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Structural modification of the antihistamine azatadine (10a) by replacing the N-methyl group with various carbamate groups eliminates CNS activity [51]. Loratidine (10b), the most potent compound in this series of carbamates, shows no sedation liability in experimental animals, binds selectively to peripheral histamine receptors [52], and appears likely to be approved by the NDA in the near future. Other nonsedatory Hj antihistamines that have been reported recently are temelastine (11), tazifylline (12), cetrizine (13), levocabastine (14), and epinastine (15) [53-57]. Tazifylline is reported to have 10 times the bronchodilator activity exhibited by either asternizole or terfenadine. The recent preclinical pharmacology of AHR-11325 (16) and PR 1036-654 (17) suggests that both these new compounds are potent, nonsedating, long-acting H1 antagonists [58,59]. Ebastine (18a), a structural analogue of terfenadine, has been reported to be a potent, selective, long-lasting antihistamine devoid of sedation at an oral dose of 10 mg [60]. Its mode of action is thought to be due to metabolism to the active form (18b) [61].

It is clear that these new agents will most likely benefit from being delivered locally into the respiratory tract in therapeutic concentrations that can avoid undesirable systemic side effects.

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