Two main pharmacological effects: 1) they inhibit the enzymes responsible for the breakdown of cAMP (phosphodiesterase) 2) they may impact on both the inflammatory processes and the bronchoconstriction seen in asthma - theophylline is considered a general antagonist of adenosine receptors, this is likely to contribute to its mechanism of action because adenosine (when not blocked) causes bronchoconstriction.