this study underlines the therapeutic potential of CB1 and/or CB2 receptor agonists against neurodegenerative diseases or injuries involving acute or chronic imbalances of cerebral blood flow and energy consumption.

Pretreatment with the monoacylglycerol lipase inhibitor URB602 protects from the long-term consequences of neonatal hypoxic-ischemic brain injury in rats

BACKGROUND

The endocannabinoids are emerging as natural brain protective substances that exert potentially beneficial effects in several neurological disorders by virtue of their hypothermic, immunomodulatory, vascular, antioxidant, and antiapoptotic actions. This study was undertaken to assess whether preventing the deactivation of the endocannabinoid 2-arachidonoylglycerol (2-AG) with the monoacylglycerol lipase (MAGL) inhibitor URB602 can provide neuroprotective effects in hypoxia-ischemia (HI)-induced brain injury.

METHODS

URB602 was administered into the right lateral ventricle 30 min before 7-day-old pup rats were subjected to HI. The neuroprotective effect was evaluated on postnatal day (PN) 14 or at adulthood (PN80) using behavioral and histological analyses. Activated caspase-3 expression and propidium iodide labeling were assessed as indexes of apoptotic and necrotic cell death, respectively.

RESULTS

Pretreatment with URB602 reduced apoptotic and necrotic cell death, as well as the infarct volume measured at PN14. At adulthood, URB602-treated HI animals performed better at the T-maze and the Morris maze, and also showed a significant reduction of brain damage.

CONCLUSION

These results demonstrate that a pretreatment with URB602 significantly reduces brain damage and improves functional outcome, indicating that endocannabinoid-degrading enzymes may represent an important target for neuroprotection in neonatal ischemic brain injury.

Structure-activity relationship of a new series of reversible dual monoacylglycerol lipase/fatty acid amide hydrolase inhibitors

The two endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), play independent and nonredundant roles in the body. This makes the development of both selective and dual inhibitors of their inactivation an important priority. In this work we report a new series of inhibitors of monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH). Among them, (±)-oxiran-2-ylmethyl 6-(1,1'-biphenyl-4-yl)hexanoate (8) and (2R)-(-)-oxiran-2-ylmethyl(4-benzylphenyl)acetate (30) stand out as potent inhibitors of human recombinant MAGL (IC(50) (8) = 4.1 μM; IC(50) (30) = 2.4 μM), rat brain monoacylglycerol hydrolysis (IC(50) (8) = 1.8 μM; IC(50) (30) = 0.68 μM), and rat brain FAAH (IC(50) (8) = 5.1 μM; IC(50) (30) = 0.29 μM). Importantly, and in contrast to the other previously described MAGL inhibitors, these compounds behave as reversible inhibitors either of competitive (8) or noncompetitive nature (30). Hence, they could be useful to explore the therapeutic potential of reversible MAGL inhibitors.

The design, synthesis and biological evaluation of novel URB602 analogues as potential monoacylglycerol lipase inhibitors

We have synthesised an extensive series of URB602 analogues as inhibitors of monoacylglycerol lipase (MAGL), which is the major enzyme responsible for metabolising the endocannabinoid 2-arachidonylglycerol. The recently identified crystal structure of MAGL was used in the design strategy and revealed three possible binding sites for URB602 and the proposed analogues. A test series of carbamate analogues were docked into the identified sites to predict the most favourable binding location. The synthesised analogues of URB602 explored the biological effects of isosteric replacement, ring size and substitution, para substitution of the biphenyl moiety and the incorporation of a bicyclic element. The compounds were tested for their ability to inhibit human MAGL. The carbamate analogue 16 displayed the most significant inhibitory activity, reducing MAGL activity to 26% of controls at 100 μM compared to 73% for the parent compound URB602.

Effects of endocannabinoid system modulation on cognitive and emotional behavior

Cannabis has long been known to produce cognitive and emotional effects. Research has shown that cannabinoid drugs produce these effects by driving the brain’s endogenous cannabinoid system and that this system plays a modulatory role in many cognitive and emotional processes. This review focuses on the effects of endocannabinoid system modulation in animal models of cognition (learning and memory) and emotion (anxiety and depression). We review studies in which natural or synthetic cannabinoid agonists were administered to directly stimulate cannabinoid receptors or, conversely, where cannabinoid antagonists were administered to inhibit the activity of cannabinoid receptors. In addition, studies are reviewed that involved genetic disruption of cannabinoid receptors or genetic or pharmacological manipulation of the endocannabinoid-degrading enzyme, fatty acid amide hydrolase (FAAH). Endocannabinoids affect the function of many neurotransmitter systems, some of which play opposing roles. The diversity of cannabinoid roles and the complexity of task-dependent activation of neuronal circuits may lead to the effects of endocannabinoid system modulation being strongly dependent on environmental conditions. Recent findings are reviewed that raise the possibility that endocannabinoid signaling may change the impact of environmental influences on emotional and cognitive behavior rather than selectively affecting any specific behavior.

