Roles of sildenafil in enhancing drug sensitivity in cancer.
Department of Pharmaceutical Sciences, College of Pharmacy and Allied Health Professions, St. John's University, Queens, New York, USA.
The phenomenon of multidrug resistance (MDR) has decreased the hope for successful cancer chemotherapy. The ATP-binding cassette (ABC) transporter superfamily is the largest transmembrane family. The overexpression of ABC transporters is a major determinant of MDR in cancer cells both in vitro and in vivo. Unfortunately, until recently, most of the strategies used to surmount ABC-transporter-mediated MDR have had limited success. An ideal modulator of MDR would be one that has a low liability to induce toxicity and alter the pharmacokinetic profile of antineoplastic drugs. Sildenafil, an inhibitor of cGMP-specific phosphodiesterase type 5, was found to significantly reverse ABC-transporter-mediated MDR. Our results indicate that sildenafil has differential inhibitory effects on ABC transporters: It significantly decreases the efflux activity of ABCB1 and ABCG2, but has no significant effects on ABCC1. Emerging evidence indicates that sildenafil and other phosphodiesterase type 5 inhibitors may enhance the sensitivity of certain types of cancer to standard chemotherapeutic drugs.