Previous studies have demonstrated that doxorubicin (DOX) encapsulated in polyisohexylcyano-acrylate nanospheres (NS-DOX) circumvented the resistance of breast cancer cells overexpressing P-glycoprotein (Pgp). Another protein is involved in multidrug resistance phenotype, the multidrug resistance associated protein (MRP1). We report that NS-DOX overcomes multidrug resistance in breast cancer cells overexpressing MRP1. Taking into account that anthracyclines are conjugated to or co-transported with glutathione by MRP1, these data suggest that probably due to ion pair formation (NS-DOX), MRP1 could not transport the anthracycline. Pgp is probably able to transport the ion pair drug complex and the mechanisms of drug resistance reversion in Pgp expressing cells need to be further elucidated.