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ANTIHISTAMINIC
AGENTS
Prepared by: Komal Pramod Sathe
College: HSBPVTs GOI College of Pharmacy, Kashti
Roll No: 117
HISTAMINE
 Histamine is an autocoid, which are biological chemicals
which act like local hormone, have brief duration and near
their site of synthesis.
 Histamine was first discovered by Sir Henry Dale.
 Histamine is present in all mammalian tissues, stored in
inactive bound form and release as a result of antigen-
antibody reaction.
Structure of Histamine
Histamine is nitrogenous base. It is composed of an imidazole ring,
ethylamine side chain.
 Functions:
1. Mediator of inflammation and local immune responses
2. Regulating physiological function in the gut
3. Acts as a neurotransmitter.
Histamine biosynthesis:
Storage:
• Stored in mast cells in complex with heparin (Anticoagulant)
• Stored in basophils in complex with chondroitin.
Histamine as stored in mast cells are found in almost everywhere: Skin
& the mucosal cells of the bronchi, intestine, urinary tract & within the
neurons of CNS.
Release:
Histamine is released in response to wide variety of immune (antigen-
antibody) & non immune (bacterial products, cholinergic effects, physical
effect) stimuli.
Metabolism:
Released histamine is rapidly inactivated by metabolism via 2 pathways:
 Histamine receptors & their distribution in
the body:
Receptors Location Treatment
(antagonist)
H1 Throughout the body, specifically in smooth
muscles of bronchi, on vascular endothelial
cells, in the heart & the CNS.
Allergies, nausea, sleep
disorders
H2 Mainly in gastric parietal cells,
Low level: Vascular smooth muscle,
neutrophils, CNS, heart, uterus.
Stomach ulcers
H3 Found mostly in the CNS,
High level: Thalamus, Caudate nucleus &
cortex,
Low level: Small intestine, testis & prostate.
Unknown
H4 In the components of Immune system such
as the spleen, thymus & leukocytes.
Autoimmune diseases like
rheumatoid arthritis.
Antihistaminic agents
Pharmaceutical agents that inhibit the action of histamine are known as
antihistaminic agents.
 They block the attachment of histamine to histamine receptors.
 Inhibits the enzymatic activity of histidine decarboxylase.
 First antihistaminic agent: Piperoxan
H1- Antagonists
Mechanism of action:
• H1 antihistamines act as inverse agonist that combine with & stabilize the inactive form of
the H1 receptor, shifting the equilibrium towards the inactive state.
• They effectively antagonize the actions of histamine at H1 receptors.
• Some H1 antihistamines can also block histamine release. The concentration are
considerably higher than those required to produce significant histamine receptor
blockade. The antihistamines do not however, block antibody production or antigen-
antibody interactions.
Classification of H-1 antagonists:
A) First generation antihistamines (Classical):
1. Aminoalkyl ethers (Ethanolamines):
e.g. Diphenhydramine hydrochloride, Dimenhydrinate, Doxylamine Succinate, Clemastine
fumarate, Diphenylpyraline hydrochloride.
2. Ethylenediamines:
e.g. Tripelennamine hydrochloride.
3. Piperazines (Cyclizines):
e.g. Chlorcyclizine hydrochloride, Meclizine hydrochloride, Buclizine hydrochloride.
4. Propylamines (Monoaminopropyl OR Alkylamine derivatives):
e.g. Chlorpheniramine Maleate, Triprolidine hydrochloride, Phenidamine tartrate.
5. Phenothiazines:
e.g. Promethazine hydrochloride, Trimeperazine Tartrate.
6. Dibenzocycloheptenes & Dibenzocycloheptanes:
e.g. Cyproheptadine hydrochloride, Azatidine maleate.
B) Second- generation Antihistamines:
e.g. Astemizole, Loratadine, Cetirizine, Levocetrazine.
Inhibition of Histamine release: Mast cell stabilizers
e.g. Cromolyn sodium.
