Selective inhibitors of monoamine oxidase (MAO-A and MAO-B) as probes of its catalytic site and mechanism

Piperine from the Fruits of Piper longum with Inhibitory Effect on Monoamine Oxidase and Antidepressant-Like Activity

A bioassay-guided isolation of the ethanol extract from the fruits of Piper longum yielded a known piperidine alkaloid, piperine, as a monoamine oxidase (MAO) inhibitor. Piperine showed an inhibitory effect against MAO-A (IC50 value: 20.9 microM) and MAO-B (IC50 value: 7.0 microM). Kinetic analyses by a Lineweaver-Burk plot clearly indicated that piperine competitively inhibited MAO-A and MAO-B with Ki values of 19.0+/-0.9 microM and 3.19+/-0.5 microM, respectively. The inhibition by piperine was found to be reversible by dialysis of the incubation mixture. In addition, the immobility times in the tail suspension test were significantly reduced by piperine, similar to that of the reference antidepressant fluoxetine, without accompanying changes in ambulation when assessed in an open-field. These results suggest that piperine possesses potent antidepressant-like properties that are mediated in part through the inhibition of MAO activity, and therefore represent a promising pharmacotherapeutic candidate as an antidepressant agent.

1-methyl-2-undecyl-4(1H)-quinolone as an irreversible and selective inhibitor of type B monoamine oxidase

The inhibitory compound of monoamine oxidase (MAO) activity was isolated from the CH(2)Cl(2) fraction of the fructus of Evodia rutaecarpa and identified as 1-methyl-2-undecyl-4(1H)-quinolone (1). Compound 1 showed a selective inhibition of type B MAO (MAO-B) activity with the IC(50) value of 15.3 microM using a substrate kynuramine, but did not inhibit type A MAO (MAO-A) activity. The kinetic analysis using Lineweaver-Burk plots indicated that compound 1 competitively inhibited MAO-B activity with the K(i) value of 9.91 microM. The inhibition of MAO-B by compound 1 was found to be irreversible by dialysis of the incubation mixture. These results suggest that compound 1 is a potent irreversible inhibitor of MAO-B, and may regulate catecholamine content in the neurons.

Synthesis and biological evaluation of enantiomerically pure pyrrolyl-oxazolidinones as a new class of potent and selective monoamine oxidase type A inhibitors

Due to the key role played by monoamine oxidases (MAOs) in the metabolism of neurotransmitters, MAO inhibitors (MAOIs) represent an useful tool for the treatment of several neurological diseases. Among selective MAOIs, MAO-A inhibitors (e.g. clorgyline) are used as antidepressant and antianxiety drugs and are claimed to protect neuronal cells against apoptosis, and selective MAO-B inhibitors (e.g. L-deprenyl) can be used in the treatment of Parkinson's disease either alone or in combination with L-DOPA. However, they engender covalent bonds with the active site of the enzyme and induce irreversible inhibition; moreover, they tend to lose their initial selectivity at high dosages or with repeated administrations. Phenyloxazolidinones belong to third-generation-MAOIs, characterized by a selective and reversible inhibition of the enzyme. Among these molecules, the most representative are toloxatone and befloxatone, two selective and reversible MAO-A inhibitors used in therapy as antidepressant drugs. Going on our searches on CNS potentially active compounds containing a pyrrole moiety we prepared 3-(1H-pyrrol-1-yl)-2-oxazolidinones (1) and isomeric 3-(1H-pyrrol-2-and -3-yl)-2-oxazolidinones (2 and 3) as anti-MAO agents. Such derivatives resulted selective and reversible MAO-A inhibitors. The most potent compound is (R)-5-methoxymethyl-3-(1H-pyrrol-1-yl)-2-oxazolidinone (1b), endowed with very high potency (K(iMAO-A) = 4.9 nM) and A-selectivity (A-selectivity = 10,200, about 116-fold greater than that of befloxatone).

