PRKG1 and breast cancer: Furthermore, the COX inhibitor sulindac sulfide was shown to down-regulate cGMP-selective PDE5 and induce cGMP accumulation and PKG activation, thus leading to apoptosis and cell growth inhibition in human SK-BR-3, ZR75-1, and MDA-MB-231 breast cancer cells (IC50 60–85 μmol/L).