Antihistamine agents:The term antihistamine historically has referred to drugs that antagonize the actions of histamine at H1 receptors rather than H2 receptors.
H1 –antagonist : block and opposite effect. H1 antagonists may be defined as drugs that competitively inhibit the action of histamine on tissues containing H1 receptors.
Classification of H1 – receptors antagonist: the H1 antagonist are now commonly subdivided into two broad groups-
- First generation or classical antihistamines
- Tricyclie dibenzoxipins.
- Ethanolamines : E.G, carbinoxamine maleate, clemastine fumerate, diphenhydramine HCl, dimenhydranate.
- Ethylenediamines : E.g; pyrilamine maleate, tripepennamine HCL/ citrate.
- Alkylamines : E.g; chlorophenaramine maleate, bromophencramine maleate.
- Piperazines : E.g; hydroxyzine hcl, hydroxyzine pamoate, cyclizine hcl, cyclizine lactate, meclizine hcl.
- Phenothiazines : Eg ; promithazine hcl.
- Piperidines : E.g; cyproheptadine hcl, phenindamine tartrate.
Second generation agents (non-sedating) .
- Alkylamines : E.g; acrivastine.
- Piperazines : E.g cetirizine hcl
- Pthalazinones : E.g, azelastine hcl.
- Piperidines : E.g levocabastine hcl, loratidine, desloralidine, ebastine, mizolastine, fexotenadine.
Pharmacological consideration of H1 antagonist:
- Sneezing
- Rhinorrtea
- Itching of eye , nose throat
- Hay fener
- Pollinosis
- Chronic, idiopathic, urticaria.
H2- receptors blockers : Are those druges which inhibit effect are not acceptable.
E.g, Ranitidine , cimetidine , famotidine, nizatidine.
The action of histamine by blocking H2 receptors.
In many of the first generation or classical antihistamines the terminal nitrogen atom is a simple dim ethyl amino moiety. In all classes the amino moiety is basic with PKa ranging from 8.5 to 10 and thus is presumed to be prorogated when bound on the receptors. The moiety is also important in the development of stable solid dosage forms through salt formation.
In some series branching of the carbon chain results in reduced antihistamine activity. There are exceptions however as evidenced by promethazine, which has greater activity than its non branched counter part.