Transcription of MDR1

Modes of drug resistance in tumor cells. Tumor cell populations are indicated by colored geometries. Black arrows indicate the passage of time over which drug is administered.

A. Intrinsic drug resistance. The tumor is predominantly resistant to drug treatment, perhaps due to inherent properties of the tumor cells (red circles).

B. Constitutive (acquired) resistance. Within the population of tumor cells there exists, in addition to drug-sensitive cells (blue squares), a subpopulation of cells that, by virtue of mutation, have acquired an MDR phenotype (red circles). These MDR cells survive initial therapy and repopulate as a resistant tumor.

C. Transient (acquired) resistance. A subset of cells (red checked squares) within the tumor cell population are able to survive drug treatment temporarily by overexpressing the MDR1 gene. In laboratory experiments, this resistance is transient; the significance of this resistance in the clinical setting is still under investigation.