H1 Antihistamines: Current Status and Future Directions

Table 2 H₁ Antihistamines: Chemical and Functional Classification

Functional Class
Chemical Class	First Generation	Second Generation
Alkylamines	Brompheniramine, chlorpheniramine, dimethindene,*,^‡ pheniramine,^‡ triprolidine	Acrivastine*
Piperazines	Buclizine, cyclizine, hydroxyzine,* meclizine, oxatomide**	Cetirizine,* levocetirizine*
Piperidines	Azatadine, cyproheptadine, diphenylpyraline, ketotifen^‡	Astemizole, bilastine, desloratadine,* ebastine,** fexofenadine,* levocabastine,^‡ loratadine,* mizolastine, olopatadine,^‡ rupatadine, terfenadine,*
Ethanolamines	Carbinoxamine, clemastine, dimenhydrinate, diphenhydramine, doxylamine, phenyltoloxamine**	--
Ethylenediamines	Antazoline, pyrilamine, tripelennamine	--
Phenothiazines	Methdilazine, promethazine	--
Other	Doxepin^§	Azelastine,^‡ emedastine,^‡ epinastine^‡

*Acrivastine is related tripolidine. Cetirizine is a metabolite of hydroxyzine, levocetirizine is an enantiomer of cetirizine, desloratadine is a metabolite of loratadine, and fexofenadine is a metabolite of terfenadine.
**In the United States, these H₁ antihistamines are not yet approved, have never been approved, or have had approval withdrawn.
^‡The H₁ antihistamines azelastine, emedastine, epinastine, ketotifen, levocabastine, and olopatadine are available in ophthalmic formulations; and azelastine, dimethindene, levocabastine, and olopatadine are available in intranasal formulations. In some countries, azelastine, dimethindene, ketotifen, and olopatadine are also available in oral formulations.
^§Doxepin has H₁ and H₂ antihistamine activities and is also classified as a tricyclic antidepressant.
Adapted from Simons [2].

ISSN: 1939-4551