ABSTRACT. It has been suggested that selected COX inhibitors can overcome multidrug resistance through the inhibition of ATP-binding cassette-transporter proteins thereby enhancing the inhibitory effect of doxorubicin on human tumor growth and promoting the actions of cytostatics. However, their effect on lung cancer and the molecular mechanisms involved in the overcoming of multidrug resistance are unclear. In the present study, the ability of meloxicam, a COX-2-specific inhibitor to enhance doxorubicin-mediated inhibition was investigated in human A549 lung cancer in vivo and in vitro. In order to unravel the molecular mechanisms involved in doxorubicin accumulation, we measured the levels of multidrug resistance-associated protein (MRP)-transporter protein activity and expression by western blotting, since this has been implicated in meloxicam action as well as in chemoresistance. We found that, in A549 cells, meloxicam could increase intracellular accumulation of doxorubicin, a substrate for MRP, through inhibition of cellular export. Western blot analysis indicated that meloxicam reduced 14549 COX-2 inhibitor as a lung cancer therapeutic ©FUNPEC-RP www.funpecrp.com.br Genetics and Molecular Research 14 (4): 14548-14560 (2015) the expression of MRP1 and MRP4. The results reported in the present study demonstrate for the first time that the specific COX-2 inhibitor meloxicam can increase the intracellular accumulation of doxorubicin and enhance doxorubicin-induced cytotoxicity in A549 cancer cells by reducing the expression of MRP1 and MRP4.