Drug retention, efflux, and resistance in tumor cells.

Multiple drug resistance (MDR) in tumor cells has been related to the expression of transport proteins which alter cellular drug transport and distribution. Three different genes (mdr, MRP, and LRP) and their products have been implicated in MDR. Several fluorescent dyes have been used to monitor the effect of these transport proteins on drug retention, as well as for screening of drugs which block drug efflux and thus enhance cellular drug retention and cytotoxicity. The present review summarizes current knowledge about MDR phenotypic markers and techniques available for study of MDR by flow cytometry.