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,[1] selective for the subtypes PDE5, PDE6, PDE9 and PDE11.

[2][3] Zaprinast inhibits the growth of asexual blood-stage malaria parasites (P. falciparum) in vitro with an ED50 value of 35 μM, and inhibits PfPDE1, a P. falciparum cGMP-specific phosphodiesterase, with an IC50 value of 3.8 μM.

[4] Zaprinast has also been shown to activate the orphan G-protein coupled receptor known as GPR35, both in rats and humans,[5] and to inhibit the mitochondrial pyruvate carrier.

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