Flow Cytometric Monitoring of Fluorescent Drug Retention and Efflux

Laser flow cytometry has been used for monitoring cellular retention of fluorescent drugs such as fluorescent anticancer antibiotics (e.g., doxorubicin) and fluorochromes used for the detection of cellular drug efflux and resistance (e.g., rhodamine 123, Hoechst 33342). Multi parametric flow cytometry can be used for identification of tumor cell subpopulations based on their drug retention profiles with or without the presence of an efflux blocker. This rapid proce dure can be used for identification of tumor cells with the drug-resistance phenotype based on drug efflux as well as for efflux blockers that may block efflux of a chemotherapeutic agent and thus increase cellular retention and sensitivity. It has been reported recently that some of the bone marrow stem cells (SP cells) efflux the Hoechst 33342 fluorochrome and thus can be rapidly identified by comparing red vs blue fluorescence in the presence or absence of an efflux blocker such as verapamil. The present chapter discusses some of the flow cytometric methods used for the study of cellular drug retention and the artifacts that may arise in such analysis.