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.

Nigral cell loss produced by infusion of isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or 1-methyl-4-phenylpyridinium (MPP+) are potential endogenous neurotoxins causing nigral cell death from Parkinson's disease. We now report the effects of 7 days unilateral supranigral infusion in rats of four isoquinoline derivatives, namely N-n-propylisoquinolinium (N-Pr-IQ+), N-methyl-6,7-dimethoxyisoquinolinium (N-Me-6,7-diOMe-IQ+), 6,7-dimethoxy-1-styryl-3,4-dihydroisoquinoline (6,7-diOMe-1-S-3,4-DHIQ) and 1,2,3,4-tetrahydroisoquinoline (THIQ) compared to MPP+. MPP+ (33 nmol/24h)-infused rats showed a marked reduction in motor activity and displayed ipsilateral postural asymmetry. Administration of apomorphine or (+)-amphetamine to these animals produced robust contralateral and ipsilateral rotations, respectively. In contrast, rats infused with the isoquinoline derivatives (150 nmol/24h) did not show spontaneous or drug-induced motor changes. Infusion of MPP+ decreased the number of tyrosine hydroxylase (TH)-positive cells in the ipsilateral substantia nigra pars compacta (SNc) by approximately 90%. Infusion of N-Me-diOme-IQ+ and THIQ produced approximately 42% and 20% ipsilateral SNc cell loss, respectively, but N-Pr-IQ+ and 6,7-diOMe-1-S-3,4-DHIQ did not alter SNc cell numbers. MPP+ markedly depleted the dopamine (DA, 95%), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) content of the ipsilateral striatum. N-Me-diOMe-IQ+ and THIQ also reduced the DA content of the ipsilateral striatum by approximately 39% and 20% respectively, but N-Pr-IQ+ and 6,7-diOme-1-S-3,4-DHIQ did not deplete striatal DA content. The isoquinoline derivatives slightly reduced (N-Me-diOMe-IQ+ and THIQ) or had no effect (N-Pr-IQ+ and 6,7-diOMe-1-S-3,4-DHIQ) on DOPAC or HVA levels. In conclusion, some isoquinoline derivatives that are substrates for the dopamine re-uptake system and inhibitors of mitochondrial function, are toxic to nigral dopaminergic neurones. Chronic exposure to endogenous or exogenous isoquinoline derivatives might contribute to cell death in Parkinson's disease.

Inhibition of [3H]dopamine uptake into striatal synaptosomes by isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP+) may be endogenous neurotoxins causing nigral cell death in Parkinson's disease. These compounds inhibit mitochondrial function but, like MPP+, require accumulation in dopaminergic neurones via the dopamine reuptake system to exert toxicity. We, now, examine the substrate affinity of 14 neutral and quaternary isoquinoline derivatives (7 isoquinolines, 2 dihydroisoquinolines and 5 1,2,3,4-tetrahydroisoquinolines) for the dopamine reuptake system by their ability to inhibit the uptake of [3H]dopamine into rat striatal synaptosomes. Ten isoquinoline derivatives and MPP+ inhibited [3H]dopamine uptake in a concentration-dependent manner. Only 5 isoquinoline derivatives produced 50% inhibition of [3H]dopamine uptake (IC50 = 8.0-50.0 microM), none of which were as potent as MPP+ (IC50 = 0.33 microM). These findings suggest that isoquinoline derivatives are moderate to poor substrates for the dopamine reuptake system and that high concentrations of, or prolonged exposure to, isoquinoline derivatives may be necessary to cause neurodegeneration.

Effects of isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on mitochondrial respiration.

Isoquinoline derivatives exert 1-methyl-4-phenylpyridinium (MPP+)-like activity as inhibitors of complex I and alpha-ketoglutarate dehydrogenase activity in rat brain mitochondrial fragments. We now examine the ability of 19 isoquinoline derivatives and MPP+ to accumulate and inhibit respiration in intact rat liver mitochondria, assessed using polarographic techniques. None of the compounds examined inhibited respiration supported by either succinate + rotenone or tetramethylparaphenylenediamine (TMPD) + ascorbate. However, with glutamate + malate as substrates, 15 isoquinoline derivatives and MPP+ inhibited state 3 and, to a lesser extent, state 4 respiration in a time-dependent manner. None of the isoquinoline derivatives were more potent than MPP+. 6,7-Dimethoxy-1-styryl-3,4-dihydroisoquinoline uncoupled mitochondrial respiration. Qualitative structure-activity relationship studies revealed that isoquinolinium cations were more active than isoquinolines in inhibiting mitochondrial respiration; these, in turn, were more active than dihydroisoquinolines and 1,2,3,4-tetrahydroisoquinolines. Three-dimensional quantitative structure-activity relationship studies using Comparative Molecular Field Analysis showed that the inhibitory potency of isoquinoline derivatives was determined by steric, rather than electrostatic, properties of the compounds. A hypothetical binding site was identified that may be related to a rate-limiting transport process, rather than to enzyme inhibition. In conclusion, isoquinoline derivatives are less potent in inhibiting respiration in intact mitochondria than impairing complex I activity in mitochondrial fragments. This suggests that isoquinoline derivatives are not accumulated by mitochondria as avidly as MPP+. The activity of charged and neutral isoquinoline derivatives implicates both active and passive processes by which these compounds enter mitochondria, although the quaternary nitrogen moiety of the isoquinolinium cations favours mitochondrial accumulation and inhibition of respiration. These findings suggest that isoquinoline derivatives may exert mitochondrial toxicity in vivo similar to that of MPTP/MPP+.

