Enhancement of antitumor effects of doxorubicin with phosphodiesterase-5 inhibitors

Background. Doxorubicin (DOX) is a chemotherapeutic agent that is highly effective for treatment of various hematological and solid malignancies. Phosphodiesterase-5 (PDE-5) inhibitors including sildenafil (SIL), tadalafil (TAD) and vardenafil (VARD) are a class of drugs used for treatment of erectile dysfunction. These drugs have been shown to have powerful anti-ischemic effects and protect against DOX-induced cardiomyopathy. The present study examined the effect of PDE-5 inhibition on enhancing the anti-cancer effects of DOX.;Methods and Results. Ovarian cancer UCI-101 and A2780, sarcoma, OSA-1, breast cancer MDA-MB231 and prostate cancer PC-3 cell lines were treated with SIL, DOX or combination of DOX plus SIL. Cell viability was assessed using CellTiter 96RTM Cell Proliferation Assay. DOX-induced cell killing was enhanced with SIL in all cell lines. Similar enhancement of cell killing was observed with VARD or TAD plus DOX. Apoptosis as assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining was higher after treatment with DOX plus SIL as compared to DOX alone in A2780, UCI-101 and OSA-1 cells. SIL administration alone had no effect on cell viability or apoptosis in all cell lines. SIL plus DOX enhanced the activity of caspase-3 as compared to DOX alone. The expression of antiapoptotic protein Bcl-2 was decreased with the DOX and DOX + SIL treatments. Additionally pro-death protein Bax was induced with concomitant increase in Bax/Bcl-2 ratio favoring apoptosis. Reactive oxygen species (ROS) also increased more in response to DOX plus SIL as compared to DOX alone. N-mercaptopropionyl glycine (MPG), a putative antioxidant, attenuated the enhanced cytotoxicity of DOX and SIL. Additionally N(G)-nitro-L-arginine methyl ester, a non-specific nitric oxide synthase inhibitor partially blocked the enhancement of cytotoxicity with SIL+DOX. Using a tumor xenograph model in athymic nude mice DOX plus SIL provided a larger reduction in final tumor weight than DOX administered alone.;Conclusion. PDE-5 inhibition enhances in vitro and in vivo cytotoxicity of DOX in cancer cells. The increased cell killing occurs through mechanisms involving apoptosis, nitric oxide and augmentation in the levels of ROS. These results suggest a potentially novel use of the PDE-5 inhibitors as adjuvant in enhancing the chemotherapeutic efficacy of DOX.