Selectivity For Monoamine Oxidase B Research Articles

Design, synthesis, in silico, and in vitro evaluation of novel benzyloxybenzene substituted (S)-α-amino amide derivatives as cholinesterases and monoaminoxidases inhibitor.

In this study, a series of novel benzyloxybenzene substituted (S)-α-amino acid methyl esters and their amide derivatives were synthesized and evaluated for their inhibitory actions against acetylcholinesterase(AChE), butyrylcholinesterase(BChE), monoamine oxidase A(MAO-A), and monoamine oxidase B(MAO-B). The synthetic strategy was based on starting from benzyl bromide (5) and 4-hydroxybenzaldehyde (6). The reaction of 5 and 6 in the presence of K2 CO3 gave benzyloxybenzaldehyde 7. Benzyloxybenzene substituted (S)-α-amino acid methyl esters 11, 12, 13, (±)-19, and (±)-20 were obtained from the reaction of L-amino acid methyl esters with benzyloxybenzaldehyde (7) followed by in situ reduction with NaBH4 . The reaction of (S)-11, (S)-12, 13, (±)-19, and (±)-20 with excess ammonia gave amides (S)-14, (S)-15, 16, (±)-21, and (±)-22. The in vitro inhibitory activities of compounds against MAO-A,MAO-B, AChE, and BChEwere investigated. Within the α-amino acid methyl ester series, 13 (21.32 ± 0.338 µM) showed selectivity by inhibiting the MAO-B better than MAO-A. 13 emerged as the most active member of this series, exhibiting a 12-fold selectivity for MAO-B. 14 (4.501 ± 0.295 µM) demonstrated a pronounced selectivity for MAO-A over MAO-B, with a selectivity ratio of 110-fold. In addition, it was determined that compound 15 (95.65 ± 3.09 µM) had high selectivity for BChE inhibition. 21was demonstrated the most potent inhibition (18.36 ± 1.36 µM) against AChE.

Monoamine oxidase B activatable red fluorescence probe for bioimaging in cells and zebrafish

A real-time and specific for the detection of Monoamine Oxidase B (MAO-B) to investigate the MAO-B-relevant disease development and treatment process is urgently desirable. Here, we utilized MAO-B to catalyze the conversion of propylamino groups to aldehyde groups, which was then quickly followed by a β-elimination process to produce fluorescent probes (FNJP) that may be used to detect MAO-B in vitro and in vivo. The FNJP probe possesses unique properties, including favorable reactivity (Km = 10.8 μM), high cell permeability, and NIR characteristics (λem = 610 nm). Moreover, the FNJP probe showed high selectivity for MAO-B and was able to detect endogenous MAO-B levels from a mixed population of NIH-3 T3 and HepG2 cells. MAO-B expression was found to be increased in cells under lipopolysaccharide-stimulated cellular oxidative stress in neuronal-like SH-SY5Y cells. In addition, the visualization of FNJP for MAO-B activity in zebrafish can be an effective tool for exploring the biofunctions of MAO-B. Considering these excellent properties, the FNJP probe may be a powerful tool for detecting MAO-B levels in living organisms and can be used for accurate clinical diagnoses of related diseases.

Why Monoamine Oxidase B Preferably Metabolizes N-Methylhistamine over Histamine: Evidence from the Multiscale Simulation of the Rate-Limiting Step.

