Table of Contents
Introduction
Monoamine oxidase inhibitors (MAO inhibitors) are a class of antidepressant drugs used in the treatment of major depressive disorder, atypical depression, and certain anxiety disorders. They act by inhibiting monoamine oxidase (MAO), an enzyme responsible for the breakdown of key neurotransmitters such as serotonin, norepinephrine, and dopamine. By preventing degradation of these monoamines, MAO inhibitors increase their availability in the central nervous system, improving mood and emotional regulation.
Mechanism of Action (Step-wise)
- Monoamine oxidase (MAO) is an enzyme located in mitochondria of neurons and other cells.
- It exists in two isoforms: MAO-A and MAO-B.
- MAO-A primarily metabolizes serotonin, norepinephrine, and tyramine.
- MAO-B primarily metabolizes dopamine and phenylethylamine.
- MAO inhibitors bind to MAO enzymes and inhibit their activity (either reversibly or irreversibly).
- This prevents breakdown of monoamine neurotransmitters.
- As a result, levels of serotonin, norepinephrine, and dopamine increase in synaptic terminals.
- Increased neurotransmitter availability enhances synaptic transmission.
- This leads to antidepressant and mood-stabilizing effects.
A key exam point is that MAO inhibitors increase monoamine levels by preventing their degradation, unlike SSRIs which inhibit reuptake.
Pharmacokinetics
MAO inhibitors are administered orally and are well absorbed from the gastrointestinal tract. Many traditional MAO inhibitors (e.g., phenelzine, tranylcypromine) are irreversible inhibitors, meaning their effects persist even after the drug is cleared, until new enzyme is synthesized. They undergo hepatic metabolism and have variable half-lives, but their pharmacodynamic effects last longer due to enzyme inhibition. Some newer agents are selective for MAO-B and may be reversible.
Clinical Uses
MAO inhibitors are primarily used in treatment-resistant depression and atypical depression. They may also be used in anxiety disorders such as social anxiety disorder. Selective MAO-B inhibitors such as selegiline are used in Parkinson disease because they increase dopamine levels. MAO inhibitors are typically reserved for patients who do not respond to first-line antidepressants due to dietary restrictions and potential drug interactions.
Adverse Effects
MAO inhibitors are associated with several important adverse effects. Orthostatic hypotension, weight gain, and sexual dysfunction are common. A major concern is the “cheese reaction,” where ingestion of tyramine-rich foods leads to hypertensive crisis due to excessive accumulation of tyramine. Drug interactions are also critical, as combining MAO inhibitors with SSRIs or other serotonergic drugs can lead to serotonin syndrome, a potentially life-threatening condition.
Comparative Analysis
| Feature | MAO Inhibitors | SSRIs | TCAs |
|---|---|---|---|
| Mechanism | Inhibit monoamine breakdown | Inhibit serotonin reuptake | Inhibit NE & serotonin reuptake |
| Neurotransmitters affected | Serotonin, NE, dopamine | Mainly serotonin | Serotonin and NE |
| Dietary restrictions | Required | Not required | Not required |
| Risk of hypertensive crisis | High (with tyramine) | Low | Low |
| Risk of serotonin syndrome | High (with interactions) | Moderate | Moderate |
| Use | Resistant depression | First-line depression | Alternative |
MAO inhibitors differ from SSRIs and TCAs in that they increase neurotransmitter levels by preventing degradation rather than blocking reuptake. While effective, their use is limited by dietary restrictions and risk of severe drug interactions.
MCQs
- MAO inhibitors act by:
a) Blocking serotonin receptors
b) Inhibiting monoamine oxidase
c) Increasing dopamine breakdown
d) Blocking calcium channels
Answer: b) Inhibiting monoamine oxidase
- MAO-A primarily metabolizes:
a) Dopamine only
b) Serotonin and norepinephrine
c) Acetylcholine
d) GABA
Answer: b) Serotonin and norepinephrine
- MAO-B primarily metabolizes:
a) Dopamine
b) Serotonin
c) Histamine
d) Acetylcholine
Answer: a) Dopamine
- MAO inhibitors increase levels of:
a) Sodium
b) Monoamines
c) Calcium
d) Chloride
Answer: b) Monoamines
- The “cheese reaction” is due to:
a) Lactose intolerance
b) Tyramine accumulation
c) Dopamine deficiency
d) Calcium overload
Answer: b) Tyramine accumulation
- MAO inhibitors are used in:
a) Hypertension
b) Depression
c) Asthma
d) Diabetes
Answer: b) Depression
- Which condition is treated with MAO-B inhibitors?
a) Parkinson disease
b) Asthma
c) Kidney failure
d) Hyperthyroidism
Answer: a) Parkinson disease
- A major drug interaction risk is:
a) Hypoglycemia
b) Serotonin syndrome
c) Hypercalcemia
d) Hypokalemia
Answer: b) Serotonin syndrome
- MAO inhibitors increase neurotransmitters by:
a) Increasing synthesis
b) Blocking degradation
c) Blocking receptors
d) Increasing excretion
Answer: b) Blocking degradation
- MAO inhibitors are usually reserved for:
a) First-line therapy
b) Resistant cases
c) Pediatric use only
d) Emergency use
Answer: b) Resistant cases
- Orthostatic hypotension is a:
a) Rare effect
b) Common adverse effect
c) Not associated
d) Only in children
Answer: b) Common adverse effect
- Combining MAO inhibitors with SSRIs can cause:
a) Hypertension only
b) Serotonin syndrome
c) Hypotension only
d) Renal failure
Answer: b) Serotonin syndrome
FAQs
What is the mechanism of action of MAO inhibitors?
They inhibit monoamine oxidase, preventing breakdown of neurotransmitters like serotonin, norepinephrine, and dopamine.
What is the difference between MAO-A and MAO-B?
MAO-A metabolizes serotonin and norepinephrine, while MAO-B primarily metabolizes dopamine.
Why are MAO inhibitors not first-line drugs?
Due to dietary restrictions and risk of serious drug interactions.
What causes the cheese reaction?
Tyramine accumulation leading to hypertensive crisis.
What is serotonin syndrome?
A potentially life-threatening condition caused by excessive serotonergic activity.
Which MAO inhibitors are used in Parkinson disease?
Selective MAO-B inhibitors such as selegiline.
References
Goodman & Gilman’s The Pharmacological Basis of Therapeutics – Antidepressants
https://accessmedicine.mhmedical.com/book.aspx?bookid=3191
Katzung: Basic and Clinical Pharmacology – Antidepressants
https://accessmedicine.mhmedical.com/content.aspx?bookid=3382
Tripathi: Essentials of Medical Pharmacology – Antidepressants
https://www.jaypeedigital.com
Harrison’s Principles of Internal Medicine – Depression
https://accessmedicine.mhmedical.com
