Hj-Receptor agonists are gaining renewed interest as potential antiasthmatic drugs. Such compounds, when given in adequate dosage, not only protect significantly against antigen and exercise-induced asthma but also produce bronchodilation comparable with that seen after inhaled isoproterenol and salbutamol. As in the case of bronchodilator receptors, few attempts have been made to determine the distribution of histamine receptors in the human respiratory tract. It is well known that histamine is used in tests of bronchial reactivity. Histamine receptors are generally thought to reside in the large airways because centrally deposited aerosolized histamine results in significant histamine receptor-mediated responses. However, Ryan et al. [48] have not confirmed this, although it is not known whether adequate differences between peripherally and centrally deposited aerosols was achieved in their studies. Previously, the drawback to using the antihistamines included their marked individual variation in response between patients, specificity of histamine receptor antagonism, and dose and route given. More importantly, the Hr receptor antagonists were unsuitable for diurnal use because of their marked sedation as a consequence of central nervous system (CNS) uptake. Advances in this drug discovery area [49] have included the development of Hj antagonists with no H2-, a-, b-receptor activity, no demonstrable anticholinergic and anti-5-hydroxytryptamine activity, and no ability to cross the blood-brain barrier. The prototype drug terfenadine (8) represents the first example of this new class of nonsedating antihistamine possessing the previously mentioned characteristics and exhibits no significant sedation in dosages up to 200 mg three times daily [50]. Terfenadine and the related drug astemizole (9) represent the only two drugs of this class marketed in the United States.

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