N-Acyl pyrazoles: Effective and tunable inhibitors of serine hydrolases

Endocannabinoid Hydrolase Inhibitors: Potential Novel Anxiolytic Drugs

Over the past decade, the idea of targeting the endocannabinoid system to treat anxiety disorders has received increasing attention. Previous studies focused more on developing cannabinoid receptor agonists or supplementing exogenous cannabinoids, which are prone to various adverse effects due to their strong pharmacological activity and poor receptor selectivity, limiting their application in clinical research. Endocannabinoid hydrolase inhibitors are considered to be the most promising development strategies for the treatment of anxiety disorders. More recent efforts have emphasized that inhibition of two major endogenous cannabinoid hydrolases, monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), indirectly activates cannabinoid receptors by increasing endogenous cannabinoid levels in the synaptic gap, circumventing receptor desensitization resulting from direct enhancement of endogenous cannabinoid signaling. In this review, we comprehensively summarize the anxiolytic effects of MAGL and FAAH inhibitors and their potential pharmacological mechanisms, highlight reported novel inhibitors or natural products, and provide an outlook on future directions in this field.

Development of Potent and Selective Monoacylglycerol Lipase Inhibitors. SARs, Structural Analysis, and Biological Characterization

New potent, selective monoacylglycerol lipase (MAGL) inhibitors based on the azetidin-2-one scaffold ((±)-5a-v, (±)-6a-j, and (±)-7a-d) were developed as irreversible ligands, as demonstrated by enzymatic and crystallographic studies for (±)-5d, (±)-5l, and (±)-5r. X-ray analyses combined with extensive computational studies allowed us to clarify the binding mode of the compounds. 5v was identified as selective for MAGL when compared with other serine hydrolases. Solubility, in vitro metabolic stability, cytotoxicity, and absence of mutagenicity were determined for selected analogues. The most promising compounds ((±)-5c, (±)-5d, and (±)-5v) were used for in vivo studies in mice, showing a decrease in MAGL activity and increased 2-arachidonoyl-sn-glycerol levels in forebrain tissue. In particular, 5v is characterized by a high eudysmic ratio and (3R,4S)-5v is one of the most potent irreversible inhibitors of h/mMAGL identified thus far. These results suggest that the new MAGL inhibitors have therapeutic potential for different central and peripheral pathologies.

Scale-up of Sodium Persulfate Mediated, Nitroxide Catalyzed Oxidative Functionalization Reactions

BACKGROUND

Oxidation is a valuable tool in preparative organic chemistry. Oxoammonium salts and nitroxides have proven valuable as reagents and catalysts in this endeavor.

OBJECTIVE

The objective of this study is to scale up the oxidative amidation, ester formation, and nitrile formation using nitroxide as an organocatalyst.

METHODS

Oxidative functionalization reactions were scaled from the 1 mmol to the 1 mole level. Sodium persulfate was used as the primary oxidant, and a nitroxide was employed as a catalyst. The products of the reactions were isolated in analytically pure form by extraction with no need for column chromatography.

RESULTS

The oxidative amidation and esterification of aldehydes can be scaled up from 1 mmol to 1 mole effectively, with comparable product yields being obtained at each increment. This work shows that conditions developed on a small scale can be transferred to a larger scale without reoptimization. The oxidative functionalization of aldehydes to prepare nitriles is not amenable to direct scale-up due to the concomitant formation of significant quantities of the corresponding carboxylic acid, thereby compromising the product yield.

CONCLUSION

Two of the three oxidative transformations studied here can be scaled up successfully from the 1 mmol to the 1 mole level.

Covalent fragment approaches targeting non-cysteine residues

Covalent fragment approaches combine advantages of covalent binders and fragment-based drug discovery (FBDD) for target identification and validation. Although early applications focused mostly on cysteine labeling, the chemistries of available warheads that target other orthosteric and allosteric protein nucleophiles has recently been extended. The range of different warheads and labeling chemistries provide unique opportunities for screening and optimizing warheads necessary for targeting non-cysteine residues. In this review, we discuss these recently developed amino-acid-specific and promiscuous warheads, as well as emerging labeling chemistries, which includes novel transition metal catalyzed, photoactive, electroactive, and noncatalytic methodologies. We also highlight recent applications of covalent fragments for the development of molecular glues and proteolysis-targeting chimeras (PROTACs), and their utility in chemical proteomics-based target identification and validation.

