Direct effects of second-generation H1-receptor antagonists on the activation of human basophils

Histamine: A mediator of inflammation

Histamine and its receptors, including the recently discovered receptors (H(3) and H(4)), novel sources of histamine, and the place of histamine in mediator networks continue to be areas of great interest. The 4 major subtypes of histamine receptors, H(1) to H(4), differ in their location, second messengers, and histamine-binding characteristics. In addition, it would appear that different histamine receptor agonists and antagonists bind to different portions of the receptor complex. A fifth receptor subtype, the intracellular H(IC), has only been defined by its location within cell types that are not traditionally associated with histamine. In airway tissue, most cells express at least 1 subtype of histamine receptor; however, blockade of these receptors does not completely abolish the inflammatory response. In addition, some H(1)-antihistamines might also exert anti-inflammatory effects by pathways independent of H(1)-receptor binding. Studies of selected second-generation H(1)-antihistamines have shown that these agents inhibit the release of certain cytokines from basophils, acting at a point downstream from the calcium signaling pathway. It has not yet been determined whether this action represents a class effect.

Beyond the histamine receptor: effect of antihistamines on mast cells

Many antihistamines exhibit inhibition of mediator release from mast cells and basophils, in in vitro studies in addition to H1 antagonism. The underlying mechanism is unclear but is unrelated to H1-receptor antagonism. Clinical studies of antihistamins in antigen challenge and seasonal allergy demonstrate reduction of mast cell mediators in nasal lavage. It is not known what mechanism(s) underly these observations, although the concentrations required in in vitro studies suggests that a direct effect on mast cells is unlikely. Furthermore, the therapeutic contribution of this effect is difficult to assess because of concomitant clinically significant H1 antagonism. This and other potential anti-allergic effects may enhance the therapeutic benefit of antihistamines and long-term studies are underway to explore this possibility.

A review of loratadine in the treatment of patients with allergic bronchial asthma

Patients with seasonal or allergic bronchial asthma experience an immediate allergic response caused by allergen-specific immunoglobulin E-mediated histamine release. The release of histamine and other chemical mediators may trigger airway hyperresponsiveness and exaggerated bronchoconstriction, characteristic features of allergic bronchial asthma. Traditional antihistamines have demonstrated only moderate activity of short duration against this disease. In contrast, loratadine, a potent, nonsedating, histamine-1-receptor antagonist with activity in seasonal and perennial allergic rhinitis, has demonstrated effective control of asthma symptoms, improved pulmonary function, and long duration of action in patients with allergic bronchial asthma. This review summarizes preclinical evidence for the antiallergic activity of loratadine and the results of clinical studies with oral loratadine in patients with allergic bronchial asthma.

Inhibitory effect of cetirizine on cytokine-enhanced in vitro eosinophil survival

BACKGROUND

Cetirizine is an antihistamine that inhibits in vivo eosinophil influx into the inflamed airways following allergen challenge, and in vitro eosinophil chemotaxis and adhesion. Since eosinophils are proposed to have an important role in the pathophysiology of asthma and allergic disease, the effects of cetirizine on eosinophil function may be a mechanism of this agent's therapeutic regulation of the allergic reaction.

OBJECTIVE

To determine the effect of cetirizine on in vitro eosinophil survival.

METHODS

Using human eosinophils isolated from patients with allergic rhinitis, the cells were cultured in vitro for 48 to 72 hours with medium, cetirizine, or dexamethasone in the presence of IL-5, IL-3, or GM-CSF. Eosinophil survival was assessed by trypan blue exclusion.

RESULTS

In the presence of IL-5, but not GM-CSF or IL-3 100 microM cetirizine significantly inhibited eosinophil survival at 48 and 72 hours; the magnitude of this inhibition was dependent on cytokine concentration. Although cetirizine significantly suppressed cytokine promotion of eosinophil survival, it was not as potent as dexamethasone.

CONCLUSIONS

Although the in vitro concentration of cetirizine was required to be quite high, cetirizine may affect in vivo airway inflammation through its inhibition of IL-5-dependent eosinophil survival.

Inhibition of leukotriene synthesis by azelastine

BACKGROUND

Azelastine, oxatomide, and ketotifen are used for patients with allergic diseases. These drugs inhibit the release of chemical mediators including the leukotrienes; however, the mechanism involved is unclear.

OBJECTIVE

To clarify the mechanism of inhibition, we investigated the effects of three drugs on the function of phospholipase A2, 5-lipoxygenase, leukotriene C4 synthase, and leukotriene A4 hydrolase, which are all catabolic enzymes involved in synthesizing leukotriene C4 and leukotriene B4 in rat basophilic leukemia (RBL)-1 cells.

METHODS AND RESULTS

The production of leukotriene C4 and leukotriene B4 was measured by high performance liquid chromatography (HPLC). All three drugs inhibited the production of leukotriene C4 and leukotriene B4 when cells were stimulated with A23187. All three drugs also inhibited the A23187-stimulated release of 3H-arachidonic acid from membrane phospholipids. Azelastine inhibited the production of leukotriene C4, but not leukotriene B4, when either arachidonic acid or leukotriene A4 free acid was used as the substrate in our cell free system. Oxatomide and ketotifen did not inhibit the synthesis of either leukotriene C4 or leukotriene B4 in the same cell free study.

CONCLUSION

Results indicated that oxatomide and ketotifen inhibit the production of leukotriene C4 and leukotriene B4 by inhibiting phospholipase A2 activity, whereas, azelastine inhibits the leukotriene C4 production by inhibiting phospholipase A2 and leukotriene C4 synthase.