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Medicine > Antihistamine > Flashcards

Antihistamine Flashcards

1
Q

Histamine - Definition

A

Histamine is a naturally occurring amine found in most tissues. Forms: Inactive (bound in granules) and active (free histamine).

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2
Q

Histamine - Sources

A

Location: Found in granules within mast cells. Also found in: Basophils and some neurons.

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3
Q

Histamine - Release Mechanisms

A

Interaction of antigen with igE antibodies on the mast cell surface

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4
Q

Histamine - Function in Body Epithelia

A

Location: Epithelia of the gut, respiratory tract, and skin. Role: Inflammatory responses to foreign substances.

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5
Q

Histamine - Function in Glands

A

Secretory Role: Involves gastric, intestinal, lachrymal, and salivary secretions.

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6
Q

Histamine - Mast Cells Near Blood Vessels

A

Role: Regulates microcirculation by releasing histamine during local inflammation.

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7
Q

Histamine - Local Hormone

A

Histamine functions as a local hormone, acting near its release site.

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8
Q

Histamine - Inactivation

A

Mechanism: Rapid inactivation by deamination and methylation.

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9
Q

Histamine - Effect on Smooth Muscle

A

Action: Generally causes contraction of smooth muscles. Exception: Arterioles, where histamine causes dilation.

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10
Q

Histamine - Bronchospasm

A

Action: Causes bronchospasm, especially in asthma and allergies.

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11
Q

Histamine - Effect on Blood Vessels

A

Vasodilation: Dilates arterioles, lowering blood pressure. Mechanism: Involves nitric oxide (NO) release from vascular endothelium.

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12
Q

Histamine - Capillary Permeability

A

Action: Increases capillary permeability, leading to edema (swelling).

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13
Q

Histamine - Skin Response

A

Effect: Causes itching (pruritus) and is involved in the flush, wheal, and flare reaction.

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14
Q

Histamine - Effect on Gastric Secretion

A

Action: Stimulates secretion of gastric acid and pepsin.

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15
Q

Histamine Receptors - Overview

A

Types: H1, H2, H3 receptors. All are G-protein-coupled receptors.

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16
Q

Histamine Receptors - H1

A

Function: Mediates pro-inflammatory effects. Involved in: Allergy symptoms like itching, bronchospasm, and vasodilation.

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17
Q

Histamine Receptors - H2

A

Function: Mediates gastric acid release. Location: Found primarily in stomach lining.

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18
Q

Histamine Receptors - H3

A

Function: Feedback inhibition for histamine and other neurotransmitters. Location: Found in the brain and nerve endings.

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19
Q

Methods to Counteract Histamine - Overview

A
  1. Using drugs with opposite effects (e.g., adrenaline). 2. Blocking histamine binding (receptor antagonists). 3. Using inverse agonists to block H1 receptor effects. 4. Preventing histamine release (e.g., glucocorticoids).
20
Q

Physiological Antagonism

A

Example: Adrenaline counters histamine by activating α and β adrenoceptors.

21
Q

Histamine Receptor Antagonism

A

Mechanism: Receptor antagonists prevent histamine from binding to its receptors.

22
Q

Inverse Agonism

A

Mechanism: Drugs act as inverse agonists, stabilizing the H1 receptor in an inactive state.

23
Q

Drugs that Prevent Histamine Release

A

Examples: Glucocorticoids, sodium cromoglycate. Action: Prevent release of histamine from mast cells.

24
Q

Antihistamines - Definition

A

H1 receptor antagonists that block histamine action at H1 receptors.

25
Q

Antihistamines - Mechanism of Action

A

Binding: Bind to H1 receptor and stabilize it in an inactive state. Effect: Block components of the triple response (flush, wheal, flare) and prevent hypotension.

26
Q

Antihistamines - No Effect on H2

A

Limitation: H1 antihistamines do not affect H2-mediated gastric acid secretion.

27
Q

Antihistamines in Asthma

A

Limitation: H1 antihistamines have negligible effects on asthma since cysteinyl leukotrienes are the predominant vasoconstrictors.

28
Q

First-Generation Antihistamines - Overview

A

Characteristic: Penetrate the CNS, causing sedation. Receptors: Also block muscarinic, α1, and serotonin receptors.

29
Q

First-Generation Antihistamines - Drugs

A

Examples: Diphenhydramine, Chlorpheniramine, Promethazine, Cyclizine.

30
Q

First-Generation Antihistamines - Side Effects

A

CNS Effects: Sedation, dizziness, incoordination. Antimuscarinic Effects: Dry mouth, blurred vision. Precaution: Avoid operating machinery or driving due to sedation.

31
Q

Second-Generation Antihistamines - Overview

A

Characteristic: More polar, less CNS penetration, reduced sedation. Selectivity: More selective for H1 receptors.

32
Q

Second-Generation Antihistamines - Drugs

A

Examples: Cetirizine, Levocetirizine, Loratadine, Fexofenadine, Desloratadine.

33
Q

Pharmacokinetics - Absorption

A

Absorption: Antihistamines are well absorbed from the gut. Availability: Some available as parenteral and topical (ophthalmic and intranasal) preparations.

34
Q

Pharmacokinetics - Distribution

A

First-Generation: Distributed in all tissues, including the CNS. Second-Generation: Poor CNS penetration, less sedation.

35
Q

Pharmacokinetics - Metabolism

A

First-Generation: Metabolized by CYP enzymes. Second-Generation: Cetirizine and levocetirizine are excreted primarily in urine.

36
Q

Antihistamines in Children

A

Metabolism: Antihistamines are eliminated more rapidly in children than adults.

37
Q

Antihistamines in Breastfeeding

A

Effect: Excreted in breast milk, causing sedation in infants.

38
Q

Duration of Action

A

First-Generation: Duration of action is 4–6 hours. Second-Generation: Longer-acting.

39
Q

Clinical Indications - Allergic Reactions

A

Use: Symptomatic relief in allergic reactions and anaphylaxis.

40
Q

Clinical Indications - Motion Sickness

A

Drug Example: Promethazine is effective for motion sickness.

41
Q

Clinical Indications - Common Cold

A

Effect: Used for symptomatic relief of rhinorrhea.

42
Q

Clinical Indications - Insomnia

A

Use: First-generation antihistamines used due to their sedative properties.

43
Q

Adverse Effects - First-Generation

A

Sedation: More frequent with first-generation drugs. Other Effects: Dizziness, tinnitus, and incoordination; driving should be avoided.

44
Q

Adverse Effects - Antimuscarinic

A

Effects: Dry mouth, blurred vision, urinary retention (seen with first-generation).

45
Q

Adverse Effects - Second-Generation

A

Tolerability: Generally well-tolerated. QT Prolongation: Terfenadine (withdrawn due to this side effect).

Medicine (7 decks)

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