In vivo and ex vivo inhibitory effects of loratadine on histamine release in patients with allergic rhinitis

Background: The aim of this study was to evaluate the in vivo and ex vivo effects of the H₁-antagonist loratadine on histamine release. Methods: The study was designed as a double-blind, crossover trial. Ten patients with allergic rhinitis due to Dermatophagoides pteronyssinus were treated with loratadine (10 mg daily p.o.) and with placebo for 1 week, with a 2-week interval between the two treatments. Nasal lavages with saline solution were done before and after challenge with the relevant allergen at the end of treatments with loratadine and placebo. Venous blood was taken after treatments, and basophil histamine release induced by anti-IgE (10 μg/ml), N-formyl-methionyl-leucyl-phenylalanine (fMLP, 1 μM), and Ca²⁺ ionophore A23187 (1 μM) was evaluated by an automated fluorometric method. Results: Treatment with loratadine attenuated early antigen-induced nasal obstruction, rhinorrhea, and itching. Nasal symptoms were accompanied by a significant histamine release in the nasal lavages collected 5 min after stimulation when the patients received placebo (median 4 ng/ml, range 1-28; P <0.05). After treatment with loratadine, histamine release in the 5-min postchallenge lavages was almost abrogated (median 0.5 ng/ml, range 0-3; P <0.01 vs placebo). Median anti-IgE-induced histamine release from basophils was 41.9% (range 27.8-79.2) after placebo and 30.0% (range 1.7-73.3, P <0.05) after loratadine. Active treatment exerted an inhibitory effect also on basophil histamine release induced by fMLP and Ca²⁺ ionophore A23187. Conclusions: Treatment for 1 week with loratadine reduces allergen-induced nasal symptoms and inhibits in vivo and ex vivo histamine release in patients with allergic rhinitis.