Possible Coal Tar Therapy Mechanism for Eczema Identified

Larry Hand

January 25, 2013

Researchers in the Netherlands have pinpointed a possible molecular mechanism by which the ancient remedy of coal tar may be a therapy for atopic dermatitis (AD), or eczema, according to an article published online January 25 in the Journal of Clinical Investigation.

Ellen H. van den Bogaard, MSc, from the Department of Dermatology, Radboud University Nijmegen Medical Centre, the Netherlands, and colleagues conducted experiments first using human skin models from patients with AD and healthy volunteers and then using skin-equivalent models.

They found that coal tar activates the aryl hydrocarbon receptor (AHR), which in turn induces expression of the filaggrin protein and promotes epidermal development. The process restores skin barrier. AD is associated with loss-of-function mutations in the skin barrier gene filaggrin.

"Our study is the first to demonstrate the beneficial therapeutic effects of AHR activation on epidermal differentiation and barrier function in a skin disease [such as AD]," the researchers write.

In addition, they note, "we made the exciting observation that coal tar normalizes histopathological and molecular hallmarks of AD."

Although coal tar has been used as a topical therapy to reduce inflammation and itch for more than 2000 years, the researchers write, the fact that it contains more than 10,000 various organic compounds has "hampered" the investigation into its active ingredients and mechanisms.

AD is a common inflammation skin disorder that affects about 20% of children in developed countries and often develops in children aged 2 years or younger.

In an accompanying commentary, W.H. Irwin McClean, PhD, from the Centre for Dermatology and Genetic Medicine, University of Dundee, United Kingdom, and Alan D. Irvine, MD, from the Department of Paediatric Dermatology, Our Lady's Children's Hospital, Dublin, Ireland, write, "[T]hese new insights identify the AHR as a key regulatory pathway and therefore a potential drug target for AD."

They point out that past associations between dioxin and coal tar exposure on the AHR "will likely need to be resolved by further experimental work in order to convince regulatory authorities to reassess the viability and safety of potential drugs that act on this pathway." They conclude, however, that the current study "nevertheless sets the scene for an exciting new rational drug design program in this important common disease."

This research was supported by a grant from the Nijmegen Institute for Infection, Inflammation, and Immunity, Radboud University Nijmegen Medical Centre, and Deutsche Forschungsgemeinschaft. The Centre for Dermatology and Genetic Medicine is supported by Wellcome Trust. The authors have disclosed no relevant financial relationships.

J Clin Invest. Published online January 25, 2013.