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The pathogenesis of allergic rhinitis is complex, involving not only histamine and mast cell-derived tryptase, but also eosinophil- and neutrophil-derived mediators, cytokines, and intercellular cell adhesion molecules (ICAM-1). It is surprising that antihistamines, which block only one component of the process, have proved so effective in the management of allergic rhinitis. Research has therefore focused on whether antihistamines have additional pharmacological activities. In vitro studies have shown that high concentrations of second generation antihistamines can block inflammatory mediator release from basophils and mast cells, and reduce ICAM-1 expression in epithelial cell lines. In vivo studies have also shown an effect on the allergen-induced inflammatory reaction; both oral and intranasal antihistamines cause a reduction in nasal symptoms and inflammatory cell influx. Oral terfenadine and cetirizine and intranasal levocabastine and azelastine have also demonstrated a lowering of ICAM-1 expression on epithelial cells. With regard to clinical efficacy, topical levocabastine (0.5 mg/mL eye drop solution and 0.5 mg/mL nasal spray) was shown to be more effective than oral terfenadine (60 mg twice daily) in relieving ocular itch (P = 0.02) and reducing nasal symptoms in allergic rhinoconjunctivitis. In a further study, levocabastine eye drops were as effective and well tolerated as sodium cromoglycate in seasonal allergic rhinitis. Intranasal azelastine (0.28 mg twice daily) showed a trend for superior relief of rhinorrhoea and nasal obstruction compared with oral terfenadine (60 mg twice daily). In addition, intranasal azelastine (0.28 mg twice daily) resulted in significant reductions in sneezing, nasal obstruction, rhinorrhoea and itching in perennial rhinitis, compared with the lower efficacy of beclomethasone dipropionate (0.1 mg twice daily). As well as benefits in efficacy, topical administration is associated with improved safety. Some antihistamines, particularly those metabolized in the liver, are associated with occasional reports of severe side-effects. It is therefore logical to administer antihistamines directly to the target organ.
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