Immunosuppressive effects of clonidine on the induction of contact sensitization in the balb/c mouse.

The clonidine transdermal therapeutic system (clonidine-TTS) has been associated with a significant incidence of allergic contact sensitization. This incidence was not predicted by premarket skin sensitization testing in animals or humans. One possible explanation lies in recent findings in guinea pigs that clonidine exposure could inhibit the elicitation of skin reactions to unrelated strong contact sensitizers. However, these studies also showed that clonidine pretreatment did not appear to affect the induction of contact sensitization. On this basis, we sought to specifically evaluate the induction phase of sensitization to clonidine as an alternative means of assessing its sensitization properties. The method selected was the assay of in situ lymphocyte proliferation in lymph nodes draining the sites of clonidine exposure, a method recently promoted as an alternative means to assess contact allergenic potential. Utilizing various induction application techniques and regimens, we were consistently unable to demonstrate clonidine's allergenic potential through such an assessment of lymphocyte proliferation. We were also unable to demonstrate sensitization by in vivo ear swelling or in vitro lymphocyte blastogenesis assay techniques. However, a subsequent assessment of the effect of clonidine exposure on the induction of sensitization to unrelated strong contact allergens demonstrated a consistent 40-70% inhibition of the proliferative response to the contact allergens oxazolone and trinitrochlorobenzene. This was similar to the degree of suppression produced by the corticosteroids fluocinonide and hydrocortisone when they were tested at 80 and 10 times lower concentrations. In addition, we observed a comparable inhibition of the ear swelling response to oxazolone. These data extend our knowledge of the immunomodulatory effects of clonidine and offer additional mechanistic insights into the failure of short-term predictive patch-test methods to detect this chemical's potential to induce allergic contact sensitization.