Discovery libraries targeting the major enzyme classes: The serine hydrolases

Two libraries of modestly reactive ureas containing either electron-deficient acyl anilines or acyl pyrazoles were prepared and are reported as screening libraries for candidate serine hydrolase inhibitors. Within each library is a small but powerful subset of compounds that serve as a chemotype fragment screening library capable of subsequent structural diversification. Elaboration of the pyrazole-based ureas provided remarkably potent irreversible inhibitors of fatty acid amide hydrolase (FAAH, apparent K_i = 100–200 pM) complementary to those previously disclosed enlisting electron-deficient aniline-based ureas.     

α-Ketoheterocycle inhibitors of fatty acid amide hydrolase: Exploration of conformational constraints in the acyl side chain

A series of α-ketooxazoles containing heteroatoms embedded within conformational constraints in the C2 acyl side chain of 2 (OL-135) were synthesized and evaluated as inhibitors of fatty acid amide hydrolase (FAAH). The studies reveal that the installation of a heteroatom (O) in the conformational constraint is achievable, although the potency of these novel derivatives is reduced slightly relative to 2 and the analogous 1,2,3,4-tetrahydronaphthalene series. Interestingly, both enantiomers (R and S) of the candidate inhibitors bearing a chiral center adjacent to the electrophilic carbonyl were found to effectively inhibit FAAH.     

Mining biologically-active molecules for inhibitors of fatty acid amide hydrolase (FAAH): Identification of phenmedipham and amperozide as FAAH inhibitors

The screening of known medicinal agents against new biological targets has been shown to be a valuable approach for revealing new pharmacology of marketed compounds. Recently, carbamate, urea and ketone inhibitors of fatty acid amide hydrolase (FAAH) have been described as promising treatments for pain, anxiety, depression and other CNS-related conditions. In order to find novel FAAH inhibitors, a focused screen of molecules containing potentially reactive moieties or having in vivo effects that are possibly relevant to the biology of FAAH was conducted. These studies revealed phenmedipham 13 and amperozide 14 to be inhibitors of human FAAH, with an IC₅₀ of 377 nM and 1.34 μM, respectively.     

Identification and optimization of soluble epoxide hydrolase inhibitors with dual potency towards fatty acid amide hydrolase

Multi-target inhibitors have become increasing popular as a means to leverage the advantages of poly-pharmacology while simplifying drug delivery. Here, we describe dual inhibitors for soluble epoxide hydrolase (sEH) and fatty acid amide hydrolase (FAAH), two targets known to synergize when treating inflammatory and neuropathic pain. The structure activity relationship (SAR) study described herein initially started with t-TUCB (trans-4-[4-(3-trifluoromethoxyphenyl-l-ureido)-cyclohexyloxy]-benzoic acid), a potent sEH inhibitor that was previously shown to weakly inhibit FAAH. Inhibitors with a 6-fold increase of FAAH potency while maintaining high sEH potency were developed by optimization. Interestingly, compared to most FAAH inhibitors that inhibit through time-dependent covalent modification, t-TUCB and related compounds appear to inhibit FAAH through a time-independent, competitive mechanism. These inhibitors are selective for FAAH over other serine hydrolases. In addition, FAAH inhibition by t-TUCB appears to be higher in human FAAH over other species; however, the new dual sEH/FAAH inhibitors have improved cross-species potency. These dual inhibitors may be useful for future studies in understanding the therapeutic application of dual sEH/FAAH inhibition.     

Structure–activity relationship studies on 1-heteroaryl-3-phenoxypropan-2-ones acting as inhibitors of cytosolic phospholipase A2α and fatty acid amide hydrolase: replacement of the activated ketone group by other serine traps

Cytosolic phospholipase A₂α (cPLA₂α) and fatty acid amide hydrolase (FAAH) are serine hydrolases. cPLA₂α is involved in the generation of pro-inflammatory lipid mediators, FAAH terminates the anti-inflammatory effects of endocannabinoids. Therefore, inhibitors of these enzymes may represent new drug candidates for the treatment of inflammation. We have reported that certain 1-heteroarylpropan-2-ones are potent inhibitors of cPLA₂α and FAAH. The serine reactive ketone group of these compounds, which is crucial for enzyme inhibition, is readily metabolized resulting in inactive alcohol derivatives. In order to obtain metabolically more stable inhibitors, we replaced this moiety by α-ketoheterocyle, cyanamide and nitrile serine traps. Investigations on activity and metabolic stability of these substances revealed that in all cases an increased metabolic stability was accompanied by a loss of inhibitory potency against cPLA₂α and FAAH, respectively.     

