Zaprinast
Zaprinast was an unsuccessful clinical drug candidate that was a precursor to the chemically related PDE5 inhibitors, such as sildenafil (Viagra), which successfully reached the market. It is a phosphodiesterase inhibitor, selective for the subtypes PDE5, PDE6, PDE9 and PDE11. IC50 values are 0.76, 0.15, 29.0, and 12.0 μM, respectively.