CALMODULIN INHIBITOR TRIFLUOPERAZINE IN COMBINATION WITH DOXORUBICIN INDUCES THE SELECTION OF TUMOR-CELLS WITH THE MULTIDRUG-RESISTANT PHENOTYPE

Trifluoperazine (TFP) is effective in modulating DNA damage/repair in doxorubicin (DOX) treated cells. In the present study we have characte rised the resistance phenotype of parental sensitive L1210 mouse leuka emia cells (L1210/S) adapted to grow in the presence of 0.017 mum DOX + 5 mum TFP (L1210/DT). Although with prolonged exposure, 0.017muM DOX alone produced <35% cell kill in L1210/S cells, similar cytotoxicity was achieved at 0.43 mum DOX in L1210/S cells selected in the presence of 0.017 muM DOX + 5 muM TFP. L1210/DT cells were >30-fold resistant to DOX following a 3 h drug exposure in a soft agar colony assay. In c ontrast, DOX sensitivity in cells adapted to grow in 5 muM TFP alone w as comparable to L1210/S cells. Resistance to other inhibitors of topo isomerase II in L1210/DT cells was >30-fold to epotoside and >6-fold t o amsacrine. The levels of the 170 kDa and 180 kDa isoforms of topoiso merase II in an immunoblot were comparable between the L1210/S and L12 10/DT cells. Cross resistance to vincristine in the L1210/DT cells was accompanied by the overexpression of plasma membrane P-glycoprotein. Although a 1.5-2-fold decrease in accumulation of etoposide and DOX wa s observed in the L1210/DT cells, drug levels for equivalent DNA damag e in the alkaline elution assay were >5-fold higher in the L1210/DT ve rsus L1210/S cells. No abrogation in the modulating effects of TFP on DOX, VP-16 or amasacrine induced cytotoxicity was apparent in the L121 0/DT cells. Results suggest that: (a) TFP in combination with low conc entrations DOX can induce the selection of cells with the multidrug re sistant phenotype; and (b) characteristics of cells selected for resis tance to DOX or DOX plus TFP are comparable.