Covalent inhibitors of fatty acid amide hydrolase: a rationale for the activity of piperidine and piperazine aryl ureas

Recently, covalent drugs have attracted great interest in the drug discovery community, with successful examples that have demonstrated their therapeutic effects. Here, we focus on the covalent inhibition of the fatty acid amide hydrolase (FAAH), which is a promising strategy in the treatment of pain and inflammation. Among the most recent and potent FAAH inhibitors (FAAHi), there are the cyclic piperidine and piperazine aryl ureas. FAAH hydrolyzes efficiently the amide bond of these compounds, forming a covalent enzyme-inhibitor adduct. To rationalize this experimental evidence, we performed an extensive computational analysis centered on piperidine-based PF750 (1) and piperazine-based JNJ1661010 (2), two potent lead compounds used to generate covalent inhibitors as clinical candidates. We found that FAAH induces a distortion of the amide bond of the piperidine and piperazine aryl ureas. Quantum mechanics/molecular mechanics ΔE(LUMO-HOMO) energies indicate that the observed enzyme-induced distortion of the amide bond favors the formation of a covalent FAAH-inhibitor adduct. These findings could help in the rational structure-based design of novel covalent FAAHi.

Cellular viability effects of fatty acid amide hydrolase inhibition on cerebellar neurons

The endocannabinoid anandamide (ANA) participates in the control of cell death inducing the formation of apoptotic bodies and DNA fragmentation. The aim of this study was to evaluate whether the ANA degrading enzyme, the fatty acid amide hydrolase (FAAH), would induce cellular death. Experiments were performed in cerebellar granule neurons cultured with the FAAH inhibitor, URB597 (25, 50 or 100 nM) as well as endogenous lipids such as oleoylethanolamide (OEA) or palmitoylethanolamide (PEA) and cellular viability was determined by MTT test. Neurons cultured with URB597 (25, 50 or 100 nM) displayed a decrease in cellular viability. In addition, if cultured with OEA (25 nM) or PEA (100 nM), cellular death was found. These results further suggest that URB597, OEA or PEA promote cellular death.

The discovery and development of inhibitors of fatty acid amide hydrolase (FAAH)

A summary of the discovery and advancement of inhibitors of fatty acid amide hydrolase (FAAH) is presented.

Mechanism of inhibition of fatty acid amide hydrolase by sulfonamide-containing benzothiazoles: long residence time derived from increased kinetic barrier and not exclusively from thermodynamic potency

Fatty acid amide hydrolase (FAAH) has emerged as a potential target for developing analgesic, anxiolytic, antidepressant, sleep-enhancing, and anti-inflammatory drugs, and tremendous efforts have been made to discover potent and selective inhibitors of FAAH. Most known potent FAAH inhibitors described to date employ covalent mechanisms, inhibiting the enzyme either reversibly or irreversibly. Recently, a benzothiazole-based analogue (1) has been described possessing a high potency against FAAH yet lacking a structural feature previously known to interact with FAAH covalently. However, covalent inhibition of FAAH by 1 has not been fully ruled out, and the issue of reversibility has not been addressed. Confirming previous reports, 1 inhibited recombinant human FAAH (rhFAAH) with high potency with IC(50) ~2 nM. It displayed an apparently noncompetitive and irreversible inhibition, titrating rhFAAH stoichiometrically within normal assay times. The inhibition appeared to be time dependent, but the time dependence only improved potency by a small degree (from ~8 to ~2 nM). However, mass spectrometric analyses of the reaction mixture failed to reveal any cleavage product or covalent adduct and showed full recovery of the parent compound, ruling out covalent, irreversible inhibition. Dialysis revealed recovery of enzyme activity from enzyme-inhibitor complex over a prolonged time (>10 h), demonstrating that 1 is indeed a reversible, albeit slowly dissociating inhibitor of FAAH. Molecular docking indicated that the sulfonamide group of 1 could form hydrogen bonds with several residues involved in catalysis, thereby mimicking the transition state. The long residence time displayed by 1 does not appear to derive exclusively from great thermodynamic potency and is consistent with an increased kinetic energy barrier that prevents dissociation from happening quickly.

Biphenyl-3-yl alkylcarbamates as fatty acid amide hydrolase (FAAH) inhibitors: steric effects of N-alkyl chain on rat plasma and liver stability

Secondary alkylcarbamic acid biphenyl-3-yl esters are a class of Fatty Acid Amide Hydrolase (FAAH) inhibitors, which include the reference compounds URB597 and URB694. Given the intrinsic reactivity of the carbamate group, the in vivo potency of these molecules in rats is strongly affected by their hydrolysis in plasma or hepatic metabolism. In the present study, in vitro chemical and metabolic stability assays (rat plasma and rat liver S(9) fraction) were used to investigate the structure-property relationships (SPRs) for a focused series of title compounds, where lipophilicity and steric hindrance of the carbamate N-substituent had been modulated. The resulting degradation rates indicate that a secondary or tertiary alkyl group at the carbamate nitrogen atom increases hydrolytic stability towards rat plasma esterases. The calculated solvent accessible surface area (SASA) of the carbamate fragment was employed to describe the differences observed in rate constants of hydrolysis in rat plasma (log k(plasma)), suggesting that stability in plasma increases if the substituent exerts a shielding effect on the carbamate carbonyl. Stability in rat liver S(9) fraction is increased when a tertiary carbon is bound to the carbamate nitrogen atom, while other steric effects showed complex relationships with degradation rates. The SPRs here described may be applied at the pharmacokinetic optimization of other classes of carbamate FAAH inhibitors.

Administration of URB597, oleoylethanolamide or palmitoylethanolamide increases waking and dopamine in rats

Background

Oleoylethanolamide (OEA) and palmitoylethanolamide (PEA) are amides of fatty acids and ethanolamine named N-acylethanolamines or acylethanolamides. The hydrolysis of OEA and PEA is catalyzed by the fatty acid amide hydrolase (FAAH). A number of FAAH inhibitors that increase the levels of OEA and PEA in the brain have been developed, including URB597. In the present report, we examined whether URB597, OEA or PEA injected into wake-related brain areas, such as lateral hypothalamus (LH) or dorsal raphe nuclei (DRN) would promote wakefulness (W) in rats.

