PDE5A and cancer: Phosphodiesterase type 5 (PDE5) inhibitors (e.g., sildenafil and vardenafil) either mono-treatment or combination with cancer therapeutic drugs have been suggested to display anti-proliferative and apoptotic activities in vitro and suppress tumor growth in vivo against several cancers, such as prostate cancer, lung cancer, breast cancer, colon adenocarcinoma, neuroblastoma, and oral cancer [5,6,7,8,9].