Heterogeneity of anthracycline retention and response to efflux blockers in human tumors.

Rapid cellular efflux of certain natural products used in cancer chemotherapy leads to reduced cytotoxicity and resistance. Multiple Drug Resistance (MDR) gene related glycoproteins are believed to act as the drug efflux pump. Several non-cancer therapy drugs (e.g., verapamil, phenothiazines) compete for the p-glycoprotein pump and thus can block efflux of a chemotherapeutic agent and overcome cellular resistance. Anthracyclines, such as adriamycin, are intrinsically fluorescent and thus their cellular retention can be determined by laser flow cytometry. This method has been used to study heterogeneity in cellular retention of anthracyclines and response to efflux blockers in human tumor cells. These studies have led to generation of clinical protocols where laser flow cytometry is used to monitor and overcome heterogeneity in drug retention of tumor cells from patients treated with a combination of adriamycin and efflux blockers.