Appraisal of the validity of histamine-induced wheal and flare to predict the clinical efficacy of antihistamines.

Antihistaminic drugs have been used successfully for many years in the treatment of allergic diseases. Second-generation antihistamines have fewer sedating side effects than first-generation agents, and the number of newer drugs available for clinical use is growing. Various methods have been used to assess antihistaminic activity, the most popular of which is the epicutaneous histamine-induced wheal and flare. This test relies on the ability of epicutaneously injected histamine to bring about the wheal and flare, a neurovascular response that involves reflex vasodilation (flare) and local swelling caused by plasma extravasation (wheal). Antihistamines have been compared on the basis of their ability to block the histamine-induced wheal and flare in the skin. Results of these trials have been applied to predict the global antiallergic efficacy of various antihistamines. The review has examined the reliability of suppression of the histamine wheal and flare reaction in the skin to predict an antihistamine's clinical efficacy in two common allergic diseases seasonal allergic rhinitis and chronic idiopathic urticaria. Although histamine is one mediator in the allergic response in the skin and nasal mucosa, many other agents are important modulators of the allergic response. In addition, the major structural and functional differences that exist between the nasal mucosa and the skin affect the type of local response. These manifest themselves as differences between the responses to antigen and histamine challenge in the skin and the nose. The allergic responses in these tissues are not simply the consequence of one chemical but are the result of a cascade of interactions among various cells and mediators. The clinical manifestations of these complex interactions obviously cannot be fully replicated by injection of one chemical mediator, histamine, into the outer layer of the skin. Studies with antihistamines have shown that certain drugs, such as cetirizine, are more suppressive than others (loratadine, terfenadine) in controlling the histamine-induced wheal and flare reaction in the skin. When the clinical efficacy of these medications is compared in clinical trials in seasonal allergic rhinitis and chronic idiopathic urticaria, all are equally efficacious in controlling symptoms. Although the histamine-induced wheal and flare reaction can serve as a useful clinical pharmacologic test to assess dose-response relations for an antihistamine, its lack of correlation with clinical responses among antihistamines indicates that this model should not be used to predict or compare clinical efficacies of antihistamines in seasonal allergic rhinitis and chronic idiopathic urticaria.