Pharmacology for Dentistry

Histamine and Antihistaminic Agents 219

dissociates more slowly from the H 1 -
receptor than the (S)-enantiomer.

It is well absorbed with an oral
bioavailability of 85%. Onset of action occurs
within one hour.

Indications for levocetirizine remain the
same as those of cetirizine.

Adverse reactions include headache,
fatigue etc.

FEXOFENADINE

It is a pharmacologically active
metabolite of terfenadine, is a non sedating
antihistaminic with selective peripheral H 1
receptor antagonist activity.

Fexofenadine inhibited antigen-induced
bronchospasm and histamine release from mast
cells. No anticholinergic or alpha adrenergic-
receptor blocking effects were observed.
Moreover, no sedative or other CNS effects
were observed. Fexofenadine does not cross the
blood-brain barrier. It inhibits skin wheal and
flare responses produced by histamine
injection. Following single and twice daily oral
administration, antihistaminic effects occurred
within 1 hour, achieved a maximum at 2-3
hours, and lasted a minimum of 12 hours.

Fexofenadine hydrochloride is rapidly

absorbed into the body following oral

administration, with Tmax occurring at

approximately 1-3 hours post dose. It is 60-

70% plasma protein bound.

Side effects include headache, fatigue,

drowsiness, nausea, tachycardia, palpita-

tions, dry mouth, GIT disturbances, taste

disturbances, photosensitivity, dysmenor-

rhoea and menstrual disorders.

The newer antihistaminics agents e.g.

Azelastine inhibits histamine release and also

inflammatory reactions evoked by

leukotrienes and plasma activating factor

(PAF). It is used for seasonal and perennial

allergic rhinitis in the form of nasal spray.

Mizolastine, a non-sedating antihistaminic is

used in allergic rhinitis and urticaria. Ebastine,

another non-sedating antihistaminic is used

in nasal and skin allergies.

H 2 -RECEPTOR ANTAGONISTS

As discussed earlier, H 2 receptors are

responsible for histamine induced gastric

acid secretion. H 2 -receptors antagonists such

as cimetidine, ranitidine, famotidine etc. are

used in the treatment of peptic ulcer and are

discussed in chapter ‘Antiulcer agents’.