Detection of P-glycoprotein expression by tumoral cells with NBDL-CsA, a fluorescent derivative of cyclosporin A.

The P-glycoprotein (P-gp) molecules which are expressed on multidrug-resistant (MDR) tumor cells efflux a variety of anti-cancer drugs, such as doxorubicin. Though first described as an inhibitor of P-gp function, cyclosporin A (CsA) was more recently shown to behave as a substrate of the P-gp pump. The retention of [3H]CsA was reduced in MDR cells of the human leukemic CEM cell subline, in comparison with the drug-sensitive parental (Par) subline. MDR-CEM cell treatment by the P-gp blockers restored the [3H]CsA retention to the control Par-CEM cell levels. Using a novel fluorescent CsA derivative, [N-epsilon-(4-nitrobenzofurazan-7-yL)-D-Lys8] cyclosporin (NBDL-CsA), we now show that MDR cells can be distinguished from Par cells both at the cell population level (in microculture) and at the single cell level (by use of flow cytometry).