What is a Monoamine Oxidase Inhibitor?

Introduction

Monoamine oxidase inhibitors (MAOIs) are a class of drugs that inhibit the activity of one or both monoamine oxidase enzymes: monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B).

They are best known as highly efficacious antidepressants, as well as effective therapeutic agents for panic disorder and social phobia. They are particularly effective in treatment-resistant depression and atypical depression. They are also used in the treatment of Parkinson’s disease and several other disorders.

Reversible inhibitors of monoamine oxidase A (RIMAs) are a subclass of MAOIs that selectively and reversibly inhibit the MAO-A enzyme. RIMAs are used clinically in the treatment of depression and dysthymia. Due to their reversibility, they are safer in single-drug overdose than the older, irreversible MAOIs, and weaker in increasing the monoamines important in depressive disorder. RIMAs have not gained widespread market share in the United States.

New research into MAOIs indicates that much of the concern over their supposed dangerous dietary side effects stems from misconceptions and misinformation, and that they are still underutilised despite demonstrated efficacy. New research also questions the validity of the perceived severity of dietary reactions, which has been based on outdated research. Despite this, many psychiatrists, who have little or no knowledge of and experience with monoamine oxidase inhibitors (and are thus unaware of their significant benefits), still reserve them as a last line of treatment, used only when other classes of antidepressant drugs (for example, selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants) have failed.

Brief History

MAOIs started off due to the serendipitous discovery that iproniazid was a potent MAO inhibitor (MAOI). Originally intended for the treatment of tuberculosis, in 1952, iproniazid’s antidepressant properties were discovered when researchers noted that the depressed patients given iproniazid experienced a relief of their depression. Subsequent in vitro work led to the discovery that it inhibited MAO and eventually to the monoamine theory of depression. MAOIs became widely used as antidepressants in the early 1950s. The discovery of the 2 isoenzymes of MAO has led to the development of selective MAOIs that may have a more favourable side-effect profile.

The older MAOIs’ heyday was mostly between the years 1957 and 1970. The initial popularity of the ‘classic’ non-selective irreversible MAO inhibitors began to wane due to their serious interactions with sympathomimetic drugs and tyramine-containing foods that could lead to dangerous hypertensive emergencies. As a result, the use by medical practitioners of these older MAOIs declined. When scientists discovered that there are two different MAO enzymes (MAO-A and MAO-B), they developed selective compounds for MAO-B, (for example, selegiline, which is used for Parkinson’s disease), to reduce the side-effects and serious interactions. Further improvement occurred with the development of compounds (moclobemide and toloxatone) that not only are selective but cause reversible MAO-A inhibition and a reduction in dietary and drug interactions. Moclobemide, was the first reversible inhibitor of MAO-A to enter widespread clinical practice.

A transdermal patch form of the MAOI selegiline, called Emsam, was approved for use in depression by the US Food and Drug Administration (FDA) on 28 February 2006.

Medical Uses

MAOIs have been found to be effective in the treatment of panic disorder with agoraphobia, social phobia, atypical depression or mixed anxiety disorder and depression, bulimia, and post-traumatic stress disorder, as well as borderline personality disorder, and obsessive compulsive disorder (OCD). MAOIs appear to be particularly effective in the management of bipolar depression according to a retrospective-analysis from 2009. There are reports of MAOI efficacy in OCD, trichotillomania, body dysmorphic disorder, and avoidant personality disorder, but these reports are from uncontrolled case reports.

MAOIs can also be used in the treatment of Parkinson’s disease by targeting MAO-B in particular (therefore affecting dopaminergic neurons), as well as providing an alternative for migraine prophylaxis. Inhibition of both MAO-A and MAO-B is used in the treatment of clinical depression and anxiety.

MAOIs appear to be particularly indicated for outpatients with dysthymia complicated by panic disorder or hysteroid dysphoria.

Newer MAOIs such as selegiline (typically used in the treatment of Parkinson’s disease) and the reversible MAOI moclobemide provide a safer alternative and are now sometimes used as first-line therapy.

