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Astemizole is a long-acting, selective histamine H1-receptor antagonist. Receptor binding studies in animals demonstrated that at pharmacological doses, Hismanal (astemizole (withdrawn from us market)) occupies peripheral H1-receptors but does not reach H1-receptors in the brain. Whole body autoradiographic studies in rats, radiolabel tissue distribution studies in dogs and radioligand binding studies of guinea pig brain H1-receptors have shown that Astemizole does not readily cross the blood-brain barrier. Screening studies in rats at effective antihistaminic doses showed no anticholinergic effects. Studies in humans using the recommended dosage regimens have not been performed to determine whether Astemizole is associated with a different frequency of anticholinergic effects than therapeutic doses of other antihistamines.
The absorption of AstemizoleAstemizole is reduced by 60% when taken with meals. In single oral dose studies, Astemizole was rapidly absorbed from the gastrointestinal tract; peak plasma concentrations of unchanged Astemizole were reached within one hour. Due to extensive first pass metabolism and significant tissue distribution, plasma concentrations of unchanged drug were low. Elimination of unchanged Astemizole occurred with a half-life of approximately one day. Elimination of Astemizole plus hydroxylated metabolites, considered together to represent the pharmacologically active fraction in plasma, was biphasic with half-lives of 20 hours for the distribution phase and 7-11 days for the elimination phase. The pharmacokinetics of Astemizole plus hydroxylated metabolites are dose proportional following single doses of 10 to 30 mg.
Following chronic administration, steady state plasma concentrations of Astemizole plus hydroxylated metabolites (mainly desmethylastemizole) were reached within four to eight weeks; concentrations of the metabolites are substantially higher than those of unchanged Astemizole. Astemizole plus hydroxylated metabolites decayed biphasically with an initial half-life of 7-9 days, with plasma concentrations being reduced by 75% within this phase, and with a terminal half-life of about 19 days. The initial phase (t1/2= 7-9 days) appears to determine the time to reach steady state plasma concentrations of Astemizole plus hydroxylated metabolites. Steady state plasma concentrations of unchanged Astemizole were reached by 6 days (with a range of 6-9 days); unchanged Astemizole was eliminated from plasma with a half-life of approximately 2 days (with a range of 1-2.5 days).
Interpatient variability in pharmacokinetic parameters may be greater in patients with liver disease as compared to normal subjects. Systemic evaluation of the pharmacokinetics in patients with hepatic or renal dysfunction has not been performed.
The in vitro plasma protein binding of unchanged Astemizole (100 ng/ml) was 96.7% with 2.3% being found as free drug in the plasma water. In human blood with an astemizole concentration of 100 ng/ml, 61.5% of astemizole was bound to the plasma proteins, with 36.2% being distributed to the blood cell fraction. The concentration of astemizole found in the blood was the same as that found in the plasma fraction of the blood. Binding studies for the astemizole metabolite(s) which achieve much higher concentrations than astemizole under chronic dosing conditions have not been conducted.
Last reviewed on RxList: 12/8/2004
This monograph has been modified to include the generic and brand name in many instances.
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