Histamine levels in the human brain are controlled by rather peculiar metabolic pathways. In the first step, histamine is enzymatically methylated at its imidazole Nτ atom, and the produced N-methylhistamine undergoes an oxidative deamination catalyzed by monoamine oxidase B (MAO-B), as is common with other monoaminergic neurotransmitters and neuromodulators of the central nervous system. The fact that histamine requires such a conversion prior to oxidative deamination is intriguing since MAO-B is known to be relatively promiscuous towards monoaminergic substrates; its in-vitro oxidation of N-methylhistamine is about 10 times faster than that for histamine, yet this rather subtle difference appears to be governing the decomposition pathway. This work clarifies the MAO-B selectivity toward histamine and N-methylhistamine by multiscale simulations of the rate-limiting hydride abstraction step for both compounds in the gas phase, in aqueous solution, and in the enzyme, using the established empirical valence bond methodology, assisted by gas-phase density functional theory (DFT) calculations. The computed barriers are in very good agreement with experimental kinetic data, especially for relative trends among systems, thereby reproducing the observed MAO-B selectivity. Simulations clearly demonstrate that solvation effects govern the reactivity, both in aqueous solution as well as in the enzyme although with an opposing effect on the free energy barrier. In the aqueous solution, the transition-state structure involving histamine is better solvated than its methylated analog, leading to a lower barrier for histamine oxidation. In the enzyme, the higher hydrophobicity of N-methylhistamine results in a decreased number of water molecules at the active side, leading to decreased dielectric shielding of the preorganized catalytic electrostatic environment provided by the enzyme. This renders the catalytic environment more efficient for N-methylhistamine, giving rise to a lower barrier relative to histamine. In addition, the transition state involving N-methylhistamine appears to be stabilized by the surrounding nonpolar residues to a larger extent than with unsubstituted histamine, contributing to a lower barrier with the former.

First-in-Humans Evaluation of 18F-SMBT-1, a Novel 18F-Labeled Monoamine Oxidase-B PET Tracer for Imaging Reactive Astrogliosis.

Reactive gliosis, characterized by reactive astrocytes and activated microglia, contributes greatly to neurodegeneration throughout the course of Alzheimer disease (AD). Reactive astrocytes overexpress monoamine oxidase B (MAO-B). We characterized the clinical performance of 18F-(S)-(2-methylpyrid-5-yl)-6-[(3-fluoro-2-hydroxy)propoxy]quinoline (18F-SMBT-1), a novel MAO-B PET tracer as a potential surrogate marker of reactive astrogliosis. Methods: Seventy-seven participants-53 who were elderly and cognitively normal, 7 with mild cognitive impairment, 7 with AD, and 10 who were young and cognitively normal-were recruited for the different aspects of the study. Older participants underwent 3-dimensional magnetization-prepared rapid gradient-echo MRI and amyloid-β, tau, and 18F-SMBT-1 PET. To ascertain 18F-SMBT-1 selectivity to MAO-B, 9 participants underwent 2 18F-SMBT-1 scans, before and after receiving 5 mg of selegiline twice daily for 5 d. To compare selectivity, 18F-THK5351 studies were also conducted before and after selegiline. Amyloid-β burden was expressed in centiloids. 18F-SMBT-1 outcomes were expressed as SUV, as well as tissue ratios and binding parameters using the subcortical white matter as a reference region. Results: 18F-SMBT-1 showed robust entry into the brain and reversible binding kinetics, with high tracer retention in basal ganglia, intermediate retention in cortical regions, and the lowest retention in cerebellum and white matter, which tightly follows the known regional brain distribution of MAO-B (R 2 = 0.84). More than 85% of 18F-SMBT-1 signal was blocked by selegiline across the brain, and in contrast to 18F-THK5351, no residual cortical activity was observed after the selegiline regimen, indicating high selectivity for MAO-B and low nonspecific binding. 18F-SMBT-1 also captured the known MAO-B increases with age, with an annual rate of change (∼2.6%/y) similar to the invitro rates of change (∼1.9%/y). Quantitative and semiquantitative measures of 18F-SMBT-1 binding were strongly associated (R 2 > 0.94), suggesting that a simplified tissue-ratio approach could be used to generate outcome measures. Conclusion: 18F-SMBT-1 is a highly selective MAO-B tracer, with low nonspecific binding, high entry into the brain, and reversible kinetics. Moreover, 18F-SMBT-1 brain distribution matches the reported invitro distribution and captures the known MAO-B increases with age, suggesting that 18F-SMBT-1 can potentially be used as a surrogate marker of reactive astrogliosis. Further validation of these findings with 18F-SMBT-1 will require examination of a much larger series, including participants with mild cognitive impairment and AD.

The Role of Chirality of [18F]SMBT-1 in Imaging of Monoamine Oxidase-B.

