Inhibition of rat aorta semicarbazide-sensitive amine oxidase by 2-phenyl-3-haloallylamines and related compounds

The inhibition of semicarbazide-sensitive amine oxidase (SSAO) in rat aorta homogenates by some 2-phenyl-3-haloallylamines has been studied. Derivatives containing a fluorine atom were approximately three times more potent than the corresponding 3-chloroallylamines. These halogen-containing compounds were irreversible inhibitors of SSAO after preincubation with aorta homogenates; kinetic evidence for an initial competitive, reversible interaction (Ki around 0.4-0.6 microM) was found with two compounds (MDL 72145 and 72274). A similar Ki (approx. 0.7 microM) was obtained with 2-phenylallylamine (MDL 72200). However, this compound which lacks a halogen atom was a reversible inhibitor, even after preincubation. The use of a spectrophotometric assay to measure H2O2 production from amine metabolism demonstrated that MDL 72200 was a substrate (Km = 1.4 microM) for SSAO, with a Vmax approximately five times smaller than that of benzylamine (Km = 8.1 microM). Of particular interest in this study is the finding that (E)-2-phenyl-3-chloroallylamine (MDL 72274) is highly selective as an inhibitor of SSAO, compared with MAO-A or B activities, and may be a useful compound for investigating the importance of SSAO in animal tissues.

Monoamine receptor sensitivity changes following chronic administration of MDL 72394, a site-directed inhibitor of monoamine oxidase

(E)-beta-Fluoromethylene-m-tyrosine (MDL 72394) is not per se an inhibitor of monoamine oxidase (MAO) but is a substrate of aromatic L-amino acid decarboxylase (AADC) which liberates the potent MAO inhibitor (E)-beta-fluoromethylene-m-tyramine (MDL 72392). When co-administered to animals with the peripherally selective AADC inhibitor, carbidopa, MDL 72394 inhibited MAO selectively in the brain. Chronic (14 days plus 3 days withdrawal) administration of 0.5 mg/kg per day p.o. MDL 72394, 0.1 mg/kg per day p.o. MDL 72394 combined with 10 mg/kg per day p.o. carbidopa or 50 mg/kg per day p.o. pargyline produced equivalent inhibition of rat brain MAO and decreased the binding of [3H]clonidine and [3H]RX 781094 to the alpha 2-adrenoceptor and of [3H]dihydroalprenolol to the beta-adrenoceptor without changing binding of [3H]prazosin to the alpha 1-adrenoceptor. The locomotor depressant effect of clonidine was attenuated without attenuation of the hypotensive effect in rats treated chronically with the MAO inhibitors. Neither the sensitivity of the alpha 2-autoreceptor nor of the alpha 2-heteroreceptor was decreased in brain slices. However, the sensitivity of adenylate cyclase to activation by both noradrenaline and isoprenaline was significantly reduced. The number of 5-HT2 and 5-HT1A binding sites was decreased: the 5-HT1B binding sites remained unchanged. The effect of chronic MAO inhibitor treatment on 5-HT1A receptors was associated with a decrease in the behavioural response to 8-hydroxy-2-(di-n-propylamino)tetralin and the decrease in 5-HT2 binding was related to a small reduction in the sensitivity of the inositol phosphate system to stimulation by 5-HT. The lack of effect of chronic MAO treatment on the 5-HT autoreceptor measured in cortical slices corresponded to a lack of effect on the 5-HT1B binding site except that chronic administration of pargyline produced a small but significant decrease in 5-HT autoreceptor sensitivity. Overall, the data show that chronic administration of MDL 72394 has a profile of effects on central monoamine receptor binding and function similar to that seen following chronic administration of a number of clinically effective antidepressants.

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.

Inhibition of monoamine oxidase selectively in brain monoamine nerves using the bioprecursor (E)-beta-fluoromethylene-m-tyrosine (MDL 72394), a substrate for aromatic L-amino acid decarboxylase

