Antihistamines Flashcards
Pharmacological effects of Histamine: Which receptors are found in the Nervous System and what is the effect of their simulation?
H1 - pain and itching
Pharmacological effects of Histamine: Which receptors are found in the Cardiovascular System and what is the effect of their simulation?
H1 & H2 - decreased BP, increased HR
H2 - increased contractility, increased pacemaker rate
Pharmacological effects of Histamine: Which receptors are found in the Bronchial smooth muscle and what is the effect of their simulation?
H1 - bronchoconstriction
Pharmacological effects of Histamine: Which receptors are found in the GIT smooth muscle and what is the effect of their simulation?
H1 - contraction, diarrhea
Pharmacological effects of Histamine: Which receptors are found in the Secretory tissue (stomach) and what is the effect of their simulation?
H2 - increased gastric acid secretion
Pharmacological effects of Histamine: Which receptors are found in the Triple response (capillaries) and what is the effect of their simulation?
H1 +++++, H2
= Wheal & flare response, 3 cell types involved:
1. Smooth muscle in microcirculation
2. Capillary and venular endothelium
3. Sensory nerve endings
= increased dilation of small vessels, increased permeability, edema and itch
Pharmacological effects of Histamine: Which receptors are found in the Exocrine Secretion and what is the effect of their simulation?
H1 & H2 - Production of nasal and bronchial mucous
Pharmacological effects of Histamine: Which receptors are found in the Central Nervous System and what is the effect of their simulation?
H1 - Excitation (arousal, wakefulness) + control of food and water intake
H2 + H3 - Inhibitory
Antihistamines are divided into what groups?
1st, 2nd, and 3rd generation
What are the characteristics of the 1st generation?
Strong sedative effects, block autonomic receptors.
enter CNS readily
What are the characteristics of the 2nd generation?
Less sedation and less complete distribution into the CNS , some are cardiotoxic
What are the characteristics of the 3rd generation?
Non-sedative and Non-cardiotoxic
Pharmacokinetics: How rapidly are the drugs absorbed?
Rapidly absorbed after oral administration
- peak blood concentration after 1-2 hours
Pharmacokinetics: How are several of the second generation antihistamines metabolized?
By the enzyme CYP3A4 which can lead to drug interactions
Pharmacokinetics: What is the duration of action?
Between 4-6 hours
- but Meclizine and several 2nd generation antihistamines have longer duration of action: 12-24 hours
Pharmacokinetics: What is the lipid solubility?
Newer generations are less lipid soluble than the older generations
What is the active metabolite of Hydroxyzine?
Cetirizine
What is the active metabolite of Terfenadine?
Fexofenadine
What is the active metabolite of Loratadine?
Desloratadine
What is the mechanism of action of the antihistamines?
Block the action of histamine by reversible competitive antagonism at H1-receptors, little effect on H2 or H3 receptors.
- Histamine-induced contraction of bronchial or GIT smooth muscle is completely blocked
- Effect on gastric acid secretion & heart are unmodified
Other effects of antihistamines (only 1st generation) not caused by H-1 receptor blockage on the CNS (H1)
- Sedation, some useful as sleep acids
- May cause excitation in children
- Toxic doses = stimulation, agitation, convulsions, coma
Other effects of antihistamines (only 1st generation) not caused by H-1 receptor blockage on the muscarinic cholinergic-receptor block
- Prevents motion sickness, decreased effectiveness when motion sickness is already present
- anti-vertigo
- anti-Parkinsonism
- decreased secretions (peripheral)
Other effects of antihistamines (only 1st generation) not caused by H-1 receptor blockage on alpha-adrenoreceptor block
Hypotension (promethazine)
Other effects of antihistamines (only 1st generation) not caused by H-1 receptor blockage on serotonin receptor block
- appetite stimulation / weight gain
Cyproheptadine
Other effects of antihistamines (only 1st generation) not caused by H-1 receptor blockage on Na channel block
Local anesthesia
Other effects of antihistamines (only 1st generation) not caused by H-1 receptor blockage: which drug inhibits mast cell release of histamine?
Cetirizine (active metabolite of hydroxyzine)
Which antihistamines should be used for allergic reactions?
More the second and third generation antihistamines
Which antihistamines should be used for allergic rhinitis
- Systemic antihistamines - mild cases with / without systemic nasal decongestants (pseudoephedrine), 2nd, 3rd generation less sedation
- Topical preparations = LEVOCABASTINE, AZELASTIN - seasonal allergic rhinitis
What is Azelastine used for?
Topical preparation antihistamine
- allergic rhinitis
- blocks H-1 receptors
- inhibits the release of inflammatory mediators from mast cells
What is Levocabastine used for?
Topical preparation antihistamine
- long acting
- short-term symptomatic treatment of seasonal allergic rhinitis
Which antihistamines should be used for atopic dermatitis?
Diphenhydramine
- 1st generation, sedative and itching effects
Which antihistamines should be used for Hay fever?
Alkylamines and second generation
- want to minimize sedation
In anaphylaxis, antihistamines are a secondary treatment to what?
Adrenaline
Which antihistamines should be used as antiemetics (nausea and vomiting)?
- Doxylamine*
- Cyclizine*
- Buclizine
Which antihistamines should be used for motion sickness and vestibular disturbances (vertigo)?
1st Generation for prevention of motion sickness: 1. Diphenhydramine 2. Promethazine 3. Cyclizine (& Meclizine)
What drugs should be used with antihistamines for more effective control of motion sickness and vestibular disturbances (vertigo)?
Ephedrine or amphetamine
Which antihistamines should be used for postoperative medication?
Promethazine
- sedation, antimuscarinic & antiemetic
Which antihistamines should be used for sedation?
- Diphenhydramine*
- Doxylamine
= induces sleep, may not however maintain sleep
Which antihistamines should be used as an antipruritic?
Diphenyhydramine
- reduces itching
Which antihistamines should be used as an appetite stimulant?
Cyproheptadine
Which antihistamines should be used for rhinorrhea in colds and flu?
This is primarily the antimuscarinic activity of the 1st generation antihistamines
= drying of secretions, specifically nasal secretions
(also cause dry mouth due to the same effect)
How are antihistamines used for rhinorrhea in colds and flu usually found?
In polycomponent cold and flu preparations as they:
- increase tenacity of secretions
- interfere with normal ciliary function
- cause drowsiness
Which antihistamine is the antidote to “serotonin syndrome”?
Cyproheptadine
Which antihistamines should be used for anti-Parkinsonism?
- Promethazine
2. Dophenhydramine
Which antihistamines should be used as antitussives?
Peripherally acting antihistamines (1st generation) - block cholinergic nerve impulses = dry up secretions = mucous plug formation
- PROMETHAZINE + DIPHENHYDRAMINE
When should antihistamines not be used as antitussives?
Not for productive coughs or asthmatics
Antihistamines as antitussives: Promethazine
Causes sedation
Antihistamines as antitussives: Chlorpheniramine
Causes less sedation than promethazine, however still causes sedation
Antihistamines as antitussives: Diphenhydramine
Potent antitussive and sedative
Antihistamines as antitussives: Mepyramine maleate
Less sedating
Antihistamines as antitussives: Tripolidine
Acts for up to 12 hours
Which antihistamines should be used for local anaesthesia?
- Diphenhydramine
- Promethazine
- when patients are allergic to the usual anaesthetics
