Intranasal antihistamines for allergic rhinitis: mechanism of action.

Although antihistamines have been in common use for treatment of allergic diseases including rhinitis for >60 years and topical therapy of the respiratory tract has been common for centuries, it is only in the past few years that topical intranasal antihistamine therapy has been widely used for treatment of allergic rhinitis. Much research has been done over the past several years showing broad anti-inflammatory effects of these medications, involving many different pathways. Effects have been shown on mediators including histamine, leukotrienes, platelet-activating factor, and substance P, as well as on cytokines, adhesion molecules, and chemokines involved in chemotaxis. It is significant that these effects have been seen at clinically relevant concentrations of the topical drugs, as opposed to the situation with oral antihistamines where anti-inflammatory effects are generally found only at concentrations much higher than that achieved with routine dosing. Clinically, it appears that this delivery of high local concentrations allows for other pharmacologic activity to be expressed. These anti-inflammatory actions may be part of the reason why these drugs also are effective in relieving many symptoms of nonallergic rhinitis, where histamine has much less of a role and where oral antihistamines have traditionally been of minimal help.