Transgenic mice that express the human multidrug-resistance gene in bone marrow enable a rapid identification of agents that reverse drug resistance.

The development of preclinical models for the rapid testing of agents that circumvent multidrug resistance in cancer is a high priority of research on drug resistance. A common form of multidrug resistance in human cancer results from expression of the MDR1 gene, which encodes a Mr 170,000 glycoprotein that functions as a plasma membrane energy-dependent multidrug efflux pump. We have engineered transgenic mice that express this multidrug transporter in their bone marrow and demonstrated that these animals are resistant to leukopenia by a panel of anticancer drugs including anthracyclines, vinca alkaloids, etoposide, taxol, and actinomycin D. Differential leukocyte counts indicate that both neutrophils and lymphocytes are protected. Drugs such as cisplatin, methotrexate, and 5-fluorouracil, which are not handled by the multidrug transporter, produce bone marrow suppression in both normal and transgenic mice. The resistance conferred by the MDR1 gene can be circumvented in a dose-dependent manner by simultaneous administration of agents previously shown to be inhibitors of the multidrug transporter in vitro, including verapamil isomers, quinidine, and quinine. Verapamil and quinine, both at levels suitable for human trials that produced only partial sensitization of the MDR1-transgenic mice, were fully sensitizing when used in combination. We conclude that MDR1-transgenic mice provide a rapid and reliable system to determine the bioactivity of agents that reverse multidrug resistance in animals.