Side Effects

Hypertensive Crisis

People taking MAOIs generally need to change their diets to limit or avoid foods and beverages containing tyramine, which is found in products such as cheese, soy sauce, and salami. If large amounts of tyramine are consumed, they may suffer a hypertensive crisis, which can be fatal. Examples of foods and beverages with potentially high levels of tyramine include animal liver and fermented substances, such as alcoholic beverages and aged cheeses. Excessive concentrations of tyramine in blood plasma can lead to hypertensive crisis by increasing the release of norepinephrine (NE), which causes blood vessels to constrict by activating alpha-1 adrenergic receptors. Ordinarily, MAO-A would destroy the excess NE; when MAO-A is inhibited, however, NE levels get too high, leading to dangerous increases in blood pressure.

RIMAs are displaced from MAO-A in the presence of tyramine, rather than inhibiting its breakdown in the liver as general MAOIs do. Additionally, MAO-B remains free and continues to metabolise tyramine in the stomach, although this is less significant than the liver action. Thus, RIMAs are unlikely to elicit tyramine-mediated hypertensive crisis; moreover, dietary modifications are not usually necessary when taking a reversible inhibitor of MAO-A (i.e. moclobemide) or low doses of selective MAO-B inhibitors (e.g. selegiline 6 mg/24 hours transdermal patch).

Drug Interactions

The most significant risk associated with the use of MAOIs is the potential for drug interactions with over-the-counter, prescription, or illegally obtained medications, and some dietary supplements (e.g. St. John’s wort, tryptophan). 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 (epinephrine) dosage should be reduced by 75%, and duration is extended).

Tryptophan supplements should not be consumed with MAOIs as the potentially fatal serotonin syndrome may result.

MAOIs should not be combined with other psychoactive substances (antidepressants, painkillers, stimulants, including prescribed, OTC and illegally acquired drugs, etc.) except under expert care. Certain combinations can cause lethal reactions, common examples including SSRIs, tricyclics, MDMA, meperidine, tramadol, and dextromethorphan. Drugs that affect the release or reuptake of epinephrine, norepinephrine, or dopamine typically need to be administered at lower doses due to the resulting potentiated and prolonged effect. MAOIs also interact with tobacco-containing products (e.g. cigarettes) and may potentiate the effects of certain compounds in tobacco. This may be reflected in the difficulty of smoking cessation, as tobacco contains naturally occurring MAOI compounds in addition to the nicotine.

While safer than general MAOIs, RIMAs still possess significant and potentially serious drug interactions with many common drugs; in particular, they can cause serotonin syndrome or hypertensive crisis when combined with almost any antidepressant or stimulant, common migraine medications, certain herbs, or most cold medicines (including decongestants, antihistamines, and cough syrup).

Ocular alpha-2 agonists such as brimonidine and apraclonidine are glaucoma medications which reduce intraocular pressure by decreasing aqueous production. These alpha-2 agonists should not be given with oral MAOIs due to the risk of hypertensive crisis.

Withdrawal

Antidepressants including MAOIs have some dependence-producing effects, the most notable one being a discontinuation syndrome, which may be severe especially if MAOIs are discontinued abruptly or too rapidly. The dependence-producing potential of MAOIs or antidepressants in general is not as significant as benzodiazepines, however. Discontinuation symptoms can be managed by a gradual reduction in dosage over a period of weeks, months or years to minimise or prevent withdrawal symptoms.

MAOIs, as with most antidepressant medication, may not alter the course of the disorder in a significant, permanent way, so it is possible that discontinuation can return the patient to the pre-treatment state. This consideration complicates prescribing between a MAOI and a SSRI, because it is necessary to clear the system completely of one drug before starting another. One physician organisation recommends the dose to be tapered down over a minimum of four weeks, followed by a two week washout period. The result is that a depressed patient will have to bear the depression without chemical help during the drug-free interval. This may be preferable to risking the effects of an interaction between the two drugs.

Interactions

The MAOIs are infamous for their numerous drug interactions, including the following kinds of substances:

  • Substances that are metabolised by monoamine oxidase, as they can be boosted by up to several-fold.
  • Substances that increase serotonin, norepinephrine, or dopamine activity, as too much of any of these neurochemicals can result in severe acute consequences, including serotonin syndrome, hypertensive crisis, and psychosis, respectively.

