Monitoring of cellular resistance to cancer chemotherapy.

Cellular resistance to a broad spectrum of natural products used as antitumor drugs is believed to be a major cause for the failure of chemotherapy. Flow cytometry has been used for monitoring the expression of drug resistance markers, determining accumulation of fluorescent drugs, and for screening of drugs that enhance chemosensitivity by blocking efflux and enhancing drug retention. This article reviews recent developments in our understanding of the multiple drug resistance phenotype and the use of flow cytometry for the study of drug efflux and its modulation in human tumor cells.