(E)-beta-Fluoromethylene-m-tyrosine (FMMT) is a dual-enzyme-activated inhibitor of monoamine oxidase (MAO). The compound is not an inhibitor per se but is decarboxylated by aromatic L-amino acid decarboxylase (AADC) to yield a potent enzyme-activated irreversible inhibitor of MAO, (E)-beta-fluoromethylene-m-tyramine, which shows some selectivity for inhibition of MAO type A. Decarboxylation of FMMT was demonstrated in vitro using hog kidney AADC and in vivo in rats by the ability of alpha-monofluoromethyldopa (MFMD), a potent inhibitor of AADC, to prevent MAO inhibition produced by FMMT. In isolated synaptosomes, FMMT was decarboxylated by AADC, and, furthermore, the compound was actively transported into these isolated nerve endings. An active transport into the CNS has also been demonstrated in vivo by performing competition experiments with leucine. To demonstrate that FMMT is preferentially decarboxylated within monoamine nerves of the CNS, the nigrostriatal 3,4-dihydroxyphenylethylamine (dopamine) pathway of rats was unilaterally lesioned with 6-hydroxydopamine or infused with MFMD. Under these conditions, MAO inhibition produced by orally administered FMMT in the striatum ipsilateral to the lesion or infusion was markedly attenuated. Combination of FMMT with an inhibitor of extracerebral AADC, such as carbidopa, protected peripheral organs against the MAO inhibitory effects and concomitantly enhanced MAO inhibition in the CNS. Such combinations had a greatly reduced propensity to augment the cardiovascular effects of intraduodenally administered tyramine, when compared with FMMT given alone or with clorgyline, a selective inhibitor of MAO type A. The results obtained with FMMT suggest the possibility of achieving selective inhibition of MAO within monoamine nerves of the CNS and, further, suggest that combination of FMMT with an inhibitor of extracerebral AADC will reduce the propensity of this inhibitor to produce adverse interactions with tyramine.

The functional consequences of inhibition of monoamine oxidase type B: comparison of the pharmacological properties of L-deprenyl and MDL 72145

The pharmacological properties of two selective inhibitors of monoamine oxidase (MAO) type B, L-deprenyl and MDL 72145 [(E)-2-(3,4-dimethoxyphenyl)-3-fluoroallylamine, HCl], have been investigated in rats and mice in relation to their effects on MAO. Selective inhibition of MAO B achieved following 18 h pretreatment with L-deprenyl and/or MDL 72145 did not per se lead to prominent pharmacological activity; no effects were seen in the mouse "Behavioural Despair" test, hypothermia induced by reserpine in mice was neither prevented nor reversed and there was no change in the cardiovascular responsiveness of the pithed rat to tyramine, noradrenaline or stimulation of the spinal sympathetic outflow. L-Deprenyl differed from MDL 72145 in that short term treatment with this drug caused positive effects in the "Behavioural Despair" test, reversal of reserpine hypothermia, indirect sympathomimetic stimulation of blood pressure and heart rate in the pithed rat and ipsilateral rotation in rats with unilateral nigro-striatal lesions. Qualitatively similar effects were seen with dexamphetamine. The marked difference between the pharmacological effects of MDL 72145 and L-deprenyl despite equivalent inhibition of MAO B suggests that many of the pharmacological actions of L-deprenyl result from its amphetamine-like sympathomimetic properties. MDL 72145 can, therefore, be considered a more reliable tool with which to explore the functional importance of MAO B inhibition in experimental animals and man.

Effect of chronic apomorphine on the development of denervation supersensitivity

We hypothesised that it would be possible to prevent the development of post-synaptic dopamine receptor supersensitivity to 6-hydroxydopamine lesions of the nigro-striatal tract in rats if they were constantly infused with the dopamine agonist apomorphine. Using osmotic minipumps to infuse apomorphine for 15 days in unilaterally lesioned rats, it was possible to delay the development of supersensitivity of the lesioned side for 9 days but not to prevent its eventual appearance. At the same time, evidence for the development of subsensitivity of presynaptic dopamine receptors of the intact side following chronic infusion of apomorphine was inferred from the production of rotations directed towards the lesioned side.

Enzyme-activated irreversible inhibitors of monoamine oxidase: phenylallylamine structure-activity relationships

Seventeen 2-aryl-3-haloallylamine derivatives were prepared and evaluated as inhibitors of monoamine oxidase (MAO, EC 1.4.3.4). The synthesis of these compounds was achieved from either alpha-methylstyrene or ring-substituted phenylacetic acid derivatives. With one exception, these 2-arylallylamines were found to be enzyme-activated, irreversible inhibitors of MAO. The most potent inhibitors were ring-substituted derivatives of (E)-2-phenyl-3-fluoroallylamine with IC50 values ranging from 10(-6) to 10(-8) M. Selectivity for the A and B form of MAO was found to depend on the nature of aromatic ring substitution. In general, hydroxyl substitution favored the inactivation of the A form of MAO, while very selective B inhibitors were obtained when the aromatic ring was substituted with a 4-methoxy group. (E)-2-(4-Methoxyphenyl)-3-fluoroallylamine and (E)-2-(3,4-dimethoxyphenyl)-3-fluoroallylamine proved to be in vitro as selective for the B form of MAO as deprenyl.