Piperazine and piperidine carboxamides and carbamates as inhibitors of fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL)

The key hydrolytic enzymes of the endocannabinoid system, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), are potential targets for various therapeutic applications. In this paper, we present more extensively the results of our previous work on piperazine and piperidine carboxamides and carbamates as FAAH and MAGL inhibitors. The best compounds of these series function as potent and selective MAGL/FAAH inhibitors or as dual FAAH/MAGL inhibitors at nanomolar concentrations. This study revealed that MAGL inhibitors should comprise leaving-groups with a conjugate acid pK_a of 8–10, while diverse leaving groups are tolerated for FAAH inhibitors.     

Design,synthesis, and biological evaluation of a series of piperazine ureas as fatty acid amide hydrolase inhibitors

A series of piperazine ureas were designed, synthesized, and evaluated for their potential as novel orally efficacious fatty acid amide hydrolase (FAAH) inhibitors for the treatment of neuropathic and inflammatory pain. We carried out an optimization study of compound 5 to improve its in vitro FAAH inhibitory activity, and identified the 2-pyrimidinylpiperazine derivative 21d with potent inhibitory activity, favorable DMPK profile and brain permeability. Compound 21d showed robust and dose-dependent analgesic efficacy in animal models of both neuropathic and inflammatory pain.     

Novel mechanistic class of fatty acid amide hydrolase inhibitors with remarkable selectivity

Fatty acid amide hydrolase (FAAH) is an integral membrane enzyme that degrades the fatty acid amide family of signaling lipids, including the endocannabinoid anandamide. Genetic or pharmacological inactivation of FAAH leads to analgesic, anti-inflammatory, anxiolytic, and antidepressant phenotypes in rodents without showing the undesirable side effects observed with direct cannabinoid receptor agonists, indicating that FAAH may represent an attractive therapeutic target for treatment of pain, inflammation, and other central nervous system disorders. However, the FAAH inhibitors reported to date lack drug-like pharmacokinetic properties and/or selectivity. Herein we describe piperidine/piperazine ureas represented by N-phenyl-4-(quinolin-3-ylmethyl)piperidine-1-carboxamide (PF-750) and N-phenyl-4-(quinolin-2-ylmethyl)piperazine-1-carboxamide (PF-622) as a novel mechanistic class of FAAH inhibitors. PF-750 and PF-622 show higher in vitro potencies than previously established classes of FAAH inhibitors. Rather unexpectedly based on the high chemical stability of the urea functional group, PF-750 and PF-622 were found to inhibit FAAH in a time-dependent manner by covalently modifying the enzyme's active site serine nucleophile. Activity-based proteomic profiling revealed that PF-750 and PF-622 were completely selective for FAAH relative to other mammalian serine hydrolases. We hypothesize that this remarkable specificity derives, at least in part, from FAAH's special ability to function as a C(O)-N bond hydrolase, which distinguishes it from the vast majority of metabolic serine hydrolases in mammals that are restricted to hydrolyzing esters and/or thioesters. The piperidine/piperazine urea may thus represent a privileged chemical scaffold for the synthesis of FAAH inhibitors that display an unprecedented combination of potency and selectivity for use as potential analgesic and anxiolytic/antidepressant agents.     

Macamides and their synthetic analogs: Evaluation of in vitro FAAH inhibition

Maca (Lepidium meyenii), a traditional food crop of the Peruvian Andes is now widely touted as a dietary supplement. Among the various chemical constituents isolated from the plant are a unique series of non-polar, long-chain fatty acid N-benzylamides known as macamides. We have synthesized 11 of the 19 reported macamides and have tested each as potential inhibitors of the human enzyme, fatty acid amide hydrolase (FAAH). The five most potent macamides were FAAH inhibitors (IC₅₀ = 10–17 μM). These amides were derivatives of oleic, linoleic and linolenic acids and benzylamine or 3-methoxybenzylamine. Of the three compounds evaluated in a pre-incubation time study, two macamides were not irreversible inhibitors of FAAH. The third, a carbamate structurally related to macamides, was shown to be an irreversible inhibitor of FAAH (IC₅₀ = 0.153 μM).     