(S)-(2-Methylpyrid-5-yl)-6-[(3-[18F]fluoro-2-hydroxy)propoxy]quinoline ([18F]SMBT-1) was recently developed as a novel class of selective and reversible monoamine oxidase-B (MAO-B) tracers for in vivo imaging of reactive astrogliosis via positron emission tomography. To investigate the effect of the chirality of [18F]SMBT-1 on tracer performance, we synthesized (S)-[18F]6 ([18F]SMBT-1) and (R)-[18F]6 and compared their binding properties, pharmacokinetics, and metabolism. (S)-6 showed higher binding affinity to MAO-B and lower binding affinity to MAO-A than (R)-6, demonstrating a higher selectivity ratio (MAO-B/MAO-A). A pharmacokinetic study in mice demonstrated that both (S)-[18F]6 and (R)-[18F]6 showed sufficient initial brain uptake. However, (S)-[18F]6 was cleared significantly faster from the body. An abundant sulfoconjugate metabolite M2 was observed in plasma for (S)-[18F]6 but not for (R)-[18F]6. In vitro sulfation assays confirmed that (S)-6 was more reactive than (R)-6, consistent with the in vivo findings. Mefenamic acid, a selective sulfotransferase 1A1 (SULT1A1) inhibitor, strongly inhibited the in vitro sulfation of (S)-6 by mouse liver fractions, human liver cytosol fractions, and human recombinant SULT1A1 enzyme. Genetic polymorphisms of SULT1A1 did not affect the sulfation of (S)-6 in vitro. In conclusion, (S)-[18F]6 had a more favorable binding affinity and binding selectivity for MAO-B than (R)-[18F]6. Additionally, (S)-[18F]6 also possessed better pharmacological and metabolic properties than (R)-[18F]6. These results suggest that (S)-[18F]6 ([18F]SMBT-1) is a promising candidate for application in the imaging of MAO-B in vivo.

Revealing the role of fluorine pharmacophore in chalcone scaffold for shifting the MAO-B selectivity: investigation of a detailed molecular dynamics and quantum chemical study

The development of highly selective monoamine oxidase-B (MAO-B) inhibitors has great therapeutic benefit in treatment of various neurodegenerative disorders. Recent study documented that shifting of fluorine atom from para to ortho position on the phenyl B ring of heteroaryl chalcones shown a remarkable shift in the selectivity and potency between MAO-A and MAO-B isoforms. Despite the large plethora of the design of new selective MAO-B inhibitors, the current paper illustrates the role and orientation of fluorine atom with remarkable MAO-B selectivity of three compounds (O23, O24 and O25), which differ from all other substituents encountered in the chalcone scaffolds is recently reported by our group. Conformational analyses of differential inhibitory effects of O23, O24 and O25 on MAO-A and MAO-B, differential analyses of complementary interactions at MAO-A/-B active sites and differential analysis of affinity binding and per-residue energy contributions are calculated by molecular dynamics study. Density functional theory based electronic structure calculations were employed with special emphasis to electrostatic potential and frontier molecular orbitals. Results of the current study can be used for lead modification and a new insight for the development of novel fluorinated chalcones for the treatment of various neurodegenerative disorders. Communicated by Ramaswamy H. Sarma

Deuterium Kinetic Isotope Effect Studies of a Potential in Vivo Metabolic Trapping Agent for Monoamine Oxidase B

Visualizing the in vivo activity of monoamine oxidase B (MAO-B) is a valuable tool in the ongoing investigation of astrogliosis in neurodegeneration. Existing strategies for imaging changes in MAO enzyme expression or activity have utilized the irreversible suicide inhibitors or high-affinity reversibly binding inhibitors as positron emission tomography (PET) ligands. As an alternative approach, we developed 4-methyl-7-[(1-[11C]methyl-1,2,3,6-tetrahydropyridin-4-yl)oxy]-2 H-chromen-2-one ([11C]Cou) as a metabolic trapping agent for MAO-B. Trapping of [11C]Cou in rhesus monkey brain demonstrated MAO-B selectivity. In this work, we have attempted to improve on the in vivo pharmacokinetics of [11C]Cou by using the deuterium kinetic isotope effect (KIE) to slow the MAO-B-mediated oxidation step and thus reduce the rate of trapping in brain tissues. However, in vitro assays of enzyme kinetics and in vivo PET imaging of pharmacokinetics in primate brain showed no effects of deuterium substitution on the tetrahydropyridine ring of [11C]Cou. The results are possibly due to masking of the KIE by a second step in the overall metabolism of the new imaging agent.

