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 "μ"M, respectively. Zaprinast inhibits the growth of asexual blood-stage malaria parasites ("P. falciparum") "in vitro" with an ED50 value of 35 "μ"M, and inhibits PfPDE1, a "P. falciparum" cGMP-specific phosphodiesterase, with an IC50 value of 3.8 "μ"M.  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.