Allegra, Claritin, Zyrtec, etc are all allergy medicines treating similar symptoms with different active ingredients. How do they work differently?

Question

Allegra, Claritin, Zyrtec, etc are all allergy medicines treating similar symptoms with different active ingredients. How do they work differently?

In: Biology

Answer 1

These three drugs all target the H1 histamine receptor. Allegra (fexofenadine) and Zyrtec (cetirizine) are both selective antagonists; they occupy/block the active site of the H1 receptor, which prevents histamine from binding and triggering downstream effects. Claritin (loratadine) is an inverse agonist, meaning it does bind and trigger a kind of response through the H1 receptor, but this response actually dampens downstream effects.

E: to use the old lock-and-key analogy for receptors and their ligands, fexofenadine and cetirizine’s mechanism of action would be to tape the keyhole shut, or stick something in it that prevents the normal key from entering the lock or being turned. Loratadine is a key that is inserted and turned much like the normal one, but somehow closes the lock even further.