Drug resistance in cancer: a multifactorial problem.

Drug resistance is an important problem in the treatment of patients with cancer. Tumors become resistant not only to the drugs used initially, but also to those to which they have not yet been exposed. Multiple mechanisms contribute to drug resistance. Many of them are inter-related or independent of each other, but may exist simultaneously in cancer cells or subpopulations of cells, producing an overall drug-resistant phenotype. Consequently, clinical reversal of drug resistance may ultimately require intervention at several different sites in the tumor cell. In the future, the use of DNA microarray technology in drug resistance in cancer will yield insight into the mechanisms of drug resistance and the rational design of more effective strategies to circumvent resistance.