Astemizole competes with histamine for binding at H_-receptor sites in the GI tract, uterus, large blood vessels, and bronchial muscle. This reversible binding of astemizole to H_-receptors suppresses the formation of edema, flare, and pruritus resulting from histaminic activity. As the drug does not readily cross the blood-brain barrier and preferentially binds at H_ receptors in the peripehery rather than within the brain, CNS depression is minimal. Astemizole may also act on H_-receptors, producing adverse effects.
Astemizole is a second generation H1-receptor antagonist. It does not significantly cross the blood brain barrier and therefore does not cause drowsiness or CNS depression at normal doses.
Almost completely metabolized in the liver and primarily excreted in the feces.
LD50=2052mg/kg in mice