Inhibitory effects of clonidine on the allergen-induced wheal-and-flare reactions in patients with extrinsic asthma

We investigated the possibility that clonidine, an alpha 2-adrenoceptor agonist, can reduce the wheal-and-flare reactions induced by intradermal injections of allergen in patients with extrinsic asthma. Ten adult subjects with asthma with positive skin tests to one or several pollens were selected. They received, in random order and double-blind manner, clonidine (two doses, each 75 micrograms) or placebo for 3 days, and then, after a 1-week washout period, they crossed over to the other treatment for 3 days. Treatment with clonidine reduced the area of wheal-and-flare reaction induced by allergen without significantly changing the blood pressure or the plasma cortisol level. There was a drop in the histamine content of leukocytes and in the number of eosinophils in peripheral blood after allergen challenge during the placebo treatment, whereas clonidine prevented these changes. The results suggest that treatment with clonidine can reduce the inflammatory reactions induced by allergens in subjects with extrinsic asthma.