Methodology and Principal Findings

Male Wistar rats (250–300 g) were implanted for sleep studies with electrodes to record the electroencephalogram and electromyogram as well as a cannulae aimed either into LH or into DRN. Sleep stages were scored to determine W, slow wave sleep (SWS) and rapid eye movement sleep (REMS). Power spectra bands underly neurophysiological mechanisms of the sleep-wake cycle and provide information about quality rather than quantity of sleep, thus fast Fourier transformation analysis was collected after the pharmacological trials for alpha (for W; α = 8–12 Hz), delta (for SWS; δ = 0.5–4.0 Hz) and theta (for REMS; θ = 6.0–12.0 Hz). Finally, microdialysis samples were collected from a cannula placed into the nucleus accumbens (AcbC) and the levels of dopamine (DA) were determined by HPLC means after the injection of URB597, OEA or PEA. We found that microinjection of compounds (10, 20, 30 µg/1 µL; each) into LH or DRN during the lights-on period increased W and decreased SWS as well as REMS and enhanced DA extracellular levels.

Conclusions

URB597, OEA or PEA promoted waking and enhanced DA if injected into LH or DRN. The wake-promoting effects of these compounds could be linked with the enhancement in levels of DA and indirectly mediated by anandamide.

Reversible competitive α-ketoheterocycle inhibitors of fatty acid amide hydrolase containing additional conformational constraints in the acyl side chain: orally active, long-acting analgesics

A series of α-ketooxazoles containing conformational constraints in the C2 acyl side chain of 2 (OL-135) were examined as inhibitors of fatty acid amide hydrolase (FAAH). Only one of the two possible enantiomers displayed potent FAAH inhibition (S vs R enantiomer), and their potency is comparable or improved relative to 2, indicating that the conformational restriction in the C2 acyl side chain is achievable. A cocrystal X-ray structure of the α-ketoheterocycle 12 bound to a humanized variant of rat FAAH revealed its binding details, confirmed that the (S)-enantiomer is the bound active inhibitor, shed light on the origin of the enantiomeric selectivity, and confirmed that the catalytic Ser241 is covalently bound to the electrophilic carbonyl as a deprotonated hemiketal. Preliminary in vivo characterization of the inhibitors 12 and 14 is reported demonstrating that they raise brain anandamide levels following either intraperitoneal (ip) or oral (po) administration indicative of effective in vivo FAAH inhibition. Significantly, the oral administration of 12 caused dramatic accumulation of anandamide in the brain, with peak levels achieved between 1.5 and 3 h, and these elevations were maintained over 9 h. Additional studies of these two representative members of the series (12 and 14) in models of thermal hyperalgesia and neuropathic pain are reported, including the demonstration that 12 administered orally significantly attenuated mechanical (>6 h) and cold (>9 h) allodynia for sustained periods consistent with its long-acting effects in raising the endogenous concentration of anandamide.

Discovery and molecular basis of potent noncovalent inhibitors of fatty acid amide hydrolase (FAAH)

Fatty acid amide hydrolase (FAAH), an amidase-signature family member, is an integral membrane enzyme that degrades lipid amides including the endogenous cannabinoid anandamide and the sleep-inducing molecule oleamide. Both genetic knock out and pharmacological administration of FAAH inhibitors in rodent models result in analgesic, anxiolytic, and antiinflammatory phenotypes. Targeting FAAH activity, therefore, presents a promising new therapeutic strategy for the treatment of pain and other neurological-related or inflammatory disorders. Nearly all FAAH inhibitors known to date attain their binding potency through a reversible or irreversible covalent modification of the nucleophile Ser241 in the unusual Ser-Ser-Lys catalytic triad. Here, we report the discovery and mechanism of action of a series of ketobenzimidazoles as unique and potent noncovalent FAAH inhibitors. Compound 2, a representative of these ketobenzimidazoles, was designed from a series of ureas that were identified from high-throughput screening. While urea compound 1 is characterized as an irreversible covalent inhibitor, the cocrystal structure of FAAH complexed with compound 2 reveals that these ketobenzimidazoles, though containing a carbonyl moiety, do not covalently modify Ser241. These inhibitors achieve potent inhibition of FAAH activity primarily from shape complementarity to the active site and through numerous hydrophobic interactions. These noncovalent compounds exhibit excellent selectivity and good pharmacokinetic properties. The discovery of this distinctive class of inhibitors opens a new avenue for modulating FAAH activity through nonmechanism-based inhibition.

Fluoride-mediated capture of a noncovalent bound state of a reversible covalent enzyme inhibitor: X-ray crystallographic analysis of an exceptionally potent α-ketoheterocycle inhibitor of fatty acid amide hydrolase

Two cocrystal X-ray structures of the exceptionally potent α-ketoheterocycle inhibitor 1 (K(i) = 290 pM) bound to a humanized variant of rat fatty acid amide hydrolase (FAAH) are disclosed, representing noncovalently and covalently bound states of the same inhibitor with the enzyme. Key to securing the structure of the noncovalently bound state of the inhibitor was the inclusion of fluoride ion in the crystallization conditions that is proposed to bind the oxyanion hole precluding inhibitor covalent adduct formation with stabilization of the tetrahedral hemiketal. This permitted the opportunity to detect important noncovalent interactions stabilizing the binding of the inhibitor within the FAAH active site independent of the covalent reaction. Remarkably, noncovalently bound 1 in the presence of fluoride appears to capture the active site in the same "in action" state with the three catalytic residues Ser241-Ser217-Lys142 occupying essentially identical positions observed in the covalently bound structure of 1, suggesting that this technique of introducing fluoride may have important applications in structural studies beyond inhibiting substrate or inhibitor oxyanion hole binding. Key insights to emerge from the studies include the observations that noncovalently bound 1 binds in its ketone (not gem diol) form, that the terminal phenyl group in the acyl side chain of the inhibitor serves as the key anchoring interaction overriding the intricate polar interactions in the cytosolic port, and that the role of the central activating heterocycle is dominated by its intrinsic electron-withdrawing properties. These two structures are also briefly compared with five X-ray structures of α-ketoheterocycle-based inhibitors bound to FAAH recently disclosed.

