What Does Multidrug Resistance (MDR) Expression Mean in the Clinic?

For 15 years, an overflowing literature has been published about MDR-1 gene expression in tumor cell lines and in cancerous tissues at various stages of disease and treatment (chemotherapy-naive, during treatment and at relapse). However, the clinical significance of this particular feature, if it seemed obvious in the 1980s as a factor responsible for the development of chemoresistance, is currently reconsidered. MDR-1 gene expression seems to be, at least in some instances, a hallmark of tumor cell aggressiveness and of chemoresistance rather than its cause, the mechanisms of which are probably far more complex. The failure of MDR reversal trials might result from the misunderstood or overvalued role of MDR expression in cancer cells rather than from a lack of control of pharmacological parameters. This review summarizes recent data and hypothesis about the expression of P-170 and its clinical significance in some important human tumor types, suggesting that it should rather be considered in the future as an adverse prognostic factor.