Inhibitors alter the spectrum and redox properties of monoamine oxidase A

Monoamine oxidase A (MAO A) catalyses the oxidation of both neurotransmitter and ingested amines. The mechanism of catalysis involves the covalently bound FAD cofactor. Although substrates and inhibitors alter the thermodynamic and kinetic properties of the flavin, how the ligands interact with the flavin is unknown. This work characterises the spectral changes that occur on inhibitor binding to MAO A and examines how the binding influences the flavin. The inhibitors, D-amphetamine, harmine, tetrindole, and befloxatone all induce similar (but not identical) changes in the spectrum of MAO A, consistent with stacking of inhibitor with the flavin in the active site. D-Amphetamine, harmine, and tetrindole stabilise the semiquinone form of FAD during reduction of MAO A by dithionite and no further reduction of these inhibitor-MAO A complexes has been observed. In contrast, semiquinone is never observed during reduction of the befloxatone-MAO A complex. Instead, partial reduction directly to the FADH(2) form occurs extremely slowly. Thus, inhibitor binding has a strong, structure-dependent influence on the environment of the flavin that alters its electronic properties.

An MCASE approach to the search of a cure for Parkinson's Disease

BACKGROUND

Parkinson's disease is caused by a dopamine deficiency state in the fore brain area. Dopamine receptor agonists, MAO-B inhibitors, and N-Methyl-D-Aspartate (NMDA) receptor antagonists are known to have antiparkinson effect. Levodopa, a dopamine structural analog, is the best currently available medication for the treatment of Parkinsons disease. Unfortunately, it also induces side effects upon long administration time. Thus, multidrug therapy is often used, in which various adjuvants alleviate side effects of levodopa and enhance its antiparkinsonian action.

RESULTS

Computer models have been created for three known antiparkinson mechanisms using the MCASE methodology. New drugs for Parkinsons disease can be designed on the basis of these models. We also speculate that the presence of biophores belonging to different groups can be beneficial and designed some potential drugs along this line. The proposed compounds bear pharmacophores of MAO-B inhibitors, dopamine agonists and NMDA antagonists, which could synergistically enhance their antiparkinson effect.

CONCLUSIONS

The methodology could readily be expanded to other endpoints where drugs with multiple activity mechanisms would be desirable.

3-(1H-Pyrrol-1-yl)-2-oxazolidinones as reversible, highly potent, and selective inhibitors of monoamine oxidase type A

3-(1H-Pyrrol-1-yl)-2-oxazolidinones 1aminus signi have been synthesized as pyrrole analogues of toloxatone (Humoryl), an antidepressant agent belonging to the 3-phenyl-2-oxazolidinone class, and their monoamine oxidase (MAO) type A and B inhibitory activities have been evaluated. The majority of 1aminus signi showed inhibitory activity against the A isoform of the enzyme higher than that exerted against the MAO-B, the sole exception being the (S)-5-aminomethylderivative 1d. (R)-5-Methoxymethyl-3-(1H-pyrrol-1-yl)-2-oxazolidinone 1b, the most potent among test derivatives, was 78-fold more potent than toloxatone.

A 3D QSAR study of monoamino oxidase-B inhibitors using the chemical function based pharmacophore generation approach

A molecular modelling study was performed using the CATALYST software package on a dataset of 100 thiosemicarbazide and thiazole derivatives acting as MAO-B irreversible inhibitors in order to, (i) better elucidate the possible role of the ligand features which are significant for binding and (ii) generate chemical features based pharmacophore models which were subsequently used as 3D queries for database searching. Based on known MAO-B inhibitors, pharmacophore hypotheses were created in order to find similarities between the thiazoles and thiosemicarbazides and identify the key sub-structures most likely to be significant for high MAO-B inhibitory activity.

Region-dependent effects of acute and chronic tranylcypromine in vivo on [3H]2-BFI binding to brain imidazoline I(2) sites

An imidazoline I(2) site has been localised to monoamine oxidase. However, in vitro studies of the effect of monoamine oxidase inhibitors on imidazoline I(2)-site radioligand binding have produced conflicting findings. Using the technique of autoradiography, we examined the effect of in vivo administration of the irreversible monoamine oxidase inhibitor tranylcypromine on binding of the imidazoline I(2) site-specific ligand [3H]2-(-2-benzofuranyl)-2-imidazoline ([3H]2-BFI) in four rat brain nuclei which are known to possess a high density of imidazoline I(2) sites, together with cerebral cortex and cerebellum which show weaker binding. A single acute pre-treatment with tranylcypromine significantly increased imidazoline I(2) site-specific binding in four regions: arcuate nucleus, interpeduncular nucleus, pineal gland and area postrema, but effects in cortical areas and cerebellum were not significant. The extent of the increase was proportional to the control binding in each region. In contrast, five daily treatments with the same dose of tranylcypromine significantly reduced [3H]2-BFI binding in these same areas. The potential role of monoamine oxidase isoforms in these changes is discussed.

