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Monoamine oxidase inhibitor
Monoamine oxidase inhibitors (MAOIs) are a class of powerful antidepressant drugs prescribed for the treatment of depression. They are particularly effective in treating atypical depression, and have also shown efficacy in helping people who want to quit smoking.
Due to potentially lethal dietary and drug interactions, MAOIs had been reserved as a last line of defense, used only when other classes of antidepressant drugs (for example selective serotonin reuptake inhibitors and tricyclic antidepressants) have been tried unsuccessfully. Recently, however, a patch form of the drug selegiline, called Emsam, was developed. It was approved for use by the FDA on February 28 2006. When applied transdermally the drug does not enter the gastro-intestinal system as it does when taken orally, thereby decreasing the dangers of dietary interactions associated with MAOI pills.
In the past they were prescribed for those resistant to tricyclic antidepressant therapy, but newer MAOIs are now sometimes used as first-line therapy. They are also used for treating agoraphobia or social anxiety. Currently, the availability of selegiline and moclobemide provides a safer alternative, although these substances are not always as effective as their predecessors.
MAO inhibitors can also be used in the treatment of Parkinson's disease (by affecting dopaminergic neurons), as well as providing an alternative for migraine prophylaxis.
Mode of action
MAOIs act by inhibiting the activity of monoamine oxidase preventing the breakdown of monoamine neurotransmitters, which increases their availability. There are two isoforms of monoamine oxidase, MAO-A and MAO-B. MAO-A preferentially deaminates serotonin, melatonin, epinephrine and norepinephrine. MAO-B preferentially deaminates phenylethylamine and trace amines. Dopamine is equally deaminated by both types. Many formulations have forms of fluoride attached to assist in permeating the blood-brain barrier, which is suspected as a factor in pineal gland effects.
The early MAOIs inhibited monoamine oxidase irreversibly. When they react with monoamine oxidase, they permanently deactivate it, and the enzyme cannot function until it has been replaced by the body, which can take about two weeks. A few newer MAOIs, notably moclobemide, are reversible, meaning that they are able to detach from the enzyme to facilitate usual anabolism of the substrate. The level of inhibition in this way is governed by the respective concentrations of the substrate and the MAOI.
In addition to reversibility, MAOIs differ by their selectivity of the MAO receptor. Some MAOIs inhibit both MAO-A and MAO-B equally, other MAOIs have been developed that target one over the other.
MAO-A inhibition reduces the breakdown of primarily serotonin, epinephrine, and norepinephrine and thus has a higher risk of serotonin syndrome and/or a hypertensive crisis. Tyramine is also broken down by MAO-A and excessive build up of it is not good, so diet must be monitored for intake.
MAO-B inhibition reduces the breakdown mainly of dopamine and phenethylamine so there are no dietary restrictions associated with this. Two such drugs, selegiline and rasagiline have been approved by the FDA without dietary restrictions, except in high dosage treatment where they lose their selectivity. 
When ingested orally, MAOIs inhibit the catabolism of dietary amines. Sufficient intestinal MAO-A inhibition can lead to hypertensive crisis, when foods containing tyramine are consumed (so-called "cheese syndrome"), or hyperserotonemia if foods containing tryptophan are consumed. The amount required to cause a reaction exhibits great individual variation and depends on the degree of inhibition, which in turn depends on dosage and selectivity.
The exact mechanism by which tyramine causes a hypertensive reaction is not well understood, but it is assumed that tyramine displaces norepinephrine from the storage vesicles. This may trigger a cascade in which excessive amounts of norepinephrine can lead to a hypertensive crisis. Another theory suggests that proliferation and accumulation of catecholamines causes hypertensive crises.
Tyrosine is the precursor to catecholamines, not tyramine. Tyramine is a breakdown product of tyrosine. In the gut and during fermentation tyrosine, an amino acid, is decarboxylated to tyramine. Ordinarily, tyramine is deaminated in the liver to an inactive metabolite, but when the hepatic MAO (primarily MAO-A) is inhibited, the "first-pass" clearance of tyramine is blocked and circulating tyramine levels can climb. Elevated tyramine competes with tyrosine for transport across the blood-brain barrier (via aromatic amino acid transport) where it can then enter adrenergic nerve terminals. Once in the cytoplasmic space, tyramine will be transported via the vesicular monoamine transporter (VMAT) into synaptic vesicles thereby displacing norepinephrine. The mass transfer of norepinephrine from its vesicular storage space into the extracellular space via mass action can precipitate the hypertensive crisis. Hypertensive crises can sometimes result in stroke or cardiac arrhythmia if not treated. This risk is generally not present with RIMAs. Both kinds of intestinal MAO inhbition can cause hyperpyrexia, nausea and psychosis if foods high in levodopa are consumed.
Chronic use of MAOIs may provide some antidepressant effects that are thought to be mediated by metabolism of tyramine to octopamine, a reaction catalyzed by phenyl-N-methyl transferase that normally converts dopamine to norepinephrine. Octopamine may then act as a "false transmitter" in that it is stored and released like the endogenous transmitter norepinephrine. However, it is a poor agonist of postsynaptic adrenoceptors while retaining agonist activity at presynaptic autoreceptors. This action reduces adrenergic transmission by diminishing postsynaptic receptor activation and by a presynaptic autoinhibitory effect. Finally, octopamine may serve as an agonist at a novel "trace amine" receptor expressed at low levels throughout the brain.
Examples of foods and drinks with potentially high levels of tyramine include fermented substances, such as Chianti and other aged wines, and aged cheeses. Liver is also a well-known source. (See a list of foods containing tyramine). Examples of levodopa-containing foods include broad beans. These diet restrictions are not necessary for those taking selective MAO-B inhibitors.
It deserves separate mention that some meat extracts and yeast extracts (Bovril, Marmite, Vegemite) contain extremely high levels of tyramine, and should not be used with these medications.
When MAOIs were first introduced, these risks were not known, and over the following four decades, fewer than 100 people have died from hypertensive crisis. Presumably due to the sudden onset and violent appearance of the reaction, MAOIs gained a reputation for being so dangerous that, for a while, they were taken off the market in America entirely. It is now known that, used as directed under the care of a qualified psychiatrist, this class of drugs remains a safe alternative for intermediate- to long-term use.
The most significant risk associated with the use of MAOIs, is the potential for interactions with over-the-counter and prescription medicines, illicit drugs and certain supplements (e.g. St. John's Wort). It is vital that a doctor supervise such combinations to avoid adverse reactions. For this reason, many users carry an MAOI-card, which lets emergency medical personnel know what drugs to avoid. (E.g. adrenaline dosage should be reduced by 75%, and duration is extended)
MAOIs should not be combined with other psychoactive substances (antidepressants, illicit drugs, painkillers, stimulants, etc.) except under expert care. Certain combinations can cause lethal reactions, common examples including SSRIs, tricyclics, MDMA, meperidine, tramadol and dextromethorphan. Agents with actions on epinephrine, norepinephrine or dopamine must be administered at much lower doses due to potentiation and prolonged effect. Purely opiate-acting analgesics, such as morphine and buprenorphine may be used safely with MAOIs, but may require a dosage adjustment.
List of MAOIs
Monoamine oxidase inhibitors include:
pervalidus obduro, sold as Orexis, for ED, cautions user not to take coincidentally with any MAOI.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Monoamine_oxidase_inhibitor". A list of authors is available in Wikipedia.|