Piperidine and piperazine inhibitors of fatty acid amide hydrolase targeting excitotoxic pathology

FAAH inhibitors offer safety advantages by augmenting the anandamide levels “on demand” to promote neuroprotective mechanisms without the adverse psychotropic effects usually seen with direct and chronic activation of the CB1 receptor. FAAH is an enzyme implicated in the hydrolysis of the endocannabinoid N-arachidonoylethanolamine (AEA), which is a partial agonist of the CB1 receptor. Herein, we report the discovery of a new series of highly potent and selective carbamate FAAH inhibitors and their evaluation for neuroprotection. The new inhibitors showed potent nanomolar inhibitory activity against human recombinant and purified rat FAAH, were selective (>1000-fold) against serine hydrolases MGL and ABHD6 and lacked any affinity for the cannabinoid receptors CB1 and CB2. Evaluation of FAAH inhibitors 9 and 31 using the in vitro competitive activity-based protein profiling (ABPP) assay confirmed that both inhibitors were highly selective for FAAH in the brain, since none of the other FP-reactive serine hydrolases in this tissue were inhibited by these agents. Our design strategy followed a traditional SAR approach and was supported by molecular modeling studies based on known FAAH cocrystal structures. To rationally design new molecules that are irreversibly bound to FAAH, we have constructed “precovalent” FAAH-ligand complexes to identify good binding geometries of the ligands within the binding pocket of FAAH and then calculated covalent docking poses to select compounds for synthesis. FAAH inhibitors 9 and 31 were evaluated for neuroprotection in rat hippocampal slice cultures. In the brain tissue, both inhibitors displayed protection against synaptic deterioration produced by kainic acid-induced excitotoxicity. Thus, the resultant compounds produced through rational design are providing early leads for developing therapeutics against seizure-related damage associated with a variety of disorders.     

N-aryl 2-aryloxyacetamides as a new class of fatty acid amide hydrolase (FAAH) inhibitors

Fatty acid amide hydrolase (FAAH) is a promising target for the development of drugs to treat neurological diseases. In search of new FAAH inhibitors, we identified 2-(4-cyclohexylphenoxy)-N-(3-(oxazolo[4,5-b]pyridin-2-yl)phenyl)acetamide, 4g, with an IC₅₀ of 2.6?µM as a chemical starting point for the development of potent FAAH inhibitors. Preliminary hit-to-lead optimisation resulted in 2-(4-phenylphenoxy)-N-(3-(oxazolo[4,5-b]pyridin-2-yl)phenyl)acetamide, 4i, with an IC₅₀ of 0.35?µM.     

Heteroaryl urea inhibitors of fatty acid amide hydrolase: Structure–mutagenicity relationships for arylamine metabolites

The structure–activity relationships for a series of heteroaryl urea inhibitors of fatty acid amide hydrolase (FAAH) are described. Members of this class of inhibitors have been shown to inactivate FAAH by covalent modification of an active site serine with subsequent release of an aromatic amine from the urea electrophile. Systematic Ames II testing guided the optimization of urea substituents by defining the structure–mutagenicity relationships for the released aromatic amine metabolites. Potent FAAH inhibitors were identified having heteroaryl amine leaving groups that were non-mutagenic in the Ames II assay.     

Unsymmetrical non-adamantyl N,N′-diaryl urea and amide inhibitors of soluble expoxide hydrolase

Incorporation of an adamantyl group in prototypical soluble expoxide hydrolase (sEH) inhibitors afforded improved enzyme potency. We explored replacement of the adamantyl group in unsymmetrical ureas and amides with substituted aryl rings to identify equipotent and metabolically stable sEH inhibitors. We found that aryl rings, especially those substituted in the para position with a strongly electron withdrawing substituent, afforded enzyme IC₅₀ values comparable to the adamantyl compounds in an ether substituted, unsymmetrical N,N′-diaryl urea or amide scaffold.     