A) First generation antihistamines:
1. Aminoalkyl ethers (Ethanolamines):
2. Ethylenediamines:
3. Piperazines (Cyclizines):
4. Propylamines:
5. Phenothiazines:
6. Dibenzocycloheptenes & Dibenzocycloheptanes:
Azatidine Maleate
B) Second- generation Antihistamines:
Levocetirizine
Inhibition of Histamine Release:
Mast cell stabilizers:
Thank You

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Antihistamines

  • 1. ANTIHISTAMINIC AGENTS Prepared by: Komal Pramod Sathe College: HSBPVTs GOI College of Pharmacy, Kashti Roll No: 117
  • 2. HISTAMINE  Histamine is an autocoid, which are biological chemicals which act like local hormone, have brief duration and near their site of synthesis.  Histamine was first discovered by Sir Henry Dale.  Histamine is present in all mammalian tissues, stored in inactive bound form and release as a result of antigen- antibody reaction.
  • 3. Structure of Histamine Histamine is nitrogenous base. It is composed of an imidazole ring, ethylamine side chain.  Functions: 1. Mediator of inflammation and local immune responses 2. Regulating physiological function in the gut 3. Acts as a neurotransmitter.
  • 4. Histamine biosynthesis: Storage: • Stored in mast cells in complex with heparin (Anticoagulant) • Stored in basophils in complex with chondroitin. Histamine as stored in mast cells are found in almost everywhere: Skin & the mucosal cells of the bronchi, intestine, urinary tract & within the neurons of CNS.
  • 5. Release: Histamine is released in response to wide variety of immune (antigen- antibody) & non immune (bacterial products, cholinergic effects, physical effect) stimuli.
  • 6. Metabolism: Released histamine is rapidly inactivated by metabolism via 2 pathways:
  • 7.  Histamine receptors & their distribution in the body:
  • 8. Receptors Location Treatment (antagonist) H1 Throughout the body, specifically in smooth muscles of bronchi, on vascular endothelial cells, in the heart & the CNS. Allergies, nausea, sleep disorders H2 Mainly in gastric parietal cells, Low level: Vascular smooth muscle, neutrophils, CNS, heart, uterus. Stomach ulcers H3 Found mostly in the CNS, High level: Thalamus, Caudate nucleus & cortex, Low level: Small intestine, testis & prostate. Unknown H4 In the components of Immune system such as the spleen, thymus & leukocytes. Autoimmune diseases like rheumatoid arthritis.
  • 9. Antihistaminic agents Pharmaceutical agents that inhibit the action of histamine are known as antihistaminic agents.  They block the attachment of histamine to histamine receptors.  Inhibits the enzymatic activity of histidine decarboxylase.  First antihistaminic agent: Piperoxan
  • 10. H1- Antagonists Mechanism of action: • H1 antihistamines act as inverse agonist that combine with & stabilize the inactive form of the H1 receptor, shifting the equilibrium towards the inactive state. • They effectively antagonize the actions of histamine at H1 receptors. • Some H1 antihistamines can also block histamine release. The concentration are considerably higher than those required to produce significant histamine receptor blockade. The antihistamines do not however, block antibody production or antigen- antibody interactions.
  • 11. Classification of H-1 antagonists: A) First generation antihistamines (Classical): 1. Aminoalkyl ethers (Ethanolamines): e.g. Diphenhydramine hydrochloride, Dimenhydrinate, Doxylamine Succinate, Clemastine fumarate, Diphenylpyraline hydrochloride. 2. Ethylenediamines: e.g. Tripelennamine hydrochloride. 3. Piperazines (Cyclizines): e.g. Chlorcyclizine hydrochloride, Meclizine hydrochloride, Buclizine hydrochloride. 4. Propylamines (Monoaminopropyl OR Alkylamine derivatives): e.g. Chlorpheniramine Maleate, Triprolidine hydrochloride, Phenidamine tartrate. 5. Phenothiazines: e.g. Promethazine hydrochloride, Trimeperazine Tartrate. 6. Dibenzocycloheptenes & Dibenzocycloheptanes: e.g. Cyproheptadine hydrochloride, Azatidine maleate.
  • 12. B) Second- generation Antihistamines: e.g. Astemizole, Loratadine, Cetirizine, Levocetrazine. Inhibition of Histamine release: Mast cell stabilizers e.g. Cromolyn sodium.
  • 13. A) First generation antihistamines: 1. Aminoalkyl ethers (Ethanolamines):
  • 14.
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  • 18.
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  • 22.
  • 23. 6. Dibenzocycloheptenes & Dibenzocycloheptanes:
  • 25. B) Second- generation Antihistamines:
  • 27. Inhibition of Histamine Release: Mast cell stabilizers: