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Pharmacology II - Winter Quarter > Lecture 25 - H1 and H2 Histamine Antagonists > Flashcards

Lecture 25 - H1 and H2 Histamine Antagonists Flashcards

1
Q

First Generation H1 Antagonists - Sedating Antihistamines

A

Doxylamine, Promethazine, and Hydroxyzine

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2
Q

First Generation H1 Antagonists - Moderately-Sedating Antihistamines

A

Diphenhydramine, Dimenhydrinate, Chlorpheniramine maleate, and Meclizine

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3
Q

Second Generation (Non-sedating) H1 Antagonists

A

Loratadine/Desloratidine, Certirizine, Fexofenadine, and Azelastine

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4
Q

H2 Antagonists

A

Cimetidine (Inhibit microsomal metabolism of drugs and anti-estrogen at high levels), Famotidine, and Nizatidine

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5
Q

Histamine functions

A
  • Mediator of physiological responses to tissue or cellular injury
  • Mediator of inflammatory responses and allergic reaction
  • Regulates cell growth and repair
  • Regulate gastric acid secretion
  • Neurotransmitter in the CNS
  • Possibly regulate cardiac functions
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6
Q

Storage and release of histamine

A

Stored primarily in mast cells and lesser extent basophils.
- Release mediated through antigen binding to surface IgE antibodies
- Free histamine binds to H1 receptors to cause pharmocological effects

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7
Q

Triple response of histamine

A

1) “Red spot” localization - dilation of blood vessels
2) “Flare” - dilation of neighboring arterioles
3) “Wheal” - increased capillary permeability

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8
Q

Gastric Acid Secretion by Histamine

A

Histamine is a key mediator of acid secretion in response to stress, vagal stimulation, gastrin, and cholinergic. Histamine acts on H2 receptors on the parietal cells of the GI. This affects the release of HCl from these cells.

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9
Q

Physiological effects of histamine

A
  • Angioedema: Swelling of the tongue and upper neck
  • Dilation of small blood vessels results in flushing, lower peripheral resistance, and fall in blood pressure.
  • Increase in capillary permeability causes leakage of fluid and protein into the extravascular space.
  • Stimulation of peripheral nerve endings results in pain, burning, and itching.
  • HISTAMINE SHOCK (Explanation on another slide)
  • BRONCHIAL CONSTRICTION (Explanation on another slide)
  • GASTRIC ACID SECRETION (Explanation on another slide)
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10
Q

Histamine shock

A

Large doses or releases during anaphylaxis causes a pronounced drop in blood pressure due to intense vasodilation and leakage of fluid into the extravascular space. Resembles traumatic, septic, or hemorrhagic shock and may be life threatening.

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11
Q

Bronchial Constriction by Histamine

A
  • Causes bronchoconstriction
  • Amplified effect in asthmatics, who are very sensitive to histamine.
  • NOTE: Asthmatic and anaphylactic bronchospasm not effectively treated by antihistamines alone due to other mechanisms; use sympathomimetics or methylxanthines with antihistamines.
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12
Q

Types of Histamine Receptors

A

H1 receptor
- Primarily in the skin, blood vessels, heart, airway, and CNS
- Mediates rapid vasodilation, increased capillary permeability, irritation of peripheral nerve endings, bronchoconstriction.
- “Antihistamines” block H1 for allergies, rhinitis, etc.

H2 receptor
- Gastrointestinal tract, heart, brain, and various blood vessels
- Mediate gastric acid secretion
- Antagonists (like cimetidine) reduce gastric acid secretion and commonly used for peptic ulcer disease.
- Can mediate some effects of histamine during immediate hypersensitivity reactions.

Also H3 and H4

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13
Q

Pharmacologic Effects of H1 histamine blockers

A

Antagonize most H1-mediated effects of histamine:
-Pain
-Itch
-Flare
-Vasodilation
-Increased vascular permeability
-Congestion

DOES NOT PREVENT RELEASE OF HISTAMINE OR OTHER MEDIATORS OF INFLAMMATION AND ALLERGY.

DOES NOT REVERSE ANAPHYLACTIC BRONCHOSPASM

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14
Q

Therapeutic Uses of H1 Antihistamine

A
  • Treatment of allergies
  • Treatment of common cold
  • Use as Antiemetics
  • Use as Sedative and Sleep Aids
  • Antisecretory agents
  • Treat Parkinsons Disease
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15
Q

H1 Antihistamine for allegies

A
  • Hay fever (seasonal) rhinitis
  • Relief of sneezing, wheezing, itching of eye, nose, throat, rhinorrhea
  • Effective in certain types of allergic dermatitis (urticaria) - certain agents, particularly diphenhydramine are included in topical “anti-itch” meds.
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16
Q

H1 Antihistamine for common cold

A
  • Antihistamines alleviate nasal irritation
  • DO NOT ALTER course of cold.
17
Q

H1 Antihistamine as antiemetics

A
  • Dimenhydrinate, meclizine, etc. used to prevent + treat motion sickness and vertigo.
  • Antihistamines can treat nausea and vomiting during pregnancy
  • Cyclizine, promethazine, and hydroxyzine may have more general antiemetic properties.
18
Q

H1 Antihistamine as sedatives and sleep aids

A
  • Most produce sedation.
  • Hydroxyzine used as a sedative.
  • Diphenhydramine and doxylamine: over the counter
19
Q

H1 Antihistamine as antisecretory agent

A
  • Decrease respiratory secretions, especially diphenhydramine
20
Q

H1 Antihistamine to treat Parkinsons

A
  • Anticholinergic activity (diphenhydramine) can treat certain parkinsonism
21
Q

Side effects and Toxicities of H1 antihistamines

A

1) Anticholinergic side effects: dry mouth, dry hot skin, constipation, urine retention, loss of visual accommodation, etc.
2) Sedation, drowsiness, confusion, amnesia, behavioral disturbances at therapeutic doses.
For 1 and 2, pronounced in elderly patients; second generation H1 have no sedation
3) Paradoxical CNS stimulation for some, especially children. ADHD patients should use 2nd gen as it doesn’t cross BBB
4) Possible teratogenic effects; avoid if pregnant
5) Allergic reactions - topical use
6) Lower seizure threshold

22
Q

Acute poisoning-overdose w/ H1 antihistamines

A

1) More likely with children
2) Similar to those of atropine poisoning:
– excitation
– hallucinations
– ataxia
– incoordination
– convulsion
– muscular tremors
– uncontrollable clonic/tonic jerky motions
– fixed dilated pupils
– flushed face
– fever
– coma
– cardio-respiratory collapse and death.
3) Treatment with cholinesterase inhibitor PHYSOSTIGMINE + anti-seizure cardiovasc meds if needed

23
Q

Drug interactions with H1 antihistamines

A
  • Potentiation of effects of CNS depressants, alcohol, barbiturates, opioids, benzodiazepines.
  • Terfenadine or Astemizole with erythromycin, ketoconazole, or itraconazole = life threatening arrhythmias
  • Antihistamine can interfere with allergy testing
24
Q

Pharmacologic effects of H2 antihistamines

A
  • Inhibition of gastric acid secretion by histamine in response to vagal stimulation, gastrin secretion (including Zollinger - Ellison syndrome), and stress
  • Little effects on H1 receptors
25
Q

Treatment capabilities of H2 antihistamines

A
  • Duodenal and gastric ulcers
  • Gastroesophageal reflex
  • Zollinger-Ellison syndrome
  • Preoperatively to lessen aspiration damage
  • Prevention of stress ulcers

Pharmacology II - Winter Quarter (5 decks)

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