Mass spectrometry as test bench for medicinal chemistry studies

This review describes how mass spectrometry can be used as a powerful test bench to obtain information on the biological activity of target compounds. Considering that mass spectrometry is based on the chemical reactivity of the analytes, it is possible to investigate the stability of the active compounds, to predict their behaviour in the environment of interest, and to obtain structure-reactivity relationships for new molecules of pharmacological interest. Electron ionization and metastable ion studies give evidence of the correlation between the mutagenic properties of a series of aryl and heteroaryl triazenes and mass spectrometric data. A linear relationship between the energetics of C(O)-O bond cleavage of some carbamic acid O-aryl esters and their FAAH inhibition activity has been proved by electrospray-ionization ion-trap mass spectrometry. An inverse correlation between the stability and cytotoxic activity of some copper complexes has been clearly established by electrospray-ionization mass spectrometry. Moreover, because of the sensitivity and specificity of mass spectrometry, it has been possible to determine and characterize impurities that in some cases can be the real bioactive compound.

Recent advances in the discovery and evaluation of fatty acid amide hydrolase inhibitors

IMPORTANCE OF THE FIELD

Cannabis has been used for both medicinal and recreational purposes since ancient times. Although cannabinoid-based medicines hold great promise in several challenging therapeutic areas such as pain management and mode control, their development has been hampered by psychoactive and other CNS-related side effects. The identification of fatty acid amide hydrolase (FAAH), a key enzyme responsible for the degradation of endocannabinoids, has brought in tremendous opportunities in that inhibition of FAAH leads to local elevation of endocannabinoids under certain stimuli, thus, avoiding the side effects from global activation of cannabinoid receptors by exogenous cannabimimetic compounds. The search for selective FAAH inhibitors has thus become a strong focus in current drug discovery.

AREAS COVERED IN THIS REVIEW

This review summarizes our current understanding of FAAH including its structure, catalytic mechanism and biological functions with emphases on its role in the regulation of endocannabinoids and other signaling lipids. The review then highlights the most recent discovery and biological activities of different classes of FAAH inhibitors. Last, the review discusses challenges and potential drawbacks in the development of FAAH inhibitor-based therapy.

WHAT THE READER WILL GAIN

Readers will have an overview of FAAH and obtain a rationale on FAAH as an attractive therapeutic target for the development of medicines for treating pain, inflammation, anxiety and other diseases. More importantly, readers will gain knowledge on various newly established FAAH inhibitor scaffolds and their development potentials, and such information will hopefully stimulate ideas for the designing of new inhibitors with superior activity profiles. The discussions on the potential challenges in developing FAAH inhibitors will impose more caution in the decision-making process, thus, lowering the possibility of late stage failure.

TAKE HOME MESSAGE

FAAH is an attractive target for modulating the endocannabinoid system, thus, treating many disease conditions including pain and mode control without the CNS side effects associated with cannabis usage. In recent years, tremendous effort has been focused in the FAAH inhibitor research field, and consequently many novel chemical templates have been discovered. FAAH hydrolyzes several important signaling lipids, but the long-term effects of FAAH inhibition in humans remain to be seen. While it is challenging to identify the right molecule with the right level of intervention of the FAAH function for treating a disease condition, it is possible to avoid mechanism-related undesired effects. With the entry of several compounds into clinical trials, FAAH inhibitor-based medicines are on the horizon.

A new group of oxime carbamates as reversible inhibitors of fatty acid amide hydrolase

A series of oxime carbamates have been identified as potent inhibitors of fatty acid amide hydrolase (FAAH), an important regulatory enzyme of the endocannabinoid signaling system. Kinetic analysis indicates that they behave as non-competitive, reversible inhibitors, and show remarkable selectivity for FAAH over the other components of the endocannabinoid system.

Crystal structure of fatty acid amide hydrolase bound to the carbamate inhibitor URB597: discovery of a deacylating water molecule and insight into enzyme inactivation

The endocannabinoid system regulates a wide range of physiological processes including pain, inflammation, and cognitive/emotional states. URB597 is one of the best characterized covalent inhibitors of the endocannabinoid-degrading enzyme fatty acid amide hydrolase (FAAH). Here, we report the structure of the FAAH-URB597 complex at 2.3 A resolution. The structure provides insights into mechanistic details of enzyme inactivation and experimental evidence of a previously uncharacterized active site water molecule that likely is involved in substrate deacylation. This water molecule is part of an extensive hydrogen-bonding network and is coordinated indirectly to residues lining the cytosolic port of the enzyme. In order to corroborate our hypothesis concerning the role of this water molecule in FAAH's catalytic mechanism, we determined the structure of FAAH conjugated to a urea-based inhibitor, PF-3845, to a higher resolution (2.4 A) than previously reported. The higher-resolution structure confirms the presence of the water molecule in a virtually identical location in the active site. Examination of the structures of serine hydrolases that are non-homologous to FAAH, such as elastase, trypsin, or chymotrypsin, shows a similarly positioned hydrolytic water molecule and suggests a functional convergence between the amidase signature enzymes and serine proteases.