MDL 72145, an enzyme-activated irreversible inhibitor with selectivity for monoamine oxidase type B

MDL 72145, (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroallylamine hydrochloride, was designed and synthesised as a potential enzyme-activated irreversible inhibitor of monoamine oxidase (MAO). In vitro, the compound displayed time-dependent pseudo-first-order irreversible inhibitory characteristics with high selectivity for the B form of rat brain mitochondrial MAO. At 10 degrees C the Ki and tau 50 values for the B enzyme were 40 microM and 1.7 min, respectively, while these same kinetic constants for the A enzyme were 131 microM and 14.5 min, respectively. Selective protection against inactivation of the two forms of MAO by MDL 72145 was obtained by preincubating the enzyme with suitable concentrations of the selective A and B substrates, 5-hydroxytryptamine and benzylamine.

A method for measuring monoamine turnover in animals using an irreversible inhibitor of aromatic L-amino acid decarboxylase, DL-alpha-mono fluoromethyldopa

DL-alpha-monofluoromethyldopa (MFMD) is a potent enzyme-activated irreversible inhibitor of aromatic L-amino acid decarboxylase which, when given to rats or mice at 100 and 250 mg/kg i.p., respectively, causes a linear accumulation of L-DOPA and 5HTP and an exponential decline of noradrenaline, dopamine, and 5HT concentrations in the brain. Rates of change of these parameters can be used to calculate the turnover of the three principle monoamine neurotransmitters. Experiments with haloperidol (1 mg/kg s.c.) and the central 5HT agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (0.25 mg/kg s.c.), have been performed to validate the use of MFMD to measure monoamine turnover. MFMD has several advantages over classical methods for the determination of comparative turnovers using enzyme inhibitors.

High-performance liquid chromatographic analysis of S-adenosylmethionine and its metabolites in rat tissues: interrelationship with changes in biogenic catechol levels following treatment with L-dopa

A method is described for the simultaneous analysis of S-adenosylmethionine (SAM) and its metabolites, S-adenosylhomocysteine (SAH) and decarboxylated S-adenosylmethionine along with the natural polyamines, putrescine, spermidine and spermine. The separation is obtained by a reversed-phase ion-pair liquid chromatographic procedure with gradient elution followed by dual detection. The UV absorbance at 254 nm is used for the analysis of SAM and of the SAM metabolites, whereas the polyamines and some major amino acids, e.g., methionine, tyrosine and tryptophan, are analyzed by fluorescence detection after UV-cell derivatization with o-phthalaldehyde. A separate ion-pair reversed-phase high-performance liquid chromatographic (HPLC) procedure using isocratic elution and electrochemical detection is employed to analyse in the same tissue extracts the catechols and 5-hydroxyindoles, 3,4-dihydroxyphenylalanine (DOPA), dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, 4-hydroxy-3- methoxyphenylalanine , tryptophan, 5-hydroxytryptophan, serotonin and 5- hydroxyindolacetic acid. The sample preparation for the two HPLC procedures requires only homogenization of the tissues in perchloric acid and centrifugation before injection onto the column. The two chromatographic procedures have been applied to study the interrelationship, in various tissues of rats, between the SAM and SAH levels and the biogenic catechols after different treatments with L-DOPA alone or in combination with alpha- monofluoromethyl -DOPA, a potent enzyme-activated irreversible inhibitor of aromatic L-amino acid decarboxylase.

A comparison of the effects of reversible and irreversible inhibitors of aromatic L-amino acid decarboxylase on the half-life and other pharmacokinetic parameters of oral L-3,4-dihydroxyphenylalanine

The effects of the irreversible, enzyme-activated, aromatic L-amino acid decarboxylase (AADC) inhibitors alpha-monofluoromethyl-beta-(3,4-dihydroxyphenyl)alanine (MFMD), alpha-difluoromethyl-beta-(3,4-dihydroxyphenyl)alanine (DFMD), and alpha-monofluoromethyl-beta-(2,3-dihydroxyphenyl)alanine (MDL 72. 163) on the serum t1/2 and other pharmacokinetic parameters of co-administered L-3,4-dihydroxyphenylalanine (L-DOPA) were compared to those of the reversible inhibitor, carbidopa in rats. MFMD and MDL 72. 163, administered as their DL-racemic mixtures, produced increases in the t1/2 and bioavailability of co-administered L-DOPA comparable to that produced by a 10-fold larger dose of carbidopa administered as the active L-enantiomer; increasing the dose of MDL 72. 163 did not further increase the t1/2 or bioavailability of L-DOPA. DFMD produced smaller increases of both t1/2 and bioavailability at higher doses. Concomitant decreases in the ClT and aVD were observed with all four inhibitors. Although AADC inhibition reduced the magnitude of the increases in serum dopamine levels following L-DOPA administration, no reduction in serum 3,4-dihydroxyphenylacetic acid levels was observed. The failure of the irreversible inhibitors to produce a larger increase in the t1/2 of L-DOPA than is produced by carbidopa is suggested to reflect the existence of alternative pathways of L-DOPA metabolism.

