Abstract Nonsteroidal anti-inflammatory drugs (NSAIDs) and cyclooxygenase (COX)-2 selective inhibitors display promising antineoplastic activity, but the gastrointestinal, renal, and cardiovascular toxicities that result from COX inhibition limit their clinical utility for cancer chemoprevention. Previous studies suggest that the pro-apoptotic activity of these drugs may be due to a COX-independent mechanism involving cyclic guanosine monophosphate (cGMP) phosphodiesterase (PDE) inhibition. However, the specific PDE isozyme(s) responsible has not yet been identified. Here we used the NSAID sulindac sulfide (SS) to further study cGMP PDE as a potential molecular target of the NSAIDs for breast cancer chemoprevention. SS inhibited growth and induced apoptosis of human SK-BR-3 and MDA-MB-231 breast tumor cells, while normal human mammary epithelial cells (HMEC) were less sensitive. SS also inhibited cGMP PDE activity from tumor cell lysates at concentrations comparable to those required for apoptosis, while appreciably higher concentrations were necessary to inhibit activity from HMEC lysates or cAMP PDE activity from any of the cell lysates. Additionally, SS increased intracellular cGMP concentrations and induced protein kinase G (PKG) activity in MDA-MB-231 cells as determined by ELISA and increased levels of phosphorylated VASP, a known PKG substrate, respectively. Moreover, pretreatment with the guanylyl cyclase inhibitor LY83583 desensitized the breast tumor cells to the growth-inhibitory activity of SS, likely by suppressing intracellular cGMP levels. The guanylyl cyclase activator NOR-3 and the cGMP PDE inhibitors MY5445 and trequinsin inhibited tumor cell growth, while the adenylyl cyclase activator forskolin and other PDE inhibitors had no effect. HMEC were found to express several cGMP hydrolyzing PDE isozymes (PDE1, 5, and 9), while the breast tumor cells expressed predominantly PDE5, which was confirmed to be sensitive to inhibition by SS using a recombinant enzyme. This work demonstrates that inhibition of PDE5A, elevation of cGMP, and activation of PKG are responsible for the pro-apoptotic effects of SS in human breast tumor cells and that differential expression of PDE isozymes between normal and tumor cells may explain the tumor selectivity of NSAIDs. These data also suggest that it may be feasible to develop NSAID derivatives with increased selectivity for inhibiting PDE5 and decreased COX-inhibitory activity, which would potentially improve safety and efficacy for breast cancer chemoprevention. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1041.