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.     

The evaluation of isatin analogues as inhibitors of monoamine oxidase

The small molecule, isatin, is a well-known reversible inhibitor of the monoamine oxidase (MAO) enzymes with IC₅₀ values of 12.3 and 4.86 μM for MAO-A and MAO-B, respectively. While the interaction of isatin with MAO-B has been characterized, only a few studies have explored structure–activity relationships (SARs) of MAO inhibition by isatin analogues. The current study therefore evaluated a series of 14 isatin analogues as in vitro inhibitors of human MAO-A and MAO-B. The results indicated good potency MAO inhibition for some isatin analogues with five compounds exhibiting IC₅₀ < 1 μM. 4-Chloroisatin ( 1b ) and 5-bromoisatin ( 1f ) were the most potent inhibitors with IC₅₀ values of 0.812 and 0.125 μM for MAO-A and MAO-B, respectively. These compounds were also found to be competitive inhibitors of MAO-A and MAO-B with K_i values of 0.311 and 0.033 μM, respectively. Among the SARs, it was interesting to note that C5-substitution was particularly beneficial for MAO-B inhibition. MAO inhibitors are established drugs for the treatment of neuropsychiatric and neurodegenerative disorders, while potential new roles in prostate cancer and cardiovascular disease are being investigated.     

Methylene blue and serotonin toxicity: inhibition of monoamine oxidase A (MAO A) confirms a theoretical prediction

BACKGROUND AND PURPOSE: Monoamine oxidase inhibitors (MAOI) are known to cause serotonin toxicity (ST) when administered with selective serotonin reuptake inhibitors (SSRI). Methylene blue (methylthionium chloride, MB), a redox dye in clinical use, has been reported to precipitate ST in patients using SSRI. MB was assessed for MAO inhibition and so for its potential to precipitate ST. EXPERIMENTAL APPROACH: Inhibition of purified human MAO was quantified using kinetic assays and visible spectral changes to study the interactions of MB with MAO A. KEY RESULTS: MB was a potent (tight binding) inhibitor for MAO A. It also inhibited MAO B but at much higher concentration. Interactions of MB with the active site of MAO A were confirmed by its action both as an oxidising substrate and as a one-electron reductant. CONCLUSIONS AND IMPLICATIONS: MB is a potent reversible inhibitor of MAO A with implications for gut uptake of amines when administered orally. At concentrations reported in the literature after intravenous administration, MAO B would be partially inhibited but MAO A would be completely inhibited. This inhibition of MAO A would be expected to lead to perturbations of 5-hydroxytryptamine metabolism and hence account for ST occurring when administered to patients on SSRI treatment.     

Selective inhibition of monoamine oxidase type B by MDL 72145 increases the central effects of L-dopa without modifying its cardiovascular effects.

The potential of a new, potent, irreversible and selective inhibitor of monoamine oxidase type B, (E)-2-(3,4-dimethoxyphenyl)-3-fluorallyamine (MDL 72145), to augment the effects of L-DOPA in an animal model which reproduces the biochemical defect of Parkinson's disease has been evaluated. In rats bearing unilateral 6-hydroxydopamine lesions of the nigro-striatal dopamine pathways, both MDL 72145 and clorgyline, a selective inhibitor of MAO A, augmented the contralateral turning response to L-DOPA combined with carbidopa. The potential of inhibitors of MAO to interact adversely in the periphery with L-DOPA was investigated in the pithed rat; L-DOPA was given either intravenously or intraduodenally. Clorgyline consistently potentiated L-DOPA when given 18 h before testing. Neither MDL 72145 nor the selective inhibitor of MAO B, L-deprenyl, augmented the cardiovascular effects of intraduodenally administered L-DOPA. The data provide no reason to suppose that MDL 72145 would be very different in clinical use from L-deprenyl which is both effective and well-tolerated as an adjunct to the L-DOPA-based therapy of Parkinson's disease.     

Monoamine oxidase in pancreatic islets,exocrine pancreas,and liver from rats. Characterization with clorgyline,deprenyl, pargyline,tranylcypromine, and amezinium

