Topical glucocorticoids, commonly used to manage canine allergic skin disease, are the first-line choice for managing acute flares of atopic dermatitis. The advantages of using topical glucocorticoids are direct drug application to the target area, maintenance of high doses at the target site, and fewer noticeable systemic adverse effects. In humans, increased absorption of topical glucocorticoids in inflamed skin versus normal skin has been documented.
This study used an in vitro model to study the penetration kinetics of normal and affected canine skin. Fresh cadaver skin was collected from 5 dogs with clinical signs compatible with flea allergy dermatitis. Investigators sampled both normal and affected skin from each dog, and the transdermal penetration of radiolabeled hydrocortisone was measured over 30 hours. The pooled data confirmed increased permeability of affected skin compared with normal skin. Penetration was approximately twice as great in affected skin, although the ratio of hydrocortisone flux through lesional and nonlesional skin varied from 1:1.5 to 10:1 within individual dogs. The finding that lesional skin in 2 of 5 affected dogs was less permeable than nonlesional skin was unexpected.
This study revealed 2 points of clinical importance: First, inflammation enhances penetration of glucocorticoids in the skin, which becomes important when recommending these products for use on skin and in ears. In clinically normal dogs, otic glucocorticoid administration can alter adrenal and liver tests,1,2 which is important to remember when recommending topical glucocorticoids as first-line treatment in acute flare of allergic skin disease. Second, in 2 dogs, penetration kinetics were unexpectedly different—the skin was less permeable. This may explain why some dogs do not respond as expected to certain topical drugs.—Karen Moriello, DVM, DACVD