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PMC 622 > Antihistamines > Flashcards

Antihistamines Flashcards

1
Q

Histamine Overview

A
  1. Biologically active amine
  2. Hydrophilic
  3. Unevenly distributed in animal kingdom
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2
Q

Histamine Synthesis

A
  1. Histidine converted to Histamine via L-histidine decarboxylase
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3
Q

Histamine Metabolism

A
  1. Ring Methylation: form N-methylhistamine via histamine N-methyltransferase followed by oxidative deamination to N-methylimidazole acetic acid by MAO
  2. Oxidative Deamination and conjugation to ribose via nonspecific diamine oxidase yielding imidazoleacetic acid riboside
  3. Excreted in urine
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4
Q

Major Source of Histamine

A
  1. Chief storage site = mast cells
  2. In blood: basophils: slow turnover rate
  3. Non-mast cells: epidermis, gastric mucosa, neurons within the CNS: rapid turnover rate
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5
Q

Immunologic Histamine Release

A
  1. IgE bind and activate mast cells leading to immediate hypersensitivity reaction and allergic response
  2. Prostaglandins and leukotrienes are also mast cell mediators
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6
Q

Non-immunologic Release of Histamine

A
  1. Organic bases
  2. Tubocurarine, succinylcholine, morphine
  3. Kallidin and substance P
  4. Polymyxin B
  5. Anaphylotoxins: C3a and C5a
  6. Inflammatory mediators
  7. Gastric Enterochromafin like cells: ACh and Gastrin mediate histamine release
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7
Q

H1 Receptor

A
  1. Smooth muscle, endothelium, brain
  2. Gq
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8
Q

H2 Receptor

A
  1. Gastric mucosa, cardiac muscle, mast cells, brain
  2. Gs
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9
Q

H3 Receptor

A
  1. Presynaptic:brain, myenteric plexus
  2. Gi
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10
Q

H4 Receptor

A
  1. Eosinophils, Neutrophils, and CD4 T-Cells
  2. Gi
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11
Q

CV Effects of Histamine: Blood Vessels

A
  1. Vasodilation, Decreased BP, Flushing
  2. Reflex tachycardia
  3. Indirect: small doses of histamine—H1 receptor on endothelium = release of NO and Prostacyclin
  4. Direct: Higher dose of histamine—H2 receptor on VSMC
  5. Increased capillary permeability: H1 mediated, contraction of endothelial cells, and responsible for urticaria
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12
Q

CV Effects of Histamine: Heart

A
  1. Increased HR and Contraction
  2. Indirect: vasodilation —> reflex
  3. Direct: H2 receptors on cardiac muscle
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13
Q

Lung Effects of Histamine: Smooth Muscle

A
  1. Bronchoconstriction in asthmatics: H1 mediation on smooth muscle
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14
Q

GI Effects of Histamine

A
  1. Increased gastric acid secretion
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15
Q

Neural Effects of Histamine

A
  1. Sensory nerve stimulation: pruritus and pain and itching: H1 mediated
  2. Neurotransmitter in CNS: Increases wakefulness and inhibits appetite via H1
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16
Q

Cutaneous Effects of Histamine

A
  1. Triple response
  2. Red Spot: Vasodilation (H1 and H2)
  3. Flare: Axon reflex and indirect vasodilation (H1)
  4. Wheal: edematous exudation and capillary permeability (H1)
17
Q

Histamine Antagonists

A
  1. Physiological antagonists
  2. Release inhibitors
  3. Histamine receptor antagonists
18
Q

H1 Receptor Antagonists

A
  1. First generation: Chlorpheniramine, Diphenhydramine, Tripelennamine, Hydroxyzine, Promethazine
  2. Second generation: Loratadine, Fexofenadine, Cetirizine, Azelastine
19
Q

Pharmacokinetics: H1 Antagonists

A
  1. Rapidly absorbed, peak blood concentrations in 1-2 hrs
  2. Widely distributed throughout body: 1st generation rapidly enters CNS
  3. Extensive metabolism by microsomal systems in liver
  4. Several 2nd generation agents metabolized by CYP3A4
20
Q

H1 Antagonist Effects

A
  1. Inhibit more rapid vasodilator effects mediated by H1 activation on endothelial cells
  2. Strongly block increased capillary permeability and formation of edema and wheal
  3. Partially block histamine induced increase in HR by blocking H1 receptor mediated arterial VD
  4. Little reduction in bronchoconstriction
  5. No gastric acid suppression
  6. Some relief from atopic and contact dermatitis
  7. Suppress both flare component of triple response and itching caused by intradermal injection
  8. Antimuscarinic Effects and motion sickness prevention
21
Q

H1 Antagonist 1st Generation ADR’s

A
  1. CNS Stimulation at conventional doses
  2. CNS depression at therapeutic doses, potentiated by alcohol and CNS depressants
  3. Antimuscarinic effects: dry mouth and urinary retention
  4. GI: nausea, vomiting, diarrhea or constipation, anorexia
22
Q

H1 Antagonist Indications

A
  1. Acute rhinitis, urticaria, conjunctivitis
  2. Systemic anaphylaxis
  3. Motion sickness prophylaxis (1st Generation: dimenhydrinate, promethazine)
  4. Sedation
23
Q

H2 Receptor Antagonists

A
  1. Ranitidine, Famotidine, Nizatidine, Cimetidine
24
Q

H2 Receptor Antagonists: GI Effects

A
  1. Decreased Gastric Acid Secretion by inhibition of H2 receptors on parietal cells
25
Q

H2 Receptor Antagonists: Pharmacokinetics

A
  1. Significant renal excretion
  2. Cimetidine inhibits CYP450
  3. Increased gastric pH may alter vitamin/mineral absorption
26
Q

H2 Receptor Antagonists: ADR’s

A
  1. HA, GI, Fatigue, Drowsiness
  2. Cytopenias and CNS (rare)
27
Q

H2 Receptor Antagonists: Indications

A
  1. Promote peptic and duodenal ulcer healing
  2. Treat uncomplicated GERD
  3. Prophylaxis of stress ulcers

PMC 622 (3 decks)

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