Such substances that can react with MAOIs include:

  • Phenethylamines: 2C-B, mescaline, phenethylamine (PEA), etc.
    • Amphetamines: amphetamine, MDMA, dextroamphetamine, methamphetamine, DOM, etc.
  • Tryptamines: DMT (MAOIs prevent oxidisation of DMT in the digestive tract, which renders it biologically inert. This allows it to be absorbed in the stomach and small intestine, allowing one to experience the effects of DMT by taking it orally i.e. by Ayahuasca. This anti-oxidation effect can also be observed when administering DMT by inhalation, and it can serve to potentiate the length of the experience.)
  • Norepinephrine, and/or dopamine reuptake inhibitors:
    • Serotonin-norepinephrine reuptake inhibitors (SNRIs): desvenlafaxine, duloxetine, milnacipran, venlafaxine.
    • Norepinephrine-dopamine reuptake inhibitors (NDRIs): amineptine, bupropion, methylphenidate, nomifensine.
    • Norepinephrine reuptake inhibitors (NRIs): atomoxetine, mazindol, reboxetine.
    • Tricyclic antidepressants (TCAs): amitriptyline, butriptyline, clomipramine, desipramine, dosulepin, doxepin, imipramine, lofepramine, nortriptyline, protriptyline, trimipramine.
    • Tetracyclic antidepressants (TeCAs): amoxapine, maprotiline.
    • Phenylpiperidine derivative opioids: meperidine/pethidine, tramadol, methadone, fentanyl, dextropropoxyphene, propoxyphene.
    • Others: brompheniramine, chlorpheniramine, cocaine, cyclobenzaprine, dextromethorphan (DXM), ketamine, MDPV, nefazodone, phencyclidine (PCP), pheniramine, sibutramine, trazodone
  • Serotonin, norepinephrine, and/or dopamine releasers: 4-methylaminorex (4-MAR), amphetamine, benzphetamine, cathine, cathinone, diethylcathinone, ephedrine, levmetamfetamine, lisdexamfetamine, MDMA (“Ecstasy”), methamphetamine, pemoline, phendimetrazine, phenethylamine (PEA), phentermine, propylhexedrine, pseudoephedrine, phenylephrine, tyramine.
  • Local and general anaesthetic in surgery and dentistry, in particular those containing epinephrine. There is no universally taught or accepted practice regarding dentistry and use of MAOIs such as phenelzine, and therefore it is vital to inform all clinicians, especially dentists, of the potential effect of MAOIs and local anaesthesia. In preparation for dental work, withdrawal from phenelzine is specifically advised; since this takes two weeks, however, it is not always a desirable or practical option. Dentists using local anaesthesia are advised to use a non-epinephrine anaesthetic such as mepivacaine at a level of 3%. Specific attention should be paid to blood pressure during the procedure, and the level of the anaesthetic should be regularly and appropriately topped-up, for non-epinephrine anaesthetics take longer to come into effect and wear off faster. Patients taking phenelzine are advised to notify their psychiatrist prior to any dental treatment.
  • Certain other supplements may exhibit below-therapeutic-level MAOI activity: Hypericum perforatum (“St John’s wort”), inositol, Rhodiola rosea, S-adenosyl-L-methionine (SAMe).
  • Antibiotics such as linezolid.
  • Other monoamine oxidase inhibitors.

Mechanism of Action

MAOIs act by inhibiting the activity of monoamine oxidase, thus preventing the breakdown of monoamine neurotransmitters and thereby increasing 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 phenethylamine and certain other trace amines; in contrast, MAO-A preferentially deaminates other trace amines, like tyramine, whereas dopamine is equally deaminated by both types.

Reversibility

The early MAOIs covalently bound to the monoamine oxidase enzymes, thus inhibiting them irreversibly; the bound enzyme could not function and thus enzyme activity was blocked until the cell made new enzymes. The enzymes turn over approximately every two weeks. A few newer MAOIs, a notable one being moclobemide, are reversible, meaning that they are able to detach from the enzyme to facilitate usual catabolism of the substrate. The level of inhibition in this way is governed by the concentrations of the substrate and the MAOI.

