Inhibition of rat brain monoamine oxidase activities by psoralen and isopsoralen: implications for the treatment of affective disorders

Psoralen and isopsoralen, furocoumarins isolated from the plant Psoralea corylifolia L., were demonstrated to exhibit in vitro inhibitory actions on monoamine oxidase (MAO) activities in rat brain mitochondria, preferentially inhibiting MAO-A activity over MAO-B activity. This inhibition of enzyme activities was found to be dose-dependent and reversible. For MAO-A, the IC50 values are 15.2 +/- 1.3 microM psoralen and 9.0 +/- 0.6 microM isopsoralen. For MAO-B, the IC50 values are 61.8 +/- 4.3 microM psoralen and 12.8 +/- 0.5 microM isopsoralen. Lineweaver-Burk transformation of the inhibition data indicates that inhibition by both psoralen and isopsoralen is non-competitive for MAO-A. The Ki values were calculated to be 14.0 microM for psoralen and 6.5 microM for isopsoralen. On the other hand, inhibition by both psoralen and isopsoralen is competitive for MAO-B. The Ki values were calculated to be 58.1 microM for psoralen and 10.8 microM for isopsoralen. These inhibitory actions of psoralen and isopsoralen on rat brain mitochondrial MAO activities are discussed in relation to their toxicities and their potential applications to treat affective disorders.     

Rasagiline [N-propargyl-1R(+)-aminoindan], a selective and potent inhibitor of mitochondrial monoamine oxidase B

1. Rasagiline [N-propargyl-1R(+)-aminoindan], was examined for its monoamine oxidase (MAO) A and B inhibitor activities in rats together with its S(-)-enantiomer (TVP 1022) and the racemic compound (AGN-1135) and compared to selegiline (1-deprenyl). The tissues that were studied for MAO inhibition were the brain, liver and small intestine. 2. While rasagiline and AGN1135 are highly potent selective irreversible inhibitors of MAO in vitro and in vivo, the S(-) enantiomer is relatively inactive in the tissues examined. 3. The in vitro IC(50) values for inhibition of rat brain MAO activity by rasagiline are 4.43+/-0.92 nM (type B), and 412+/-123 nM (type A). The ED(50) values for ex vivo inhibition of MAO in the brain and liver by a single dose of rasagiline are 0.1+/-0.01, 0.042+/-0.0045 mg kg(-1) respectively for MAO-B, and 6.48+/-0.81, 2.38+/-0.35 mg kg(-1) respectively for MAO-A. 4. Selective MAO-B inhibition in the liver and brain was maintained on chronic (21 days) oral dosage with ED(50) values of 0.014+/-0.002 and 0.013+/-0.001 mg kg(-1) respectively. 5. The degree of selectivity of rasagiline for inhibition of MAO-B as opposed to MAO-A was similar to that of selegiline. Rasagiline was three to 15 times more potent than selegiline for inhibition of MAO-B in rat brain and liver in vivo on acute and chronic administration, but had similar potency in vitro. 6. These data together with lack of tyramine sympathomimetic potentiation by rasagiline, at selective MAO-B inhibitory dosage, indicate that this inhibitor like selegiline may be a useful agent in the treatment of Parkinson's disease in either symptomatic or L-DOPA adjunct therapy, but lack of amphetamine-like metabolites could present a therapeutic advantage for rasagiline.     

Monoamine oxidase inhibitory components from the roots ofSophora flavescens

In our search for monoamine oxidase (MAO) inhibitors from natural resources, we found that the methanol extract of the roots of Sophora flavescens showed an inhibitory effect on mouse brain monoamine oxidase (MAO). Bioactivity-guided isolation of the extract yielded two known flavonoids, formononetin (1) and kushenol F (2), as active compounds along with three inactive compounds, oxymatrine (3), trifolirhizin (4), and beta-sitosterol (5). Formononetin (1) and kushenol F (2) showed significant inhibitory effects on MAO in a dose-dependent manner with IC50 values of 13.2 and 69.9 microM, respectively. Formononetin (1) showed a slightly more potent inhibitory effect against MAO-B (IC50: 11.0 microM) than MAO-A (IC50: 21.2 microM). Kushenol F (2) also preferentially inhibited the MAO-B activity than MAO-A activity with the IC50 values of 63.1 and 103.7 microM, respectively.     