Total GABA and homocarnosine in CSF as indices of brain GABA concentrations

Treatment of rats with gamma-vinyl-GABA, an inhibitor of GABA-transaminase, caused a dose- and time-related increase in brain GABA concentrations concomitant with increases in the concentrations of total GABA and homocarnosine in the CSF. At 18 h after treatment both CSF parameters correlated significantly with brain GABA concentrations. However, only total GABA in CSF accurately reflected brain GABA concentrations as a function of time after treatment and is therefore the preferred index.

Chronic elevation of brain GABA by gamma-vinyl GABA treatment does not alter the sensitivity of GABAergic or dopaminergic receptors in rat CNS

Rat brain GABA levels were elevated chronically by daily administration of gamma-vinyl GABA, an enzyme-activated, irreversible inhibitor of GABA:2-oxo-glutarate aminotransferase (GABA-T; EC2.6.1.19). Following various periods of drug treatment and withdrawal, the sensitivity of dopamine and GABA receptors in the CNS was determined by biochemical and behavioral evaluations. In contrast to chronic haloperidol treatment, none of the treatment schedules with gamma-vinyl GABA had any significant effect on parameters such as apomorphine induced locomotor activity, [3H] spiperone binding or dopamine-stimulated adenylate cyclase in the corpus striatum; nor did gamma-vinyl GABA treatment affect [3H] GABA binding or GABA-activated [3H] diazepam binding in the cerebral cortex. Moreover, co-administration of gamma-vinyl GABA and haloperidol did not alter the ability of the neuroleptic to induce supersensitivity in the striatal dopaminergic system. Thus, it appears that, in contrast to reported studies using chronic administration of other less specific GABA-T inhibitors such as gamma-acetylenic GABA, amino-oxyacetic acid and isonicotinic acid hydrazide or direct GABA agonists such as THIP (4,5,6,7-tetrahydroisoxazolo (5,4-c-)-pyridin-3-ol) or kojic amine, gamma-vinyl GABA does not alter the sensitivity of the striatal dopaminergic system.

Value of monoamine metabolite determinations in CSF as an index of their concentrations in rat brain following various pharmacological manipulations

Using reversed-phase high performance liquid chromatography with electrochemical detection it is possible to measure concomitantly the concentration of several monoamines, their metabolites and aminoacid precursors in 100 microliters of rat cerebrospinal fluid. To study the quantitative relationship between CSF and brain, alterations in brain monoamines and monoamine metabolites were effected by treatment with L-DOPA or L-5HTP administered with or without concomitant inhibition of extracerebral aromatic amino acid decarboxylase and by treatment with alpha-monofluoromethyldopa, probenecid, haloperidol, or probenecid plus haloperidol. The concentrations of the monoamine metabolites, 5-hydroxyindoleacetic acid, 3,4-dihydroxyphenylacetic acid, and homovanillic acid as well as of the L-DOPA metabolite, 3-methoxy-4-hydroxyphenylalanine in the cerebrospinal fluid were linearly correlated with the concentrations of these metabolites in the brain. However, these correlations need to be interpreted cautiously, since the slopes of the individual regression lines obtained after different pharmacological treatments differed significantly.

Effects of gamma-vinyl GABA on food intake of rats

Gamma-vinyl GABA, an inhibitor of GABA-transaminase, produced a dose-related reduction of food intake in rats, after both single, (125-1000 mg/kg IP or 500 mg/kg PO) and repeated (250 mg/kg/day IP) administration. No tolerance was observed to the effect of repeated injections. Catecholamine and indoleamine systems in the CNS do not appear to be implicated in this anorexic effect. Combination of gamma-vinyl GABA with amphetamine (2.5 mg/kg) enhanced the anorexic effects of the latter compound whilst attenuating its stimulant effects. The data suggest an important role for GABA in the control of food intake.