Studies on synthetic approaches to 1H- and 2H-indazolyl derivatives

Synthetic approaches designed to provide 1H- and 2H-indazolyl derivatives of potential biological interest are reported. Special emphasis has been placed on the characterization of indazolylpyridinium products generated from reactions between indazole and 4-chloro-1-methylpyridinium iodide under various conditions. A stable mixture consisting of 3 parts of the 1H-isomer 9 to 1 part of the 2H-isomer 10 was obtained at room temperature in the presence of the base 2,2,6,6-tetramethylpiperidine (TMP). The same reaction at 60 degrees C gave only the 1H-isomer 9. At 100 degrees C in the absence of TMP only the 2H-isomer 10 was formed. The isomerization of 10 to 9 was found to proceed quantitatively at 60 degrees C but only in the presence of TMP. The effects of temperature and base on the course of these reactions are rationalized in terms of kinetic and thermodynamic parameters.

Cardiovascular sympathomimetic amine interactions in rats treated with monoamine oxidase inhibitors and the novel oxazolidinone antibiotic linezolid

Linezolid (PNU-100766) is a new gram-positive oxazolidinone antibiotic that is effective at in vitro concentrations < or =4 microg/ml and in vivo doses < or =10 mg/kg. Because linezolid also competitively inhibits human monoamine oxidase-A (MAO-A; Ki = 55 microM), we monitored its effects on the cardiovascular responses to tyramine and amine cold remedies in comparison with standard MAO inhibitors. In anesthetized rats, the pressor response to 16 microg i.v. tyramine was potentiated by the MAO-A inhibitors clorgyline (0.1-1.0 mg/kg i.v.) and moclobemide (5.0-50 mg/kg p.o.), but not by the MAO-B inhibitor selegiline (0.15-15 mg/kg p.o.). Fifteen milligrams per kilogram intravenous linezolid weakly potentiated i.v. tyramine independent of changes in alpha-adrenoceptor reactivity, but this effect was not enhanced chronically (90-100 mg/kg/day). In conscious rats, 30 mg/kg/day oral linezolid (8 microg/ml plasma concentration) minimally affected the pressor response to 20 mg/kg oral tyramine, whereas 100 mg/kg/day linezolid (20 microg/ml plasma concentration) moderately potentiated this response similar to 3 mg/kg per day moclobemide. Linezolid's tyramine potentiation was reversible, attenuated by food, and independent of pseudoephedrine, phenylpropanolamine, and dextromethorphan interactions. These studies demonstrate that high-dose linezolid only moderately potentiates the cardiovascular effects of tyramine and validate these models for evaluating such MAO inhibitory interactions.

Inhibition of monoamine oxidases by functionalized coumarin derivatives: biological activities, QSARs, and 3D-QSARs

A large series of coumarin derivatives (71 compounds) were tested for their monoamine oxidase A and B (MAO-A and MAO-B) inhibitory activity. Most of the compounds acted preferentially on MAO-B with IC(50) values in the micromolar to low-nanomolar range; high inhibitory activities toward MAO-A were also measured for sulfonic acid esters. The most active compound was 7-[(3, 4-difluorobenzyl)oxy]-3,4-dimethylcoumarin, with an IC(50) value toward MAO-B of 1.14 nM. A QSAR study of 7-X-benzyloxy meta-substituted 3,4-dimethylcoumarin derivatives acting on MAO-B yielded good statistical results (q(2)() = 0.72, r(2)() = 0.86), revealing the importance of lipophilic interactions in modulating the inhibition and excluding any dependence on electronic properties. CoMFA was performed on two data sets of MAO-A and MAO-B inhibitors. The GOLPE procedure, with variable selection criteria, was applied to improve the predictivity of the models and to facilitate the graphical interpretation of results.

