Development of Novel Monoamine Oxidase-B (MAO-B) Inhibitors with Reduced Blood-Brain Barrier Permeability for the Potential Management of Noncentral Nervous System (CNS) Diseases

Discovery of C-3 isoxazole substituted thiochromone S,S-dioxide derivatives as potent and selective inhibitors for monoamine oxidase B (MAO-B)

Developing new scaffolds for highly potent and selective inhibitors of human Monoamine Oxidase B (hMAO-B) is a crucial objective in enhancing the efficacy and safety in the clinical treatment of neurodegenerative diseases. In this study, we have identified a series of C-3 isoxazole-substituted thiochromone S,S-dioxide derivatives that exhibit strong inhibitory activity against hMAO-B. The strategy of oxidizing thiochromone to thiochromone S,S-dioxide solves the key defect of extreme insolubility observed for thiochromone analogues. In addition, the sulfone group contributes extra hydrogen(H)-bonding interactions with Tyr435, which significantly increases the activity of thiochromone S,S-dioxide derivatives against hMAO-B. Furthermore, the presence of isoxazole group provides potential H-bonding interaction and electrostatic interaction with the residue of Tyr326, while the rigid aryl ring introduces a potential steric conflict with Phe208 of hMAO-A to improve both potency and selectivity. In our investigations, several compounds (9c, 10c, 10e, 10g, 10l and 10m) demonstrate remarkable single-digit nanomolar potency. These compounds exhibit favorable cytotoxicity profiles in both differentiated SH-SY5Y and HVSMC cells, without apparent cardiotoxic effects. Moreover, compounds 10e and 10h do not lead to an increase in ROS levels in differentiated SH-SY5Y cells, further demonstrating their potential as safe and effective hMAO-B inhibitors. These findings indicate that the C-3 isoxazole substituted thiochromone S,S-dioxide analogues are potential leading compounds for the development of selective inhibitors with high potency.

GPR35 and mediators from platelets and mast cells in neutrophil migration and inflammation

Neutrophil recruitment from circulation to sites of inflammation is guided by multiple chemoattractant cues emanating from tissue cells, immune cells, and platelets. Here, we focus on the function of one G-protein coupled receptor, GPR35, in neutrophil recruitment. GPR35 has been challenging to study due the description of multiple ligands and G-protein couplings. Recently, we found that GPR35-expressing hematopoietic cells respond to the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). We discuss distinct response profiles of GPR35 to 5-HIAA compared to other ligands. To place the functions of 5-HIAA in context, we summarize the actions of serotonin in vascular biology and leukocyte recruitment. Important sources of serotonin and 5-HIAA are platelets and mast cells. We discuss the dynamics of cell migration into inflamed tissues and how multiple platelet and mast cell-derived mediators, including 5-HIAA, cooperate to promote neutrophil recruitment. Additional actions of GPR35 in tissue physiology are reviewed. Finally, we discuss how clinically approved drugs that modulate serotonin uptake and metabolism may influence 5-HIAA-GPR35 function, and we speculate about broader influences of the GPR35 ligand-receptor system in immunity and disease.

New Indole-3-Propionic Acid and 5-Methoxy-Indole Carboxylic Acid Derived Hydrazone Hybrids as Multifunctional Neuroprotectors

In light of the known neuroprotective properties of indole compounds and the promising potential of hydrazone derivatives, two series of aldehyde-heterocyclic hybrids combining those pharmacophores were synthesized as new multifunctional neuroprotectors. The obtained derivatives of indole-3-propionic acid (IPA) and 5-methoxy-indole carboxylic acid (5MICA) had good safety profiles: Hemolytic effects < 5% (200 μM) and IC₅₀ > 150 µM were found in the majority of the SH-SY5Y and bEnd3 cell lines. The 2,3-dihydroxy, 2-hydroxy-4-methoxy, and syringaldehyde derivatives of 5MICA exhibited the strongest neuroprotection against H₂O₂-induced oxidative stress in SH-SY5Y cells and 6-OHDA-induced neurotoxicity in rat-brain synaptosomes. All the compounds suppressed the iron-induced lipid peroxidation. The hydroxyl derivatives were also the most active in terms of deoxyribose-degradation inhibition, whereas the 3,4-dihydroxy derivatives were able to decrease the superoxide-anion generation. Both series of compounds showed an increased inhibition of hMAO-B, with greater expression detected in the 5MICA hybrids. The in vitro BBB model with the bEnd3 cell line showed that some compounds increased the permeability of the endothelial monolayer while maintaining the tight junctions. The combined results demonstrated that the derivatives of IPA and 5MICA showed strong neuroprotective, antioxidant, MAO-B inhibitory activity and could be considered as prospective multifunctional compounds for the treatment of neurodegenerative disorders.

