Design and Synthesis of Highly Potent and Specific ABHD6 Inhibitors

Fine-tuning than complete disruption of 2-arachidonoylglycerol (2-AG) metabolism in the brain represents a promising pharmacological approach to limit potential untoward effects associated with complete blockade of monoacylglycerol lipase (MGL), the primary hydrolase of 2-AG. This could be achieved through a/b-hydrolase domain containing 6 (ABHD6) inhibition, which will provide a smaller and safer contribution to 2-AG regulation in the brain. Pharmacological studies with ABHD6 inhibitors have recently been reported, where modulation of ABHD6 activity either through CB1R-dependent or CB1R-independent processes showed promise in preclinical models of epilepsy, neuropathic pain and inflammation. Furthermore in the periphery, ABHD6 modulates 2-AG and other fatty acid monoacylglycerols (MAGs) and is implicated in Type-2 diabetes, metabolic syndrome and potentially other diseases. Herein, we report the discovery of single-digit nanomolar potent and highly specific ABHD6 inhibitors with >1000-fold selectivity against MGL and FAAH. The new ABHD6 inhibitors provide early leads to develop therapeutics for neuroprotection and the treatment of inflammation and diabetes.

1-, 2- and 3-AG as substrates of the endocannabinoid enzymes and endogenous ligands of the cannabinoid receptor 1

2-Arachidonoylglycerol (2-AG) is the most potent and abundant endocannabinoid that acts as a full agonist at the cannabinoid 1 (CB1) and 2 (CB2) receptors. It serves as a substrate for several serine hydrolases, including monoacylglycerol lipase (MGL), α/β hydrolase domain 6 (ABHD6) and fatty acid amide hydrolase (FAAH). However, 2-AG's rapid conversion to 1-AG (the S stereoisomer) and 3-AG (the R stereoisomer) complicates in vivo signaling. Here, we present the interaction profiles of 2-AG and its isomerization products, 1- and 3-AG, with the endocannabinoid MGL, ABHD6 and FAAH enzymes as well as the CB1 receptor. The 1- and 3-AG enantiomers are less prone to isomerization, and their affinities to endocannabinoid enzymes and potencies at CB1 receptor are quite different compared to 2-AG. Although MGL is the principal hydrolytic enzyme of 2-AG, 3-AG (the R isomer) appears to be the best substrate for hMGL. Contrarily, 1-AG (the S isomer) demonstrates the worst substrate profile, indicating that the stereochemistry of 1(3)-monoacylglycerols is very important for MGL enzyme. On the other hand, both 1- and 3-AG (the sn1 monoacylglycerols) are efficiently hydrolyzed by hABHD6 without preference, while 2-AG (the sn2 monoacylglycerol) has the lowest rate of hydrolysis. FAAH, the principal hydrolytic enzyme for arachidonoylethanolamide (anandamide, AEA), catalyzes the hydrolysis of all three isomers with similar efficiencies. In a functional cAMP assay at CB1 receptor, all three isomers behaved as agonists, with 2-AG being the most potent, followed by 3-AG then 1-AG. The presented data provides stereochemical insights to design chemically stable AG analogs with preferential stability against enzymes of interest.

Design and Structure-Activity Relationships of Isothiocyanates as Potent and Selective N-Acylethanolamine-Hydrolyzing Acid Amidase Inhibitors

N-Acylethanolamines are signaling lipid molecules implicated in pathophysiological conditions associated with inflammation and pain. N-Acylethanolamine acid amidase (NAAA) favorably hydrolyzes lipid palmitoylethanolamide, which plays a key role in the regulation of inflammatory and pain processes. The synthesis and structure-activity relationship studies encompassing the isothiocyanate pharmacophore have produced potent low nanomolar inhibitors for hNAAA, while exhibiting high selectivity (>100-fold) against other serine hydrolases and cysteine peptidases. We have followed a target-based structure-activity relationship approach, supported by computational methods and known cocrystals of hNAAA. We have identified systemically active inhibitors with good plasma stability (t_1/2 > 2 h) and microsomal stability (t_1/2 ∼ 15-30 min) as pharmacological tools to investigate the role of NAAA in inflammation, pain, and drug addiction.

Design and synthesis of cyanamides as potent and selective N-acylethanolamine acid amidase inhibitors

N-acylethanolamine acid amidase (NAAA) inhibition represents an exciting novel approach to treat inflammation and pain. NAAA is a cysteine amidase which preferentially hydrolyzes the endogenous biolipids palmitoylethanolamide (PEA) and oleoylethanolamide (OEA). PEA is an endogenous agonist of the nuclear peroxisome proliferator-activated receptor-α (PPAR-α), which is a key regulator of inflammation and pain. Thus, blocking the degradation of PEA with NAAA inhibitors results in augmentation of the PEA/PPAR-α signaling pathway and regulation of inflammatory and pain processes. We have prepared a new series of NAAA inhibitors exploring the azetidine-nitrile (cyanamide) pharmacophore that led to the discovery of highly potent and selective compounds. Key analogs demonstrated single-digit nanomolar potency for hNAAA and showed >100-fold selectivity against serine hydrolases FAAH, MGL and ABHD6, and cysteine protease cathepsin K. Additionally, we have identified potent and selective dual NAAA-FAAH inhibitors to investigate a potential synergism between two distinct anti-inflammatory molecular pathways, the PEA/PPAR-α anti-inflammatory signaling pathway,^1-4 and the cannabinoid receptors CB1 and CB2 pathways which are known for their antiinflammatory and antinociceptive properties.^5-8 Our ligand design strategy followed a traditional structure-activity relationship (SAR) approach and was supported by molecular modeling studies of reported X-ray structures of hNAAA. Several inhibitors were evaluated in stability assays and demonstrated very good plasma stability (t_1/2 > 2 h; human and rodents). The disclosed cyanamides represent promising new pharmacological tools to investigate the potential role of NAAA inhibitors and dual NAAA-FAAH inhibitors as therapeutic agents for the treatment of inflammation and pain.

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.

Short-term regulation of acetyl CoA carboxylase: is the key enzyme in long-chain fatty acid synthesis regulated by an existing physiological mechanism?

Acetyl CoA carboxylase, the rate-limiting enzyme in regulating fatty acid synthesis, is thought to be controlled by allosteric effectors, its state of aggregation, covalent modulation and protein inhibitors. It is still obscure whether citrate, a positive allosteric effector, and long-chain fatty acyl CoA esters, negative allosteric effectors, function physiologically to regulate acetyl CoA carboxylase activity. New evidence from several laboratories reveals that the covalent phosphorylation may not involve regulation of acetyl CoA carboxylase activity. Protein inhibitors from liver cytosol and a peptide from fat cells were found to regulate acetyl CoA carboxylase both in vivo and in vitro. Coenzyme A, guanosine 5-monophosphate and phosphatidylinositol 4,5-bisphosphate may have an indirect effect, but certainly no direct involvement, on carboxylase activity.