Harmaline found in Peganum harmala, Banisteriopsis caapi, and Passiflora incarnata is a reversible inhibitor of monoamine oxidase A (RIMA).

Selectivity

In addition to reversibility, MAOIs differ by their selectivity of the MAO enzyme subtype. Some MAOIs inhibit both MAO-A and MAO-B equally, other MAOIs have been developed to target one over the other.

MAO-A inhibition reduces the breakdown of primarily serotonin, norepinephrine, and dopamine; selective inhibition of MAO-A allows for tyramine to be metabolised via MAO-B. Agents that act on serotonin if taken with another serotonin-enhancing agent may result in a potentially fatal interaction called serotonin syndrome or with irreversible and unselective inhibitors (such as older MAOIs), of MAO a hypertensive crisis as a result of tyramine food interactions is particularly problematic with older MAOIs. Tyramine is broken down by MAO-A and MAO-B, therefore inhibiting this action may result in its excessive build-up, so diet must be monitored for tyramine intake.

MAO-B inhibition reduces the breakdown mainly of dopamine and phenethylamine so there are no dietary restrictions associated with this. MAO-B would also metabolize tyramine, as the only differences between dopamine, phenethylamine, and tyramine are two phenylhydroxyl groups on carbons 3 and 4. The 4-OH would not be a steric hindrance to MAO-B on tyramine. Selegiline is selective for MAO-B at low doses, but non-selective at higher doses.

List of MAO Inhibiting Drugs

Marketed MAOIs

  • Nonselective MAO-A/MAO-B inhibitors.
    • Hydrazine (antidepressant).
      • Isocarboxazid (Marplan).
      • Hydracarbazine.
      • Phenelzine (Nardil).
    • Non-hydrazines.
      • Tranylcypromine (Parnate, Jatrosom).
  • Selective MAO-A inhibitors.
    • Bifemelane (Alnert, Celeport) (available in Japan).
    • Moclobemide (Aurorix, Manerix).
    • Pirlindole (Pirazidol) (available in Russia).
  • Selective MAO-B inhibitors.
    • Rasagiline (Azilect).
    • Selegiline (Deprenyl, Eldepryl, Emsam, Zelapar).
    • Safinamide (Xadago).

Linezolid is an antibiotic drug with weak, reversible MAO-inhibiting activity.

Methylene blue, the antidote indicated for drug-induced methemoglobinemia, among a plethora of other off-label uses, is a highly potent, reversible MAO inhibitor.

MAOIs that have been Withdrawn from the Market

  • Nonselective MAO-A/MAO-B inhibitors:
    • Hydrazines.
      • Benmoxin (Nerusil, Neuralex).
      • Iproclozide (Sursum).
      • Iproniazid (Marsilid, Iprozid, Ipronid, Rivivol, Propilniazida) (discontinued worldwide except for France).
      • Mebanazine (Actomol).
      • Nialamide (Niamid).
      • Octamoxin (Ximaol, Nimaol).
      • Pheniprazine (Catron).
      • Phenoxypropazine (Drazine).
      • Pivalylbenzhydrazine (Tersavid).
      • Safrazine (Safra) (discontinued worldwide except for Japan).
    • Non-hydrazines.
      • Caroxazone (Surodil, Timostenil).
  • Selective MAO-A inhibitors:
    • Minaprine (Cantor).
    • Toloxatone (Humoryl).

List of RIMAs

  • Marketed pharmaceuticals:
    • Moclobemide (Aurorix, Manerix).
  • Other pharmaceuticals.
    • Brofaromine (Consonar).
    • Caroxazone (Surodil, Timostenil).
    • Eprobemide (Befol).
    • Methylene blue.
    • Metralindole (Inkazan).
    • Minaprine (Cantor).
    • Pirlindole (Pirazidol).
  • Naturally occurring RIMAs in plants:
    • Curcumin (selectivity for MAO-A and reliability of research on curcumin are disputed).
    • Harmaline.
    • Harmine.
  • Research compounds:
    • Amiflamine (FLA-336).
    • Befloxatone (MD-370,503).
    • Cimoxatone (MD-780,515).
    • Esuprone.
    • Sercloremine (CGP-4718-A).
    • Tetrindole.
    • CX157 (TriRima).