3-Heterocycle-phenyl N-alkylcarbamates as FAAH inhibitors: design, synthesis and 3D-QSAR studies

Carbamates are a well-established class of fatty acid amide hydrolase (FAAH) inhibitors. Here we describe the synthesis of meta-substituted phenolic N-alkyl/aryl carbamates and their in vitro FAAH inhibitory activities. The most potent compound, 3-(oxazol-2yl)phenyl cyclohexylcarbamate (2 a), inhibited FAAH with a sub-nanomolar IC(50) value (IC(50)=0.74 nM). Additionally, we developed and validated three-dimensional quantitative structure-activity relationships (QSAR) models of FAAH inhibition combining the newly disclosed carbamates with our previously published inhibitors to give a total set of 99 compounds. Prior to 3D-QSAR modeling, the degree of correlation between FAAH inhibition and in silico reactivity was also established. Both 3D-QSAR methods used, CoMSIA and GRID/GOLPE, produced statistically significant models with coefficient of correlation for external prediction (R(2) (PRED)) values of 0.732 and 0.760, respectively. These models could be of high value in further FAAH inhibitor design.

Synthesis and evaluation of paracetamol esters as novel fatty acid amide hydrolase inhibitors

Fatty acid amide hydrolase (FAAH) is the key hydrolytic enzyme for the endogenous cannabinoid receptor ligand anandamide. The synthesis and evaluation for their FAAH inhibitory activities of a series of 18 paracetamol esters are described. Structure-activity relationship studies indicated that the ester (33) with a 2-(4-(2-(trifluoromethyl)pyridin-4-ylamino)phenyl)acetic acid substituent was the most potent analogue in this series. The compound inhibited FAAH activity in a competitive manner with a K(i) value of 0.16 microM. The compound was also able to inhibit the FAAH activity in rat basophilic leukemia cells as assessed by measuring either the hydrolysis of anandamide, the FAAH-dependent cellular accumulation of anandamide, or the FAAH-dependent recycling of tritium to the cell membranes. The compound also inhibited the activity of monoacylglycerol lipase (MGL), the enzyme responsible for the hydrolysis of the endogenous cannabinoid receptor ligand 2-arachidonoylglycerol, with an IC(50) value of 1.9 microM. It is concluded that the compound may be a useful template for the design of potent novel inhibitors of FAAH.

Characterization of tunable piperidine and piperazine carbamates as inhibitors of endocannabinoid hydrolases

Monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) are two enzymes from the serine hydrolase superfamily that degrade the endocannabinoids 2-arachidonoylglycerol and anandamide, respectively. We have recently discovered that MAGL and FAAH are both inhibited by carbamates bearing an N-piperidine/piperazine group. Piperidine/piperazine carbamates show excellent in vivo activity, raising brain endocannabinoid levels and producing CB1-dependent behavioral effects in mice, suggesting that they represent a promising class of inhibitors for studying the endogenous functions of MAGL and FAAH. Herein, we disclose a full account of the syntheses, structure-activity relationships, and inhibitory activities of piperidine/piperazine carbamates against members of the serine hydrolase family. These scaffolds can be tuned for MAGL-selective or dual MAGL-FAAH inhibition by the attachment of an appropriately substituted bisarylcarbinol or aryloxybenzyl moiety, respectively, on the piperidine/piperazine ring. Modifications to the piperidine/piperazine ring ablated inhibitory activity, suggesting a strict requirement for a six-membered ring to maintain potency.

X-ray crystallographic analysis of alpha-ketoheterocycle inhibitors bound to a humanized variant of fatty acid amide hydrolase

Three cocrystal X-ray structures of the alpha-ketoheterocycle inhibitors 3-5 bound to a humanized variant of fatty acid amide hydrolase (FAAH) are disclosed and comparatively discussed alongside those of 1 (OL-135) and its isomer 2. These five X-ray structures systematically probe each of the three active site regions key to substrate or inhibitor binding: (1) the conformationally mobile acyl chain-binding pocket and membrane access channel responsible for fatty acid amide substrate and inhibitor acyl chain binding, (2) the atypical active site catalytic residues and surrounding oxyanion hole that covalently binds the core of the alpha-ketoheterocycle inhibitors captured as deprotonated hemiketals mimicking the tetrahedral intermediate of the enzyme-catalyzed reaction, and (3) the cytosolic port and its uniquely important imbedded ordered water molecules and a newly identified anion binding site. The detailed analysis of their key active site interactions and their implications on the interpretation of the available structure-activity relationships are discussed providing important insights for future design.