Does homocarnosine mediate the dyskinetic movements induced by gaba-transaminase inhibitors

Unilateral application of gamma-aminobutyric acid (GABA) antagonists on the motor cortex of conscious rats produces myoclonic movements. Paradoxically, the same behaviour can be observed with high concentrations of some GABA-transaminase (GABA-T) inhibitors. Since the GABA conjugate homocarnosine is increased in the brain following GABA-T inhibition and since homocarnosine is known to displace [3H]-GABA from its binding sites at high concentration, we investigated whether homocarnosine might explain the dyskinetic movements produced by these GABA-T inhibitors. We found that homocarnosine produces dyskinesia similar to that observed with GABA antagonists and GABA-T inhibitors when applied directly to the cortex. However, this property of homocarnosine is unlikely to be the basis of the dyskinetic effect of GABA-T inhibitors since we found no relationship between brain homocarnosine levels and the appearance of abnormal movements following GABA-T inhibition.

Simultaneous determination of 3,4-dihydroxyphenylalanine, 5-hydroxytryptophan, dopamine, 4-hydroxy-3-methoxyphenylalanine, norepinephrine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, serotonin, and 5-hydroxyindoleacetic acid in rat cerebrospinal fluid and brain by high-performance liquid chromatography with electrochemical detection

A method using reversed-phase ion-pair high-performance liquid chromatography with electrochemical detection for the simultaneous determination of tryptophan (TRP), 3,4-dihydroxyphenylalanine (DOPA), and their metabolites in whole brain, small-brain parts, and cerebrospinal fluid of rats has been developed. The sample preparation requires only homogenization in perchloric acid and centrifugation before injection onto the column. With a LiChrosorb RP-18 (10 micrometer) column and a mobile phase consisting of a phosphate (NaH2PO4, 0.1 M)-methanol mixture with octylsulfonate (2.6 x 10(-3) M) at pH 3.35 and 26 degrees C, the separation of DOPA, dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, 4-hydroxy-3-methoxyphenylalanine, TRP, 5-hydroxytryptophan (5-HTP), serotonin, and 5-hydroxyindoleacetic acid was achieved. The method has been applied to study the effect of alpha-monofluoromethyldopa alone and in combination with L-DOPA or L-5-HTP, on the catechol and 5-OH indole levels in brain and CSF of the rat.

Mechanism of the antihypertensive action of DL-alpha-monofluoromethyldopa: implications for the role of the sympathetic nervous system in maintenance of elevated blood pressure in spontaneously hypertensive rats

We investigated the mechanism of the antihypertensive action of DL-alpha-monofluoromethyldopa (MFMD), an irreversible inhibitor of L-aromatic amino acid decarboxylase, in spontaneously hypertensive rats (SHR). MFMD, 25 mg/kg i.p. daily for 3 days, reduced mean arterial blood pressure by 47 +/- 11 mm Hg (n = 6; p less than 0.001). The effect was associated with marked attenuation of the responses to stimulation of the whole sympathetic outflow in pithed preparations and with substantial reductions in the norepinephrine concentrations of hearts and portal veins. Similar functional and biochemical deficits were produced in age-matched, Wistar--Kyoto, normotensive rats, but blood pressure in these animals fell only by an average of 8 +/- 4 mm Hg (n = 5; NS). Blood pressure of SHR, lowered by MFMD, was restored to pretreatment levels by infusion with dopamine (0.1 mg/kg/min for 5 min), and there was a concomitant return towards normal of both the peripheral sympathetic transmitter stores and the response to stimulation of the nerves supplying the cardiovascular system. Neither the dopamine nor the norepinephrine concentrations of the brain, depleted by treatment with MFMD, were altered following infusion of dopamine. These results provide direct evidence that attenuation of sympathetic function arising from depletion of the peripheral transmitter store is the mechanism by which MFMD lowers blood pressure in SHR. They further lend strong support to the view that hyperactivity of the sympathetic nervous system plays a primary role in the maintenance of the elevated blood pressure in the genetically hypertensive rat.

Further studies on the inhibition of monoamine synthesis by monofluoromethyldopa

1 alpha-Monofluoromethyldopa (MFMD, RMI 71963), a potent and selective enzyme-activated irreversible inhibitor of aromatic L-amino acid decarboxylase produces a substantial and long-lasting decrease in the catecholamine content of mouse brain, heart and kidney. 2 Single doses of MFMD reduce the 5-hydroxytryptamine concentration of mouse brain without altering the tryptophan concentration. 3 In animals treated with MFMD, peripheral but not brain noradrenaline is restored within 1 h to control levels by an intraperitoneal injection of dopamine.