Synthesis and quantitative structure-activity relationship of fatty acid amide hydrolase inhibitors: modulation at the N-portion of biphenyl-3-yl alkylcarbamates

Potential Therapeutic Targets to Modulate the Endocannabinoid System in Alzheimer's Disease

Alzheimer's disease (AD), the most common neurodegenerative disease (NDD), is characterized by chronic neuronal cell death through progressive loss of cognitive function. Amyloid beta (Aβ) deposition, neuroinflammation, oxidative stress, and hyperphosphorylated tau proteins are considered the hallmarks of AD pathology. Different therapeutic approaches approved by the Food and Drug Administration can only target a single altered pathway instead of various mechanisms that are involved in AD pathology, resulting in limited symptomatic relief and almost no effect in slowing down the disease progression. Growing evidence on modulating the components of the endocannabinoid system (ECS) proclaimed their neuroprotective effects by reducing neurochemical alterations and preventing cellular dysfunction. Recent studies on AD mouse models have reported that the inhibitors of the fatty acid amide hydrolase (FAAH) and monoacylglycerol (MAGL), hydrolytic enzymes for N-arachidonoyl ethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), respectively, might be promising candidates as therapeutical intervention. The FAAH and MAGL inhibitors alone or in combination seem to produce neuroprotection by reversing cognitive deficits along with Aβ-induced neuroinflammation, oxidative responses, and neuronal death, delaying AD progression. Their exact signaling mechanisms need to be elucidated for understanding the brain intrinsic repair mechanism. The aim of this review was to shed light on physiology and pathophysiology of AD and to summarize the experimental data on neuroprotective roles of FAAH and MAGL inhibitors. In this review, we have also included CB1R and CB2R modulators with their diverse roles to modulate ECS mediated responses such as anti-nociceptive, anxiolytic, and anti-inflammatory actions in AD. Future research would provide the directions in understanding the molecular mechanisms and development of new therapeutic interventions for the treatment of AD.

Activity of FAAH-Inhibitor JZP327A in an Experimental Rat Model of Migraine

Increased anandamide levels via fatty acid amide hydrolase (FAAH) inhibition can decrease the pronociceptive responses and inflammatory mediators in animal models of migraine. Here, we profile the pharmacological activity of the FAAH inhibitor JZP327A, a chiral 1,3,4-oxadiazol-2(3H)-one compound, in the mediation of spontaneous and nocifensive behaviour in the animal models of migraine based on nitroglycerin (NTG) administration. JZP327A (0.5 mg/kg, i.p.) or vehicle was administered to male rats 3 h after NTG (10 mg/kg, i.p.) or NTG vehicle injection. The rats were then exposed to the open field test and an orofacial formalin test 1 h later. The levels of endocannabinoids and lipid-related substances, and the expression of pain and inflammatory mediators were evaluated in cranial tissues and serum. The findings show that JZP327A did not affect NTG-induced changes in the spontaneous behaviour of rats, while it inhibited NTG-induced hyperalgesia at the orofacial formalin test. Furthermore, JZP327A dramatically decreased the gene expression of calcitonin gene-related peptide (CGRP), tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) in the trigeminal ganglia and medulla-pons, while it did not change endocannabinoids or lipids levels nor CGRP serum levels in the same tissues. These data suggest an anti-hyperalgesic role for JZP327A in the NTG model, which is mediated by the inhibition of the inflammatory cascade of events. This activity does not seem mediated by a change in the levels of endocannabinoids and lipid amides.

Inhibition of Fatty Acid Amide Hydrolase (FAAH) Regulates NF-kb Pathways Reducing Bleomycin-Induced Chronic Lung Inflammation and Pulmonary Fibrosis

The deadly interstitial lung condition known as idiopathic pulmonary fibrosis (IPF) worsens over time and for no apparent reason. The traditional therapy approaches for IPF, which include corticosteroids and immunomodulatory drugs, are often ineffective and can have noticeable side effects. The endocannabinoids are hydrolyzed by a membrane protein called fatty acid amide hydrolase (FAAH). Increasing endogenous levels of endocannabinoid by pharmacologically inhibiting FAAH results in numerous analgesic advantages in a variety of experimental models for pre-clinical pain and inflammation. In our study, we mimicked IPF by administering intratracheal bleomycin, and we administered oral URB878 at a dose of 5 mg/kg. The histological changes, cell infiltration, pro-inflammatory cytokine production, inflammation, and nitrosative stress caused by bleomycin were all reduced by URB878. Our data clearly demonstrate for the first time that the inhibition of FAAH activity was able to counteract not only the histological alteration bleomycin-induced but also the cascade of related inflammatory events.

