No abstract

Engagement of the mast cell high-affinity receptor for immunoglobulin E (IgE) Allergen-induced activation of the high-affinity receptor for IgE (FcRI) in mast cells and basophils plays a pivotal role in the initiation of allergic and inflammatory reactions. As a result of receptor engagement, mast cells and basophils release a variety of inflammatory mediators, such as histamine, arachidonic acid metabolites, and cytokines (1, 2). The activation of protein tyrosine kinases (PTKs) is one of the early and critical signaling events following FcRI engagement (3). Since FcRI belongs to the family of multisubunit antigen receptors that do not possess intrinsic PTK activity in their structure (1), the recruitment and activation of non-receptor PTKs is crucial for FcRI-mediated signal transduction (4).,FcRI is a tetrameric receptor composed of an ␣ subunit, a ␤ subunit, and a homodimer of disulfide-linked ␥ subunits (5). The cytoplasmic domains of ␤ and ␥ subunits contain sequences known as the immunoreceptor tyrosine-based activation motifs (ITAMs) (6, 7). The phosphorylation of the tyrosines of the ITAM allows the interaction of ITAMcontaining receptors with signaling proteins that have phosphotyrosine-recognizing Src homology 2 (SH2) domains (8, 9).

The effect of azelastine on the release of leukotriene C₄ and D₄ (LTC₄ and LTD₄), and the antagonistic action of the drug against the leukotrienes were determined by using in vitro tests and compared with those of ketotifen and chlorpheniramine. Azelastine inhibited LTC₄ and LTD₄ release from guinea pig lung fragments passively sensitized with homologous anti-ovalbumin IgG1-b antibody. The 50% inhibitory concentration (IC₅₀) of azelastine was 6.4 × 10^––5M for a 15-min preincubation or 4.7 × 10^––5M for a 30-min preincubation. Ketotifen and chlorpheniramine were inhibitory only at the highest concentration tested (3 × 10^––4M), giving inhibitions of 35.6 and 21.3%, respectively. Azelastine also inhibited calcium ionophore A23187-induced release of leukotrienes from human polymorphonuclear leukocytes; the IC₅₀ values were 3.6 × 10^––5M for 15 min and 2.3 × 10^––6M for 30 min of preincubation. Ketotifen and chlorpheniramine were inhibitory only after a 30-min preincubation, with IC₅₀ values of 2.1 × 10^––5 and 5.9 × 10^––5M, respectively. The potent inhibition by azelastine might be partly a result of the inhibition of 5-lipoxygenase, since 5-hydroxyeicosatetraenoic acid formation in rat basophilic leukemia cell homogenate was inhibited by azelastine. Pretreatment of guinea pig ileum with azelastine antagonized LTC₄- and LTT-Vinduced contraction of the ileum with IC₅₀ values of 7.0 × 10^––6 and 1.1 × 10^––5M, respectively. Azelastine (with a 50% relaxation concentration of 6.0 × 10^––6M) relaxed the ileum contracted by LTC₄. These antagonistic actions of azelastine were 2–3 times more potent than those of ketotifen. The present results suggest that the anti-allergic action of azelastine may be based on not only inhibition of the release of mediators (including LTC₄/D₄ and histamine), but also antagonism against leukotrienes and histamine.