What is Tranylcypromine?

Introduction

Tranylcypromine (sold under the trade name Parnate among others) is a monoamine oxidase inhibitor (MAOI); more specifically, tranylcypromine acts as nonselective and irreversible inhibitor of the enzyme monoamine oxidase (MAO).

It is used as an antidepressant and anxiolytic agent in the clinical treatment of mood and anxiety disorders, respectively.

Tranylcypromine is a propylamine formed from the cyclisation of amphetamine’s side chain; therefore, it is classified as a substituted amphetamine.

Brief History

Tranylcypromine was originally developed as an analogue of amphetamine. Although it was first synthesized in 1948, its MAOI action was not discovered until 1959. Precisely because tranylcypromine was not, like isoniazid and iproniazid, a hydrazine derivative, its clinical interest increased enormously, as it was thought it might have a more acceptable therapeutic index than previous MAOIs.

The drug was introduced by Smith, Kline and French in the United Kingdom in 1960, and approved in the United States in 1961. It was withdrawn from the market in February 1964 due to a number of patient deaths involving hypertensive crises with intracranial bleeding. However, it was reintroduced later that year with more limited indications and specific warnings of the risks.

Medical Uses

Tranylcypromine is used to treat major depressive disorder, including atypical depression, especially when there is an anxiety component, typically as a second-line treatment. It is also used in depression that is not responsive to reuptake inhibitor antidepressants, such as the SSRIs, TCAs, or bupropion.

Contraindications

Contraindications include:

  • Porphyria.
  • Cardiovascular or cerebrovascular disease.
  • Pheochromocytoma.
  • Tyramine, found in several foods, is metabolized by MAO. Ingestion and absorption of tyramine causes extensive release of norepinephrine, which can rapidly increase blood pressure to the point of causing hypertensive crisis.
  • Concomitant use of serotonin-enhancing drugs, including SSRIs, serotonergic TCAs, dextromethorphan, and meperidine may cause serotonin syndrome.
  • Concomitant use of MRAs, including fenfluramine, amphetamine, and pseudoephedrine may cause toxicity via serotonin syndrome or hypertensive crisis.
  • L-DOPA given without carbidopa may cause hypertensive crisis.

Dietary Restrictions

Tyramine is a common component in many foods, and is normally rapidly metabolised by MAO-A. Individuals not taking MAOIs may consume at least 2 grams of tyramine in a meal and not experience an increase in blood pressure, whereas those taking MAOIs such as tranylcypromine may experience a sharp increase in blood pressure following consumption of as little as 10 mg of tyramine, which can lead to hypertensive crisis.

Foods containing tyramine include aged cheeses, cured meats, tofu and certain red wines. Some, such as yeast extracts, contain enough tyramine to be potentially fatal in a single serving. Spoiled food is also likely to contain dangerous levels of tyramine.

Adverse Effects

Incidence of Adverse Effects

  • Very common (>10% incidence) adverse effects include:
    • Dizziness secondary to orthostatic hypotension (17%).
  • Common (1-10% incidence) adverse effects include:
    • Tachycardia (5-10%).
    • Hypomania (7%).
    • Paresthesia (5%).
    • Weight loss (2%).
    • Confusion (2%).
    • Dry mouth (2%).
    • Sexual function disorders (2%).
    • Hypertension (1-2 hours after ingestion) (2%).
    • Rash (2%).
    • Urinary retention (2%).
  • Other (unknown incidence) adverse effects include:
    • Increased/decreased appetite.
    • Blood dyscrasias.
    • Chest pain.
    • Diarrhoea.
    • Oedema.
    • Hallucinations.
    • Hyperreflexia.
    • Insomnia.
    • Jaundice.
    • Leg cramps.
    • Myalgia.
    • Palpitations.
    • Sensation of cold.
    • Suicidal ideation.
    • Tremor.

Of note, there has not been found to be a correlation between sex and age below 65 regarding incidence of adverse effects.

Tranylcypromine is not associated with weight gain and has a low risk for hepatotoxicity compared to the hydrazine MAOIs.