Design and synthesis of novel pipernonaline derivatives as anti-austerity agents against human pancreatic cancer PANC-1 cells

Pipernonaline (1), one of the components of the spice pepper, preferentially reduced the survival of human pancreatic cancer PANC-1 cells under nutrient-deprived conditions witha PC₅₀ value of 7.2 μM, suggesting that1couldpotentially lead to the development ofnew anticanceragents basedon theanti-austerity strategy. We have synthesized a total of 31 pipernonaline derivatives, revealing clear structure-activity relationships. Compound 9, which showed the strongest preferential cytotoxicity among synthesized derivatives, inhibited Akt activation and cancer cell migration, making it an extremely promising candidate compound for new pancreatic cancer agents based on the anti-austerity strategy.

Fatty Acid Amide Hydrolase (FAAH) Inhibition Plays a Key Role in Counteracting Acute Lung Injury

Acute lung injury (ALI) is a group of lung illnesses characterized by severe inflammation, with no treatment. The fatty acid amide hydrolase (FAAH) enzyme is an integral membrane protein responsible for the hydrolysis of the main endocannabinoids, such as anandamide (AEA). In pre-clinical pain and inflammation models, increasing the endogenous levels of AEA and other bioactive fatty acid amides (FAAs) via genetic deletion or the pharmacological inhibition of FAAH produces many analgesic benefits in several different experimental models. To date, nobody has investigated the role of FAAH inhibition on an ALI mouse model. Mice were subjected to a carrageenan injection and treated orally 1 h after with the FAAH inhibitor URB878 dissolved in a vehicle consisting of 10% PEG-400, 10% Tween-80 and 80% saline at different doses: The inhibition of FAAH activity was able to counteract not only the CAR-induced histological alteration, but also the cascade of related inflammatory events. URB878 clears the way for further studies based on FAAH inhibition in acute lung pathologies.

Identifying FAAH Inhibitors as New Therapeutic Options for the Treatment of Chronic Pain through Drug Repurposing

Chronic pain determines a substantial burden on individuals, employers, healthcare systems, and society. Most of the affected patients report dissatisfaction with currently available treatments. There are only a few and poor therapeutic options-some therapeutic agents are an outgrowth of drugs targeting acute pain, while others have several serious side effects. One of the primary degradative enzymes for endocannabinoids, fatty acid amide hydrolase (FAAH) attracted attention as a significant molecular target for developing new therapies for neuropsychiatric and neurological diseases, including chronic pain. Using chemical graph mining, quantitative structure-activity relationship (QSAR) modeling, and molecular docking techniques we developed a multi-step screening protocol to identify repurposable drugs as FAAH inhibitors. After screening the DrugBank database using our protocol, 273 structures were selected, with five already approved drugs, montelukast, repaglinide, revefenacin, raloxifene, and buclizine emerging as the most promising repurposable agents for treating chronic pain. Molecular docking studies indicated that the selected compounds interact with the enzyme mostly non-covalently (except for revefenacin) through shape complementarity to the large substrate-binding pocket in the active site. A molecular dynamics simulation was employed for montelukast and revealed stable interactions with the enzyme. The biological activity of the selected compounds should be further confirmed by employing in vitro and in vivo studies.

