Histamine and Anti-Histaminic Drugs- Part IV : Anti-histaminic Drugs

In the concluding part of this series of blogs based on histamine and anti-histaminic drugs, i would like to disclose few informative facts about histamine antagonists.

4) Histamine Antagonists (Anti-histaminic drugs)

Histamine antagonists or anti-histaminics may be divided into 3 broad categories based upon the type of receptor for which they have affinity.

4.1) H1 antagonists-

4.1.1) Pharmacological Roles—

a) Anti-allergic and anti-asthamatic role:

Since, histamines have the activity of bronchoconstriction, especially worsening the condition of allergic diseases like asthma during initial phase, anti-histaminics prevent this activity and impedes bronchoconstriction caused by histamine.

b) Anti-pruritic Action:

Anti-histaminic drugs prevent the pruritis and itching caused by histamine at sensory nerve endings. Hence, it reduces the irritation by minimizes the binding of histamine to H1 receptors at these ends.

c) Effect on Central Nervous System (CNS):

This category of drugs induce depression, sedative effect etc. Some individuals experience insomnia and restlessness by these drugs. Overdose induced toxicity may result in convulsions and excitement in the patients.

d) Other antagonizing actions:

They effectively block histamine induced effects like bronchoconstriction, smooth muscle contraction, vasodilatation, adrenaline release etc.

4.1.2) Examples of H1 antagonists:

4.1.2.1) Diphenhydramine (25-50 mg oral; highly sedative)

4.1.2.2) Meclizine (25-50 mg oral; moderately sedative)

4.1.2.3) Buclizine (25-50 mg oral; moderately sedative)

4.1.2.4) Promethazine (25-50 mg oral; highly sedative)

4.1.2.5) Pheniramine (25-50 mg oral; moderately sedative)

4.1.2.6) Chlorpheniramine (2-4 mg oral; mild sedative)

4.1.2.7) Cyclizine (25-50 mg oral; mild sedative)

4.1.2.8) Terfenadine (60-180 mg oral; 2nd generation drug )

4.1.2.9) Fexofenadine (120-180 mg oral; 2nd generation drug)

4.1.2.10) Cetirizine (10 mg oral; 2nd generation drug)

4.1.2.11) Loratidine (10 mg oral; 2nd generation drug)

4.1.2.12) Desloratidine (5 mg oral; 2nd generation drug)

4.1.3) Few 2nd generation drugs- Some anti-histaminics don’t have the ability to penetrate the blood-brain barrier and induce CNS response resulting in sedative action. Such drugs have been categorized as 2nd generation anti-histaminic drugs

4.1.3.1) Terfenatidine- It has high specificity for H1 receptors and acts by blockinf the K+ ion channels. This often produces torsades-de-pointes and ultimately resulting in cardiac arrhythmia.

4.1.3.2) Fexofenatidine- It’s an active metabolite of Terfenatidine and doesn’t blocks delayed rectifier K+ channels and is largely free from arrhythmic effect.

4.1.3.3) Loratidine- It is a H 1 antagonist and lacks CNS depressive activity and sedation. It is also well tolerated and doesn’t give rise to atopic dermatitis.

4.1.3.4) Cetirizine- It has a high affinity for peripheral H 1 receptors and penetrates Blood-Brain-Barrier poorly. In high doses, it causes somnolence.

4.2) H2 antagonists:

This class of drugs has specific affinity for H2 receptors, as the name indicates. However, they are largely used for gastric dysfunctions and pathology like ulcerative conditions in stomach etc. They function by acting as a competitive inhibitor in binding to H2 receptors. However, they are associated with few adverse drug reactions like constipation, hypotension etc.

4.2.1) Examples of H2 antagonist:

4.2.1.1) Ranitidine
4.2.1.2) Cimetidine
4.2.1.3) Famotidine

4.3) H3 antagonists:

Not much has been known about action of H3 antagonist and these drugs have a very little clinical utility.

4.3.1)Examples of H3 antagonists:

a) Thioperamide

b) Burimamide

I hope you might have enjoyed reading these blogs based on histamine and anti-histaminics and also found them informative simultaneously.