Interaction of psychotropic drugs with monoamine oxidase in rat brain

Literature observations indicate that some psychotropic drugs may have inhibitory activity towards monoamine oxidase (MAO). This study was undertaken to assess the potency, isozyme selectivity and mechanism of inhibition of representative first- and second-generation antidepressant drugs towards rat brain MAO-A and MAO-B. Five tricyclic antidepressants (imipramine, trimipramine, clomipramine, amitriptyline and doxepine) and three selective serotonin reuptake inhibitors (fluoxetine, fluvoxamine and citalopram) were examined. They showed inhibitory activity towards MAO-A and MAO-B, with clear selectivity for MAO-B (Ki in the micromolar range). Their mechanism of inhibition was competitive towards MAO-B and of a mixed competitive type towards MAO-A. The results suggest that some of the drugs examined might also contribute an MAO inhibitory effect in chronically treated patients.     

Synthesis, molecular modeling studies, and selective inhibitory activity against monoamine oxidase of 1-thiocarbamoyl-3,5-diaryl-4,5-dihydro-(1H)- pyrazole derivatives

A novel series of 1-thiocarbamoyl-3,5-diaryl-4,5-dihydro-(1H)-pyrazole derivatives have been synthesized and investigated for the ability to inhibit selectively the activity of the A and B isoforms of monoamine oxidase (MAO). All the synthesized compounds show high activity against both the MAO-A and the MAO-B isoforms with Ki values between 27 and 4 nM and between 50 and 1.5 nM, respectively, except for a few derivatives whose inhibitory activity against MAO-B was in the micromolar range. Knowing that stereochemistry may be an important modulator of biological activity, we performed the semipreparative chromatographic enantioseparation of the most potent, selective, and chiral compounds. The separated enantiomers were then submitted to in vitro biological evaluation. The selectivity of the (-)-(S)-1 enantiomer against MAO-B increases twice and a half, while the selectivity of the (-)-(S)-4 enantiomer against MAO-A triples. Both the MAO-A and MAO-B isoforms respectively of the 1O5W and 1GOS models deposited in the Protein Data Bank were considered in the computational study. The docking study was carried out using several computational approaches with the aim of proposing possible binding modes of the MAO enantioselective compounds 1 and 4.     

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.     

Inhibition of brain mitochondrial monoamine oxidases by the endogenous compound 5-hydroxyoxindole

5-Hydroxyoxindole is a recently identified endogenous compound. Its physiological role remains unclear but certain evidence exists, that it may share some regulatory properties with isatin, a known endogenous inhibitor of monoamine oxidase (MAO) type B (MAO-B). In this study several oxidized indoles were tested for their in vitro inhibition of MAO type A (MAO-A) and B of rat brain non-synaptic mitochondria. 5-Hydroxyoxindole was less potent MAO-A inhibitor (IC50 56.8 microM) than isatin (31.8 microM) and especially 5-hydroxyisatin (6.5 microM), but it was the only highly selective MAO-A inhibitor among the all compounds studied (IC50 MAO-A:IC50 MAO-B = 0.044). Thus, the in vitro data suggest that MAO-A may represent potential target for 5-hydroxyoxindole.     

Inhibition of rat brain monoamine oxidase activity by cerebral anti-ischemic agent, ifenprodil

Ifenprodil, which is clinically used as a cerebral vasodilator, inhibited rat brain type A (MAO-A) and type B (MAO-B) monoamine oxidase activity. It did not, however, affect rat lung semicarbazide-sensitive amine oxidase. The degree of inhibition of either form of MAO was not changed by 30 min preincubation of the enzyme preparations at 37 degrees C with ifenprodil. Modes of inhibition of MAO-A and MAO-B by ifenprodil were competitive towards oxidation of their respective substrates, 5-hydroxytryptamine and benzylamine, with Ki values of 75 microM for inhibition of MAO-A and 110 microM for inhibition of MAO-B.     