Rational Design 2-Hydroxypropylphosphonium Salts as Cancer Cell Mitochondria-Targeted Vectors: Synthesis, Structure, and Biological Properties

It has been shown for a wide range of epoxy compounds that their interaction with triphenylphosphonium triflate occurs with a high chemoselectivity and leads to the formation of (2-hydroxypropyl)triphenylphosphonium triflates 3 substituted in the 3-position with an alkoxy, alkylcarboxyl group, or halogen, which were isolated in a high yield. Using the methodology for the disclosure of epichlorohydrin with alcohols in the presence of boron trifluoride etherate, followed by the substitution of iodine for chlorine and treatment with triphenylphosphine, 2-hydroxypropyltriphenylphosphonium iodides 4 were also obtained. The molecular and supramolecular structure of the obtained phosphonium salts was established, and their high antitumor activity was revealed in relation to duodenal adenocarcinoma. The formation of liposomal systems based on phosphonium salt 3 and L-α-phosphatidylcholine (PC) was employed for improving the bioavailability and reducing the toxicity. They were produced by the thin film rehydration method and exhibited cytotoxic properties. This rational design of phosphonium salts 3 and 4 has promising potential of new vectors for targeted delivery into mitochondria of tumor cells.

Synthesis of New Steroidal Carbamates with Plant-Growth-Promoting Activity: Theoretical and Experimental Evidence

A priority of modern agriculture is to use novel and environmentally friendly plant-growth promoter compounds to increase crop yields and avoid the indiscriminate use of synthetic fertilizers. Brassinosteroids are directly involved in the growth and development of plants and are considered attractive candidates to solve this problem. Obtaining these metabolites from their natural sources is expensive and cumbersome since they occur in extremely low concentrations in plants. For this reason, much effort has been dedicated in the last decades to synthesize brassinosteroids analogs. In this manuscript, we present the synthesis and characterization of seven steroidal carbamates starting from stigmasterol, β-sitosterol, diosgenin and several oxygenated derivatives of it. The synthesis route for functionalization of diosgenin included epoxidation and epoxy opening reactions, reduction of carbonyl groups, selective oxidation of hydroxyl groups, among others. All the obtained compounds were characterized by ¹H and ¹³C NMR, HRMS, and their melting points are also reported. Rice lamina inclination test performed at different concentrations established that all reported steroidal carbamates show plant-growth-promoting activity. A molecular docking study evaluated the affinity of the synthesized compounds towards the BRI1-BAK1 receptor from Arabidopsis thaliana and three of the docked compounds displayed a binding energy lower than brassinolide.

Design, microwave-assisted synthesis, biological evaluation and molecular modeling studies of 4-phenylthiazoles as potent fatty acid amide hydrolase inhibitors

Endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), are endogenous lipids that activate cannabinoid receptors. Activation of these receptors produces anti-inflammatory and analgesic effects. Fatty acid amide hydrolase (FAAH) is a membrane enzyme that hydrolases endocannabinoids; thus, inhibition of FAAH represents an attractive approach to develop new therapeutics for treating inflammation and pain. Previously, potent rat FAAH inhibitors containing 2-naphthyl- and 4-phenylthiazole scaffolds were identified, but up to the present time, very little structure-activity relationship studies have been performed on these moieties. We designed and synthesized several analogs containing these structural motifs and evaluated their inhibition potencies against human FAAH enzyme. In addition, we built and validated a homology model of human FAAH enzyme and performed docking experiments. We identified several inhibitors in the low nanomolar range and calculated their ADME predicted values. These FAAH inhibitors represent promising drug candidates for future preclinical in vivo studies.

A perspective review on fatty acid amide hydrolase (FAAH) inhibitors as potential therapeutic agents

Fatty acid amide hydrolase (FAAH) is an important enzyme creditworthy of hydrolyzing endocannabinoids and related-amidated signalling lipids, discovery of which has pioneered novel arena of pharmacological canvasses to unwrap its curative potency in various diseased circumstances. It presents contemporary basis for understanding molecules regulating and mediating inflammatory reactions, pain, anxiety, depression, and neurodegeneration. FAAH inhibitors form vital approach for discovery of therapeutic agents that are concerned with local elevation of endocannabinoids under certain stimuli, debarring adverse/unwanted secondary effects from global activation of cannabinoid receptors by exogenous cannabimimetics. During past decades, several molecules with excellent potency developed through tailor-made approaches entered into clinical trials, but none could reach market. Hence, hunt for novel, non-toxic and selective FAAH inhibitors are on horizon. This review summarizes present perception on FAAH in conjunction with its structure, mechanism of catalysis and biological functions. It also foregrounds recent development of molecules belonging to diverse chemical classes as potential FAAH inhibitors bobbing up from in-depth chemical, mechanistic and computational studies published since 2015-November 2019, focusing on their potency. This review will assist readers to obtain rationale on FAAH as potential target for addressing various disease conditions, acquiring significant knowledge on recently established inhibitor scaffolds and their development potentials. New technologies including MD-MM simulations and 3D-QSAR studies allow mechanistic characterization of enzyme. Assessment of in-vitro and in-vivo efficacy of existing FAAH inhibitors will facilitate researchers to design novel ligands utilizing modern drug design methods. The discussions will also impose precaution in decision making process, quashing possibility of late stage failure.

