The development of monoamine oxidase (MAO) inhibitors has followed a winding path. At the origin was the finding of E. A. Zeller that iproniazid, a hydrazine derivative designed to improve the tuberculostatic action of isoniazid, caused potent inhibition of MAO in vitro and in vivo. The inhibitor also exerted in vivo activity in the brain, subsequent pharmacological research indicating that this inhibition led to changes of cerebral functions [1]. Although the drug had no overt behavioral effects in animals when given alone, it modified the action of reserpine, one of the first modern psychotropic drugs. Thus, administration of iproniazid prior to reserpine unexpectedly antagonized the decreases in locomotor activity and in 5-hydroxy-tryptamine (5 HT) levels induced by reserpine in the brain, the animals sometimes even showing locomotor stimulation [1]. Since one serious side effect of reserpine in man was known to be mental depression, the antagonism of reserpine by iproniazid hinted at the possibility that this drug might be of use in the treatment of mood disorders. However, the actual breakthrough for the development of iproniazid as an antidepressant came from clinical observations. Various investigators had reported psychostimulant side effects of the MAO inhibitor in patients with tuberculosis and other chronic diseases.

The new generation of monoamine oxidase inhibitors.

The acetylenic selective monoamine oxidase (MAO) type B suicide inhibitor selegiline (previously called L-deprenyl) has proved to be a useful adjuvant to levodopa therapy and monotherapy of Parkinson's disease (PD). Selegiline is readily absorbed from the gastrointestinal tract and rapidly enters the brain and spinal cord following oral administration. The drug binds to brain regions with a high MAO-B content, such as the thalamus, the striatum, the cortex, and the brainstem. It is extensively metabolized in humans, mainly in the liver, to form desmethylselegiline and methamphetamine, which are further metabolized to amphetamine. Eighty-six percent of the 10-mg dose was recovered in the urine within 24 hours. These data suggest that accumulation of metabolites does not occur. Although not all features of its anti-PD action are known, studies using brain obtained at autopsy from patients who had been treated with 10 mg of selegiline showed that selective inhibition of MAO-B, with the concomitant increase of phenylethylamine and dopamine (DA) but not of serotonin or noradrenaline, in the basal ganglia may be regarded as its mode of action. The protective effects afforded by selegiline in PD, resulting in a delayed need for levodopa therapy, have been variously interpreted in terms of the involvement of an endogenous neurotoxin or an oxygen free radical mechanism (oxidative stress) in the development of PD. However, although many different hypotheses have been advanced and recent findings have emphasized the significance of oxidative stress in the pathogenesis of the disease, the cause of chronic nigral cell death and the underlying mechanisms remain, as yet, elusive. Therefore, there is no clear knowledge regarding an understanding of the reported effects of selegiline on the progression of PD. Nevertheless, selegiline might be expected to have some protective effects in reducing the production of potentially neurotoxic compounds resulting in the MAO-catalyzed oxidation of DA. In addition, some evidence suggests both an indirect (via induction of radical-scavenging enzymes) and a direct antioxidant function for selegiline. On the other hand, the reported protective effect of selegiline might also receive a contribution from the diminished potentiation of the N-methyl-D-aspartate receptor by the polyamine binding site. Finally, the effects of selegiline might also involve preventing, or perhaps to some extent reversing, the decline in resistance normally associated with cellular aging because of its neurotrophine-like action. However, even in the early clinical stage of PD, the sequence of events leading to nigral cell death may be too far advanced for selegiline to exhibit its maximum potential.

Pharmacology of selegiline

New directions in monoamine oxidase A and B selective inhibitors and substrates.