Molecular determinants of MAO selectivity in a series of indolylmethylamine derivatives: biological activities, 3D-QSAR/CoMFA analysis, and computational simulation of ligand recognition

A series of indolylmethylamine derivatives were assayed toward MAO-A and MAO-B inhibition. The K(i) values of these compounds are in the range from 0.8 to >10(6) nM for MAO-A or from 0.75 to 476000 nM for MAO-B. The most selective MAO-A or MAO-B inhibitors elicit a ratio of K(i) in the order of 1500 or 1000, respectively. Comparison of MAO-A and MAO-B CoMFA models showed that both the steric and electrostatic properties at the 5 position of the indole ring are determinant for MAO selectivity. Computational simulations of the complex between this part of the ligand and Phe-208 of MAO-A or Ile-199 of MAO-B, experimentally identified as responsible for substrate selectivity, allowed us to further characterize the nature of these enzyme-inhibitor interactions.

Determination of the human cytochrome P450 isoforms involved in the metabolism of zolmitriptan

1. Zolmitriptan was extensively metabolized by freshly isolated human hepatocytes to a number of components including the three main metabolites observed in vivo (N-desmethyl-zolmitriptan, zolmitriptan N-oxide and the indole acetic acid derivative). In contrast, metabolism of zolmitriptan by human hepatic microsomes was extremely limited with only small amounts of the N-desmethyl and indole ethyl alcohol metabolites being produced. 2. Furafylline, a selective inhibitor of CYP1A2, almost completely abolished the hepatocellular metabolism of zolmitriptan and markedly inhibited formation of the N-desmethyl metabolite in microsomes. Chemical inhibitors, selective against other major human cytochrome P450 (CYP2C9, 2C19, 2D6 and 3A4), had no obvious effects. In addition, expressed human CYP1A2 was the only cytochrome P450 to form the N-desmethyl metabolite. 3. N-desmethyl-zolmitriptan was extensively metabolized by both human hepatocytes and microsomes. The indole acetic acid and ethyl alcohol derivatives were the major metabolites formed by hepatocytes, whereas only the indole ethyl alcohol derivative was produced by microsomes. Metabolism of N-desmethyl-zolmitriptan was not inhibited by cytochrome P450-selective chemical inhibitors nor was it observed following incubation with expressed human cytochrome P450. Clorgyline, a selective inhibitor of monoamine oxidase A (MAO-A), markedly inhibited the microsomal formation of the indole ethyl alcohol derivative. 4. Primary metabolism of zolmitriptan is dependent mainly on CYP1A2, whereas MAO-A is responsible for further metabolism of N-desmethyl-zolmitriptan, the active metabolite. Since the in vivo clearance of zolmitriptan is primarily dependent on metabolism, interactions with drugs that induce or inhibit CYP1A2 or MAO-A may be anticipated.

Relevance of benzyloxy group in 2-indolyl methylamines in the selective MAO-B inhibition

1. Previous studies with indolyl derivatives as monoamine oxidase (MAO) inhibitors have shown the relevance of the indole structure for recognition by the active site of this enzyme. We now report a new series of molecules with structural features which determine the selectivity of MAO inhibition. 2. A benzyloxy group attached at position 5 of the indole ring is critical for this selective behaviour. Amongst all of these benzyloxy-indolyl methylamines, N-(2-propynyl)-2-(5-benzyloxyindol)methylamine FA-73 was the most potent MAO-B 'suicide' inhibitor studied. 3. The Ki values for MAO-A and MAO-B were 800+/-60 and 0.75+/-0.15 nM, respectively. These data represent a selectivity value of 1066 for MAO-B, being 48 times more selective than L-deprenyl (Ki values of 376+/-0.032 and 16.8+/-0.1 nM for MAO A and MAO-B, respectively). The IC50 values for dopamine uptake in striatal synaptosomal fractions from rats were 150+/-8 microM for FA-73 and 68 +/- 10 microM for L-deprenyl whereas in human caudate tissue the IC50 values were 0.36+/-0.015 microM for FA-73 and 0.10+/-0.007 microM for L-deprenyl. Moreover, mouse brain MAO-B activity was 90% ex vivo inhibited by both compounds 1 h after 4 mg kg(-1) administration, MAO-A activity was not affected. 4. These novel molecules should provide a better understanding of the active site of monoamine oxidase and could be the starting point for the design of further selective, non-amphetamine-like MAO-B inhibitors with therapeutic potential for the treatment of neurological disorders.

