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Cetirizine
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Histamine
H1 receptor
- allergic
reactions H2 receptor
- gastric acid secretion
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Antihistamines
- Compete
with histamine and block histamine H1 receptor
- Conventional antihistamines may also block other receptors, i.e.,
muscarinic, serotinergic, dopaminergic & adrenergic, causing adverse
effects such as dryness of mouth, etc.
- Crossing of blood brain barrier leads to CNS effects, e.g., sedation
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Cetirizine
- Selective
antihistamine, with action chiefly on histamine H1 receptor
- Negligible effect on muscarinic, serotinergic, dopaminergic & adrenergic
receptors
- Poor ability to cross blood brain barrier
- Hence better tolerability than older antihistamines
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Phases
of Allergy
Early
Phase
Type I (anaphylactic) reaction results in release of histamine and
other substances, resulting in inflammatory manifestations of allergy
Late Phase
Other factors including eosinophil activity, result in release of
chemical mediators which produce cytotoxic and other effects and further
aggravate the inflammatory response
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Actions
of Cetirizine
On
Early Phase
Blocks histamine receptors, hence inhibits early phase reaction
Late Phase
Inhibits eosinophil activity, hence suppresses late phase reaction
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Advantages
of Cetirizine
Thus cetirizine
is superior to older antihistamines :
1. Efficacy :
Inhibition of early and late phase allergic responses, whereas older
anhistamines inhibit early reaction only
2. Tolerability :
Selectivity of action avoids atropine like adverse effects and sedation,
which are commonly encountered with older agents
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Pharmacokinetics
- Rapidly
absorbed from G.I. Tract
- Peak plasma concentrations in 1 hour
- Good distribution in tissues, including respiratory tract and skin
- Poor penetration into CSF
- Half life - 11 hours approx.
- Excreted primarily via urine, and mainly as unchanged drug
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Indications
- Seasonal
and Perennial Allergic Rhinitis
- Chronic Idiopathic Urticaria
- Allergic Conjunctivitis
- Atopic Dermatitis
- Pruritus
- Adjunct to Antiasthmatic Therapy
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Dosage
Adults
Usual dosage 10 mg once daily In severe cases, 10 mg b.d.
In renal impairment,
5 mg o.d.
Children (> 6 years)
5-10 mg once daily
Children (2-5 years)
2.5-5 mg once daily
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Adverse Effects
- Well tolerated
- Drowsiness, headache, dry mouth, G.I.effects may occur
- No important drug interactions
Contraindications
- Hypersensitivity Precautions
- Renal impairment (reduce dosage),
- Pregnancy (inadequate data)
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Salient
Features
- Selective
antihistamine
- Blocks both early and late phase allergic reactions
- Avoids adverse effects of older antihistamines
- Non sedating
- Convenient once daily dosage
- Effective in allergic rhinitis and conjunctivitis, urticaria, atopic
dermatitis and pruritus and as adjunct in bronchial asthma
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Cetirizine
+ Pseudoephedrine
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Pseudoephedrine
- Widely used
nasal decongestant
- Alpha adrenergic receptor agonist
- Produces vasoconstriction leading to reduction in mucosal oedema
and swelling
- Less adverse effects (e.g., CNS effects, elevated BP and palpitation)
than ephedrine
- Less rebound congestion
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Pharmacokinetics
of pseudoephedrine
- Rapidly absorbed
from G.I. Tract
- Well distributed in the body
- Some metabolism in liver (1%)
- Excreted unchanged in urine
- Half life 5-8 hours
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Rationale
of combination
- Cetirizine
is a highly effective antihistamine, but is more suited for preventing
attacks of allergic rhinitis
- Pseudoephedrine relieves nasal congestion thus producing symptomatic
relief
- This combination will do both
- relieve symptoms and prevent attacks
- thus is an ideal agent
- In perennial rhinitis, nasal congestion is often a consistent feature.
A combination such as this will be particularly useful here
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Indication
Allergic
Rhinitis
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Dosage
Fixed
dose combinations are available as Cetirizine 5 mg + Pseudoephedrine
120 mg
Dosage
1 tablet 12 hourly
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Adverse
Effects & Precautions
- Pseudoephedrine
can cause palpitations, headache, dizziness, restlessness, anxiety,
tremor, insomnia, hypertension
- Concomitant administration of pseudoephedrine with sympathomimetic
decongestants, appetite suppressants and MAO inhibitors can cause
increased BP and hypertensive crisis
- Antihypertensive activity of some antihypertensives e.g., methylodopa,
reserpine and beta blockers may be countered by pseudoephedrine
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