Monoamine oxidase inhibitory constituents from the fruits ofCudrania tricuspidata

A methylene chloride soluble fraction of the fruits of Cudrania tricuspidata significantly inhibited the mouse brain monoamine oxidase (MAO). Three known prenylated isoflavones were isolated and identified by activity-guided fractionation. Gancaonin A (1), 4'-O-methylalpinumisoflavone (2), and alpinumisoflavone (3) inhibited MAO activity in a concentration-dependent manner with IC50 values of 19.4, 23.9, and 25.8 microM, respectively. Of these, gancaonin A (1) showed a selective and potent inhibitory effect against MAO-B (IC50 0.8 microM) than MAO-A (IC50 >800 microM). The kinetic analysis using Lineweaver-Burk plots indicated that gancaonin A (1) competitively inhibited MAO-B.     

Stereoselectivity and isoenzyme selectivity of monoamine oxidase inhibitors. Enantiomers of amphetamine, N-methylamphetamine and deprenyl

The enantiomers of amphetamine, N-methylamphetamine and deprenyl were studied, using a solubilised rat liver mitochondrial monoamine oxidase (MAO) preparation, as competitive inhibitors of MAO-A and MAO-B (5-hydroxytryptamine and beta-phenylethylamine as substrate respectively). Only in the case of deprenyl enantiomers inhibiting MAO-B was a preference shown towards the [R]-configuration enantiomer justifying the use of [R]-(-)-deprenyl (as compared to the racemate) for the specific inhibition of MAO-B. Recalculation of the observed Ki values in terms of the base form of the inhibitor indicated that the activity of all enantiomers fell within a narrow, approximately 25-fold range when inhibiting MAO-B. The selectivity of inhibition of MAO-B by [R]-(-)-deprenyl cannot therefore be attributed to any specific structural features of the MAO-B isoenzyme form but rather to a lack of affinity of this enantiomer towards MAO-A.     

Methylpiperate derivatives from Piper longum and their inhibition of monoamine oxidase

We have previously reported that piperine, a known piperidine alkaloid from Piper longum, competitively inhibited mouse brain MAO-A and MAO-B activities. Piperine also showed in vivo antidepressant-like activity against the tail suspension test. In the present study, we further expanded on the identification of MAO inhibitors from the fruit of P. longum. Activity-guided fractionation of a methylene chloride soluble extract led to the isolation of three known piperine-related compounds, methylpiperate (1), guineensine (2), and piperlonguminine (3). Of these, methylpiperate (1) and guineensine (2) showed significant MAO inhibitory activities with IC50 values of 3.6 and 139.2 microM, respectively. Furthermore, methylpiperate (1) exhibited a selective inhibitory effect against MAO-B (IC50 value: 1.6 microM) than MAO-A (IC50 value: 27.1 microM). The kinetic study using the Lineweaver-Burk plots analysis suggested that methylpiperate (1) competitively inhibits MAO-A and MAO-B activities with the Ki values of 23.5 and 1.3 microM, respectively.     

Chromone, a privileged scaffold for the development of monoamine oxidase inhibitors

Two series of novel chromone derivatives were synthesized and investigated for their ability to inhibit the activity of monoamine oxidase. The SAR data indicate that chromone derivatives with substituents in position 3 of γ-pyrone nucleus act preferably as MAO-B inhibitors, with IC(50) values in the nanomolar to micromolar range. Almost all chromone 3-carboxamides display selectivity toward MAO-B. Identical substitutions on position 2 of γ-pyrone nucleus result in complete loss of activity in both isoforms (chromones 2-12 except 3 and 5). Notably, chromone (19) exhibits an MAO-B IC(50) of 63 nM, greater than 1000-fold selectivity over MAO-A, and behaves as a quasi-reversible inhibitor. Docking experiments onto the MAO binding of the most active compound highlight different interaction patterns among the isoforms A and B. The differential analysis of the solvation effects among the chromone isomers gave additional insight about the superior outline of the 3-substituted chromone derivatives.     

Monoamine oxidase A and B inhibitors

This patent update relates to monoamine oxidase inhibitors, covers the patent activity in this field for the last 3 - 5 years and focuses on the most relevant filings. Patents are classified according to the type of MAO inhibition: selective MAO-A and MAO-B inhibitors, and mixed and reversible MAO-A and MAO-B inhibitors.     

