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Antihistaminic agents (Med Chem:- 2) Notes Download

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Histamine, a biologically active substance potentiates the inflammatory and immune responses of the body. It also regulates the physiological functions in thegut, and behaves as neurotransmitter. Anti-histaminic agents (or histamine antagonists) are the drugs that antagonise the action of histamine. On the basis of the type of H receptor targeted, antihistamines are divided into:1) H1-Antihistamines: They are used for treating allergicreactions and disorders mediated by mast cells. H1-antihistamines are sub-divided into two generations. The first generation H1-antihistamines have a central effect so are used as sedatives. The second generation H1-antihistamines have low central effects so are used as anti-allergenic drugs.2) H2-Antihistamines: They can reduce the production of stomach acid by reversibly blocking the H2-histamine receptors found in the parietal cells of gastric mucosa; thus, they are used in gastric reflux diseases.The pregnant women and children should avoid using most H1- and H2-antihistamines. First-generation H1-antihistamines are contraindicated in patients having angle-closure glaucoma and pyloric stenosis.1.1.2. HistamineHistamines are nitrogen containing organic compounds belonging to the group of amines. Histamines are produced in almost all the cells (present in an animal) during a local immune response. They regulate various physiological functions of the gut. In addition, histamines have also be en known to play a role in neurotransmission. Release of histamines is the initiating factor of any inflammatory response. Histamines are synthesised and released by basophils and mast cells (found in the nearby connective tissues) on stimulation (as a part of an immune response against foreign pathogens). They cause increased vascular (capillary) permeability for WBCs and other proteins to facilitate adequate invasion of foreign bodies within the tissues.

The details given in the chapter can be summarised as follows:
1) Histamine, a biologically active substance potentiates the inflammatory and
immune responses of the body.
2) Anti-histaminic agents (or histamine antagonists ) are the drugs that
antagonise the action of histamine.
3) The first generation H1-antihistamines have a central effect so are used as
sedatives.
4) The second generation H1-antihistamines have low central effects so are
used as anti-allergenic drugs.
5) Histamines are nitrogen containing organic compounds belonging to the
group of amines.
6) Histamines are synthesised and released by basophils and mast cells on
stimulation.
7) Diphenhydramine is a first generation antihistamine which is mainly used
for treating seasonal allergies.
8) Dimenhydrinate is a comb ination drug as it comprises of
diphenhydramine (53-55.5%) and 8-chlorotheophylline (not less than 44 –
47%) in a salt form, calculated on the dried basis.
9) Clemastine fumarate is the fumaric acid salt of clemastine. It is an
antihistamine having antimuscarinic and moderate sedative properties.
10) Diphenylpyraline is an antihistamine used for treating allergy by competing
with histamine to bind to the H1-receptor sites found on the effector cells.
11) Tripelennamine is an ethylenediamine derivative having anti -histaminergic
property.
12) Chlorcyclizine is a first generation antihistamine belonging to
phenylpiperazine class.
13) Meclizine hydrochloride is the hydrochloride salt form of meclizine, which
is a synthetic piperazine having anti -emetic, sedative and H 1-antagonistic
properties.
14) Buclizine hydrochloride is the hydrochloride salt form of buclizine, which
is a piperazine H 1-receptor antagonist having antiemetic and anti -vertigo
properties.
15) Chlorpheniramine maleate is a H1-recpetor antagonist.
16) Triprolidine hydrochloride is obtained by the reaction between equimolar
amounts of triprolidine and hydrogen chloride.
17) Phenindamine tartrate is a phenylalkylamine sympathomimetic amine.
18) Promethazine hydrochloride is the hydrochloride salt form
of promethazine, which is a phenothiazine derivative having antihistaminic,
sedative and antiemetic properties.

19) Trimeprazine tartrate (or alimemazine) is a tartrate salt and a
phenothiazine derivative, which is used as an antipruritic agent.
20) Cyproheptadine is a first generation antihistamine wh ich is used as an
appetite stimulant and for treating allergic rhinitis and urticaria.
21) Azatadine maleate is a first-generation antihistamine. It is the dimaleate salt
of azatadine.
22) Astemizole is a long-acting, non-sedating second generation antihistamine.
23) Loratadine is an azatadine derivative and a second generation H 1-receptor
antagonist.
24) Cetirizine (or zyrtec) is an orally active second generation H1-antagonist.
25) Levocetirizine is a third generation non-sedative antihistamine.
26) Cromolyn sodium is the sodium salt form of cromolyn, which is a mast cell
stabiliser having anti-inflammatory activity.
27) Cimetidine is a histamine congener and competitively inhibits the binding of
histamine to histamine H2-receptors.
28) Famotidine is a competitive H 2-receptor antagonist and its main
pharmacodynamic effect is the inhibition of gastric secretion.
29) Ranitidine is a non -imidazole blocker of those histamine receptors which
mediate gastric secretion (H2-receptors).
1.5. EXERCISE
1.5.1. True or False
1) Histamine, a biologically active substa nce potentiates the inflammatory and immune
responses of the body.
2) The first gene-antihistamines have a central effect so are used as hypnotics.
3) The second generation H1-antihistamines have low central effects so are used as
sedatives.
4) Histamines are nit rogen containing organic compounds belonging to the group of
amine
5) Diphenhydramine is a first generation antihistamine which is mainly used for
treating seasonal allergies.
6) Chlorcyclizine is a second generation antihistamine belonging to phenylpiperazine
class.
7) Buclizine hydrochloride is the hydrochloride salt form of buclizine, which is
a piperazine H2-receptor antagonist.
8) Chlorpheniramine maleate is a H1-recpetor antagonist.
9) Cimetidine is a histamine congener and competitively inhibits the binding of
histamine to histamine H2-receptors.
1.5.2. Fill in the Blanks
10) Anti-histaminic agents are the drugs that antagonise the action of ________.
11) Histamines are synthesised and released by _______ and __________ on
stimulation.

1.5.3. Very Short Answer Type Questions
1) Give the structure of histamine
2) What are the types of histamine receptors?
3) Give the structure of diphenhydramine.
4) What is the mechanism of action of clemastine fumarate?
5) Enlist uses of doxylamine succinate.
6) Give the mechanism of action of levocetrizine.
1.5.4. Short Answer Type Questions
1) Write a short note on histamine.
2) Explain histamine receptors and their distribution in the body.
3) Write a note on recent development of antihistaminic agents.
4) Give the synthesis of cimetidine.
5) Write a short note on ranitidine.
1.5.5. Long Answer Type Questions
1) Explain H 1-antagonists and give mechanism of action , uses a nd structure of
following drugs:
i) Doxylamine succinate
ii) Clemastine fumarate
iii) Meclizine hydrochloride
iv) Chlorpheniramine
v) Levocetrizine
2) Explain antihistaminic agents in detail and give its SAR.
3) Write about H 2-antagonists in d etails and mechanism of action of action, uses of
following drugs:
i) Cimetidine
ii) Famotidine
iii) Ranitidine

Description

Subject:- Medicinal chemistry 2

Semester:- Sem 5

Course:- Bachelor of pharmacy

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