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I've been researching H1-antihistamines for personal reasons. And I'm trying to find out if H1-antihistamines are sorted based on pharmacology.

I understand that H1-antihistamines have up to three generations. And with each new generation, the drugs get safer and have less side effects.

According to Wikipedia, the antihistamines are also classified 5 classes based on their chemical structures.

H1-Antagonist

But further research shows that each antihistamine works differently in suppressing allergic responses and possesses varying potency.

Loratadine, for example,

acts as a selective inverse agonist of peripheral histamine H1 receptors.

Loratadine also shows anti-inflammatory properties independent of H1 receptors. The effect is exhibited through suppression of the NF-κB pathway, and by regulating the release of cytokines and chemokines, thereby regulating the recruitment of inflammatory cells.

Bepotastine, on the other hand,

is a direct H1-receptor antagonist that inhibits the release of histamine from mast cells.

And Azelastine has a triple mode of action:

Anti-histamine effect, Mast-cell stabilizing effect and Anti-inflammatory effect.

When given orally, antihistamines are also absorbed in various ways.

Loratadine, according to Wikipedia,

is given orally, is well absorbed from the gastrointestinal tract, and has rapid first-pass hepatic metabolism.

Ebastine undergoes similar absorption. But other antihistamines, notably, Cetirizine and Olopatadine, do not undergo hepatic metabolism.

Loratadine Bepotastine Azelastine Cetirizine [Olopatadine][6]

What I'm wondering is: Do the antihistamines from the same generation (second generation, for example) have official classifications that group different antihistamines with similar pharmacodynamics together?

I've scoured NIH medical articles and haven't been able to find such classification. But I'm unsure if I'm simply not thorough in my search or if this type of classification doesn't exist for medical professionals.

[6]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5391828/#:~:text=In%20the%20pharmacokinetic%20study%2C%20olopatadine,plasma%20concentration%20%5BTmax%5D).

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The following table taken from Continuous versus on-demand pharmacotherapy of allergic rhinitis: evidence and practice by Laekeman et al., lists H1 antihistamines with their respective half-life and duration of action. In this table, you can see the antihistamines are grouped based on their class: Ethanolamines, Alkylamines, Piperazines, Phenothiazines, Phtalazinones, and Piperidines.

H1 Antihistamines: class, half-life, duration of action

These classes seem to be independent of the generation of the drugs--Alkylamines and Piperazines have both 1st and 2nd gen antihistamines.

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  • The article helps! Thank you!
    – Stardust
    Jan 17, 2022 at 11:01

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