Monoamine oxidase inhibitory naphthoquinones from the roots ofLithospermum erythrorhizon

Activity-guided fractionation of a hexane-soluble extract of the roots of Lithospermum erythrorhizon, using a mouse brain monoamine oxidase (MAO) inhibition assay, led to the isolation of two known naphthoquinones, acetylshikonin and shikonin, and a furylhydroquinone, shikonofuran E. These compounds were shown to inhibit MAO with IC50 values of 10.0, 13.3, and 59.1 microM, respectively. Although no specificity for MAO-A and MAO-B was shown by acetylshikonin and shikonin, a Lineweaver-Burk plot analysis indicated that the inhibition was competitive for both MAO-A and MAO-B activity.     

Synthesis and monoamine oxidase inhibitory activity of new pyridazine-, pyrimidine- and 1,2,4-triazine-containing tricyclic derivatives

A number of condensed azines, mostly belonging to the families of indeno-fused pyridazines (1), pyrimidines (2, 3), and 1,2,4-triazines (4, 5), were synthesized and evaluated in vitro as monoamine oxidase (MAO) A and B inhibitors. Most of them showed higher inhibition potency toward MAO-B, the most effective one being 3-(3-nitrophenyl)-9H-indeno[1,2-e] [1,2,4]triazin-9-one (4c), which displayed an IC50 value of 80 nM and proved to be 10-fold more potent than its [2,1-e] fusion isomer 5. Replacing the 3-phenyl group of the known indeno[1,2-c]pyridazin-5-one MAO-B inhibitors with a flexible phenoxymethyl group enhanced the inhibitory potency. The inhibition data highlighted the importance of the aza-heterocyclic scaffold in affecting the MAO isoform selectivity. The 3-phenyl derivatives with type 1, 4, and 5 scaffolds were inhibitors of MAO-B with little or no MAO-A effect, whereas 2- or 3-phenyl derivatives of type 2 and 3 pyrimidine-containing fusion isomers inhibited both isoenzymes with a structure-dependent preference toward MAO-A.     

Monoamine oxidase inhibitory coumarins from the aerial parts ofDictamnus albus

The methanol extract from the aerial parts of Dictamnus albus was active in inhibiting monoamine oxidase (MAO) from the mouse brain. Activity-guided fractionation led to the isolation of four known coumarins, 7-(6'R-hydroxy-3', 7'-dimethyl-2'E, 7'-octadienyloxy) coumarin (1), auraptene (2), umbelliferone (3), and xanthotoxin (4), as active compounds along with an inactive alkaloid, skimmianine (5). Compounds 1 and 2 inhibited MAO activity in a concentration-dependent manner with IC50 values of 0.7 and 1.7 microM, respectively. Compounds 1 and 2 showed a slight and potently selective inhibitory effect against MAO-B (IC50 0.5 and 0.6 microM, respectively) compared to MAO-A (IC50 1.3 and 34.6 microM, respectively). According to kinetic analyses derived by Lineweaver-Burk reciprocal plots, compounds 1 and 2 exhibited a competitive inhibition to MAO-B.     

Inhibition of monoamine oxidases by haloperidol and its metabolites: pharmacological implications for the chemotherapy of schizophrenia

The effect of haloperidol and its metabolites on human platelet monoamine oxidase B (MAO-B) and human placenta monoamine oxidase A (MAO-A) in vitro has been investigated. We found that 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-pyridinium (HP⁺), 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6-tetrahydropyridine (HTP) and 4-chlorophenyl-1,2,3,6-tetrahydropyridine (CPTP) are potent inhibitors of MAO. HP⁺ appeared to be a reversible, uncompetitive and selective MAO-B inhibitor with a Ki of 0.83 µM. HTP was found to be an irreversible, uncompetitive and selective MAO-B inhibitor (Ki of 1.84 µM). CPTP inhibits both MAO-A and MAO-B. Some other haloperidol metabolites, i.e. 4-(4-chlorophenyl)-4-hydroxypyridine (CPHP), 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6-tetrahydropyridine N-oxide (HTPNO) and reduced haloperidol (RHAL), do not inhibit MAO to any appreciable degree at concentrations up to 100 µM. The results suggest that haloperidol metabolites may contribute to the reduction of platelet MAO-B activity in schizophrenic patients undergoing neuroleptic chemotherapy. An examination of the literature reveals that schizophrenic patients with low platelet MAO activity exhibit a strong association with the use of haloperidol. Other possible pharmacological implications of the inhibition of MAO activity are discussed.     