A second generation of carbamate-based fatty acid amide hydrolase inhibitors with improved activity in vivo

The fatty acid ethanolamides are a class of signaling lipids that include agonists at cannabinoid and alpha type peroxisome proliferator-activated receptors (PPARalpha). In the brain, these compounds are primarily hydrolyzed by the intracellular serine enzyme fatty acid amide hydrolase (FAAH). O-aryl carbamate FAAH inhibitors such as URB597 are being evaluated clinically for the treatment of pain and anxiety, but interactions with carboxylesterases in liver might limit their usefulness. Here we explore two strategies aimed at overcoming this limitation. Lipophilic N-terminal substitutions, which enhance FAAH recognition, yield potent inhibitors but render such compounds susceptible to attack by broad-spectrum hydrolases and inactive in vivo. By contrast, polar electron-donating O-aryl substituents, which decrease carbamate reactivity, yield compounds, such as URB694, that are highly potent FAAH inhibitors in vivo and less reactive with off-target carboxylesterases. The results suggest that an approach balancing inhibitor reactivity with target recognition produces FAAH inhibitors that display significantly improved drug-likeness.

beta-Lactams derived from a carbapenem chiron are selective inhibitors of human fatty acid amide hydrolase versus human monoacylglycerol lipase

A library of 30 beta-lactams has been prepared from (3R,4R)-3-[(R)-1'-(tbutyldimethylsilyloxy)-ethyl]-4-acetoxy-2-azetidinone, and the corresponding deacetoxy derivative, by sequential N- and O-functionalizations with various omega-alkenoyl and omega-arylalkanoyl chains. All compounds were selective inhibitors of hFAAH versus hMGL, and IC(50) values in the nanomolar range (5-14 nM) were recorded for the best representatives. From time-dependent preincubation and rapid dilution studies, and from docking analyses in a homology model of the target enzyme, a reversible mechanism of inhibition of hFAAH is proposed.

New 2-aryloxy-3-phenyl-propanoic acids as peroxisome proliferator-activated receptors alpha/gamma dual agonists with improved potency and reduced adverse effects on skeletal muscle function

The preparation of a new series of 2-aryloxy-3-phenyl-propanoic acids, resulting from the introduction of a linker into the diphenyl system of the previously reported PPARalpha/gamma dual agonist 1, allowed the identification of new ligands with improved potency on PPARalpha and unchanged activity on PPARgamma. For the most interesting stereoisomers S-2 and S-4, X-ray studies in PPARgamma and docking experiments in PPARalpha provided a molecular explanation for their different behavior as full and partial agonists of PPARalpha and PPARgamma, respectively. Due to the adverse effects provoked by hypolipidemic drugs on skeletal muscle function, we also investigated the blocking activity of S-2 and S-4 on skeletal muscle membrane chloride channel conductance and found that these ligands have a pharmacological profile more beneficial compared to fibrates currently used in therapy.

New therapeutic potential for psychoactive natural products

Much of our knowledge in neuroscience was discovered through the study of mind-altering natural products. However, although much has been learned about human physiology and basic biological processes, the underlying causes of CNS disorders and other disease states are still elusive. Based on its main past successes, the continued study of mind-altering compounds promises to yield novel agents that may be developed into medications and to identify new targets for the treatment of diseases. This Highlight describes the history of investigations into several classes of mind-altering natural products and relates recent and potential therapeutic uses for these agents.

Structure-property relationships of a class of carbamate-based fatty acid amide hydrolase (FAAH) inhibitors: chemical and biological stability

Cyclohexylcarbamic acid aryl esters are a class of fatty acid amide hydrolase (FAAH) inhibitors, which includes the reference compound URB597. The reactivity of their carbamate fragment is involved in pharmacological activity and may affect their pharmacokinetic and toxicological properties. We conducted in vitro stability experiments in chemical and biological environments to investigate the structure-stability relationships in this class of compounds. The results show that electrophilicity of the carbamate influences chemical stability, as suggested by the relation between the rate constant of alkaline hydrolysis (log k(pH9)) and the energy of the lowest unoccupied molecular orbital (LUMO). Introduction of small electron-donor substituents at conjugated positions of the O-aryl moiety increased the overall hydrolytic stability of the carbamate group without affecting FAAH inhibitory potency, whereas peripheral non-conjugated hydrophilic groups, which favor FAAH recognition, helped decrease oxidative metabolism in the liver.

Fatty acid amide hydrolase as a potential therapeutic target for the treatment of pain and CNS disorders

BACKGROUND: Fatty acid amide hydrolase (FAAH) is an integral membrane enzyme that hydrolyzes the endocannabinoid anandamide and related amidated signaling lipids. Genetic or pharmacological inactivation of FAAH produces analgesic, anti-inflammatory, anxiolytic, and antidepressant phenotypes without showing the undesirable side effects of direct cannabinoid receptor agonists, indicating that FAAH may be a promising therapeutic target. OBJECTIVES: This review highlights advances in the development of FAAH inhibitors of different mechanistic classes and their in vivo efficacy. Also highlighted are advances in technology for the in vitro and in vivo selectivity assessment of FAAH inhibitors employing activity-based protein profiling (ABPP) and click chemistry-ABPP, respectively. Recent reports on structure-based drug design for human FAAH generated by protein engineering using interspecies active site conversion are also discussed. METHODS: The literature searches of Medline and SciFinder databases were used. CONCLUSIONS: There has been tremendous progress in our understanding in FAAH and development of FAAH inhibitors with in vivo efficacy, selectivity, and drug like pharmacokinetic properties.

Benzothiophene piperazine and piperidine urea inhibitors of fatty acid amide hydrolase (FAAH)

The synthesis and structure-activity relationships (SAR) of a series of benzothiophene piperazine and piperidine urea FAAH inhibitors is described. These compounds inhibit FAAH by covalently modifying the enzyme's active site serine nucleophile. Activity-based protein profiling (ABPP) revealed that these urea inhibitors were completely selective for FAAH relative to other mammalian serine hydrolases. Several compounds showed in vivo activity in a rat complete Freund's adjuvant (CFA) model of inflammatory pain.