Monoamine oxidase (MAO) was characterized in tissue homogenates from pancreatic islets, exocrine pancreas, and liver from rats. Phenylethylamine was preferentially deaminated by pancreatic islet MAO while 5-hydroxytryptamine was preferentially deaminated by MAO from exocrine pancreas, and tyramine was a good substrate for both tissues. All three substrates were well deaminated by liver tissue. Clorgyline, a selective inhibitor of MAO-A, preferentially inhibited deamination of 5-hydroxytryptamine by all three tissue homogenates, while deprenyl, a selective inhibitor of MAO-B, preferentially inhibited deamination of phenylethylamine. In the case of pargyline, a less selective MAO-B inhibitor, the preference in favour of phenylethylamine was less pronounced. According to these results, MAO in pancreatic islets can be classified as predominantly type B enzyme species and MAO in exocrine pancreas as predominantly type A enzyme species while both types of the enzyme are present in the liver. Using the same three MAO substrates and compared with the effects of the selective enzyme inhibitors, clorgyline and deprenyl, tranylcypromine can be classified as a potent nonselective inhibitor of MAO in homogenates of all three tissues investigated with a slight preference in favour of the inhibition of the B-form of the enzyme, while in contrast amezinium can be classified as a weak nonselective inhibitor of MAO with a slight preference in favour of the inhibition of the A-form of the enzyme. All MAO inhibitors tested also inhibited insulin secretion by isolated incubated rat pancreatic islets, however only at IC50 which were two to three decimal powers higher than those necessary for the inhibition of the MAO activity, thus indicating that inhibition of MAO activity and inhibition of insulin secretion are apparently not closely related.     

Inhibition of monoamine oxidase by N-phenacyl-cyclopropylamine

N-phenacyl-cyclopropylamine hydrobromide (54761) was evaluated in vitro and in vivo as a monoamine oxidase (MAO) inhibitor in rats. In contrast to 51641, which has an o-chlorophenoxy group in place of the phenacyl group and which is a highly selective inhibitor of type A MAO, 54761 showed a slight preference as a type B MAO inhibitor, since it inhibited phenylethylamine oxidation at slightly lower concentrations than were required to inhibit serotonin oxidation in vitro by rat liver MAO. Twelve analogs of 54761 with various substituents on the phenyl ring were also studied, but none was substantially more selective than 54761 as a type B inhibitor and most were preferential type A inhibitors. When 51641 and 54761 were injected into rats and MAO activity was assayed in tissue homogenates, the oxidation of serotonin in brain, heart and liver was inhibited more by 51641 than by 54761. In contrast, the oxidation of phenylethylamine was inhibited more by 54761 than by 51641 in brain and liver. In heart, however, 51641 was a more effective inhibitor of phenylethylamine oxidation than was 54761, supporting earlier evidence that phenylethylamine is destroyed in heart mainly by type A MAO. The oxidation of exogenous [¹⁴C]phenylethylamine was inhibited in vivo more effectively by 54761, whereas the oxidation of endogenous serotonin in brain was inhibited more by 51641. Although 54761 is not as selective an inhibitor of type B MAO as some other compounds such as deprenyl, it illustrates that a large range of selectivity in MAO inhibition can exist within the N-cyclopropylamine series. Further, selective type B inhibition could be achieved in vivo 24 hr after injection of 54761 by co-administration of harmaline. Harmaline selectively protected against the inactivation of type A MAO by 54761 but permitted the inactivation of type B MAO to occur.     

Increase of brain endogenous monoamine oxidase inhibitory activity (tribulin) in experimental audiogenic seizures in rats: evidence for a monoamine oxidase A inhibiting component of tribulin.

Brain tribulin activity in rats with an inherited predisposition to audiogenic epilepsy was studied after seizures of different intensity were induced by an electric bell. Weak seizures (from 0 to 2 arbitrary units) did not produce any changes in endogenous inhibitory activity towards either monoamine oxidase (MAO) A or B. Moderate seizures were characterized by increases in both MAO A and MAO B inhibitory activity (up to 1.9-fold). Complete tonic epileptiform seizures with total areflexia (4 arbitrary units) induced further augmentation (up to 2.5-fold) of MAO A but not of MAO B inhibitory activity. This dissociation between the two inhibitory activities points to the existence of a separate MAO A-inhibiting component of brain tribulin which is different from isatin.     

Evaluation of Natural and Synthetic 1,4‐naphthoquinones as Inhibitors of Monoamine Oxidase

Previous reports have documented that 1,4‐naphthoquinones act as inhibitors of the monoamine oxidase (MAO) enzymes. In particular, fractionation of the extracts of cured tobacco leafs has led to the characterization of 2,3,6‐trimethyl‐1,4‐naphthoquinone, a non‐selective MAO inhibitor. To derive structure–activity relationships for MAO inhibition by the 1,4‐naphthoquinone class of compounds, this study investigates the human MAO inhibitory activities of fourteen structurally diverse 1,4‐naphthoquinones of natural and synthetic origin. Of these, 5,8‐dihydroxy‐1,4‐naphthoquinone was found to be the most potent inhibitor with an IC₅₀ value of 0.860 μm for the inhibition of MAO‐B. A related compound, shikonin, inhibits both the MAO‐A and MAO‐B isoforms with IC₅₀ values of 1.50 and 1.01 μm , respectively. It is further shown that MAO‐A and MAO‐B inhibition by these compounds is reversible by dialysis. In this respect, kinetic analysis suggests that the modes of MAO inhibition are competitive. This study contributes to the discovery of novel MAO inhibitors, which may be useful in the treatment for disorders such as Parkinson's disease, depressive illness, congestive heart failure and cancer.     

