This randomized, double-blind, parallel, placebo-controlled, single rising-dose study investigated the safety, tolerability, pharmacokinetic, and pharmacodynamic profiles of BI 1356 (once-daily, given orally) in healthy men. BI 1356 was well tolerated and safe up to and including a dose of 600 mg. The incidence of drug-related adverse events was equal in subjects receiving BI 1356 (30%) or placebo (31%). No clinically relevant deviations in laboratory or ECG parameters were reported. Exposure of BI 1356 increased less than proportionally from 2.5 mg to 5 mg, more than proportionally from 25 mg to 100 mg and approximately proportionally for doses from 100 mg to 600 mg. The geometric mean terminal half-life was up to 184 hours. Renal excretion was low. All doses of BI 1356 inhibited plasma dipeptidyl peptidase 4 activity. Single doses of 2.5 mg and 5 mg inhibited dipeptidyl peptidase 4 activity by 72.7% and 86.1% from baseline, respectively. The time to achieve maximum inhibition shifted with increasing doses from 3 hours (2.5 mg) to <0.7 hours (> or =200 mg). Within the dose range tested, a direct pharmacokinetic/pharmacodynamic relationship was observed. The pharmacokinetic and pharmacodynamic profile results demonstrate the potency and full 24-hour duration of action of BI 1356. Based on an estimated therapeutic dose of 5 mg, the therapeutic window of BI 1356 is expected to be >100-fold.