MK-0873 is a novel selective phosphodiesterase-4 inhibitor, which has been in development for the treatment of chronic obstructive pulmonary disease (COPD). In preclinical studies, intravenous administration of MK-0873 produced potent, dose-dependent inhibition of antigen-induced bronchoconstriction in sensitized guinea pigs. MK-0873 protected against both the early- and late-phase bronchoconstrictor response to antigen challenge in Ascaris-sensitive sheep. In healthy volunteers, MK-0873 has been shown to be safe and generally well tolerated in single oral doses up to 6 mg and in multiple oral doses up to 4 mg for 28 days. The most commonly reported adverse events were similar to those reported in other studies with drugs in its class: i.e. headaches and gastrointestinal complaints.