Three-Dimensional Quantitative Structure-Activity Relationships (3D-QSAR) on a Series of Piperazine-Carboxamides Fatty Acid Amide Hydrolase (FAAH) Inhibitors as a Useful Tool for the Design of New Cannabinoid Ligands

Fatty Acid Amide Hydrolase (FAAH) is one of the main enzymes responsible for endocannabinoid metabolism. Inhibition of FAAH increases endogenous levels of fatty acid ethanolamides such as anandamide (AEA) and thus consitutes an indirect strategy that can be used to modulate endocannabinoid tone. In the present work, we present a three-dimensional quantitative structure-activity relationships/comparative molecular similarity indices analysis (3D-QSAR/CoMSIA) study on a series of 90 reported irreversible inhibitors of FAAH sharing a piperazine-carboxamide scaffold. The model obtained was extensively validated (q² = 0.734; r² = 0.966; r²_m = 0.723). Finally, based on the information derived from the contour maps we designed a series of 10 new compounds with high predicted FAAH inhibition (predicted pIC₅₀ of the best-proposed compounds = 12.196; 12.416).

Synthesis, biological evaluation, and structure activity relationship (SAR) study of pyrrolidine amide derivatives as N-acylethanolamine acid amidase (NAAA) inhibitors

N-Acylethanolamine acid amidase (NAAA) is one of the key enzymes involved in the degradation of fatty acid ethanolamides (FAEs), especially for palmitoylethanolamide (PEA). Pharmacological blockage of NAAA restores PEA levels, providing therapeutic benefits in the management of inflammation and pain. In the current work, we showed the structure-activity relationship (SAR) studies for pyrrolidine amide derivatives as NAAA inhibitors. A series of aromatic replacements or substituents for the terminal phenyl group of pyrrolidine amides were examined. SAR data showed that small lipophilic 3-phenyl substituents were preferable for optimal potency. The conformationally flexible linkers increased the inhibitory potency of pyrrolidine amide derivatives but reduced their selectivity toward fatty acid amide hydrolase (FAAH). The conformationally restricted linkers did not enhance the inhibitor potency toward NAAA but improved the selectivity over FAAH. Several low micromolar potent NAAA inhibitors were developed, including 4g bearing a rigid 4-phenylcinnamoyl group. Dialysis and kinetic analysis suggested that 4g inhibited NAAA via a competitive and reversible mechanism. Furthermore, 4g showed high anti-inflammatory activities in lipopolysaccharide (LPS) induced acute lung injury (ALI) model, and this effect was blocked by pre-treatment with the PPAR-α antagonist MK886. We anticipate that 4g (E93) will enable a new agent to treat inflammation and related diseases.

Synthesis and biological evaluation of ferrocene-based cannabinoid receptor 2 ligands

Ferrocene analogs of known fatty acid amide hydrolase inhibitors and CB2 ligands have been synthesized and characterized spectroscopically and crystallographically. The resulting bio-organometallic isoxazoles were assayed for their effects on CB1 and CB2 receptors as well as on fatty acid amide hydrolase. None had any fatty acid amide hydrolase activity but compound 3, 5-(2-(pentyloxy)phenyl)-N-ferrocenylisoxazole-3-carboxamide, was found to be a potent CB2 ligand (Ki = 32.5 nM).

Design and synthesis of uracil urea derivatives as potent and selective fatty acid amide hydrolase inhibitors

Fatty acid amide hydrolase (FAAH) is one of the key enzymes involved in the biological degradation of endocannabinoids, especially anandamide.