Synthesis and evaluation of biaryl derivatives for structural characterization of selective monoamine oxidase B inhibitors toward Parkinson’s disease therapy

Benzyloxyphenyl moiety is a common structure of highly potent, selective and reversible inhibitors of monoamine oxidase B (MAO-B), safinamide and sembragiline. We synthesized 4-(benzyloxy)phenyl and biphenyl-4-yl derivatives including halogen substituents on the terminal aryl unit. In addition, we modified the carbon linker between amine group and the biaryl linked unit. Among synthesized compounds, 12c exhibited the most potent and selective MAO-B inhibitory effect (hMAO-B IC50: 8.9 nM; >10,000-fold selectivity over MAO-A) as a competitive inhibitor. In addition, 12c showed greater MAO-B inhibitory activity and selectivity compared to well-known MAO-B inhibitors such as selegiline, safinamide and sembragiline. In the MPTP-induced mouse model of Parkinson’s disease (PD), 12c significantly protected the tyrosine hydroxylase (TH)-immunopositive DAergic neurons and attenuated the PD-associated behavioral deficits. This study suggests characteristic structures as a MAO-B inhibitor that may provide a good insight for the development of therapeutic agents for PD.

Alkynyl–coumarinyl ethers as MAO-B inhibitors

In this study, alkynyl–coumarinyl ethers were developed as inhibitors of human monoamine oxidase B (MAO-B). A series of 31 new, ether-connected coumarin derivatives was synthesized via hydroxycoumarins, whose phenolic group at position 6, 7 or 8 was converted by means of the Mitsunobu reaction. The majority of the final products were produced from primary alcohols with a terminal alkyne group. The inhibitors were optimized with respect to the structure of the alkynyloxy chain and its position at the fused benzene ring as well as the residue at position 3 of the pyran-2H-one part. A hex-5-ynyloxy chain at position 7 was found to be particular advantageous. Among the 7-hex-5-ynyloxy-coumarins, the 3-methoxycarbonyl derivative 36 was characterized as a dual-acting inhibitor with IC50 values of less than 10nM towards MAO-A and MAO-B, and the 3-(4-methoxy)phenyl derivative 44 was shown to combine strong anti-MAO-B potency (IC50=3.0nM) and selectivity for MAO-B over MAO-A (selectivity >3400-fold).

Synthesis and biological evaluation of novel propargyl amines as potential fluorine-18 labeled radioligands for detection of MAO-B activity

The aim of this project was to synthesize and evaluate three novel fluorine-18 labeled derivatives of propargyl amine as potential PET radioligands to visualize monoamine oxidase B (MAO-B) activity.The three fluorinated derivatives of propargyl amine ((S)-1-fluoro-N,4-dimethyl-N-(prop-2-ynyl)-pent-4-en-2-amine (5), (S)-N-(1-fluoro-3-(furan-2-yl)propan-2-yl)-N-methylprop-2-yn-1-amine (10) and (S)-1-fluoro-N,4-dimethyl-N-(prop-2-ynyl)pentan-2-amine (15)) were synthesized in multi-step organic syntheses. IC50 values for inhibition were determined for compounds 5, 10 and 15 in order to determine their specificity for binding to MAO-B. Compound 5 inhibited MAO-B with an IC50 of 664±48.08nM. No further investigation was carried out with this compound. Compound 10 inhibited MAO-B with an IC50 of 208.5±13.44nM and compound 15 featured an IC50 of 131.5±0.71nM for its MAO-B inhibitory activity. None of the compounds inhibited MAO-A activity (IC50 > 2μM).The fluorine-18 labeled analogues of the two higher binding affinity compounds (10 and 15) (S)-N-(1-[18F]fluoro-3-(furan-2-yl)propan-2-yl)-N-methylprop-2-yn-1-amine (16) and (S)-1-[18F]fluoro-N,4-dimethyl-N-(prop-2-ynyl)pentan-2-amine (18) were both prepared from the corresponding precursors 9A, 9B and 14A, 14B by a one-step fluorine-18 nucleophilic substitution reaction. Autoradiography experiments on human postmortem brain tissue sections were performed with 16 and 18. Only compound 18 demonstrated a high selectivity for MAO-B over MAO-A and was, therefore, chosen for further examination by PET in a cynomolgus monkey.The initial uptake of 18 in the monkey brain was 250% SUV at 4min post injection. The highest uptake of radioactivity was observed in the striatum and thalamus, regions with high MAO-B activity, whereas lower levels of radioactivity were detected in the cortex and cerebellum. The percentage of unchanged radioligand 18 was 30% in plasma at 90min post injection.In conclusion, compound 18 is a selective inhibitor of MAO-B in vitro and demonstrated a MAO-B specific binding pattern in vivo by PET in monkey. It can, therefore, be considered as a candidate for further investigation in human by PET.

