Pharmacology for Dentistry

218 Section 5/ Autacoids

Therapeutic Uses

H 1 antagonists have a widespread value
in the symptomatic treatment of various
disorders.

1. Hypersensitivity reactions: To prevent
   allergic reactions or to treat their
   symptoms, in which histamine is the
   primary mediator, the H 1 -antagonists
   are the drugs of choice and are often
   quite effective. They are used primarily
   to treat allergic reactions produced by
   the release of histamine e.g., edematous
   states, pruritus, allergic rhinitis and
   urticaria. They are generally more
   effective in acute conditions and are
   used only for symptomatic relief.


2. Motion sickness: Drugs like promet-
   hazine, promethazine chlorotheophyl-
   linate, diphenhydramine, dimenhydri-
   nate, cyclizine and meclizine have
   value in prophylaxis of motion sickness.


3. Antivertigo: Drugs line cyclizine, cin-
   narizine, dimenhydrinate, diphenhy-
   dramine are used in the treatment of
   vertigo.


4. Antiparkinsonism: Based on
   anticholinergic property, some H 1 -
   antagonists such as diphenhydramine
   can be used in the early stages of
   treatment of parkinsonism.


5. Local anaesthetics: In patients allergic
   to procaine H 1 -antagonists, such as
   diphenhydramine and tripelennamine
   have been used successfully as local
   anaesthetic.

NEWER ANTIHISTAMINICS

CETIRIZINE

Cetirizine is a potent antihistaminic with a
low potential for drowsiness at pharmaco-

logically active doses and with additional

antiallergic properties. It is a selective H 1

antagonist with negligible effects on other

receptors and is, therefore, virtually free

from anti-cholinergic and anti-serotonin ef-

fects. Cetirizine inhibits the histaminemedi-

ated ‘early’ phase of the allergic reaction and

also reduces the migration of inflammatory

cells such as eosinophils and the release of

mediators associated with ‘late’ allergic

response.

Cetirizine is indicated for the

symptomatic treatment of perennial allergic

rhinitis, seasonal allergic rhinitis, chronic

idiopathic urticaria, conjunctivitis and

pruritus in adults and children above two

years of age.

Side effects include headache, dizziness,

drowsiness, dry mouth and gastrointestinal

discomfort.

LEVOCETIRIZINE

The second generation H 1 -receptor an-

tagonist cetirizine is a reacemate consist-

ing of equal quantities of 2 enantiomers,

levocetirizine [(R)-enantiomer] and

dextrocetirizine [(S)-enantiomer]. In vitro

and human pharmacodynamic studies

have provided evidence that levocetirizine

is the more active enantiomer, accounting

for most or all clinical antihistaminic activ-

ity of racemic cetirizine; this activity of

levocetirizine is seen at half the dose of

cetirizine.

Levocetirizine exhibit a 2-fold higher

affinity for human H 1 -receptors than

cetirizine, and is about 10 fold more potent

than the (S)-enantiomer. Levocetirizine