Fatty Acid Amide Hydrolase (FAAH), Acetylcholinesterase (AChE), and Butyrylcholinesterase (BuChE): Networked Targets for the Development of Carbamates as Potential Anti-Alzheimer's Disease Agents

The modulation of the endocannabinoid system is emerging as a viable avenue for the treatment of neurodegeneration, being involved in neuroprotective and anti-inflammatory processes. In particular, indirectly enhancing endocannabinoid signaling to therapeutic levels through FAAH inhibition might be beneficial for neurodegenerative disorders such as Alzheimer's disease, effectively preventing or slowing the progression of the disease. Hence, in the search for a more effective treatment for Alzheimer's disease, in this paper, the multitarget-directed ligand paradigm was applied to the design of carbamates able to simultaneously target the recently proposed endocannabinoid system and the classic cholinesterase system, and achieve effective dual FAAH/cholinesterase inhibitors. Among the two series of synthesized compounds, while some derivatives proved to be extremely potent on a single target, compounds 9 and 19 were identified as effective dual FAAH/ChE inhibitors, with well-balanced nanomolar activities. Thus, 9 and 19 might be considered as new promising candidates for Alzheimer's disease treatment.

A Double Whammy: Targeting Both Fatty Acid Amide Hydrolase (FAAH) and Cyclooxygenase (COX) To Treat Pain and Inflammation

Pain states that arise from non-resolving inflammation, such as inflammatory bowel disease or arthritis, pose an unusually difficult challenge for therapy because of the complexity and heterogeneity of their underlying mechanisms. It has been suggested that key nodes linking interactive pathogenic pathways of non-resolving inflammation might offer novel targets for the treatment of inflammatory pain. Nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit the cyclooxygenase (COX)-mediated production of pain- and inflammation-inducing prostanoids, are a common first-line treatment for this condition, but their use is limited by mechanism-based side effects. The endogenous levels of anandamide, an endocannabinoid mediator with analgesic and tissue-protective functions, are regulated by fatty acid amide hydrolase (FAAH). This review outlines the pharmacological and chemical rationale for the simultaneous inhibition of COX and FAAH activities with designed multitarget agents. Preclinical studies indicate that such agents may combine superior anti-inflammatory efficacy with reduced toxicity.

Potent multitarget FAAH-COX inhibitors: Design and structure-activity relationship studies

Non-steroidal anti-inflammatory drugs (NSAIDs) exert their pharmacological effects by inhibiting cyclooxygenase (COX)-1 and COX-2. Though widely prescribed for pain and inflammation, these agents have limited utility in chronic diseases due to serious mechanism-based adverse events such as gastrointestinal damage. Concomitant blockade of fatty acid amide hydrolase (FAAH) enhances the therapeutic effects of the NSAIDs while attenuating their propensity to cause gastrointestinal injury. This favorable interaction is attributed to the accumulation of protective FAAH substrates, such as the endocannabinoid anandamide, and suggests that agents simultaneously targeting COX and FAAH might provide an innovative strategy to combat pain and inflammation with reduced side effects. Here, we describe the rational design and structure-active relationship (SAR) properties of the first class of potent multitarget FAAH-COX inhibitors. A focused SAR exploration around the prototype 10r (ARN2508) led to the identification of achiral (18b) as well as racemic (29a-c and 29e) analogs. Absolute configurational assignment and pharmacological evaluation of single enantiomers of 10r are also presented. (S)-(+)-10r is the first highly potent and selective chiral inhibitor of FAAH-COX with marked in vivo activity, and represents a promising lead to discover novel analgesics and anti-inflammatory drugs.

Inhibitors of Fatty Acid Amide Hydrolase and Monoacylglycerol Lipase: New Targets for Future Antidepressants