Polyamine oxidase (PAO) is a FAD-dependent enzyme with a molecular mass of about 62 kDa, present with high activity in most tissues of vertebrates Structural requirements of a substrate for PAO are two positively charged amino groups, separated by a short carbon chain and an alkyl substituent on one or both nitrogen atoms Spermine and the monoacetyl derivatives N1-acetylspermine and N1-acetylspermidine appear to be the natural substrates Spermidine is only poorly oxidized by PAO Using O2, the substrates are oxidatively cleaved by PAO to form equimolar amounts of an amine, an aldehyde and hydrogen peroxide PAO is an integral part of the polyamine interconversion cycle, a major intracellular regulatory system, which contributes to the maintenance of polyamine homeostasis in non-proliferating cells, including brain cells Selective inactivators were used as tools in the elucidation of the functions of PAO Interestingly, even long-term inactivation of PAO did not provoke behavioral changes in experimental animals, despite considerable changes in polyamine metabolism PAO inactivation, however, improves the growth-inhibitory effects of inhibitors of polyamine biosynthetic enzymes and the antitumoral effects of some structural analogs of the polyamines

Polyamine oxidase, properties and functions.

In silico studies for the rational discovery of anticonvulsant compounds.

Milacemide (2-n-pentylaminoacetamide) is a secondary monoamine that in the brain is converted to glycinamide and glycine. This oxidative reaction was suspected to involve the reaction of monoamine oxidase (MAO). Using mitochondrial preparations from tissues that contain MAO-A and -B (rat brain and liver), MAO-A (human placenta), and MAO-B (human platelet and bovine adrenal chromaffin cell), it has been established that mitochondria containing MAO-B rather than MAO-A oxidize (H2O2 production and glycinamide formation) milacemide. The apparent Km (30-90 microM) for milacemide oxidation by mitochondrial MAO-B preparations is significantly lower than that for milacemide oxidation by mitochondrial MAO-A (approximately 1,300 microM). In vitro MAO-B (l-deprenyl and AGN 1135) rather than MAO-A (clorgyline) selectively inhibited the oxidation of milacemide. These in vitro data are matched by ex vivo experiments where milacemide oxidation was compared to oxidation of serotonin (MAO-A) and beta-phenylethylamine (MAO-B) by brain mitochondria prepared from rats pretreated with clorgyline (0.5-10 mg/kg) and l-deprenyl (0.5-10 mg/kg). Furthermore, in vivo experiment demonstrated that l-deprenyl selectively increased the urinary excretion of [14C]milacemide and the total radioactivity with a concomitant decrease of [14C]glycinamide. Such changes were not observed after clorgyline treatment, but were evident only at doses beyond clorgyline selectivity. The present data therefore demonstrate that milacemide is a substrate for brain MAO-B, and its conversion to glycinamide, further transformed to the inhibitory neurotransmitter, glycine, mediated by this enzyme may contribute to its pharmacological activities.

Formation of the Neurotransmitter Glycine From the Anticonvulsant Milacemide Is Mediated By Brain Monoamine Oxidase-b

Metabolism of a New Antiepileptic Drug (milacemide) in the Brain

2-Pentanoylaminoacetic acid and pharmaceutically suitable salts thereof are claimed for use as pharmaceuticals. The compounds are useful in producing an effect on the central nervous system, especially in the treatment of various forms of epilepsy, of disninesiae, of parkinsonism, of memory troubles, of psychic troubles and of cerebral anoxia.

Derivatives of 2-aminoacetic acid

2-pentanoylaminoacetic acid and pharmaceutically suitable salts thereof are claimed for use as pharmaceuticals. The compounds are useful in producing an effect on the central nervous system, especially in the treatment of various forms of epilepsy, of disninesiae, of parkinsonism, of memory troubles, of psychic troubles and of cerebral anoxia.

Cavalier Robert

Papers

Formation of the Neurotransmitter Glycine From the Anticonvulsant Milacemide Is Mediated By Brain Monoamine Oxidase-b

Metabolism of a New Antiepileptic Drug (milacemide) in the Brain

Derivatives of 2-aminoacetic acid

Use of 2-pentanoylaminoacetic acid and salts thereof