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Antihistamines, Cetrizine, First generation anti-histaminics, Terfenadine,…
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Cetrizine
Cetirizine is an active metabolite of first generation antihistaminic drug hydroxyzine.
All second generation antihistaminics are metabolized to active products
except cetirizine and mizolastine.
Cetirizine also possess additional anti-allergic mechanism
like inhibition of release of cytotoxic mediators from platelets
and inhibition of chemotaxis.
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Terfenadine
Terfenadine is the fastest acting antihistaminic drug.
In overdose, it blocks cardiac delayed rectifier K+ channels
and may result in polymorphic ventricular tachycardia(torsa de pointes)
manifested as prolongation of QTc interval.
Use of terfenadine with microsomal enzyme inhibitors
like ketoconazole, erythromycin, clarithromycin and itraconazole
increases the risk of arrythmia.
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Olapatadine
Olapatadine is recently approved topical H1 antihistaminic
used as nasal spray for seasonal allergic rhinitis.
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Histamine receptors
Histamine acts mainly on the H1 and H2 receptors. Recently H3(presynaptic) and H4 receptors have also been isolated.
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Triple response
Intradermal injection of histamine may result in triple response consisting of
red reaction due to vasodilation,
wheal due to exudation of fluid due to increased permeability and
flare(spreading redness due to axon reflex).
It is primarily at H1 response.
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Maintaining wakefullness
Histamine synthesized within the brain stimulates reticular activating system
and maintains wakefullness through H1 receptors.
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H4 receptors
H4 receptors are present in hematopoeitic cells like eosinophils, basophils and mast cells.
These promote chemotaxis.
Antagonists of these receptors are being developed for allergic conditions.
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H1 receptor:
H1 receptor: mediates allergy and inflammation, bronchoconstriction and promote wakefullness.
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Antagonists: cetrizine , promethazine used in allergies
H2 receptor:
H2 receptor stimulates gastric acid secretion.
Antagonist like ranitidine, famotidine used in dyspepsia.
H3 receptor:
H3 receptor: presynaptic and inhibit release of histamine.
Inverse agonist: tiprolisant used in narcolepsy.
H4 receptor:
H4 receptor: present on eosinophils, basophils and mast cells.
Promote chemotaxis.
H1 antihistaminics
These drugs act as competitive antagonists at H1 receptors.
These may be classified into first generation and second generation compounds
on the basis of CNS penetration and anticholinergic properties.
Highly sedating drugs
are diphenhydramine, dimenhydrinate, promethazine, hydroxyzine and doxepin.
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Mildly sedating drugs
are chlorpheniramine, mepyramine, cyclizine and clemastine.
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fexofenadine
Terfenadine is metabolized to an active metabolite “fexofenadine”.
It is available as separate drug that lacks K+ channel blocking property.
Astemizole
Astemizole is the slowest and longest acting agent
and possess arrythmogenic property similar to terfenadine.
Therefore it should not be administered with
ketoconazole, clarithromycin, erythromycin and itraconazole.
Loratadine
Loratadine is another long acting second generation antihistamine
and is metabolized to desloratadine(available as separate drug).
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Doxepin
Doxepin is a tricyclic antidepressant
having most potent H1 blocking action 800 times
more potent than diphenhydramine.
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Cyproheptadine
May increase appetite, cause weight gain
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