H2 antagonists and PPIs

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What is the mechanism of action of H2 receptor antagonists?

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They competitively inhibit histamine H2 receptors on gastric parietal cells, reducing acid secretion.

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Why was cimetidine associated with CNS side effects?

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It is lipophilic, crosses the blood-brain barrier, and causes confusion, depression, and anxiety, especially in the elderly

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13 Terms

1
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What is the mechanism of action of H2 receptor antagonists?

They competitively inhibit histamine H2 receptors on gastric parietal cells, reducing acid secretion.

2
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Why was cimetidine associated with CNS side effects?

It is lipophilic, crosses the blood-brain barrier, and causes confusion, depression, and anxiety, especially in the elderly

3
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What replaced cimetidine to reduce CNS side effects?

Ranitidine, which is less lipophilic and does not cross the BBB, leading to fewer CNS adverse effects.

4
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What are key side effects and risks of H2-receptor antagonists?

  • CNS effects

  • Sexual dysfunction

  • Inhibition of CYP450 enzymes → drug interactions

  • Can mask symptoms of gastric cancer

5
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What is the mechanism of action of PPIs?

PPIs irreversibly inhibit the K+ ATPase (proton pump) in gastric parietal cells, preventing acid synthesis

6
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Why are PPIs considered prodrugs?

PPIs are inactive in the bloodstream. They are activated in the acidic environment of the stomach's parietal cells.

7
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Describe the activation process of a PPI inside the parietal cell

  • diffuses into parietal cell canaliculi

  • Protonation in acidic pH traps the drug

  • Acid catalysis forms an active drug

  • Forms covalent disulphide bonds with proton pump → irreversible inhibition

8
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What are the advantages of PPIs over H2 antagonists?

  • More potent

  • Longer duration

  • Irreversible inhibition

  • Target-specific activation (only in acid-producing cells)

9
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What are the disadvantages and side effects of long-term PPI use?

  • Hypomagnesemia

  • Increased risk of bone fractures (calcium malabsorption)

  • Infections (e.g., C. difficile, pneumonia)

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How do PPIs accumulate selectively at their target site?

Due to pH-dependent activation, they activate only in the acidic canaliculi, leading to site-specific action.

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What are important prescribing considerations with PPIs?

  • Take 30–60 minutes before food

  • Use lowest effective dose

  • Monitor in long-term users for electrolyte imbalance, B12 deficiency, and bone health

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Compare the onset and duration of action for H2 antagonists vs PPIs

  • H2 antagonists: Faster onset, shorter duration, reversible

  • PPIs: Slower onset, longer duration, irreversible

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When should an H2 antagonist be used instead of a PPI?

  • For mild symptoms

  • When rapid relief is needed