One of the major problems in cancer chemotherapy is the development of drug resistance during treatment. The nature of drug resistance in cancer patients is complex. Recently, it has been found that tumor cells can acquire resistance to anticancer drugs. It is now generally accepted that drug resistance at the cellular level (cellular resistance) is also an important mechanism of drug resistance in patients.
The elucidation of the resistance mechanisms has progressed well recently owing to the application of cellular and molecular techniques in addition to the usual biological and biochemical techniques. In this article, I describe the mechanisms of cellular resistance, especially those of multidrug resistance at the molecular level, and I also discuss possible approaches to overcoming drug resistance.