ABC transporter inhibitors in reversing multidrug resistance to chemotherapy

The superfamily of human ATP-binding cassette (ABC) transporters comprises seven subfamilies (ABCA to G) with 48 members. In addition to their profound physiological and pharmacological functions, ABC transporters play important roles in instigating multidrug resistance (MDR) in cancer by mediating the efflux of many anticancer drugs, particularly, ABCB1, ABCG2 and ABCC subfamily members. Previous development of ABCB1 transporter inhibitors has provided insights into seeking novel strategies in developing new classes of compound that inhibit ABCB1 and other MDRrelated ABC transporters. We herein review and evaluate current evidence in this area, with an emphasis on experimental and investigational agents that are under preclinical and clinical tests, including tyrosine kinase inhibitors, natural products, microRNAs and novel chemical entities. New strategies targeting ABC transporters in cancer stem cells and future perspectives in this field are also discussed.