AUTACOIDS AND THEIR ANTAGONISTS Histamine and Antihistaminics: - 1. Chemistry a. Histamine is a bioamine derived principally from dietary histidine, which is decarboxylated by L-histidine decarboxylase. b. Antihistaminics (histamine antagonists) can be classified as H 1 - or H 2 - receptor antagonists. (1) H 1 - receptor antagonists, the classic antihistaminic agents, are chemically classified as ethylenediamines (e.g., pyrilamine), alkylamines (e.g., brompheniramine [Dimetapp], chlorpheniramine [Chlor-Trimeton] ), ethanolamines (e.g., diphenhydramine [Benadryl ], clemastine [Tavist] ), piperazines (e.g. , hydroxyzine [Atarax, Vistari l ], cetirizine [Zyrtec] ), phenothiazines (e.g., promethazine [Phenergan] ), dibenzocycloheptenes (cyproheptadine [Periactin] ), phthalazinones (azelastine [Optivar, Astelin]), and piperidines (e.g., azatadine [Optimine] , loratadine [Claritin], desloratadine [Clarinex] , and fexofenadine [Allegra] ). Cetirizine, azelastine, loratadine, de
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