GPR35 promotes neutrophil recruitment in response to serotonin metabolite 5-HIAA

Rapid neutrophil recruitment to sites of inflammation is crucial for innate immune responses. Here, we reveal that the G-protein-coupled receptor GPR35 is upregulated in activated neutrophils, and it promotes their migration. GPR35-deficient neutrophils are less recruited from blood vessels into inflamed tissue, and the mice are less efficient in clearing peritoneal bacteria. Using a bioassay, we find that serum and activated platelet supernatant stimulate GPR35, and we identify the platelet-derived serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) as a GPR35 ligand. GPR35 function in neutrophil recruitment is strongly dependent on platelets, with the receptor promoting transmigration across platelet-coated endothelium. Mast cells also attract GPR35+ cells via 5-HIAA. Mice deficient in 5-HIAA show a loss of GPR35-mediated neutrophil recruitment to inflamed tissue. These findings identify 5-HIAA as a GPR35 ligand and neutrophil chemoattractant and establish a role for platelet- and mast cell-produced 5-HIAA in cell recruitment to the sites of inflammation and bacterial clearance.

Monoamine Oxidase-B Inhibitor Reduction in Pro-Inflammatory Cytokines Mediated by Inhibition of cAMP-PKA/EPAC Signaling

Mucosal epithelial cell integrity is an important component of innate immunity and it protects the host from an environment rich in microorganisms. Virulence factors from Gram-negative bacteria [e.g. lipopolysaccharide (LPS)] induce significant pro-inflammatory cytokine expression. Monoamine oxidase (MAO) inhibitors reduce cytokine expression in a variety of inflammatory models and may therefore have therapeutic potential for a number of inflammatory diseases. We tested the anti-inflammatory therapeutic potential of a recently developed reversible MAO-B inhibitor (RG0216) with reduced transport across the blood–brain barrier. In an epithelial cell culture model, RG0216 significantly decreased LPS-induced interleukin (IL)-6 and IL-1β gene and protein expression and was as effective as equimolar concentrations of deprenyl (an existing irreversible MAO-B inhibitor). Hydrogen peroxide and modulating dopamine receptor signaling had no effect on cytokine expression. We showed that LPS-induced expression of IL-6 and IL-1β was cAMP dependent, that IL-6 and IL-1β expression were induced by direct cAMP activation (forskolin) and that RG0216 and deprenyl effectively reduced cAMP-mediated cytokine expression. Targeted protein kinase A (PKA) and Exchange Protein Activated by cAMP (EPAC) activation regulated IL-6 and IL-1β expression, albeit in different ways, but both cytokines were effectively decreased with RG0216. RG0216 reduction of LPS-induced cytokine expression occurred by acting downstream of the cAMP-PKA/EPAC signaling cascade. This represents a novel mechanism by which MAO-B selective inhibitors regulate LPS-induced IL-6 and IL-1β expression.

Discovery of benzamide-hydroxypyridinone hybrids as potent multi-targeting agents for the treatment of Alzheimer's disease

A novel class of benzamide-hydroxypyridinone (HPO) derivatives were innovatively designed, synthesised, and biologically evaluated as potential multitargeting candidates for the treatment of Alzheimer's disease (AD) through pharmacophores-merged approaches based on lead compounds 18d, benzyloxy phenyl analogs, and deferiprone (DFP). These hybrids possessed potent Monoamine oxidase B (MAO-B) inhibition as well as excellent iron chelation, with pFe³⁺ values ranging from 18.13 to 19.39. Among all the compounds, 8g exhibited the most potent selective MAO-B inhibitor (IC₅₀ = 68.4 nM, SI = 213). Moreover, 8g showed favourable pharmacokinetic properties and had great potential to penetrate the BBB in silico and PAMPA-BBB assay. Molecular modelling showed that 8g could adopt an extended conformation and have more enhanced interactions with MAO-B than 18d. In vitro and in vivo assays demonstrated that 8g remarkably resisted Aβ-induced oxidation and ameliorated cognitive impairment induced by scopolamine. Taken collectively, these results suggest that compound 8g is a potential multifunctional candidate for anti-AD treatment.

Mapping Chromone-3-Phenylcarboxamide Pharmacophore: Quid Est Veritas?

Chromone-3-phenylcarboxamides (Crom-1 and Crom-2) were identified as potent, selective, and reversible inhibitors of human monoamine oxidase B (hMAO-B). Since they exhibit some absorption, distribution, metabolism, and excretion (ADME)-toxicity liabilities, new derivatives were synthesized to map the chemical structural features that compose the pharmacophore, a process vital for lead optimization. Structure-activity relationship data, supported by molecular docking studies, provided a rationale for the contribution of the heterocycle's rigidity, the carbonyl group, and the benzopyran heteroatom for hMAO-B inhibitory activity. From the study, N-(3-chlorophenyl)-4H-thiochromone-3-carboxamide (31) (hMAO-B IC₅₀ = 1.52 ± 0.15 nM) emerged as a reversible tight binding inhibitor with an improved pharmacological profile. In in vitro ADME-toxicity studies, compound 31 showed a safe cytotoxicity profile in Caco-2, SH-SY5Y, HUVEC, HEK-293, and MCF-7 cells, did not present cardiotoxic effects, and did not affect P-gp transport activity. Compound 31 also protected SH-SY5Y cells from iron(III)-induced damage. Collectively, these studies highlighted compound 31 as the first-in-class and a suitable candidate for in vivo preclinical investigation.