Preparation of novel acyl pyrazoles and triazoles by means of oxidative functionalization reactions

Novel acyl pyrazoles and acyl triazoles have been prepared by means of the oxidative amidation of aldehydes in the presence of the requisite azole. Yields range from modest to good in both cases, and some limitations of the substrate scope have been discovered. Acyl pyrazoles were prepared by treatment of a mixture of aldehyde and pyrazole with an oxoammonium salt bearing the nitrate anion. In the case of acyl triazoles, the oxidative functionalization was performed using sodium persulfate as a terminal oxidant in the presence of a catalytic quantity of a nitroxide.

(3,5-Di-tert-butylphenyl)(1H-pyrazol-1-yl)methanone

The acyl pyrazole derivative (3,5-di-tert-butylphenyl)(1H-pyrazol-1-yl)methanone was prepared simply and rapidly in 86% isolated yield by means of an oxidative functionalization reaction of an aldehyde with pyrazole. A substoichiometric quantity of 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxoammonium nitrate was used as the oxidant. The reaction was performed solvent-free and in the absence of a base, making it a clean, green approach. The mixture of aldehyde, pyrazole, and the oxidant was heated at 55 °C for 3 h, and then, the product was isolated in analytically pure form via extraction with no need for column chromatography.

Novel inhibitors and activity-based probes targeting serine proteases

Serine proteases play varied and manifold roles in important biological, physiological, and pathological processes. These include viral, bacterial, and parasitic infection, allergic sensitization, tumor invasion, and metastasis. The use of activity-based profiling has been foundational in pinpointing the precise roles of serine proteases across this myriad of processes. A broad range of serine protease-targeted activity-based probe (ABP) chemotypes have been developed and we have recently introduced biotinylated and "clickable" peptides containing P1 N-alkyl glycine arginine N-hydroxy succinimidyl (NHS) carbamates as ABPs for detection/profiling of trypsin-like serine proteases. This present study provides synthetic details for the preparation of additional examples of this ABP chemotype, which function as potent irreversible inhibitors of their respective target serine protease. We describe their use for the activity-based profiling of a broad range of serine proteases including trypsin, the trypsin-like protease plasmin, chymotrypsin, cathepsin G, and neutrophil elastase (NE), including the profiling of the latter protease in clinical samples obtained from patients with cystic fibrosis.

Inactivation Mechanism of the Fatty Acid Amide Hydrolase Inhibitor BIA 10-2474

BIA 10-2474 is a time-dependent inhibitor of fatty acid amide hydrolase(FAAH) that was under clinical development for the treatment of neurological conditions when the program was terminated after one subject died and four were hospitalized with neurological symptoms during a first-in-human clinical study. The present manuscript describes the mechanism of FAAH inhibition by BIA 10-2474 as a target-specific covalent inhibition, supported by quantum mechanics and molecular modelling studies. The inhibitor incorporates a weakly reactive electrophile which, upon specific binding to the enzyme's active site, is positioned to react readily with the catalytic residues. The reactivity is enhanced on-site by the increased molarity at the reaction site and by specific inductive interactions with FAAH. In the second stage, the inhibitor reacts with the enzyme's catalytic nucleophile to form a covalent enzyme-inhibitor adduct. The hydrolysis of this adduct is shown to be unlikely under physiological conditions, therefore leading to irreversible inactivation of FAAH. The results also reveal the important role played by FAAH Thr236 in the reaction with BIA 10-2474, which is specific to FAAH and is not present in other serine hydrolases.

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

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

Novel Inhibitors and Activity-Based Probes Targeting Trypsin-Like Serine Proteases

The trypsin-like proteases (TLPs) play widespread and diverse roles, in a host of physiological and pathological processes including clot dissolution, extracellular matrix remodelling, infection, angiogenesis, wound healing and tumour invasion/metastasis. Moreover, these enzymes are involved in the disruption of normal lung function in a range of respiratory diseases including allergic asthma where several allergenic proteases have been identified. Here, we report the synthesis of a series of peptide derivatives containing an N-alkyl glycine analogue of arginine, bearing differing electrophilic leaving groups (carbamate and triazole urea), and demonstrate their function as potent, irreversible inhibitors of trypsin and TLPs, to include activities from cockroach extract. As such, these inhibitors are suitable for use as activity probes (APs) in activity-based profiling (ABP) applications.

