H1 antagonist
Class Description Examples
Ethylenediamines Ethylenediamines were the first group of clinically effective H 1 -antihistamines developed Mepyramine ( pyrilamine ) Chloropyramine Antazoline Tripelennamine
Ethanolamines Diphenhydramine was the prototypical agent in this group . Significant anticholinergic adverse effects , as well as sedation , are observed in this group but the incidence of gastrointestinal adverse effects is relatively low Diphenhydramine Carbinoxamine Doxylamine Orphenadrine Bromazine Clemastine Dimenhydrinate
Alkylamines The isomerism is a significant factor in the activity of the agents in this group . E -triprolidine , for example , is 1000-fold more potent than Z -triprolidine . This difference relates to the positioning and fit of the molecules in the histamine H 1 -receptor binding site . Alkylamines are considered to have relatively fewer sedative and gastrointestinal adverse effects , but relatively greater incidence of paradoxical central nervous system ( CNS ) stimulation Pheniramine Chlorphenamine ( chlorpheniramine ) Dexchlorpheniramine Dexbrompheniramine Brompheniramine Triprolidine Dimetindene
Piperazines These compounds are structurally related to the ethylenediamines and the ethanolamines , and produce significant anticholinergic adverse effects with the exception of hydroxyzine , which has low to no affinity for muscarinic acetylcholine receptors and therefore produces negligible anticholinergic side-effects . Compounds from this group are often used for motion sickness , vertigo , nausea , and vomiting . The second-generation H 1 -antihistamine cetirizine also belongs to this chemical group Cyclizine Chlorcyclizine Hydroxyzine Meclizine
Tricyclics and Tetracyclics These compounds differ from the phenothiazine antipsychotics in the ring-substitution and chain characteristics . They are also structurally related to the tricyclic antidepressants ( and tetracyclics ) , explaining the H 1 -antihistaminergic adverse effects of those three drug classes and also the poor tolerability profile of tricyclic H 1 -antihistamines . The second-generation H 1 -antihistamine loratadine was derived from compounds in this group Promethazine Alimemazine ( trimeprazine ) Cyproheptadine
First-generation ( non-selective ) -- Classes
The first H1-antihistamine discovered was piperoxan , by Ernest Fourneau and Daniel Bovet ( 1933 ) in their efforts to develop a guinea pig animal model for anaphylaxis at the Pasteur Institute in Paris . [ 6 ] Bovet went on to win the 1957 Nobel Prize in Physiology or Medicine for his contribution . Following their discovery , the first-generation H1-antihistamines were developed in the following decades . They can be classified on the basis of chemical structure , and agents within these groups have similar properties .
H antagonists, also called H blockers, are a class of medications that block the action of histamine at the H receptor, helping to relieve allergic reactions. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines. In common use, the term antihistamine refers only to H-antihistamines. Virtually all H-antihistamines function as inverse agonists at the histamine H-receptor, as opposed to neutral antagonists, as was previously believed.