Compositions and methods are provided for the treatment and diagnosis of diseases or conditions amenable to treatment through modulation of the synthesis or metabolism of multidrug resistance-associated protein (MRP). In accordance with referred embodiments, oligonucleotides are provided which are specifically hybridizable with nucleic acids encoding MRP. In a preferred embodiment, the oligonucleotide has at least one 2'-methoxyethoxy modification. Methods of preventing the development of multidrug resistance and of improving the efficacy of chemotherapy are also disclosed.