Toxicity to PC12 cells of isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or 1-methyl-4-phenylpyridinium (MPP+) are inhibitors of mitochondrial function and substrates for the dopamine re-uptake system, but their neuronal toxicity is unclear. In this study, the effects of exposing PC12 cells to four isoquinoline derivatives (isoquinoline, N-methylisoquinolinium, 6,7-methylenedioxyisoquinoline and 1,2,3,4-tetrahydroisoquinoline) and MPP+ (100-1000 microM) were examined. All compounds exhibited concentration-dependent toxicity as determined by lactate dehydrogenase release, but none of the isoquinoline derivatives were more toxic than MPP+. Cytotoxicity of these compounds appears to be directly correlated with their substrate affinity for the dopamine reuptake system, but not mitochondrial inhibition. Thus, the low toxicity of isoquinoline derivatives towards PC12 cells suggests that high concentrations of or prolonged exposure to these compounds may be necessary to cause the neurodegenerative changes related to Parkinson's disease.

Inhibition of complex I by isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

Mitochondrial respiratory failure secondary to complex I inhibition may contribute to the neurodegenerative process underlying nigral cell death in Parkinson's disease (PD). Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP+) may be inhibitors of complex I, and have been implicated in the cause of PD as endogenous neurotoxins. To determine the potency and structural requirements of isoquinoline derivatives to inhibit mitochondrial function, we examined the effects of 22 neutral and quaternary compounds from three classes of isoquinoline derivatives (11 isoquinolines, 2 dihydroisoquinolines, and 9 1,2,3,4-tetrahydroisoquinolines) and MPP+ on the enzymes of the respiratory chain in mitochondrial fragments from rat forebrain. With the exception of norsalsolinol and N,n-propylisoquinolinium, all compounds inhibited complex I in a time-independent, but concentration-dependent manner, with IC50s ranging from 0.36-22 mM. Several isoquinoline derivatives were more potent inhibitors of complex I than 1-methyl-4-phenylpyridinium ion (MPP+) (IC50 = 4.1 mM), the most active being N-methyl-6-methoxy-1,2,3,4-tetrahydroisoquinoline (IC50 = 0.36 mM) and 6-methoxy-1,2,3,4-tetrahydroisoquinoline (IC50 = 0.38 mM). 1,2,3,4-Tetrahydroisoquinoline was the least potent complex I inhibitor (IC50 approximately 22 mM). At 10 mM, only isoquinoline (23.1%), 6,7-dimethoxyisoquinoline (89.6%), and N-methylsalsolinol (34.8%) inhibited (P < 0.05) complex II-III, but none of the isoquinoline derivatives inhibited complex IV. There were no clear structure-activity relationships among the three classes of isoquinoline derivatives studied, but lipophilicity appears to be important for complex I inhibition. The effects of isoquinoline derivatives on mitochondrial function are similar to those of MPTP/MPP+, so respiratory inhibition may underlie their reported neurotoxicity.

Inhibition of monoamine oxidase by isoquinoline derivatives. Qualitative and 3D-quantitative structure-activity relationships.

A series of isoquinolines, N-methyl-1,2-dihydroisoquinolines, N-methyl-1,2,3,4-tetrahydroisoquinolines, 1,2,3,4-tetrahydroisoquinolines, and N-methylisoquinolinium ions were tested as inhibitors of monoamine oxidases A and B. All compounds were found to act as reversible and time-independent MAO inhibitors, often with a distinct selectivity towards MAO-A. As a class, the N-methylisoquinolinium ions were found to be the most active MAO-A inhibitors, with N-methyl-6-methoxyisoquinolinium ion emerging as a potent (IC50 = 0.81 microM) and competitive MAO-A inhibitor. Comparative molecular field analysis (CoMFA, a 3D-QSAR method) of MAO-A inhibition was performed using the data reported here and in the literature. Using the steric and lipophilic fields of the inhibitors, quantitative models with reasonable predictive power were obtained that point to the importance of steric, lipophilic, and polar interactions in modulating MAO-A inhibitory activity.

