Effectiveness of Type I interferons in the treatment of multidrug resistant osteosarcoma cells. P-glycoprotein overexpression is an important adverse prognostic marker for osteosarcoma (OS) patients, which is associated with higher risk for developing metastases as a consequence of the limited responsiveness to standard treatments of P-glycoprotein overexpressing OS cells. The use of cytokines has been advocated as a possible therapeutic approach to overcome multidrug resistance (MDR), being active on cell lines that are resistant to conventional drugs. In this study, we evaluated in vitro effects of interferons (IFNs) on MDR P-glycoprotein overexpressing OS cells. Type I IFNs, but not IFNgamma, showed tangible inhibitory effects on OS cell growth, which were higher in MDR cell lines compared to parental cells. The higher sensitivity of P-glycoprotein overexpressing cells to Type I IFNs correlates with higher expression of the activator of the transcription (STAT)-2 and (STAT)-3, two intracellular mediators of the IFNalpha and IFNbeta signaling pathways, whereas no differences were observed with respect to the expression or activation of the Type I IFN receptor and STAT-1. Exposure of OS MDR cells to Type I IFN decreased the expression of P-glycoprotein. This effect resulted in a significantly increased chemosensitivity of MDR cells to doxorubicin. Therefore, our data support the use of IFNalpha or IFNbeta in the treatment of osteosarcoma patients who overexpress P-glycoprotein in their primary tumors, and respond insufficiently to current therapeutic regimens.