Comparison of the efficacy of a phenothiazine and a bisquinaldinium calmodulin antagonist against multidrug-resistant P388 cell lines.

Dequalinium belongs to a group of cationic lipophilic drugs believed to be selectively cytotoxic to malignant cells of epithelial origin by virtue of their accumulation in mitochondria. In addition, they are potent inhibitors of calmodulin and, therefore, might sensitize multidrug-resistant cells to chemotherapeutic agents. We compared the responsiveness of multidrug-resistant cells to the effect of dequalinium with that to trifluoperazine, a potent phenothiazine inhibitor of calmodulin. In addition, we studied the effect of these drugs on the responsiveness of multidrug-resistant cell lines to doxorubicin. The effect of drugs on P388 murine leukemic cells was determined by cell counting, [3H]thymidine incorporation into DNA, or soft agar cloning. Drug accumulation was measured by fluorescence spectrophotometry. We found that multidrug-resistant lines were less sensitive than parental cell lines to the intrinsic growth inhibitory effects of dequalinium (IC50, 4.4 versus 0.3 microM in multidrug-resistant and sensitive P388 cells, respectively), whereas they were equally sensitive as the parental line to the effects of trifluoperazine. Following a 3-h exposure of P388/doxorubicin-resistant cells to 0-100 microM doxorubicin with or without either 10 microM dequalinium or 10 microM trifluoperazine, the latter increased the sensitivity to doxorubicin whereas the former had little effect (IC50 values were doxorubicin, 30 microM; doxorubicin plus dequalinium, 25 microM; doxorubicin plus trifluoperazine, 4 microM). Calmodulin prepared from resistant cells were equally sensitive to inhibition by dequalinium and trifluoperazine. P388/doxorubicin-resistant cells accumulated 4.5-fold less dequalinium than P388 cells whereas trifluoperazine was accumulated equally in both. The addition of 4 microM trifluoperazine to resistant cells exposed to 0-100 microM dequalinium completely reversed the alteration in accumulation and resistance to the dequalinium. These studies demonstrate that certain multidrug-resistant lines are cross-resistant to dequalinium and that sensitivity can be completely restored by nontoxic concentrations of trifluoperazine. The resistance appears to be due to changes in drug accumulation and not to be related to an altered sensitivity of calmodulin.