Antiallergic activity of H1-receptor antagonists assessed by nasal challenge.

Most oral drugs used for the treatment of allergic rhinitis are classified as H1-receptor antagonists, and although they represent major sales throughout the world, their mechanism of action is still poorly known. In an attempt to understand better the in vivo therapeutic effects of these drugs, a double-blind, crossover study was carried out. The study compared the effects of terfenadine and loratadine, nonsedative H1-receptor antagonists, on the immediate allergic response of the upper airways to challenge with orchard-grass pollens in 14 highly allergic subjects. Increasing numbers of pollen grains were insufflated into the nostrils, and the response of the subjects was assessed by examining symptoms and measuring the release of histamine and prostaglandin D2 in nasal secretions. Each drug was administered for a week before challenge. This study demonstrated the clinical efficacy of both drugs by comparison to that of a control day, since symptoms were observed for a significantly (p = 0.014) greater number of pollen grains. Only one patient had a significant release of histamine when they were treated with loratadine versus 10 during control day (p less than 0.0023) and six when they were treated with terfenadine (p less than 0.01). Prostaglandin D2 release occurred with a higher allergen dose when patients were treated with both drugs. This study indicates that some H1 antagonists also possess antiallergic activities.