Monoamine Oxidase Inhibitors: A Review of Their Anti-Inflammatory Therapeutic Potential and Mechanisms of Action

Chronic inflammatory diseases are debilitating, affect patients' quality of life, and are a significant financial burden on health care. Inflammation is regulated by pro-inflammatory cytokines and chemokines that are expressed by immune and non-immune cells, and their expression is highly controlled, both spatially and temporally. Their dysregulation is a hallmark of chronic inflammatory and autoimmune diseases. Significant evidence supports that monoamine oxidase (MAO) inhibitor drugs have anti-inflammatory effects. MAO inhibitors are principally prescribed for the management of a variety of central nervous system (CNS)-associated diseases such as depression, Alzheimer's, and Parkinson's; however, they also have anti-inflammatory effects in the CNS and a variety of non-CNS tissues. To bolster support for their development as anti-inflammatories, it is critical to elucidate their mechanism(s) of action. MAO inhibitors decrease the generation of end products such as hydrogen peroxide, aldehyde, and ammonium. They also inhibit biogenic amine degradation, and this increases cellular and pericellular catecholamines in a variety of immune and some non-immune cells. This decrease in end product metabolites and increase in catecholamines can play a significant role in the anti-inflammatory effects of MAO inhibitors. This review examines MAO inhibitor effects on inflammation in a variety of in vitro and in vivo CNS and non-CNS disease models, as well as their anti-inflammatory mechanism(s) of action.

Small-Molecule Inhibitors of Reactive Oxygen Species Production

Reactive oxygen species (ROS) are involved in physiological cellular processes including differentiation, proliferation, and apoptosis by acting as signaling molecules or regulators of transcription factors. The maintenance of appropriate cellular ROS levels is termed redox homeostasis, a balance between their production and neutralization. High concentrations of ROS may contribute to severe pathological events including cancer, neurodegenerative, and cardiovascular diseases. In recent years, approaches to target the sources of ROS production directly in order to develop tool compounds or potential therapeutics have been explored. Herein, we briefly outline the major sources of cellular ROS production and comprehensively review the targeting of these by small-molecule inhibitors. We critically assess the value of ROS inhibitors with different mechanisms-of-action, including their potency, mode-of-action, known off-target effects, and clinical or preclinical status, while suggesting future avenues of research in the field.

Synthesis, docking study and inhibitory activity of 2,6-diketopiperazines derived from α-amino acids on HDAC8

Diketopiperazines (DKPs) have been regarded as an important scaffold from the viewpoint of synthesis due to their biological properties for the treatment of several diseases, including cancer. In this work, two novel series of enantiomeric 2,6-DKPs derived from α-amino acids were synthesized through nucleophilic substitution and intramolecular cyclization reactions. All the compounds were docked against histone deacetylase 8 (HDAC8), which is a promising target for the development of anticancer drugs. These compounds bound into the active site of HDAC8 in a similar way to Trichostatin A (TSA), which is an HDAC8 inhibitor. This study showed that the conformation of the 2,6-DKP ring, stereochemistry, and the type of substituent on the chiral center had an important role in the binding modes. The Gibbs free energies and dissociation constants values of HDAC8-ligand complexes showed that compounds (S)-4hBn, (S)-4m, (R)-4h, and (R)-4m were more stable and affine towards HDAC8 than TSA. The inhibitory activities of 4a, (S)-4h, (S)- and (R)-4(g, l, m) were evaluated in vitro on HDAC8. It was found that compounds (R)-4g (IC₅₀ = 21.54 nM) and (R)-4m (IC₅₀ = 10.81 nM) exhibited better inhibitory activities than TSA (IC₅₀ = 28.32 nM). These results suggested that 2,6-DKPs derivatives may be promising anticancer agents for further biological studies.

Novel approaches to the discovery of selective human monoamine oxidase-B inhibitors: is there room for improvement?

Introduction: Selective monoamine oxidase-B (MAO-B) inhibitors are currently used as coadjuvants for the treatment of early motor symptoms in Parkinson's disease. They can, based on their chemical structure and mechanism of inhibition, be categorized into reversible and irreversible agents. Areas covered: This review provides a comprehensive update on the development state of selective MAO-B inhibitors describing the results, structures, structure-activity relationships (SARs) and Medicinal chemistry strategies as well as the related shortcomings over the past five years. Expert opinion: Researchers have explored and implemented new and old chemical scaffolds achieving high inhibitory potencies and isoform selectivity. Most of them were characterized and proposed as multitarget agents able to act at different levels (including AChE inhibition, H3R or A_2AR antagonism, antioxidant and chelating properties, Aβ_1-42 aggregation reduction) in the network of aetiologies of neurodegenerative disorders. These results can also be used to avoid 'cheese-reaction' effects and the occurrence of serotonergic syndrome in patients.