What is "zaprinast"

n. A drug acting as a phosphodiesterase inhibitor, unsuccessful in clinical trials.

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. IC values are 0.76, 0.15, 29.0, and 12.0 μM, respectively.

Zaprinast inhibits the growth of asexual blood-stage malaria parasites ( P. falciparum) in vitro with an ED value of 35 μM, and inhibits PfPDE1, a P. falciparum cGMP-specific phosphodiesterase, with an IC value of 3.8 μM.

Zaprinast has also been shown to activate the orphan G-protein coupled receptor known as GPR35, both in rats and humans - however the clinical significance of this has yet to be determined.