KINETICS OF DAUNORUBICIN TRANSPORT IN EHRLICH ASCITES TUMOR-CELLS WITH DIFFERENT EXPRESSION OF P-GLYCOPROTEIN - INFLUENCE OF VERAPAMIL

The classical multidrug resistance (MDR) phenotype is characterized by a decrease in the intracellular drug concentration in resistant cells as compared to sensitive cells. P-glycoprotein (P-gp) is thought to b e responsible for an active efflux of lipophilic drugs. Four Ehrlich a scites tumor cell lines selected in vivo for resistance to daunorubici n (DNR) and their sensitive counterpart were investigated. The resista nt sublines EHR2/0.1, EHR2/0.2, EHR2/0.4, and EHR2/0.8 were developed by treatment of tumor bearing mice with DNR 0.1, 0.2, 0.4, and 0.8 mg/ kg x 4 weekly, respectively. One passage from EHR2/0.1, EHR2/0.2, and EHR2/0.4 and two passages from EHR2/0.8 were investigated. Western blo t analysis showed significantly different amounts of P-gp (a 6-fold va riation). Efflux of DNR in a drug free medium was investigated with an d without presence of verapamil (VER). Eff-lux from sensitive and resi stant cells was described by mono- and bi-exponential kinetics, respec tively. In ah cases but one, a correlation between resistance, express ion of P-gp, P-gp mediated efflux capacity and effect of VER was estab lished. In passage No. 12 of EHR2/0.8, however, a high expression of P -gp was found in spite of a low degree of resistance and a low efflux capacity. In this subline the effect of VER did not correlate to the e xpression of P-gp. Active efflux seemed to be saturable and was sugges ted to constitute the major route of efflux in MDR cells. A dose-respo nse relationship was established for the effect of VER on efflux. In c onclusion, the results support that P-gp acts as a drug efflux pump. N o simple correlation, however, could be established between P-gp and d rug transport in all the investigated cell lines. Other factors which might influence transmembranous transportation of DNR are suggested. T he active efflux capacity of the cell lines seemed to determine the de gree of resistance and the sensitivity to circumvention by VER.