CAY10499, a novel monoglyceride lipase inhibitor evidenced by an expeditious MGL assay

Monoglyceride lipase (MGL) plays a major role in the metabolism of the lipid transmitter 2-arachidonoylglycerol (2-AG). This endocannabinoid is known to mediate a large number of physiological processes, and its regulation is thought to be of great therapeutic potential. However, the number of available monoglyceride lipase inhibitors is limited, mostly due to the lack of rapid and accurate pharmacological assays for the enzyme. We have developed a 96-well-format assay for MGL using a nonradiolabeled substrate, 4-nitrophenylacetate. The IC(50) values that were obtained for known inhibitors of MGL using 4-nitrophenylacetate were similar to those reported by using the radiolabeled form of an endogenous substrate, 2-oleoylglycerol. In a first small-scale screening, we identified CAY10499 as a novel monoglyceride lipase inhibitor. Thus, we report here the characterization of this submicromolar inhibitor, which acts on MGL through an unprecedented mechanism for inhibitors of this enzyme.

Biochemical and biological properties of 4-(3-phenyl-[1,2,4] thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide, a mechanism-based inhibitor of fatty acid amide hydrolase

Fatty acid amide hydrolase (FAAH) is an integral membrane enzyme within the amidase-signature family. It catalyzes the hydrolysis of several endogenous biologically active lipids, including anandamide (arachidonoyl ethanolamide), oleoyl ethanolamide, and palmitoyl ethanolamide. These endogenous FAAH substrates have been shown to be involved in a variety of physiological and pathological processes, including synaptic regulation, regulation of sleep and feeding, locomotor activity, pain and inflammation. Here we describe the biochemical and biological properties of a potent and selective FAAH inhibitor, 4-(3-phenyl-[1,2,4]thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide (JNJ-1661010). The time-dependence of apparent IC(50) values at rat and human recombinant FAAH, dialysis and mass spectrometry data indicate that the acyl piperazinyl fragment of JNJ-1661010 forms a covalent bond with the enzyme. This bond is slowly hydrolyzed, with release of the piperazinyl fragment and recovery of enzyme activity. The lack of inhibition observed in a rat liver esterase assay suggests that JNJ-1661010 is not a general esterase inhibitor. JNJ-1661010 is >100-fold preferentially selective for FAAH-1 when compared to FAAH-2. JNJ-1661010 dose-dependently increases arachidonoyl ethanolamide, oleoyl ethanolamide, and palmitoyl ethanolamide in the rat brain. The compound attenuates tactile allodynia in the rat mild thermal injury model of acute tissue damage and in the rat spinal nerve ligation (Chung) model of neuropathic pain. JNJ-1661010 also diminishes thermal hyperalgesia in the inflammatory rat carrageenan paw model. These data suggest that FAAH inhibitors with modes of action similar to JNJ-1661010 may be useful clinically as broad-spectrum analgesics.

The endocannabinoid system as a target for novel anxiolytic and antidepressant drugs

Observational studies in humans suggest that exposure to marijuana and other cannabis-derived drugs produces a wide range of subjective effects on mood tone and emotionality. These observations have their counterpart in animal studies, showing that cannabinoid agonists strongly affect emotional reactivity in directions that vary depending on dose and context. Based on these evidence, the activation of central CB(1) receptor has emerged as potential target for the development of antianxiety and antidepressant therapies. However, the variable effects of exogenous cannabinoid agonists have gradually shifted the interest to the alternative approach of amplifying the effects of endogenous cannabinoids (EC), namely anandamide (AEA) and 2-arachidonoylglycerol (2-AG), by preventing their deactivation. The enzyme fatty acid amide hydrolase (FAAH) has been the target of intense research efforts aimed at developing potent and selective inhibitors that might prolong AEA actions in vivo. Among the inhibitors developed, the compound URB597 was found to potently inhibit FAAH activity in vivo and cause brain AEA levels to increase. Interestingly, the enhanced AEA tone produced by URB597 does not result in the behavioral effects typical of a direct-acting cannabinoid agonist. Though URB597 does not elicit a full-fledged cannabinoid profile of behavioral responses, it does elicit marked anxiolytic-like and antidepressant-like effects in rats and mice. Such effects involve the downstream activation of CB(1) receptors, since they are attenuated by the CB(1) antagonist SR141716 (rimonabant). Parallel to FAAH inhibition, similar results can also be observed by pharmacologically blocking the AEA transport system, which is responsible of the intracellular uptake of AEA from the synaptic cleft. The reason why FAAH inhibition approach produces a smaller set of cannabimimetic effects might depend on the mechanism of EC synthesis and release upon neuronal activation and on the target selectivity of the drug. The mechanism of EC release is commonly referred to as "on request", since they are not synthesized and stored in synaptic vesicles, such as classical neurotransmitters, but are synthesized from membrane precursors and immediately released in the synaptic cleft following neuronal activation. The neural stimulation in specific brain areas, for example, those involved in the regulation of mood tone and/or emotional reactivity, would result in an increased EC tone in these same areas, but not necessarily in others. Therefore, inhibition of AEA metabolism activity could amplify CB(1) activation mainly where AEA release is higher. Furthermore, the inhibition of FAAH causes an accumulation of AEA but not 2-AG, which, being 200-fold more abundant than AEA in the brain, might differently modulate CB(1)-mediated behavioral responses. The evidence outlined above supports the hypothesis that the EC system plays an important role in anxiety and mood disorders and suggests that modulation of FAAH activity might be a pharmacological target for novel anxiolytic and antidepressant therapies.