Synthesis of novel N-substituted imidazolecarboxylic acid hydrazides as monoamine oxidase inhibitors

Novel 2-alkylsulfanyl-1-benzyl-5-imidazolecarboxylic acid hydrazides (15a,b) were synthesized as analogues of isocarboxazide, which is a known nonselective irreversible monoamine oxidase inhibitor and tested for monoamine oxidase A and B inhibitory activity. Neither of the compounds showed any inhibition of MAO B activity up to a high concentration of 100 microM. An MAO A activity was only slowly inhibited at this high concentration after prolonged incubation with either compound. This suggests any observed inhibition is not very specific.     

Effect of nonselective and selective inhibitors of monoamine oxidases A and B on pethidine toxicity in mice.

The LD50 of pethidine was determined in mice pretreated (4 h) either with the nonselective monoamine oxidase (MAO) inhibitor, phenelzine or with clorgyline, a selective inhibitor of MAO A or deprenyl, a selective inhibitor of MAO B. Phenelzine or combined clorgyline plus deprenyl pretreatments decreased pethidine LD50. Clorgyline or deprenyl alone did not affect pethidine toxicity. Whole brain 5-hydroxytryptamine (5-HT) concentrations were measured in the pretreated mice. 5-HT levels were approximately doubled (P less than 0.001) after phenelzine or clorgyline plus deprenyl treatment, but not after clorgyline or deprenyl given alone. These results indicate that both MAO A and MAO B need to be inhibited to increase pethidine toxicity and brain 5-HT levels. They support the involvement of 5-HT in the toxic interaction between pethidine and MAO inhibitors.     

Inactivation of purified human recombinant monoamine oxidases A and B by rasagiline and its analogues

The inactivation of purified human recombinant monoamine oxidases (MAO) A and B by rasagiline [N-propargyl-1(R)-aminoindan] and four of its analogues [N-propargyl-1(S)-aminoindan (S-PAI), 6-hydroxy-N-propargyl-1(R)-aminoindan (R-HPAI), N-methyl-N-propargyl-1(R)-aminoindan (R-MPAI), and 6-(N-methyl-N-ethyl carbamoyloxy)-N-propargyl-1(R)-aminoindan (R-CPAI)] has been investigated. All compounds tested, with the exception of R-CPAI, form stoichiometric N(5) flavocyanine adducts with the FAD moiety of either enzyme. No H(2)O(2) is produced during either MAO A or MAO B inactivation, which demonstrates that covalent addition occurs in a single turnover. Rasagiline has the highest specificity for MAO B, as demonstrated by a 100-fold higher inhibition potency (k(inact)/K(i)) compared to MAO A, with the remaining compounds exhibiting lower isozyme specificities. MAO B and MAO A are more selective for the R-enantiomer (rasagiline) compared to the S-enantiomer (S-PAI) by 2500-fold and 17-fold, respectively. Differences in UV/vis and CD spectral data of the complexes of the studied compounds with both MAO A and MAO B are interpreted in light of crystallographic data of complexes of MAO B with rasagiline and its analogues (Binda, C.; et al. J. Med. Chem. 2004, 47, 1767-1774.     

Binding of rasagiline-related inhibitors to human monoamine oxidases: a kinetic and crystallographic analysis

Monoamine oxidases A and B (MAO A and B) catalyze the degradation of neurotransmitters and represent drug targets for the treatment of neurodegenerative disorders. Rasagiline is an irreversible, MAO B-selective inhibitor that has been approved as a novel anti-Parkinson's drug. In this study, we investigate the inhibition of recombinant human MAO A and MAO B by several rasagiline analogues. Different substituents added onto the rasagiline scaffold alter the binding affinity depending on the position on the aminoindan ring and on the size of the substituent. Compounds with a hydroxyl group on either the C4 or the C6 atom inhibit both isozymes, whereas a bulkier substituent such as a carbamate is tolerated only at the C4 position. The 1.7 A crystal structure of MAO B in complex with 4-(N-methyl-N-ethyl-carbamoyloxy)-N-methyl-N-propargyl-1(R)-aminoindan shows that the binding mode is similar to that of rasagiline with the carbamate moiety occupying the entrance cavity space. 1(R)-Aminoindan, the major metabolic product of rasagiline, and its analogues reversibly inhibit both MAO A and MAO B. The crystal structure of N-methyl-1(R)-aminoindan bound to MAO B shows that its aminoindan ring adopts a different orientation compared to that of rasagiline.     

