Astemizole

Abstract

Synopsis: Astemizole¹ is an H₁-histamine receptor antagonist with a long duration of action permitting once daily administration. Its efficacy in seasonal and perennial allergic rhinitis has been convincingly demonstrated, and several comparative studies suggest that astemizole is at least as effective as some other H₁-histamine receptor antagonists. A few smaller studies have shown beneficial effects on the symptoms of allergic conjunctivitis and chronic urticaria (but not atopic dermatitis). While astemizole appears to share with other H₁-histamine receptor antagonists a tendency to increase appetite and cause weight gain after prolonged use, it offers the important advantage of an absence of significant central nervous system depression or anticholinergic effects with usual doses. Thus, astemizole offers a worthwhile improvement in side effect profile over ‘traditional’ H₁-histamine receptor antagonists, especially in patients bothered by the sedative effects of these drugs. Pharmacodynamic Properties: Because of difficulties associated with the assay procedures, the H₁-histamine receptor binding profile of astemizole in vitro remains unclear. However, a striking finding in such studies was the extremely slow dissociation of astemizole from H₁-histamine receptors. In vivo (guinea-pigs) astemizole occupied H₁-histamine receptors in the lung at very low doses while severalfold greater doses were required to achieve equivalent occupancy of cerebellum receptors; the duration of occupancy was several days. While astemizole showed some affinity for serotonergic and α₁-adrenergic receptors in vitro, it was devoid of activity on dopaminergic and muscarinic receptors. In vivo (rats) astemizole demonstrated little antiserotonergic activity and no anticholinergic activity. In healthy volunteers salivary flow rates after a 40mg dose of astemizole were no different from placebo values. In human volunteers, single doses of astemizole (40mg) significantly inhibited the weal and flare response to intradermal histamine. Long term administration (up to 3 months) to volunteers and patients with chronic urticaria increased the maximum inhibitory effect on the histamine response and did not result in tachyphylaxis. Results of studies in animals and human volunteers suggested a similar degree of activity in allergen-mediated reactions. Astemizole did not exert any consistent effect on mast cell mediator storage and release. The central nervous system depressant activity of astemizole has been assessed in volunteers by several psychomotor performance, reactivity and sedation tests. These studies showed no significant central nervous system depression following administration of astemizole (doses ranged between 10 and 60mg), while sustained release tripolidine 10mg, ketotifen 1mg or chlorpheniramine 16mg often produced significant changes in such tests. Additionally, astemizole did not potentiate the central effects of either alcohol or diazepam 10mg. Pharmacokinetics: Following oral administration, astemizole appeared to undergo extensive first-pass metabolism. Maximum plasma concentrations of astemizole and metabolites occurred 1 to 4 hours after single oral doses (10 to 40mg) and generally varied with dose. Administration of the drug with food significantly decreased bioavailability. With multiple-dose administration (10mg daily), steady-state plasma concentrations of unchanged drug were attained within 1 to 2 weeks, after which no further drug accumulation occurred. In dogs, astemizole and its active metabolite, desmethylastemizole, were distributed to the pancreas, adrenals, liver, lungs, salivary glands, kidney tissue and the testes in concentrations greater than 400 times the concomitant plasma concentration. The drug and/or its metabolites were found in the milk of lactating dogs and to a limited extent in the placenta of pregnant rats. Astemizole was 96.7% bound to plasma proteins in human volunteers. Plasma concentrations in terminal renal failure patients were not decreased by dialysis. In healthy volunteers, astemizole was rapidly and extensively metabolised to several active and inactive metabolites. Excretion of the metabolised drug was slow and was mainly via the faeces (54 to 73% of an administered dose was excreted in the faeces within 14 days). After long term administration of 10mg daily to volunteers and patients, the apparent elimination half-life of the drug and metabolites was 18 to 20 days. Therapeutic Trials: In published and unpublished double-blind trials of seasonal and perennial allergic rhinitis, astemizole (usual dosage 30mg daily for 2 to 3 days followed by 10mg once daily for up to 12 weeks) was significantly superior to placebo at achieving ‘excellent’ to ‘good’ symptom control as measured by global assessments by patients and investigators. Additionally, the use of ‘backup’ antihistamines was less frequent among astemizole-treated patients, and in many of the studies the astemizole group exhibited significantly less severe daily and mean nasal and ocular symptoms (as recorded in patient diaries and on visual analogue scales). Astemizole appeared to be at least as effective as clemastine and several other H₁-histamine receptor antagonists in alleviating the symptoms of rhinitis. In a single study astemizole was more effective than terfenadine in patients with seasonal allergic rhinitis, although some puzzling findings of the study suggest it should be interpreted cautiously. In a double-blind comparison, astemizole therapy (30mg daily for up to 7 days and then 10mg daily) for 2 weeks was significantly superior to placebo in relieving the symptoms of allergic conjunctivitis. Global assessments, both by investigators and patients, rated treatment results ‘excellent’ to ‘good’ in a greater percentage of astemizole-treated patients than placebo patients. In another double-blind comparison there was no significant difference between astemizole 10mg daily and mequitazine 5mg twice daily after 2 weeks’ therapy. However, following 6 weeks’ therapy astemizole had alleviated 2 of the 5 measured symptoms significantly more so than mequitazine, and the investigator rated the results ‘good’ to ‘excellent’ in significantly more astemizole-treated patients. When assessed subjectively on a linear analogue scale and objectively as nocturnal scratching, there was no significant difference in the magnitude of pre- and post-treatment (astemizole 10mg) itch and scratching in patients suffering from atopic eczema. Similarly, in an unpublished double-blind comparison, the efficacy of astemizole 10mg daily in relieving the symptoms of atopic dermatitis did not differ statistically from that of placebo. In contrast, astemizole 10mg daily or 30mg daily for up to 1 week, then 10mg daily for 3 more weeks, was significantly more effective than placebo in relieving the symptoms of chronic urticaria (double-blind comparisons). Side Effects: The cumulation of side effect data from over 700 astemizole-treated patients resulted in incidences of central nervous system depression and dry mouth that were little different from those reported during placebo administration. However, astemizole may share with other H₁-histamine receptor antagonists the tendency to promote increased appetite and weight gain after prolonged use. Dosage and Administration: The recommended adult dosage is 10mg once daily taken on an empty stomach. In some countries up to 30mg once daily may be given for up to 7 days (then 10mg daily) when symptoms are severe. This dosage should be halved in children aged 6 to 12 years. For younger children the dose is determined on a weight basis with the suspension formulation: 1ml (2mg)/10kg bodyweight daily.