Multiple drug resistance mechanisms in cancer.

Multiple drug resistance (multidrug resistance; MDR), a phenomenon whereby human tumours that acquire resistance to one type of therapy are found to be resistant to several other drugs that are often quite different in both structure and mode of action, has been recognised clinically for several decades. An important advance in our understanding of MDR came with the identification of P-glycoprotein and other related transporters that were expressed in some cancer cells and could recognise and catalyse the efflux of diverse anticancer drugs from cells. A second advance came from an understanding of the mechanism of programmed cell death or apoptosis, leading to MDR mediated by increased to resistance to anticancer drug-induced apoptosis. A third advance came with the finding that the proliferation of human tumours was driven by a small population of self-renewing tumour cells, focussing attention on the MDR properties of these so-called tumour stem cells rather than on the cells that comprised the majority of the tumour population. A fourth advance was the delineation of features of the tumour microenvironment, including immunosuppression, which essentially provided tumour stem cells with an MDR phenotype. Most published work on the overcoming of MDR has concentrated on inhibition of drug transporters but the complexity of mechanisms contributing demands a broad strategy for the development of methods to overcome MDR in a clinical setting.