Monoamine oxidase inhibition by selected dye compounds

Monoamine oxidase (MAO) is an important drug target as the MAO isoforms play key roles in neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, as well as in neuropsychiatric diseases such as depression. Methylene blue is an inhibitor of MAO‐A, while azure B, the major metabolite of methylene blue, and various other structural analogues retain the ability to inhibit MAO‐A. Based on this, the present study evaluated 22 dyes, many of which are structurally related to methylene blue, as potential inhibitors of human MAO‐A and MAO‐B. The results highlighted three dye compounds as good potency competitive and reversible MAO inhibitors, and which exhibit higher MAO inhibition than methylene blue: acridine orange, oxazine 170 and Darrow red. Acridine orange was found to be a MAO‐A specific inhibitor (IC₅₀ = 0.017 μM), whereas oxazine 170 is a MAO‐B specific inhibitor (IC₅₀ = 0.0065 μM). Darrow red was found to be a non‐specific MAO inhibitor (MAO‐A, IC₅₀ = 0.059 μM; MAO‐B, IC₅₀ = 0.065 μM). These compounds may be advanced for further testing and preclinical development, or be used as possible lead compounds for the future design of MAO inhibitors.     

MAO‐B Inhibition by a Single Dose of l‐Deprenyl or Lazabemide Does Not Prevent Neuronal Damage Following Focal Cerebral Ischaemia in Rats

The present study investigated the effect of postischaemic infusion of an irreversible monoamine oxidase B (MAO‐B) inhibitor, l‐deprenyl, an equipotent dose of a reversible MAO‐B inhibitor, lazabemide, or 0.9% NaCl on infarct volumes following focal cerebral ischaemia in rats. The drug doses (0.3 mg/kg) were selected to induce selective MAO‐B inhibition (45–55%), but not MAO‐A inhibition. The infarct volumes in the cortex or in the striatum did not differ between the experimental groups 72 hr after transient occlusion of the middle cerebral artery, which suggests that during ischaemia/reperfusion, suppressed oxidative stress by partial MAO‐B inhibition or MAO‐B independent mechanisms such as induction of trophic factors, does not protect against ischaemia/reperfusion damage.     

Selective inhibition of carboxylesterases by isatins, indole-2,3-diones

Carboxylesterases (CE) are ubiquitous enzymes thought to be responsible for the metabolism and detoxification of xenobiotics. Numerous clinically used drugs including Demerol, lidocaine, capecitabine, and CPT-11 are hydrolyzed by these enzymes. Hence, the identification and application of selective CE inhibitors may prove useful in modulating the metabolism of esterified drugs in vivo. Having recently identified benzil (diphenylethane-1,2-dione) as a potent selective inhibitor of CEs, we sought to evaluate the inhibitory activity of related 1,2-diones toward these enzymes. Biochemical assays and kinetic studies demonstrated that isatins (indole-2,3-diones), containing hydrophobic groups attached at a variety of positions within these molecules, could act as potent, specific CE inhibitors. Interestingly, the inhibitory potency of the isatin compounds was related to their hydrophobicity, such that compounds with clogP values of <1.25 were ineffective at enzyme inhibition. Conversely, analogs demonstrating clogP values>5 routinely yielded Ki values in the nM range. Furthermore, excellent 3D QSAR correlates were obtained for two human CEs, hCE1 and hiCE. While the isatin analogues were generally less effective at CE inhibition than the benzils, the former may represent valid lead compounds for the development of inhibitors for use in modulating drug metabolism in vivo.     

N-methyl,N-propargyl-2-aminotetralins:novel dopamine agonists with monoamine oxidase inhibiting properties