Discovery of novel spirocyclic inhibitors of fatty acid amide hydrolase (FAAH). Part 2. Discovery of 7-azaspiro[3.5]nonane urea PF-04862853, an orally efficacious inhibitor of fatty acid amide hydrolase (FAAH) for pain

Fatty acid amide hydrolase (FAAH) is an integral membrane serine hydrolase responsible for the degradation of fatty acid amide signaling molecules such as endocannabinoid anandamide (AEA), which has been shown to possess cannabinoid-like analgesic properties. Herein we report the optimization of spirocyclic 7-azaspiro[3.5]nonane and 1-oxa-8-azaspiro[4.5]decane urea covalent inhibitors of FAAH. Using an iterative design and optimization strategy, lead compounds were identified with a remarkable reduction in molecular weight and favorable CNS drug like properties. 3,4-Dimethylisoxazole and 1-methyltetrazole were identified as superior urea moieties for this inhibitor class. A dual purpose in vivo efficacy and pharmacokinetic screen was designed to be the key decision enabling experiment affording the ability to move quickly from compound synthesis to selection of preclinical candidates. On the basis of the remarkable potency, selectivity, pharmacokinetic properties and in vivo efficacy, PF-04862853 (15p) was advanced as a clinical candidate.     

Discovery of heterocyclic carbohydrazide derivatives as novel selective fatty acid amide hydrolase inhibitors: design,synthesis and anti-neuroinflammatory evaluation

Fatty acid amide hydrolase (FAAH) is a promising target for the development of drugs to treat pain, inflammation, and other central nervous system disorders. Herein, a series of novel heterocyclic carbohydrazide derivatives were firstly designed by the classic scaffold-hopping strategy. Then, multi-steps synthesis and human FAAH enzyme inhibiting activity assays were conducted. Among them, compound 26 showed strong inhibition against human FAAH with IC₅₀ of 2.8 μM. Corresponding docking studies revealed that the acyl hydrazide group of compound 26 well-occupied the acyl-chain binding pocket. It also exhibited high selectivity towards FAAH when comparing with CES2 and MAGL. Additionally, compound 26 effectively suppressed the LPS-induced neuroinflammation of microglial cells (BV2) via the reduction of interleukin-1β and tumor necrosis factor-α. Our results provided significative lead compounds for the further discovery of novel selective and safe FAAH inhibitors with potent anti-neuroinflammation activity.     

Bioisosteric substitution of adamantane with bicyclic lipophilic groups improves water solubility of human soluble epoxide hydrolase inhibitors

A series of inhibitors of the soluble epoxide hydrolase (sEH) containing lipophilic groups of natural origin (camphanyl, norcamphanyl, furan-2-yl) were developed. Inhibitory potency ranging from 0.4 nM to 2.16 μM were obtained. While having the same level of inhibitory activity bicyclic ureas are up to 10-fold more soluble than the corresponding ureas containing adamantyl or 4-trifluoromethoxyphenyl substituents. This makes them easier to formulate, more bioavailable and thus more promising as therapeutic sEH inhibitors. Endo/exo-form of compound 2b derived from l-camphor is 14-fold more potent than the corresponding analogue derived from d-camphor (IC₅₀ = 3.7 nM vs. 50.6 nM) indicating enantiomeric preference.     

Interaction of the N-(3-Methylpyridin-2-yl)amide Derivatives of Flurbiprofen and Ibuprofen with FAAH: Enantiomeric Selectivity and Binding Mode

Background

Combined fatty acid amide hydrolase (FAAH) and cyclooxygenase (COX) inhibition is a promising approach for pain-relief. The Flu-AM1 and Ibu-AM5 derivatives of flurbiprofen and ibuprofen retain similar COX-inhibitory properties and are more potent inhibitors of FAAH than the parent compounds. However, little is known as to the nature of their interaction with FAAH, or to the importance of their chirality. This has been explored here.