Cannabis and analogs of Δ⁹-tetrahydrocannabinol have been used for therapeutic purposes, but their therapeutic use remains limited because of various adverse effects. Endogenous cannabinoids have been discovered, and dysregulation of endocannabinoid signaling is implicated in the pathophysiology of major depressive disorder (MDD). Recently, endocannabinoid hydrolytic enzymes such as fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) have become new therapeutic targets in the treatment of MDD. Several FAAH or MAGL inhibitors are reported to have no cannabimimetic side effects and, therefore, are new potential therapeutic options for patients with MDD who are resistant to first-line antidepressants (selective serotonin and serotonin-norepinephrine reuptake inhibitors). In this review, we focus on the possible relationships between MDD and the endocannabinoid system as well as the inhibitors' therapeutic potential. MAGL inhibitors may reduce inflammatory responses through activation of cannabinoid receptor type 2. In the hypothalamic-pituitary-adrenal axis, repeated FAAH inhibitor administration may be beneficial for reducing circulating glucocorticoid levels. Both FAAH and MAGL inhibitors may contribute to dopaminergic system regulation. Recently, several new inhibitors have been developed with strong potency and selectivity. FAAH inhibitor, MAGL inhibitor, or dual blocker use would be promising new treatments for MDD. Further pre-clinical studies and clinical trials using these inhibitors are warranted.

Physical Nature of Fatty Acid Amide Hydrolase Interactions with Its Inhibitors: Testing a Simple Nonempirical Scoring Model

Fatty acid amide hydrolase (FAAH) is an enzyme responsible for the deactivating hydrolysis of fatty acid ethanolamide neuromodulators. FAAH inhibitors have gained considerable interest due to their possible application in the treatment of anxiety, inflammation, and pain. In the context of inhibitor design, the availability of reliable computational tools for predicting binding affinity is still a challenging task, and it is now well understood that empirical scoring functions have several limitations that in principle could be overcome by quantum mechanics. Herein, systematic ab initio analyses of FAAH interactions with a series of inhibitors belonging to the class of the N-alkylcarbamic acid aryl esters have been performed. In contrast to our earlier studies of other classes of enzyme-inhibitor complexes, reasonable correlation with experimental results required us to consider correlation effects along with electrostatic term. Therefore, the simplest comprehensive nonempirical model allowing for qualitative predictions of binding affinities for FAAH ligands consists of electrostatic multipole and second-order dispersion terms. Such a model has been validated against the relative stabilities of the benchmark S66 set of biomolecular complexes. As it does not involve parameters fitted to experimentally derived data, this model offers a unique opportunity for generally applicable inhibitor design and virtual screening.

O-(triazolyl)methyl carbamates as a novel and potent class of fatty acid amide hydrolase (FAAH) inhibitors

Inhibition of fatty acid amide hydrolase (FAAH) activity is under investigation as a valuable strategy for the treatment of several disorders, including pain and drug addiction. A number of potent FAAH inhibitors belonging to different chemical classes have been disclosed to date; O-aryl carbamates are one of the most representative families. In the search for novel FAAH inhibitors, a series of O-(1,2,3-triazol-4-yl)methyl carbamate derivatives were designed and synthesized exploiting a copper- catalyzed [3+2] cycloaddition reaction between azides and alkynes (click chemistry). Exploration of the structure-activity relationships within this new class of compounds identified potent inhibitors of both rat and human FAAH with IC50 values in the single-digit nanomolar range. In addition, these derivatives showed improved stability in rat plasma and kinetic solubility in buffer with respect to the lead compound. Based on the results of the study, the novel analogues identified can be considered to be promising starting point for the development of new FAAH inhibitors with improved drug-like properties.

Applying a multitarget rational drug design strategy: the first set of modulators with potent and balanced activity toward dopamine D3 receptor and fatty acid amide hydrolase

Combining computer-assisted drug design and synthetic efforts, we generated compounds with potent and balanced activities toward both D3 dopamine receptor and fatty acid amide hydrolase (FAAH) enzyme. By concurrently modulating these targets, our compounds hold great potential toward exerting a disease-modifying effect on nicotine addiction and other forms of compulsive behavior.

Combined quantum mechanics/molecular mechanics (QM/MM) methods in computational enzymology

Computational enzymology is a rapidly maturing field that is increasingly integral to understanding mechanisms of enzyme-catalyzed reactions and their practical applications. Combined quantum mechanics/molecular mechanics (QM/MM) methods are important in this field. By treating the reacting species with a quantum mechanical method (i.e., a method that calculates the electronic structure of the active site) and including the enzyme environment with simpler molecular mechanical methods, enzyme reactions can be modeled. Here, we review QM/MM methods and their application to enzyme-catalyzed reactions to investigate fundamental and practical problems in enzymology. A range of QM/MM methods is available, from cheaper and more approximate methods, which can be used for molecular dynamics simulations, to highly accurate electronic structure methods. We discuss how modeling of reactions using such methods can provide detailed insight into enzyme mechanisms and illustrate this by reviewing some recent applications. We outline some practical considerations for such simulations. Further, we highlight applications that show how QM/MM methods can contribute to the practical development and application of enzymology, e.g., in the interpretation and prediction of the effects of mutagenesis and in drug and catalyst design.

