Background: Clinical studies have reported overexpression of PDE5 and elevation of intracellular cyclic GMP in various types of cancer cells. ABCC5 transports intracellular cGMP out of the cells with high affinity, while PDE5 inhibitors prevent high affinity cGMP efflux by inhibiting ABCC5. Increasing intracellular cGMP levels through inhibition of PDE5 and PDE5 export activity is hypothesized ...