Inhibition of monoamine oxidase-B by 5H-indeno[1,2-c]pyridazines: biological activities, quantitative structure-activity relationships (QSARs) and 3D-QSARs.

A large series (66 compounds) of indeno[1,2-c]pyridazin-5-ones (IPs) were synthesized and tested on their monoamine oxidase-A (MAO-A) and MAO-B inhibitory activity. All of the tested compounds acted preferentially on MAO-B displaying weak (nonmeasurable IC50 values) to high (submicromolar IC50 values) activities. The most active compound was p-CF3-3-phenyl-IP (IC50 = 90 nM). Multiple linear regression analysis of the substituted 3-phenyl-IPs yielded good statistical results (q2 = 0.74; r2 = 0.86) and showed the importance of lipophilic, electronic, and steric properties of the substituents in determining inhibitory potency. Various comparative molecular field analysis studies were performed with different alignments and including the molecular lipophilicity potential. This led to a model including the steric, electrostatic and lipophilicity fields and having a good predictive value (q2 = 0.75; r2 = 0.93).

Inhibition of alpha-ketoglutarate dehydrogenase by isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

Defects in complex I and alpha-ketoglutarate dehydrogenase (alpha-KGDH) occur in the substantia nigra in Parkinson's disease (PD). Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP+) are implicated in the cause of PD as endogenous toxins and are inhibitors of complex I. However, their effects on alpha-KGDH and other mitochondrial non-respiratory chain enzymes are unknown. We have examined the effects of six isoquinoline derivatives (isoquinoline, N-methylisoquinolinium, N-n-propylisoquinolinium, 1,2,3,4-tetrahydroisoquinoline, N-methyl-1,2,3,4-tetrahydroisoquinoline and salsolinol) and MPP+ on the activities of alpha-KGDH, citrate synthase (CS) and glutamate dehydrogenase (GDH) in mitochondrial fragments from rat forebrain. None of the compounds examined had any effect on CS or GDH activity. In contrast, all isoquinoline derivatives investigated and MPP+ inhibited alpha-KGDH activity in a concentration-dependent manner with IC50s ranging from 2.0 to 18.9 mM. MPP+ was previously shown to inhibit alpha-KGDH, but this is the first report of inhibition of alpha-KGDH by isoquinoline derivatives. These findings may represent an additional mechanism contributing to mitochondrial dysfunction and cell death in Parkinson's disease.

An antifungal gamma-pyrone and xanthones with monoamine oxidase inhibitory activity from Hypericum brasiliense.

A new gamma-pyrone (hyperbrasilone), three known xanthones (1,5-dihydroxyxanthone, 5-hydroxy-1-methoxyxanthone and 6-deoxyjacareubin) and betulinic acid have been isolated from a dichloromethane extract of stems and roots of Hypericum brasiliense. Their structures were established by spectroscopic methods (UV, EI-MS, 1H and 13C NMR) and that of the gamma-pyrone was confirmed by X-ray crystallography. Hyperbrasilone and the xanthones were all antifungal against Cladosporium cucumerinum, while the three xanthones showed differing degrees of inhibition of monoamine oxidase A and B.

Screening of unsubstituted cyclic compounds as inhibitors of monoamine oxidases.

A number of unsubstituted aromatic hydrocarbons, azaheterocycles, oxaheterocycles and cyclic ketones were screened for their inhibitory potency towards monoamine oxidases (MAO; EC 1.4.3.4.) A and B. Fair activities (IC50 10-100 microM) and selectivities were found for, e.g. naphthalene, anthracene, phenanthrene, isoquinoline and acridine. The most active inhibitors are oxygen-containing compounds (e.g. coumarin, flavone, dibenzofuran, xanthene, thioxanthone and acridone), with xanthone emerging as a potent (IC50 0.8 microM) and reversible MAO-A inhibitor. All tested inhibitors seem to act in a reversible and time-independent manner.

Substituted xanthones as selective and reversible monoamine oxidase A (MAO-A) inhibitors.

1,3-Dihydroxy-2-methylxanthone (X1), its 4-chloro and 4-bromo derivatives (X1-Cl and X1-Br), and 1,3-dihydroxy-4-methylxanthone were investigated for their inhibition activities toward MAO. A hyperbolic function was derived to fit the data and to calculate IC50 values. The compounds proved to be reversible and selective inhibitors of MAO-A, with X1 displaying the highest activity (IC50 = 3.7 microM).