Reactive chemistry for covalent probe and therapeutic development

Bioactive small molecules that form covalent bonds with a target protein are important tools for basic research and can be highly effective drugs. This review highlights reactive groups found in a collection of thiophilic and oxophilic drugs that mediate pharmacological activity through a covalent mechanism of action (MOA). We describe the application of advanced proteomic and bioanalytical methodologies for assessing selectivity of these covalent agents to guide and inspire the search for additional electrophiles suitable for covalent probe and therapeutic development. While the emphasis is on chemistry for modifying catalytic serine, threonine or cysteine residues, we devote a substantial fraction of the review to a collection of exploratory reactive groups of understudied residues on proteins.

ABHD17 regulation of plasma membrane palmitoylation and N-Ras-dependent cancer growth

Multiple Ras proteins, including N-Ras, depend on a palmitoylation/depalmitoylation cycle to regulate their subcellular trafficking and oncogenicity. General lipase inhibitors such as Palmostatin M (Palm M) block N-Ras depalmitoylation, but lack specificity and target several enzymes displaying depalmitoylase activity. Here, we describe ABD957, a potent and selective covalent inhibitor of the ABHD17 family of depalmitoylases, and show that this compound impairs N-Ras depalmitoylation in human acute myeloid leukemia (AML) cells. ABD957 produced partial effects on N-Ras palmitoylation compared with Palm M, but was much more selective across the proteome, reflecting a plasma membrane-delineated action on dynamically palmitoylated proteins. Finally, ABD957 impaired N-Ras signaling and the growth of NRAS-mutant AML cells in a manner that synergizes with MAP kinase kinase (MEK) inhibition. Our findings uncover a surprisingly restricted role for ABHD17 enzymes as regulators of the N-Ras palmitoylation cycle and suggest that ABHD17 inhibitors may have value as targeted therapies for NRAS-mutant cancers.

Chemical proteomic identification of functional cysteines with atypical electrophile reactivities

Cysteine-directed covalent ligands have emerged as a versatile category of chemical probes and drugs that leverage thiol nucleophilicity to form permanent adducts with proteins of interest. Understanding the scope of cysteines that can be targeted by covalent ligands, as well as the types of electrophiles that engage these residues, represent important challenges for fully realizing the potential of cysteine-directed chemical probe discovery. Although chemical proteomic strategies have begun to address these important questions, only a limited number of electrophilic chemotypes have been explored to date. Here, we describe a diverse set of candidate electrophiles appended to a common core 6-methoxy-1,2,3,4-tetrahydroquinoline fragment and evaluate their global cysteine reactivity profiles in human cancer cell proteomes. This work uncovered atypical reactivity patterns for a discrete set of cysteines, including residues involved in enzymatic catalysis and located in proximity to protein-protein interactions. These findings thus point to potentially preferred electrophilic groups for site-selectively targeting functional cysteines in the human proteome.

Strategies for Tuning the Selectivity of Chemical Probes that Target Serine Hydrolases

Serine hydrolases comprise a large family of enzymes that have diverse roles in key cellular processes, such as lipid metabolism, cell signaling, and regulation of post-translation modifications of proteins. They are also therapeutic targets for multiple human pathologies, including viral infection, diabetes, hypertension, and Alzheimer disease; however, few have well-defined substrates and biological functions. Activity-based probes (ABPs) have been used as effective tools to both profile activity and screen for selective inhibitors of serine hydrolases. One broad-spectrum ABP containing a fluorophosphonate electrophile has been used extensively to advance our understanding of diverse serine hydrolases. Due to the success of this single reagent, several robust chemistries have been developed to further diversify and tune the selectivity of ABPs used to target serine hydrolases. In this review, we highlight approaches to identify selective serine hydrolase ABPs and suggest new synthetic methodologies that could be applied to further advance probe development.