Pharmacology: Antihistamines

Question 1

Describe the pharmacological effects of Histamine

describe in terms of H1 and H2 receptors

Answer 1

H1 Effects
Nervous system- Pain and Itching
Bronchial smooth muscle-Bronchoconstriction
GIT smooth muscle- Contraction and Diarrhoea
CNS- Arousal and wakefulness,

H1 and H2 Effects
CVS- Decreased BP and Increased HR
Capillaries- Increased permeability, Oedema, Itch, Wheal and flare response, Increased dilation of small vessels
Exocrine secretion- Increased production of nasal and bronchial mucous

H2 and H3 effects
CNS- Inhibitory

H2 Effects
CVS- Increased contractility and Increased pacemaker rate
Stomach- Increased Gastric Acid secretion

Question 2

Describe the MOA and Pharmacokinetics of Antihistamines: H1-receptor antagonists

Answer 2

Mech. Of Action
Block the effect of histamine on the H1-receptor
They do not inhibit antigen/antibody reaction nor histamine release
(NB) First generation drugs have antimuscarinic effects

Pharmacokinetics
Peak blood concentration after 1-2 hours
available in topical forms such as creams, nasal sprays and eye drops

Question 3

Describe AGENTS/Antihistamines:H1-receptor antagonists

it has 3 generations

Answer 3

1st generation
Strongly sedating and block autonomic responses
1. Chlorpheniramine
2. Cyproheptadine
3. Promethazine
4. Hydroxyzine
2nd generation
Less sedating and have some cardiotoxicity
1. Loratadine
2. Cetirizine
3. Rupatadine
3rd generation
Non-sedatinf, no cardiotoxicity
1. Fexofenadine
2. Levocetirizine
3. Desloratadine

Question 4

Describe potential side effects of First Gen of Antihistamines

Answer 4

Dry mouth
Urinary retewntion
Appetite
Weight gain
Dizziness
Postural hypotension
Prolonged QT interval
Increased Ventricular Arrhythmias
Decreased alertness, cognition, learning, memory and psychomotor perfomance

Question 5

Describe the Clincal Uses of Antihistamines

Answer 5

Clinical Uses
[1]Allergic Reactions:
1. Allergic Rhinitis
2. Chronic Urticaria
3. Atopid dermatitis
4. Hay fever
5. Acute angioedema
6. Allergic Conjuctivitis
[2] Anti-Emetic:
1. Nausea and Vomiting(doxylamine should be safe in pregnancy)
[3]Sedation
[4]Appetite stimulation
[5]Antitussive(releave cough)
[6]Rhinorrhoea in colds and flu

Question 6

Describe the drug interactions of Antihistamines

Answer 6

Drug interactions(esp. 2nd generation)
1, Hepatic (CYP3A4) enzyme inhibitors- caution with Ketoconazole, Itraconazole, Macrolides, H2 antagonist(cimetidine)
2. Drugs with Arrhythmogenic potential: widening of QRS and prolongation of QT interval
3. Hepatic(CYP3A4) enzyme inducers- Rifampicin

Question 7

effects caused by blocking other receptors

Describe the effects caused by blocking H1 receptors

describe according to drug generations

Answer 7

First generation
1. Sedation
2. Toxic doses- agitation(anxiety or nervous excitement) , convulsions, coma

2nd and 3rd generations
1. Non-sedative

Question 8

effects caused by blocking other receptors

Describe the effects caused by blocking Muscarinic Receptor block according to drug generations.

Answer 8

First Generation
1. Prevent motion-sickness
2. Anti-vertigo
3. Anti-Parkinsonism
4. Decreased peripheral secretions

NO EFFECT on 2nd and 3rd generation drugs

Question 9

effects caused by blocking other receptors

All at once, describe the effects of blocking alpha-adrenergic receptor, serotonin receptor, and sodium channels.

Answer for 1st generation drugs

Answer 9

Alpha-adrenergic block- Hypotension (promethazine)

Serotonin receptor block- Appetite stimulation and weight gain (cyproheptadine)

Sodium channels block- Local Anaesthesia