Sulfanylphthalonitrile analogues as selective and potent inhibitors of monoamine oxidase B

It has recently been reported that nitrile containing compounds frequently act as potent monoamine oxidase B (MAO-B) inhibitors. Modelling studies suggest that this high potency inhibition may rely, at least in part, on polar interactions between nitrile functional groups and polar moieties within the MAO-B substrate cavity. In an attempt to identify potent and selective inhibitors of MAO-B and to contribute to the known structure–activity relationships of MAO inhibition by nitrile containing compounds, the present study examined the MAO inhibitory properties of series of novel sulfanylphthalonitriles and sulfanylbenzonitriles. The results document that the evaluated compounds are potent and selective MAO-B inhibitors with most homologues possessing IC50 values in the nanomolar range. In general, the sulfanylphthalonitriles exhibited higher binding affinities for MAO-B than the corresponding sulfanylbenzonitrile homologues. Among the compounds evaluated, 4-[(4-bromobenzyl)sulfanyl]phthalonitrile is a particularly promising inhibitor since it displayed a high degree of selectivity (8720-fold) for MAO-B over MAO-A, and potent MAO-B inhibition (IC50=0.025μM). Based on these observations, this structure may serve as a lead for the development of therapies for neurodegenerative disorders such as Parkinson’s disease.

Design, Synthesis and Inhibitory Activities of 8-(Substituted styrol-formamido)phenyl-xanthine Derivatives on Monoamine Oxidase B

The design and synthesis of two series of 8-(substituted styrol-formamido)phenyl-xanthine derivatives are described. Their in vitro monoamine oxidase B (MAO-B) inhibition were tested and the effect of substituents on the N-7, phenyl and the substituted positions are discussed. It was observed that compound 9b displayed significant MAO-B inhibition activity and selectivity, fluorine substitution plays a key role in the selectivity of MAO-B inhibition, and the styrol-formamido group at position-3' may enhance the activity and selectivity of 8-phenyl-xanthine analogues. These results suggest that such compounds may be utilized for the development of new candidate MAO-B inhibitors for treatment of Parkinson's disease.

Inhibition of monoamine oxidase by ( E)-styrylisatin analogues

Previous studies have shown that ( E)-8-(3-chlorostyryl)caffeine (CSC) is a specific reversible inhibitor of human monoamine oxidase B (MAO-B) and does not bind to human MAO-A. Since the small molecule isatin is a natural reversible inhibitor of both MAO-B and MAO-A, ( E)-5-styrylisatin and ( E)-6-styrylisatin analogues were synthesized in an attempt to identify inhibitors with enhanced potencies and specificities for MAO-B. The ( E)-styrylisatin analogues were found to exhibit higher binding affinities than isatin with the MAO preparations tested. The ( E)-5-styrylisatin analogues bound more tightly than the ( E)-6 analogue although the latter exhibits the highest MAO-B selectivity. Molecular docking studies with MAO-B indicate that the increased binding affinity exhibited by the ( E)-styrylisatin analogues, in comparison to isatin, is best explained by the ability of the styrylisatins to bridge both the entrance cavity and the substrate cavity of the enzyme. Experimental support for this model is shown by the weaker binding of the analogues to the Ile199Ala mutant of human MAO-B. The lower selectivity of the ( E)-styrylisatin analogues between MAO-A and MAO-B, in contrast to CSC, is best explained by the differing relative geometries of the aromatic rings for these two classes of inhibitors.

