Immunomodulatory actions of xanthines and isoenzyme selective phosphodiesterase inhibitors

Theophylline and related xanthines have been used in the treatment of airways diseases such as bronchial asthma for over 50 years. The therapeutic effectiveness of this class of drugs was traditionally thought to be derived from their ability to elicit bronchodilation, however evidence is accumulating to suggest that theophylline in particular possesses immunomodulatory and anti-inflammatory activity. The molecular mechanisms of action of theophylline have not yet been clarified, although several putative mechanisms of action have been proposed. One of these suggests that theophylline via non-selective inhibition of the phosphodiesterase (cAMP) and cyclic guanosine monophosphate (cGMP) levels resulting in relaxation of airway smooth muscle and inhibition of inflammatory cell activation. To date seven different families of the PDE enzyme have been defined according to a variety of criteria including substrate specificity, sensitivity to selective inhibitors and the effect of allosteric modulators. The type IV isoenzyme is the predominant isoenzyme in most inflammatory cells. This article reviews some of the in vitro, in vivo and clinical studies which have demonstrated that theophylline and selective PDE inhibitors possess anti-inflammatory and immunomodulatory activity.