Prenylated isoflavones are suitable to specifically inhibit P-glycoprotein in multidrug-resistant cancer cells leading to an accumulation of cytotoxic compounds or therapeutic compounds in the cells while having exceptionally increased cytotoxic activity specifically towards multidrug-resistant cancer cells and while allowing for an increased activity of chemotherapeutic compounds towards which the cells are resistant.Also in accordance with the present invention is a method for specifically targeting cancer cells with multidrug-resistance as well as a method of potentiating the activity of a chemotherapeutic compound in multidrug-resistant cancer cells. In a further aspect, a kit including a prenylated isoflavone and a chemotherapeutic compound is provided.