Solid-Phase Synthesis and Insights into Structure−Activity Relationships of Safinamide Analogues as Potent and Selective Inhibitors of Type B Monoamine Oxidase

Safinamide, (S)-N2-{4-[(3-fluorobenzyl)oxy]benzyl}alaninamide methanesulfonate, which is in phase III clinical trials as an anti-Parkinson drug, and a library of alkanamidic analogues were prepared through an expeditious solid-phase synthesis and evaluated for their monoamine oxidase B (MAO-B) and monoamine oxidase A (MAO-A) inhibitory activity and selectivity. (S)-3-Chlorobenzyloxyalaninamide (8) and (S)-3-chlorobenzyloxyserinamide (13) derivatives proved to be more potent MAO-B inhibitors than safinamide (IC50 = 33 and 43 nM, respectively, vs 98 nM) but with a lower MAO-B selectivity (SI = 3455 and 1967, respectively, vs 5918). The highest MAO-B inhibitory potency (IC50 = 17 nM) and a good selectivity (SI = 2941) were displayed by (R)-21, a tetrahydroisoquinoline analogue of safinamide. Structure-affinity relationships and docking simulations pointed out strong negative steric effects of alpha-aminoamide side chains and para substituents of the benzyloxy groups and favorable hydrophobic interactions of meta substituents. The significantly diverse MAO-B affinities of a number of R and S alpha-aminoamide enantiomers, including the two rigid analogues (21) of safinamide, indicated likely enantioselective interactions at the enzymatic binding sites.

N-(6- 18F-Fluorohexyl)- N-Methylpropargylamine: A Fluorine-18-Labeled Monoamine Oxidase B Inhibitor for Potential Use in PET Studies

We have synthesized N-(6- 18F-fluorohexyl)- N-methylpropargylamine ( 18F-FHMP) as a positron emission tomography (PET) radiotracer for monoamine oxidase B (MAO-B). The radiosynthesis was carried out by a fluorine-for-bromine substitution in 30–40% radiochemical yield in specific activities of 1–2 Ci/μmol. Selectivity for MAO-B was demonstrated by the high affinity of ( R)-deprenyl (IC 50 = 6.8 nM) and lower affinity of clorgyline (IC 50 = 1.2 μM) for the inhibition of 18F-FHMP binding in vitro in rat brain homogenates. In vitro autoradiographic studies in rat brain slices showed localization of 18F-FHMP in regions such as the ependyma of the lateral ventricle, dorsal raphe, area postrema, and other regions such as the cerebellum. The specific binding observed in the autoradiograms was displaced by preincubation with ( R)-deprenyl. In in vivo experiments, the uptake of 18F-FHMP in the rat brains was high (0.10–0.20% injected dose/g). The binding of 18F-FHMP in the rat brain correlated with the general distribution of MAO-B and was displaced completely by preadministration of 10 μM ( R)-deprenyl. These results suggest that 18F-FHMP is a potential PET radiotracer for MAO-B for use in in vitro and in vivo experiments.

Kinetics and metabolism of p-tyramine during monoamine oxidase inhibition by mofegiline

The effects of monoamine oxidase B (MAO-B) inhibition by mofegiline on the pharmacokinetics of p-tyramine and its major metabolite, p-hydroxyphenylacetic acid, were investigated in 24 healthy male volunteers. p-Tyramine doses were administered before and after a 14-day treatment period of 1, 12, or 24 mg mofegiline or placebo. Normalized p-tyramine for area under the plasma concentration-time curve after treatment were not significantly different from their respective before-treatment values for any of the dose groups. The relative bioavailability of p-tyramine after treatment was not significantly different from before treatment, although a tendency to a greater bioavailability was seen with the 12 and 24 mg doses. There were no significant differences between pharmacokinetic parameters for p-hydroxyphenylacetic acid. The data suggest that mofegiline maintains its selectivity for MAO-B in the intestine and liver at doses up to and including 24 mg. Therefore these doses would not be expected to be associated with the hypertensive crises normally associated with the "cheese effect."