It is generally recommended that MAOIs be discontinued prior to anaesthesia; however, this creates a risk of recurrent depression. In a retrospective observational cohort study, patients on tranylcypromine undergoing general anaesthesia had a lower incidence of intraoperative hypotension, while there was no difference between patients not taking an MAOI regarding intraoperative incidence of bradycardia, tachycardia, or hypertension. The use of indirect sympathomimetic drugs or drugs affecting serotonin reuptake, such as meperidine or dextromethorphan poses a risk for hypertension and serotonin syndrome respectively; alternative agents are recommended. Other studies have come to similar conclusions. Pharmacokinetic interactions with anaesthetics are unlikely, given that tranylcypromine is a high-affinity substrate for CYP2A6 and does not inhibit CYP enzymes at therapeutic concentrations.

Tranylcypromine abuse has been reported at doses ranging from 120-600 mg per day. It is thought that higher doses have more amphetamine-like effects and abuse is promoted by the fast onset and short half-life of tranylcypromine.

Cases of suicidal ideation and suicidal behaviours have been reported during tranylcypromine therapy or early after treatment discontinuation.

Symptoms of tranylcypromine overdose are generally more intense manifestations of its usual effects.

Interactions

In addition to contraindicated concomitant medications, tranylcypromine inhibits CYP2A6, which may reduce the metabolism and increase the toxicity of substrates of this enzyme, such as:

  • Dexmedetomidine.
  • Nicotine.
  • TSNAs (found in cured tobacco products, including cigarettes).
  • Valproate.

Norepinephrine reuptake inhibitors prevent neuronal uptake of tyramine and may reduce its pressor effects.

Pharmacology

Pharmacodynamics

Tranylcypromine acts as a nonselective and irreversible inhibitor of monoamine oxidase. Regarding the isoforms of monoamine oxidase, it shows slight preference for the MAOB isoenzyme over MAOA. This leads to an increase in the availability of monoamines, such as serotonin, norepinephrine, and dopamine, as well as a marked increase in the availability of trace amines, such as tryptamine, octopamine, and phenethylamine. The clinical relevance of increased trace amine availability is unclear.

It may also act as a norepinephrine reuptake inhibitor at higher therapeutic doses. Compared to amphetamine, tranylcypromine shows low potency as a dopamine releasing agent, with even weaker potency for norepinephrine and serotonin release.

Tranylcypromine has also been shown to inhibit the histone demethylase, BHC110/LSD1. Tranylcypromine inhibits this enzyme with an IC50 < 2 μM, thus acting as a small molecule inhibitor of histone demethylation with an effect to de-repress the transcriptional activity of BHC110/LSD1 target genes. The clinical relevance of this effect is unknown.

Tranylcypromine has been found to inhibit CYP46A1 at nanomolar concentrations. The clinical relevance of this effect is unknown.

Pharmacokinetics

Tranylcypromine reaches its maximum concentration (tmax) within 1-2 hours. After a 20 mg dose, plasma concentrations reach at most 50-200 ng/mL. While its half-life is only about 2 hours, its pharmacodynamic effects last several days to weeks due to irreversible inhibition of MAO.

Metabolites of tranylcypromine include 4-hydroxytranylcypromine, N-acetyltranylcypromine, and N-acetyl-4-hydroxytranylcypromine, which are less potent MAO inhibitors than tranylcypromine itself. Amphetamine was once thought to be a metabolite of tranylcypromine, but has not been shown to be.

Tranylcypromine inhibits CYP2A6 at therapeutic concentrations.

Research

Tranylcypromine is known to inhibit LSD1, an enzyme that selectively demethylates two lysines found on histone H3. Genes promoted downstream of LSD1 are involved in cancer cell growth and metastasis, and several tumour cells express high levels of LSD1. Tranylcypromine analogues with more potent and selective LSD1 inhibitory activity are being researched in the potential treatment of cancers.

Tranylcypromine may have neuroprotective properties applicable to the treatment of Parkinson’s disease, similar to the MAO-B inhibitors selegiline and rasagiline. As of 2017, only one clinical trial in Parkinsonian patients has been conducted, which found some improvement initially and only slight worsening of symptoms after a 1.5 year follow-up.