Radiosynthesis and evaluation of [¹¹C-carbonyl]-labeled carbamates as fatty acid amide hydrolase radiotracers for positron emission tomography

Fatty acid amide hydrolase (FAAH) plays a key role in regulating the tone of the endocannabinoid system. Radiotracers are required to image and quantify FAAH activity in vivo. We have synthesized a series of potent FAAH inhibitors encompassing two classes of N-alkyl-O-arylcarbamates and radiolabeled eight of them with carbon-11. The [¹¹C-carbonyl]-radiotracers were evaluated in vitro and ex vivo in rats as potential FAAH imaging agents for positron emission tomography (PET). Both sets of [¹¹C]O-arylcarbamates showed good to excellent brain penetration and an appropriate regional distribution. Pretreatments with a FAAH inhibitor demonstrated that 80–95% of brain uptake of radioactivity constituted binding of the radiotracers to FAAH. Brain extraction measurements showed that binding to FAAH was irreversible and kinetically different for the two classes of carbamates. These promising results are discussed in terms of the requirements of a suitable radiotracer for the in vivo imaging of FAAH using PET.

Fluorescent penta- and hexaene fatty acids by a Wittig-Horner/elimination strategy

Molecular fluorescent probes have revolutionized biochemical and biophysical studies in the last decades, but with regard to lipids there has been a lack of combining the slim shape of saturated acyl chains with fluorescent properties. Our strategy to pentaene and hexaene fatty acids builds upon commercially available 4-(E)-decenal, which is subjected to a Wittig-Horner reaction after chlorination in α-position. DBU-mediated β-elimination of HCl proceeding the olefination establishes a highly conjugated system to which a salt-free Wittig reaction adds a final double bond leading to a good (Z)-selectivity of 83-86%. The double bond geometry can be optionally isomerized with I(2) to furnish the all-(E)-species. The five conjugated alkene moieties result in a longest-wavelength absorption maximum of about 350 nm. A red-shift to 380 nm was realized by addition of another double bond employing a common Wittig-Horner prolongation sequence. Stokes shifts of about 7300 and 7800 cm(-1), respectively, were observed.

Approximating protein flexibility through dynamic pharmacophore models: application to fatty acid amide hydrolase (FAAH)

A structure-based drug discovery method is described that incorporates target flexibility through the use of an ensemble of protein conformations. The approach was applied to fatty acid amide hydrolase (FAAH), a key deactivating enzyme in the endocannabinoid system. The resultant dynamic pharmacophore models are rapidly able to identify known FAAH inhibitors over drug-like decoys. Different sources of FAAH conformational ensembles were explored, with both snapshots from molecular dynamics simulations and a group of X-ray structures performing well. Results were compared to those from docking and pharmacophore models generated from a single X-ray structure. Increasing conformational sampling consistently improved the pharmacophore models, emphasizing the importance of incorporating target flexibility in structure-based drug design.

Computational methods for the discovery of mood disorder therapies

INTRODUCTION

Despite the significant progress, research is still needed to reveal details of the complex and dynamic chemical processes operating in the central nervous system (CNS) and their relationship to psychological effects such as mood disorders. The incidence of behavioral depression is widely spread worldwide, with an estimated 14.8 million adults diagnosed yearly in the United States alone. The efficacy of current antidepressants on 50 - 60% of patients, their slow onset of action and the prevalence of adverse side effects highlight the need for developing a new generation of improved antidepressants. Computational methods have the potential to aid in the discovery of mood modulators.

AREAS COVERED

This review contains three main sections: historical evolution of marketed antidepressants, physicochemical and structural properties of antidepressant compounds reported in the ChEMBL database and recent efforts in the design and discovery of antidepressants using computational methods. The authors provide details of the computational methods employed, from chemoinformatic analyses to molecular modeling.

