Cells selected for resistance to a single cytotoxic drug may become cross-resistant to a broad range of drugs with different structures and cellular targets. This phenomenon is called multiple drug resistance (MDR). The MDR proteins (Mdrs) are members of a highly conserved superfamily of ATP-binding cassette transport proteins.Research studies have shown that MDR1 reduces drug accumulation in cancer cells, allowing the development of drug resistance. On the other hand, MDR1 expressed in the plasma membrane of cells in the blood-brain, blood-cerebral spinal fluid, or blood-placenta barriers restricts the permeability of drugs into these organs from the apical or serosal side .
