The extended-MDR phenotype.

Cellular models have made a significant contribution to our understanding of the molecular mechanisms of resistance to chemotherapeutic drugs. However the vast majority of these models involve cell sublines with high levels of resistance generated by continuous exposure to high drug doses, and although the majority express a multidrug resistance (MDR) phenotype, they fall short of the broader drug cross resistance that is characteristic of cancers which no longer respond to treatment. Several studies have reported cell sublines which not only have the MDR phenotype and are resistant to 'natural product' lipophilic drugs, but they are also resistant to alkylating agents and antimetabolites. A common feature of these sublines is they were generated by treatment with low, clinically relevant levels of drug given intermittently. The term extended-MDR has been used to describe this type of broad drug cross resistance. Here we review those factors that promote the development of extended-MDR, the characteristics of extended-MDR sublines and the possible resistance mechanisms involved.