1. Histamines and Antihistamines
Synthesis
Pharmacology
Classification
Drugs
Brands

2. Histamine Pharmacology
Demonstrated to be a natural constituent of
mammalian tissues (1927)
Involved in inflammatory and anaphylactic reactions.
Local application causes swelling redness, and edema, mimicking a mild inflammatory reaction.
Large systemic doses leads to profound vascular
changes similar to those seen after shock or anaphylactic origin.

3. Histamine formation
Synthesized in mammalian tissues by
decarboxylation of the amino acid l-histidine.
Histamines are stored in mast cells,skin,gastric and intestinal mucosa,lungs, liver and placenta,brain, epidermis,gastric mucosa etc.

4. Conditions That Release Histamine
1.Tissue injury
2. Allergic reactions
3. Drugs and other foreign compounds

5. Histamine receptors
1. H1 receptors: mediate effects on smooth muscle leading to vasodilation, increased vascular permeability, and contraction of nonvascular smooth muscle.
2. H2 receptors: mediate histamine stimulation of
gastric acid secretion and may be involved in
cardiac stimulation.
3. H3 receptors: feedback inhibitors in CNS,
gastrointestinal tract, lung, heart.

6. Pharmacological Effects: Exocrine Glands
Gastric glands
Salivary glands
Sweat glands
Pancreas
Bronchial glands
Lacrimal glands
Secretion
All Secretions

7. Pharmacological Effects: Arterioles, Capillaries & Venules
Vasodilation
Increased permeability, (edema)
Systemic hypotension

8. Pharmacological Effects Vascular Smooth Muscles
Bronchial tree
Gastrointestinal tract
Uterus
Contraction

9. Antihistamines Daniel Bovet, Nobel Prize 1944
Synthesized first antihistamines
Compounds appeared to prevent the binding of histamine to H1 receptors through their structural similarities

10. General Mechanism of Action of Antihistamines
Blocks action of histamine at receptor
Competes with histamine for binding
Displaces histamine from receptor
Most beneficial when given early

11. Therapeutic Uses of H1 Antihistamine Blockers
1. Allergic rhinitis, relieves rhinorrhea, sneezing, and itching of eyes
and nasal mucosa.
2. Common cold: palliative, dries out the nasal mucosa. Often
combined with nasal decongestant and analgesics.
3. Allergic dermatoses: can control itching associated with insect bites.
4. Outpatient procedures for preanesthetic sedation and prevention of
nausea and vomiting
5. Antiemetic: prevention or treatment of nausea and vomiting
6. Hypnotics: limited value.
7. Other uses:
a. Reduction of tremors and muscle rigidity in Parkinson's disease
b. Treatment of migraine headaches

12. Classification of Antihistamines
First generation
1) Highly sedative.
2) Moderately sedative
3) Mild sedative
Second Generation /Non sedative antihistamines

13. First generation antihistamines
Highly sedative:-
Drugs are Diphenhydramine,Dimenhydrinate, Promethazine,Hydroxyzine etc.
Brands
Diphenhydramine:-Amydramine,Histalix Stopkcough,Viscordil,Exylin etc.
Dimenhydrinate:- Dizinil
Promethazine:- Phenergan, Histazin Histaloc etc.
Hydroxyzine:- Atarax

14. First generation antihistamines
Moderately sedative
Drugs:- Pheniramine, Cyproheptidine, Meclizine, Cinnarizine etc.
Brands:-
Pheniramine:- Avil
Meclizine:- Longifene, Diligan
Cinnarizine:- Stugeron
Cyproheptidine:- Periactin
Ketotifen:- Zaditen

15. First generation antihistamines
Mild sedatives
Drugs:- Chlorpheniramine,Dexchlorpheniramine,
Dimethindene,Triprolidine,Cyclizine, Clemastine etc.
Brands:-
Chlorpheniramine:- Allerfin, Chlorhistol,Histop etc
Dexchlorpheniramine:- Polaramine
Dimethindene:- Fenistil
Triprolidine:- Actifed,Sudafed,Sedofan etc.
Clemastine:- Tavegyl

16. Second Generation /Non Sedative Antihistamines
Drugs:- Fexofenadine,Loratidine, Desloratidine, Cetrizine, Levocetrizine,Azelastine, Mizolastine Ebastine, Rupatadine etc.
Brands:-
Fexofenadine:- Telfast
Loratidine:-Claritine,Restamine, Tidilor,clara,Lorine, etc
Desloratidine:-Aerius
Cetrizine:-Zyrtec,Zeran,Cetralon,Artiz, Alerid etc.
Levocetrizine:- Xyzal

17. Thank You
By,
Philip K Mathew
Clinical instructor
University of Nizwa