The detection of rhodamine 123 efflux at low levels of drug resistance.

Although many cell models of multidrug resistance (MDR) have been developed, most have been high-resistance models which generate up to 100-fold increases in drug resistance. However, the drug concentrations required to achieve these levels of resistance are much higher than those found in vivo. In this paper we describe the development of a cell model that reflects the resistance levels that are likely to be found clinically. We then investigated the methods used to detect MDR1 expression at these low levels of drug resistance. We demonstrated that the immunological and PCR-based methods are unable to detect increased MDR1 expression in cells with a < 5.2- and 6.5-fold increase in vinblastine resistance, respectively, in our drug-resistant sublines. The rhodamine 123 (Rh 123) efflux assay was able to discriminate the vinblastine-sensitive parent cells from all the vinblastine-resistant sublines, including cells with a 1.7-fold increase in resistance. However, this assay is non-specific to the MDR1 gene and may detect the activity of other drug efflux proteins such as the MDR-associated protein (MRP). Our results show that the Rh 123 efflux assay is able to detect the activity of drug efflux proteins such as the P-glycoprotein, MRP or other efflux systems at the low levels of drug resistance that are likely to be attained in vivo.