A series of mono- (5 and 7) and dihydroxylated (5,6 and 6,7)N-methyl,N-propargyl-2-aminotetralins were studied with respect to their dopamine agonistic and monoamine oxidase inhibitory activities. MAO inhibition was found to be reduced by hydroxylation of the aromatic ring. Among the hydroxylated compounds the 7-OH analogue was the most potent inhibitor in in vitro and ex vivo experiments. Both catecholic structures were equipotent with apomorphine as displacers of the specific in vitro binding of [3H]NPA to rat striatal homogenates. Moreover, the catecholic analogues had a potency comparable to that of apomorphine in the gamma-butyrolactone model whereas the monohydroxy analogues were less active. On the basis of their effectiveness to induce stereotypy in rats and to reverse reserpine-induced hypomotility in mice (both used as indices of postsynaptic dopamine receptor stimulation) the catecholic compounds were more potent than the monohydroxy analogues but much less active than apomorphine. Dopamine agonistic activity was also reflected in decreased HVA levels in the striatum whilst effects on striatal 3-MT levels reflected the balance between dopamine agonistic (decrease in 3-MT) and MAO inhibitory (increase in 3-MT) activity of the various compounds. It was concluded that both the mono- and dihydroxylated compounds have MAO inhibiting and dopamine agonistic activities. The MAO inhibitory activity predominated within the monohydroxy structures whereas the dopamine agonistic effect was predominant for the catecholic compounds. It would thus appear that, at least for the 2-aminotetralins, it is difficult to prepare an analogue which combines a high degree of both MAO inhibitory and DA agonistic activity.     

Behavioural hyperactivity in rats treated with selective monoamine oxidase inhibitors and LM 5008, a selective 5-hydroxytryptamine uptake blocker

The administration of 4-[2-(3-indolyl)ethyl]piperidine (LM 5008), a selective 5-hydroxytryptamine (5-HT) uptake blocker to rats pretreated with tranylcypromine (Tcp) resulted in a behavioural syndrome of locomotor hyperactivity which is indistinguishable from that following combined treatment with Tcp and L-tryptophan. A similar behavioural response was elicited by the administration of LM 5008 to rats pretreated with 5-hydroxytryptophan. The response to LM 5008 after monoamine oxidase (MAO) inhibition was abolished by pretreatment with p-chlorophenylalanine, indicating the involvement of 5-HT in producing the hyperactivity syndrome. The administration of imipramine and chlorimipramine in combination with Tcp also resulted in hyperactivity, but these drugs were much less potent than LM 5008 in producing the syndrome. In contrast to L-tryptophan, which can produce hyperactivity only after the inhibition of both type A and type B MAO, LM 5008 can elicit the syndrome after selective inhibition of MAO type A only but not after inhibition of MAO type B. The behavioural results indicate that when MAO type A is inhibited, LM 5008 treatment elicits hyperactivity by preventing the availability of 5-HT to be metabolized by MAO-B component.     

Inhibition of type a and type B monoamine oxidases by naturally occurring xanthones.

Eleven naturally occurring xanthones were tested for IN VITRO inhibition of type A and type B monoamine oxidases (MAO's), prepared by treating rat brain mitochondria with selective inhibitors. All compounds showed various potencies of inhibition for both types of MAO. 1-Hydroxy-3,8-dimethoxy-xanthone and 1,3-dihydroxy-7,8-dimethoxy-xanthone were most potent for type A MAO, while they were rather weak for type B MAO, showing that these compounds can be regarded as type A inhibitors. The xanthones bearing 1,3-dihydroxy-groups generally exhibited potent inhibition for both types. The 3-O-glucosides of xanthones were much weaker than their corresponding genuine aglycones for both types.     

1-Benzylcyclopropylamine and 1-(phenylcyclopropyl)methylamine: an inactivator and a substrate of monoamine oxidase

1-Benzylcyclopropylamine (1) and 1-(phenylcyclopropyl)methylamine (2), cyclopropane analogues of phenethylamine, were tested as inactivators for monoamine oxidase (MAO). Compound 1 is a potent competitive reversible inhibitor of the oxidation of benzylamine and also is a mechanism-based inactivator. It requires 2.3 equiv of 1 to inactivate 1 equiv of MAO. The excess equivalents of 1 are converted into benzyl vinyl ketone. A one-electron mechanism of inactivation is proposed. Compound 2 is a substrate for MAO and is converted into 1-phenylcyclopropanecarboxaldehyde without inactivation of the enzyme. Mechanistic consequences are discussed as a result of this observation.     

Different stereoselective inhibition of monoamine oxidase-B by the R- and S-enantiomers of MD 780236

MD 780236, a selective inhibitor of the B form of MAO behaves as an irreversible inhibitor in in-vitro conditions and mainly as a short-acting inhibitor in ex-vivo experiments. The enantiomer with the R absolute configuration (MD 240928) is fully reversible in ex-vivo conditions, whereas the S-enantiomer (MD 240931) has kept the irreversible component of the inhibition seen with MD 780236. The corresponding racemic alcohol derivative (MD 760548) is also a short-acting inhibitor of the B form of MAO; its S-enantiomer, which has an absolute configuration corresponding to that of MD 240928, has a selectivity towards the B form superior to that of the other enantiomer. The mechanism of the MAO inhibition by MD 780236 is discussed.