Tonapofylline
Tonapofylline is a drug which acts as a potent and selective antagonist for the adenosine A1 receptor. It was developed as a potential agent for the treatment of heart failure by increasing sodium excretion by the kidneys, and reached Phase III human clinical trials, showing reasonable efficacy with a good safety profile. However, it was ultimately not adopted for medical use, though it continues to be used in research. Its activity as a phosphodiesterase inhibitor does not appear to have been tested, though most related xanthine derivatives are phosphodiesterase inhibitors as well as adenosine receptor antagonists.