Alterations in central monoaminergic neurotransmission induced by polycyclic aromatic hydrocarbons in rats

Benzo[alpha]pyrene (B[a]P) is a product derived from incomplete combustion of organic material and is considered responsible for chemically-induced cancer in humans. In the present study, the levels of noradrenaline (NA), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) were measured in the brains of female Wistar rats 6, 12, 24 and 96 h after a single dose of B[alpha]P (50 mg kg(-1) b.w., i.p.), and also after repeated administration of B[alpha]P (50 mg kg(-1) b.w., i.p., 2 x wk, 1 mo). The brain regions studied were the striatum, hypothalamus, midbrain and cortex. Catecholamines were measured using high performance liquid chromatography (HPLC) and electrochemical detection. Significant changes were observed in the striatum where NA, DA, DOPAC were decreased after 24 h and HVA was decreased after 6 h. In contrast, no major alterations occurred in 5-HT and 5- HIAA. In the hypothalamus, a significant decrease in NA was observed after 96 h. In the midbrain, the most important change observed was the decrease in NA after 24 h. A trend toward an increase in 5-HIAA was observed in the cortex after 6 h. The results demonstrate that B[alpha]P induces alterations in the dopaminergic and serotoninergic systems throughout the brain. These alterations may lead to behavioural and hormonal disturbances.

Inhibition of monoamine oxidase-B by condensed pyridazines and pyrimidines: effects of lipophilicity and structure-activity relationships

A number of condensed pyridazines and pyrimidines were synthesized and tested for their monoamine oxidase-A (MAO-A) and MAO-B inhibitory activity. Their lipophilicity was examined by measuring partition coefficients and RP-HPLC capacity factors, revealing some peculiar electronic and conformational effects. Further insights were obtained by X-ray crystallography and a thermodynamic study of RP-HPLC retention. Structure-activity relations highlighted the main factors determining both selectivity and inhibitory potency. Thus, while most of the condensed pyridazines were reversible inhibitors of MAO-B with little or no MAO-A effects, the pyrimidine derivatives proved to be reversible and selective MAO-A inhibitors. Substituents on the diazine nucleus modulated enzyme inhibition. A QSAR analysis of X-substituted 3-X-phenyl-5H-indeno[1,2-c]pyridazin-5-ones showed lipophilicity to increase MAO-B and not MAO-A inhibitory activity.

First partial three-dimensional model of human monoamine oxidase A

A survey of the major known structural aspects of monoamine oxidase (MAO) is given and a first partial model of human MAO A is presented. This 3D model has been established using secondary structure predictions and fold recognition methods. It shows two alpha/beta domains (the FAD-binding N-terminal and central domains) and an alpha+beta domain. The C-terminal region is predicted to be responsible for anchoring the protein into the mitochondrial membrane and was not modeled. The covalent binding of the flavin cofactor to a cysteine residue is well predicted. The model is validated with experimental data from the literature and should be useful in designing new experimental studies (site-directed mutagenesis, chemical modification, specific antibodies). This first step towards the 3D structure of monoamine oxidase should contribute to a better understanding of the mechanisms of action and inhibition of this drug target in the treatment of clinical depression.

