Tonapofylline is an antagonist of adenosine A1 receptor being developed for heart failure. In the present studies, pharmacokinetic characteristics, including dose proportionality, bioavailability, and effects of gender and food, were evaluated in healthy subjects receiving single-dose tonapofylline (0.2-375 mg) in a parallel or crossover design. Following oral administration, tonapofylline concentrations mostly peaked within 3 hours and declined over time in a multiple phasic manner. Based on a power model, dose proportionality of peak concentration (C(max)), area under the time-concentration curve for all values (AUC(all)), and area under the time-concentration curve to infinity (AUC(inf)) was concluded in a clinical setting. The bioavailability of tonapofylline was 81.2% (90% confidence interval, 70.6%-93.5%). Following intravenous administration, the steady-state volume of distribution of tonapofylline was estimated to be 756 mL/kg. The total clearance of tonapofylline was low (64.8 mL/h/kg), approximately 5% of hepatic blood flow. The terminal half-life was variable within groups and ranged from 11.2 to 24.2 hours across the dose range. Female subjects showed significantly higher C(max), AUC(all), and AUC(inf) than male subjects (P < .05). Food decreased C(max) by approximately 39%, whereas it did not appear to affect AUC(all) and AUC(inf). The intersubject variability of the pharmacokinetic parameters of tonapofylline was generally less than 30%. In these studies, a single dose of tonapofylline was safe and well tolerated.