Effect of topical doxepin cream on skin testing

Abstract

Doxepin hydrochloride cream is a new tricyclic antidepressant preparation indicated for topical application in the treatment of atopic dermatitis and lichen simplex chronicus. 1 Tricyclic antidepressant compounds are potent antagonists of H a and H 2 receptors. 2 Doxepin is a competitive antagonist of the muscarinic acetylcholine receptor, which accounts for the majority of adverse effects, including blurred vision, dry mouth, and urinary retention.2, 3 Because of the adverse effect profile of the oral preparation, a topical agent was investigated. It is believed that the antipruritic effect of the cream is secondary to the local antihistaminic effect only. During clinical trials it was suggested that there was minimal percutaneous absorption of topical doxepin. 4 Plasma levels of topical doxepin ranged from 0 to 47 ng/ml, whereas plasma levels of orally administered doxepin have been reported to range from 30 to 150 ng/ml. 4 Patients with atopic dermatitis often require skin testing to assess for specific allergies. Before immediate hypersensitivity skin testing is performed, it is important to discontinue all antihistamines because they will suppress skin test responses. It is also important to determine whether and how long a medication will suppress skin test responses to avoid false-negative results. The intent of this study was to demonstrate whether and how long histamine skin test responses were suppressed after topical use of doxepin and to determine whether systemic absorption could account for the suppression.