Sensitization of multidrug-resistant human leukemia cells with MDR1-targeted antisense and inhibition of drug-mediated MDR1 induction.

Increased expression of MDR1 is strongly implicated in the appearance of chemotherapeutic drug resistance in cancer, especially hematological malignancies. We therefore examined the potential of antisense oligonucleotides to inhibit MDR1 and restore sensitivity to drug-resistant human lymphoblastic cells (CCRF-CEM). Treatment with two different phosphorothioate-modified antisense sequences as well as a DNA-RNA hybrid sequence resulted in a 30 to 45% decrease in MDR1 expression as determined by staining with the monoclonal antibody MRK16 followed by flowcytometry (FCM) analysis. Further, inhibition of MDR1 expression persisted for 3 days after removal of oligonucleotides. Increased accumulation of rhodamine 123 and nearly a three-fold sensitization of cells to vincristine paralleled the reduction in staining with MRK16. Reversed or scrambled control sequences had no effect in any of the assays. During the course of these studies, we observed a 25 to 75% increase in MRK16 staining of cells treated with the chemotherapeutic agents daunorubicin and vincristine as well as by the resistance reversal agents verapamil and cyclosporin. Treatment of cells with antisense oligonucleotides prior to exposure to daunorubicin or cyclosporin reduced the increase in MRK16 staining. These results indicate that antisense targeted to MDR1 can sensitize drug-resistant leukemia cells and suggest that antisense treatment may prevent the emergence of MDR1-mediated drug resistance.