Synthesis and selective inhibitory activity of 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives against monoamine oxidase

A novel series of 1-acetyl-3-(4-hydroxy- and 2,4-dihydroxyphenyl)-5-phenyl-4,5-dihydro-(1H)-pyrazole derivatives 1-12 have been synthesized and investigated for the ability to selectively inhibit the activity of the A and B isoforms of monoamine oxidase (MAO). The new synthesized compounds 1-12 proved to be more reversible, potent, and selective inhibitors of MAO-A than of MAO-B. Knowing that stereochemistry may be an important modulator of biological activity, we performed the semipreparative chromatographic enantioseparation of the most potent, selective, and chiral compounds, 6 and 11. The separated enantiomers were then submitted to in vitro biological evaluation while increasing their inhibitory activity and A selectivity. The (-)-6 enantiomer shows K(i(MAO-A)) = 2 nM and SI = 165 000, (+)-6 shows K(i(MAO-A)) = 6 nM and SI = 166 666, (-)-11 shows K(i(MAO-A)) = 4 nM and SI = 80 000, and (+)-11 shows K(i(MAO-A)) = 7 nM and SI = 38 571.     

The molecular pharmacology of L-deprenyl.

L-Deprenyl, the selective inhibitor of monoamine oxidase type B (MAO-B), has gained wide acceptance as a useful form of adjunct therapeutic drug in the treatment of Parkinson's disease. This review summarizes the molecular pharmacology of L-deprenyl, and the advances in our understanding of its possible mode of action in Parkinson's disease. L-Deprenyl belongs to the class of enzyme-activated irreversible inhibitors also described as 'suicide' inhibitors, because the compound acts as a substrate for the target enzyme, whose action on the compound results in irreversible inhibition. L-Deprenyl first of all forms a noncovalent complex with MAO as an initial, reversible step. The subsequent interaction of L-deprenyl with MAO leads to a reduction of the enzyme-bound flavin-adenine dinucleotide (FAD), and concomitant oxidation of the inhibitor. This oxidized inhibitor then reacts with FAD at the N-5-position in a covalent manner. The observed in vitro selectivity of L-deprenyl for MAO-B may be accounted for by differences in the affinities of the two MAO subtypes for reversible interaction with L-deprenyl, differences in the rates of reaction within the noncovalent complexes to form the irreversibly inhibited adduct, or a combination of both these factors. However, if selective inhibition is to be maintained in vivo, correct dosage schedules are critically important, since all selective MAO inhibitors described up to now lack selectivity at high doses. In experimental animals L-deprenyl is protective against the damaging effects of several neurotoxins, including the dopaminergic agents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) and the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). Beside MAO-B inhibition, which above all explains the prevention of neurotoxic action of MPTP by preventing its metabolism, L-deprenyl appears to exhibit other mechanisms of action which are independent of its action on MAO-B.     

Selectivity of clorgyline and pargyline as inhibitors of monoamine oxidases A and B in vivo in man

During 4 weeks of treatment with clorgyline, a selective MAO-A inhibitor, platelet monoamine oxidase (MAO) activity was unchanged. During a similar 4-week crossover treatment period with pargyline, a selective MAO-B inhibitor, platelet MAO activity was essentially completely inhibited in the same individuals. The differential effects of the two drugs on platelet MAO, which consists exclusively of the MAO-B form, suggests that the in vitro selectivity of clorgyline, and possibly of pargyline, on MAO-A and MAO-B may be maintained in vivo during long-term administration in man. Reductions in blood pressure, heart rate, and plasma amine oxidase activity were generally similar in magnitude during treatment with both drugs, however, suggesting that either these effects are nonspecific consequences of both MAO-A and MAO-B inhibition, or that pargyline also inhibited MAO-A activity.