Table of Contents
Introduction
Quetiapine is an atypical antipsychotic medication used in schizophrenia, bipolar disorder, and major depressive disorder. It acts on multiple neurotransmitter receptors in the central nervous system, particularly dopamine and serotonin receptors. Compared with typical antipsychotics, quetiapine has lower risk of extrapyramidal side effects because of its serotonin-dopamine antagonistic profile.
Mechanism of Action (Step-wise)
- Quetiapine acts primarily as an antagonist at dopamine D2 receptors and serotonin 5-HT2A receptors.
- Dopamine D2 receptor blockade reduces excessive dopaminergic activity in mesolimbic pathways.
- Reduction of mesolimbic dopamine activity improves positive symptoms of schizophrenia such as hallucinations and delusions.
- Serotonin 5-HT2A receptor antagonism increases dopamine release in nigrostriatal pathways.
- Increased dopamine in nigrostriatal pathways lowers the risk of extrapyramidal side effects.
- Quetiapine also blocks histamine H1 receptors.
- H1 receptor blockade contributes to sedation and weight gain.
- α1-adrenergic receptor antagonism causes vasodilation and orthostatic hypotension.
- Quetiapine has moderate muscarinic receptor antagonistic effects.
- Its active metabolite, norquetiapine, inhibits norepinephrine reuptake.
- This contributes to antidepressant effects in mood disorders.
- The overall effect is stabilization of psychotic symptoms and mood regulation.
A key exam point is that quetiapine blocks dopamine D2 and serotonin 5-HT2A receptors with lower extrapyramidal toxicity than typical antipsychotics.
Pharmacokinetics
Quetiapine is administered orally and is well absorbed. It undergoes extensive hepatic metabolism mainly through CYP3A4. The drug has an active metabolite, norquetiapine. It is excreted mainly through urine and feces. Extended-release formulations provide prolonged therapeutic effect.
Clinical Uses
Quetiapine is used in schizophrenia, bipolar mania, bipolar depression, and major depressive disorder as adjunct therapy. It is also used off-label for generalized anxiety disorder and insomnia in selected patients.
Adverse Effects
Common adverse effects include sedation, dizziness, orthostatic hypotension, weight gain, and metabolic syndrome. Quetiapine may also cause hyperglycemia and dyslipidemia. Compared with typical antipsychotics, extrapyramidal symptoms are less common.
Comparative Analysis
| Feature | Quetiapine | Haloperidol | Olanzapine |
|---|---|---|---|
| Drug class | Atypical antipsychotic | Typical antipsychotic | Atypical antipsychotic |
| Main receptors blocked | D2 + 5-HT2A | D2 | D2 + 5-HT2A |
| EPS risk | Low | High | Moderate |
| Sedation | High | Moderate | High |
| Weight gain | Moderate | Low | High |
| Mood disorder use | Yes | Limited | Yes |
Quetiapine differs from haloperidol because it has combined serotonin and dopamine antagonism with lower extrapyramidal toxicity. Compared with olanzapine, quetiapine generally causes slightly less metabolic disturbance.
MCQs
- Quetiapine is classified as a:
a) Typical antipsychotic
b) Atypical antipsychotic
c) Antidepressant only
d) Benzodiazepine
Answer: b) Atypical antipsychotic
- Quetiapine blocks which dopamine receptor?
a) D1
b) D2
c) D3
d) D5
Answer: b) D2
- Quetiapine also blocks which serotonin receptor?
a) 5-HT1A
b) 5-HT2A
c) 5-HT3
d) 5-HT4
Answer: b) 5-HT2A
- D2 blockade in mesolimbic pathways improves:
a) Positive symptoms of schizophrenia
b) Hyperglycemia
c) Renal failure
d) Hypotension
Answer: a) Positive symptoms of schizophrenia
- 5-HT2A antagonism helps reduce:
a) Sedation
b) Extrapyramidal side effects
c) Blood glucose
d) Platelet aggregation
Answer: b) Extrapyramidal side effects
- Quetiapine blocks which receptor causing sedation?
a) Dopamine receptor
b) Histamine H1 receptor
c) Muscarinic receptor only
d) NMDA receptor
Answer: b) Histamine H1 receptor
- α1 receptor blockade may cause:
a) Hypertension
b) Orthostatic hypotension
c) Hypercalcemia
d) Bradykinesia
Answer: b) Orthostatic hypotension
- Quetiapine is used in:
a) Schizophrenia
b) Hyperthyroidism
c) Asthma
d) Peptic ulcer disease
Answer: a) Schizophrenia
- A common adverse effect is:
a) Weight gain
b) Hyperactivity
c) Hypoglycemia
d) Hypernatremia
Answer: a) Weight gain
- Quetiapine is metabolized mainly by:
a) CYP2D6
b) CYP3A4
c) MAO
d) Xanthine oxidase
Answer: b) CYP3A4
- Compared with haloperidol, quetiapine has:
a) Higher EPS risk
b) Lower EPS risk
c) No D2 blockade
d) No sedation
Answer: b) Lower EPS risk
- Norquetiapine contributes to:
a) Dopamine depletion
b) Antidepressant effects
c) Sodium retention
d) Histamine release
Answer: b) Antidepressant effects
FAQs
What is the mechanism of action of quetiapine?
Quetiapine blocks dopamine D2 and serotonin 5-HT2A receptors in the CNS.
Why does quetiapine cause fewer extrapyramidal side effects?
Because serotonin 5-HT2A antagonism increases dopamine release in nigrostriatal pathways.
What disorders are treated with quetiapine?
Schizophrenia, bipolar disorder, and major depressive disorder.
Why does quetiapine cause sedation?
Due to histamine H1 receptor blockade.
Can quetiapine cause metabolic side effects?
Yes, including weight gain, dyslipidemia, and hyperglycemia.
What enzyme mainly metabolizes quetiapine?
CYP3A4.
References
Goodman & Gilman’s The Pharmacological Basis of Therapeutics – Antipsychotic Agents
https://accessmedicine.mhmedical.com/book.aspx?bookid=3191
Katzung: Basic and Clinical Pharmacology – Antipsychotic Drugs
https://accessmedicine.mhmedical.com/content.aspx?bookid=3382
Tripathi: Essentials of Medical Pharmacology – Antipsychotic Drugs
https://www.jaypeedigital.com
Harrison’s Principles of Internal Medicine – Schizophrenia and Bipolar Disorders
https://accessmedicine.mhmedical.com