Stereoselective biotransformation of the selective serotonin reuptake inhibitor citalopram and its demethylated metabolites by monoamine oxidases in human liver

Citalopram (CIT) is an antidepressive drug of the group of selective serotonin reuptake inhibitors (SSRIs). The tertiary amine CIT is given as a racemic drug, but its pharmacological activity resides mainly in S-CIT. CIT is metabolised by cytochrome P450 (CYP) to N-demethylcitalopram (DCIT) and N-didemethylcitalopram (DDCIT). The citalopram propionic acid derivative (CIT-PROP) is another, but pharmacologically inactive, metabolite, the formation of which has been poorly characterised but is postulated to occur by deamination of CIT, DCIT and/or DDCIT. The aim of the present investigation was to study the formation of the enantiomers of CIT-PROP from CIT and its two N-demethylated metabolites, DCIT and DDCIT, in an in vitro incubation system (microsomal and cytosolic fractions) obtained from human livers. The production of CIT-PROP was measured by a stereospecific HPLC method. Incubation of rac-CIT, rac-DCIT and rac-DDCIT (500 microM each, separately) in the presence (or absence) of NADP showed that CIT-PROP formation was substrate-dependent and essentially NADP-independent. Monoamine oxidases (MAO) type A and B and aldehyde oxidase were identified as the probable enzymes involved in the formation of CIT-PROP from CIT, DCIT and DDCIT. Indeed, the irreversible monoamine oxidase type A inhibitor clorgyline and the irreversible monoamine oxidase type B inhibitor selegiline (both at 0.5 microM in the incubation mixture) inhibited CIT-PROP formation, depending on the substrate, up to 70% and 88%, respectively. The participation of aldehyde oxidase in the subsequent step is suggested by the inhibition caused by menadione (50 microM) in CIT-PROP formation. Preliminary experiments suggest the presence of four unknown metabolites, probably products of deamination, which were detected in plasma and urine samples of patients treated with CIT as well as in in vitro biotransformations. Their presence confirms the importance of deamination in the biotransformation of CIT and its demethylated metabolites, especially in the brain where, in contrast to the liver, the role of cytochrome P450 appears to be low.

Probing the active sites of monoamine oxidase A and B with 1,4-disubstituted tetrahydropyridine substrates and inactivators

As part of our efforts to characterize more fully the structural features of the monoamine oxidase (MAO) A and B active sites, we have examined the substrate and inhibitor properties of several 1-methyl- and 1-cyclopropyl-4-aryl-1,2,3,6-tetrahydropyridine derivatives with the human placental A and beef liver B forms of the enzyme. We find that the 4-(2-phenylphenyl) analog 23 exhibits a high activity and selectivity for MAO-A while the 4-(3-phenylphenyl) analog 22 shows activity only with MAO-B. Selectivities similar to those of the N-methyl series are observed with a series of N-cyclopropyl mechanism based inactivators. These results support a topological analysis which attempts to identify steric factors related to the reported substrate and inhibitor selectivities of these two flavoproteins and provide a better definition of the size of the active sites of the two enzymes.

4-substituted cubylcarbinylamines: a new class of mechanism-based monoamine oxidase B inactivators

Cubylcarbinylamine (1a), (4-cyclopropylcubyl)carbinylamine (1b), and (4-phenylcubyl)carbinylamine (1c) were synthesized and shown to be time-dependent, irreversible inactivators of monoamine oxidase B (MAO B). Substrate protects the enzyme from inactivation, but beta-mercaptoethanol does not, suggesting that these compounds are mechanism-based inactivators. All three compounds were also substrates for MAO B with partition ratios ranging from 152 to 536. The 4-substituted analogues were more potent inactivators than the unsubstituted analogue, indicating a benefit to 4-substitution in this class of inactivators.

Assessment of structural requirements for the monoamine oxidase-B-catalyzed oxidation of 1,4-disubstituted-1,2,3,6-tetrahydropyridine derivatives related to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

The monoamine oxidase B (MAO-B) substrate properties and distance measurements along the N1-C4 axis of 38 1,4-disubstituted-1,2,3,6-tetrahydropyridine derivatives, including seven newly synthesized MPTP analogs, were used to define the maximum size that can be accommodated by the MAO-B active site. Only those compounds measuring less than 12 A displayed significant MAO-B substrate properties. The behavior of various 4-substituted-1-cyclopropyltetrahydropyridine analogs also is discussed in terms of this N1-C4 distance parameter in an effort to understand factors which contribute to their substrate vs inactivator properties. We conclude that this distance parameter will predict the majority of substrates vs nonsubstrates with this class of compound.