Fatty acid amide hydrolase inhibition heightens anandamide signaling without producing reinforcing effects in primates

BACKGROUND

CB(1) cannabinoid receptors in the brain are known to participate in the regulation of reward-based behaviors. However, the contribution of each of the endocannabinoid transmitters, anandamide and 2-arachidonoylglycerol (2-AG), to these behaviors remains undefined. To address this question, we assessed the effects of URB597, a selective anandamide deactivation inhibitor, as a reinforcer of drug-seeking and drug-taking behavior in squirrel monkeys.

METHODS

We investigated the reinforcing effects of the fatty acid amide hydrolase (FAAH) inhibitor URB597 in monkeys trained to intravenously self-administer Delta(9)-tetrahydrocannabinol (THC), anandamide, or cocaine and quantified brain endocannabinoid levels using liquid chromatography/mass spectrometry. We measured brain FAAH activity using an ex vivo enzyme assay.

RESULTS

URB597 (.3 mg/kg, intravenous) blocked FAAH activity and increased anandamide levels throughout the monkey brain. This effect was accompanied by a marked compensatory decrease in 2-AG levels. Monkeys did not self-administer URB597, and the drug did not promote reinstatement of extinguished drug-seeking behavior previously maintained by THC, anandamide, or cocaine. Pretreatment with URB597 did not modify self-administration of THC or cocaine, even though, as expected, it significantly potentiated anandamide self-administration.

CONCLUSIONS

In the monkey brain, the FAAH inhibitor URB597 increases anandamide levels while causing a compensatory down-regulation in 2-AG levels. These effects are accompanied by a striking lack of reinforcing properties, which distinguishes URB597 from direct-acting cannabinoid agonists such as THC. Our results reveal an unexpected functional heterogeneity within the endocannabinoid signaling system and suggest that FAAH inhibitors might be used therapeutically without risk of abuse or triggering of relapse to drug abuse.

The endogenous brain constituent N-arachidonoyl L-serine is an activator of large conductance Ca2+-activated K+ channels

The novel endocannabinoid-like lipid N-arachidonoyl L-serine (ARA-S) causes vasodilation through both endothelium-dependent and -independent mechanisms. We have analyzed the vasorelaxant effect of ARA-S in isolated vascular preparations and its effects on Ca(2+)-activated K(+) currents in human embryonic kidney cells stably transfected with the alpha-subunit of the human, large conductance Ca(+)-activated K(+) (BK(Ca)) channel [human embryonic kidney (HEK) 293hSlo cells]. ARA-S caused relaxation of rat isolated, intact and denuded, small mesenteric arteries preconstricted with (R)-(-)-1-(3-hydroxyphenyl)-2-methylaminoethanol hydrochloride (pEC(50), 5.49 and 5.14, respectively), whereas it caused further contraction of vessels preconstricted with KCl (pEC(50), 5.48 and 4.82, respectively). Vasorelaxation by ARA-S was inhibited by 100 nM iberiotoxin. In human embryonic kidney cells stably transfected with the alpha-subunit of the human BK(Ca) channel cells, ARA-S and its enantiomer, N-arachidonoyl-D-serine, enhanced the whole-cell outward K(+) current with similar potency (pEC(50), 5.63 and 5.32, respectively). The potentiation was not altered by the beta(1) subunit or mediated by ARA-S metabolites, stimulation of known cannabinoid receptors, G proteins, protein kinases, or Ca(2+)-dependent processes; it was lost after patch excision or after membrane cholesterol depletion but was restored after cholesterol reconstitution. BK(Ca) currents were also enhanced by N-arachidonoyl ethanolamide (pEC(50), 5.27) but inhibited by another endocannabinoid, O-arachidonoyl ethanolamine (pIC(50), 6.35), or by the synthetic cannabinoid O-1918 [(-)-1,3-dimethoxy-2-(3-3,4-trans-p-menthadien-(1,8)-yl)-orcinol] (pIC(50), 6.59), which blocks ARA-S-induced vasodilation. We conclude the following. 1) ARA-S directly activates BK(Ca) channels. 2) This interaction does not involve cannabinoid receptors or cytosolic factors but is dependent on the presence of membrane cholesterol. 3) Direct BK(Ca) channel activation probably contributes to the endothelium-independent component of ARA-S-induced mesenteric vasorelaxation. 4) O-1918 is a BK(Ca) channel inhibitor.

Optimization of the central heterocycle of alpha-ketoheterocycle inhibitors of fatty acid amide hydrolase

The synthesis and evaluation of a refined series of alpha-ketoheterocycles based on the oxazole 2 (OL-135) incorporating systematic changes in the central heterocycle bearing a key set of added substituents are described. The nature of the central heterocycle, even within the systematic and minor perturbations explored herein, significantly influenced the inhibitor activity: 1,3,4-oxadiazoles and 1,2,4-oxadiazoles 9 > tetrazoles, the isomeric 1,2,4-oxadiazoles 10, 1,3,4-thiadiazoles > oxazoles including 2 > 1,2-diazines > thiazoles > 1,3,4-triazoles. Most evident in these trends is the observation that introduction of an additional heteroatom at position 4 (oxazole numbering, N > O > CH) substantially increases activity that may be attributed to a reduced destabilizing steric interaction at the FAAH active site. Added heterocycle substituents displaying well-defined trends may be utilized to enhance the inhibitor potency and, more significantly, to enhance the inhibitor selectivity. These trends, exemplified herein, emerge from both enhancements in the FAAH activity and simultaneous disruption of binding affinity for competitive off-target enzymes.