EXPERT OPINION

While there have been numerous and important findings in depression research, the high cost and time spent on research into new therapies for brain disorders is a risky undertaking. Computational methodologies can be employed to speed up the discovery of new antidepressants and to detect new sources of chemical compounds with potential antidepressant activity. Compound collections containing compounds already approved in the pharmaceutical and food industries that cover the property space and complement the structural space of CNS drugs represent a promising starting point for the discovery of new antidepressant agents.

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.

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.

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.

Protein dynamics and enzyme catalysis: insights from simulations

The role of protein dynamics in enzyme catalysis is one of the most active and controversial areas in enzymology today. Some researchers claim that protein dynamics are at the heart of enzyme catalytic efficiency, while others state that dynamics make no significant contribution to catalysis. What is the biochemist - or student - to make of the ferocious arguments in this area? Protein dynamics are complex and fascinating, as molecular dynamics simulations and experiments have shown. The essential question is: do these complex motions have functional significance? In particular, how do they affect or relate to chemical reactions within enzymes, and how are chemical and conformational changes coupled together? Biomolecular simulations can analyse enzyme reactions and dynamics in atomic detail, beyond that achievable in experiments: accurate atomistic modelling has an essential part to play in clarifying these issues. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches.

Hydrolysis of 2-arachidonoylglycerol in Tetrahymena thermophila. Identification and partial characterization of a Monoacylglycerol Lipase-like enzyme

Tetrahymena thermophila is a model organism for molecular and cellular biology. Previous studies from our group showed that Tetrahymena contains major components of the endocannabinoid system, such as various endocannabinoids and FAAH. In mammalian cells the endocannabinoid 2-arachidonoylglycerol is inactivated mainly by MAGL. In this study we showed that 2-arachidonoylglycerol and 2-oleoylglycerol are hydrolyzed by the combined actions of MAGL and FAAH. MAGL-like activity was examined in the presence of FAAH specific inhibitors, URB597 or AM374 and showed optimum pH of 8-9, apparent K(M) of 14.1μM and V(max) of 5.8nmol/min×mg. The enzyme was present in membrane bound and cytosolic isoforms; molecular mass was determined at ∼45 and ∼40kDa. MAGL and FAAH could also inactivate endogenous signaling lipids, which might play an important role in Tetrahymena as suggested in mammals. Tetrahymena could be used as a model system for testing drugs targeting enzymes of the endocannabinoid system.

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.

Structural Fluctuations in Enzyme-Catalyzed Reactions: Determinants of Reactivity in Fatty Acid Amide Hydrolase from Multivariate Statistical Analysis of Quantum Mechanics/Molecular Mechanics Paths

The effects of structural fluctuations, due to protein dynamics, on enzyme activity are at the heart of current debates on enzyme catalysis. There is evidence that fatty acid amide hydrolase (FAAH) is an enzyme for which reaction proceeds via a high-energy, reactive conformation, distinct from the predominant enzyme-substrate complex (Lodola et al. Biophys. J. 2007, 92, L20-22). Identifying the structural causes of differences in reactivity between conformations in such complex systems is not trivial. Here, we show that multivariate analysis of key structural parameters can identify structural determinants of barrier height by analysis of multiple reaction paths. We apply a well-tested quantum mechanics/molecular mechanics (QM/MM) method to the first step of the acylation reaction between FAAH and oleamide substrate for 36 different starting structures. Geometrical parameters (consisting of the key bond distances that change during the reaction) were collected and used for principal component analysis (PCA), partial least-squares (PLS) regression analysis, and multiple linear regression (MLR) analysis. PCA indicates that different "families" of enzyme-substrate conformations arise from QM/MM molecular dynamics simulation and that rarely sampled, catalytically significant conformational states can be identified. PLS and MLR analyses allowed the construction of linear regression models, correlating the calculated activation barriers with simple geometrical descriptors. These analyses reveal the presence of two fully independent geometrical effects, explaining 78% of the variation in the activation barrier, which are directly correlated with transition-state stabilization (playing a major role in catalysis) and substrate binding. These results highlight the power of statistical approaches of this type in identifying crucial structural features that contribute to enzyme reactivity.

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.