Histamine, H1 Antagonists and H2 Antagonists

These flashcards cover important concepts related to histamine, H1 and H2 antagonists, including their mechanisms, differences, and clinical applications.

What is histamine biosynthesis primarily dependent on at pH 7.4?

L-Histidine predominates.

What is the initial step in an IgE mediated hypersensitivity reaction?

Exposure leads to IgE synthesis.

What are the physiological actions of H1 receptors?

Dilation of vascular smooth muscle, contraction of vascular endothelium, bronchoconstriction, and afferent sensitization.

What is the mechanism of action of H1 antagonists?

They act as competitive and reversible inhibitors or inverse agonists of H1 receptors.

What are the key differences between first and second generation H1 antagonists?

First generation is less selective and more sedating with more side effects; second generation is more selective, less sedating, and has fewer side effects.

What structural feature increases the potency of alkylamines in first generation H1 antagonists?

Halogenation at the para position.

What are the common uses of H2 antagonists?

Treatment of duodenal ulcers, gastric ulcers, pathologic hypersecretory conditions, and reflux esophagitis.

What is the clinical significance of cimetidine as an H2 antagonist?

It is a potent CYP3A4 inhibitor involved in multiple drug-drug interactions.

Why was ranitidine recalled from the market in April 2020?

Due to the presence of carcinogenic N-nitrosodimethylamine (NDMA) contaminant.

What distinguishes famotidine from cimetidine and ranitidine?

Famotidine is more potent and does not inhibit CYP enzymes.