Methodology/Principal Findings

FAAH inhibitory activity was measured in rat brain homogenates and in lysates expressing either wild-type or FAAH^T488A-mutated enzyme. Molecular modelling was undertaken using both docking and molecular dynamics. The (R)- and (S)-enantiomers of Flu-AM1 inhibited rat FAAH with similar potencies (IC₅₀ values of 0.74 and 0.99 μM, respectively), whereas the (S)-enantiomer of Ibu-AM5 (IC₅₀ 0.59 μM) was more potent than the (R)-enantiomer (IC₅₀ 5.7 μM). Multiple inhibition experiments indicated that both (R)-Flu-AM1 and (S)-Ibu-AM5 inhibited FAAH in a manner mutually exclusive to carprofen. Computational studies indicated that the binding site for the Flu-AM1 and Ibu-AM5 enantiomers was located between the acyl chain binding channel and the membrane access channel, in a site overlapping the carprofen binding site, and showed a binding mode in line with that proposed for carprofen and other non-covalent ligands. The potency of (R)-Flu-AM1 was lower towards lysates expressing FAAH mutated at the proposed carprofen binding area than in lysates expressing wild-type FAAH.

Conclusions/Significance

The study provides kinetic and structural evidence that the enantiomers of Flu-AM1 and Ibu-AM5 bind in the substrate channel of FAAH. This information will be useful in aiding the design of novel dual-action FAAH: COX inhibitors.     

Structure based design of novel irreversible FAAH inhibitors

Fatty acid amide hydrolase (FAAH) has attracted significant attention due to its promise as an analgesic target. This has resulted in the discovery of numerous chemical classes as inhibitors of this potential therapeutic target. In this paper we disclose a new series of novel FAAH irreversible azetidine urea inhibitors. In general these compounds illustrate potent activity against the rat FAAH enzyme. Our SAR studies allowed us to optimize this series resulting in the identification of compounds 13 which were potent inhibitors of both human and rat enzyme. This series of compounds illustrated good hydrolase selectivity along with good PK properties.     

Thiadiazolopiperazinyl ureas as inhibitors of fatty acid amide hydrolase

A series of thiadiazolopiperazinyl aryl urea fatty acid amide hydrolase (FAAH) inhibitors is described. The molecules were found to inhibit the enzyme by acting as mechanism-based substrates, forming a covalent bond with Ser241. SAR and PK properties are presented.     

Methylation and acetylation of 15-hydroxyanandamide modulate its interaction with the endocannabinoid system

The biological activity of endocannabinoids like anandamide (AEA) and 2-arachidonoylglycerol (2-AG) is subjected in vivo to a “metabolic control”, exerted mainly by catabolic enzymes. AEA is inactivated by fatty acid amide hydrolase (FAAH), that is inhibited competitively by hydroxyanandamides (HAEAs) generated from AEA by lipoxygenase activity. Among these derivatives, 15-HAEA has been shown to be an effective (K_i ∼0.6 μM) FAAH inhibitor, that blocks also type-1 cannabinoid receptor (CB1R) but not other components of the “endocannabinoid system (ECS)”, like the AEA transporter (AMT) or CB2R. Here, we extended the study of the effect of 15-HAEA on the AEA synthetase (NAPE-PLD) and the AEA-binding vanilloid receptor (TRPV1), showing that 15-HAEA activates the former (up to ∼140% of controls) and inhibits the latter protein (down to ∼70%). We also show that 15-HAEA halves the synthesis of 2-AG and almost doubles the transport of this compound across the membrane. In addition, we synthesized methyl and acetyl derivatives of 15-HAEA (15-MeOAEA and 15-AcOAEA, respectively), in order to check their ability to modulate FAAH and the other ECS elements. In fact, methylation and acetylation are common biochemical reactions in the cellular environment. We show that 15-MeOAEA, unlike 15-AcOAEA, is still a powerful competitive inhibitor of FAAH (K_i ∼0.7 μM), and that both derivatives have negligible interactions with the other proteins of ECS. Therefore, 15-MeOAEA is a FAAH inhibitor more selective than 15-HAEA. Further molecular dynamics analysis gave clues to the molecular requirements for the interaction of 15-HAEA and 15-MeOAEA with FAAH.