TIME-DEPENDENCE OF [H-3] VINCRISTINE ACCUMULATION BY L1210 MOUSE LEUKEMIC-CELLS - EFFECT OF P-GLYCOPROTEIN OVEREXPRESSION

Overexpression of P-glycoprotein (P-GP) accompanied by multidrug resis tance (MDR) to diverse groups of cytostatics was developed by long-ter m adaptation of mouse leukemic cell line L1210 to vincristine. Two res istant sublines of cells characterized by ID50 values for vincristine 1.05 mg/l (L1210/VCR-1) and 2.3 mg/l (L1210/VCR-2), respectively, were used. The sensitive parental cell line L1210 had the ID50 value for v incristine around 0.01 mg/l. Overexpression of P-GP induced by the ada ptation procedure was found to be accompanied by an increase in the me an cell diameter from 10.28 +/- 1.60 mu m (mean +/- S-D, n = 122) for sensitive L1210 cells to 17.82 +/- 2.59 mu m (n = 120) and 37.26 +/- 5 .72 mu m (n = 121) for L1210/VCR-1 and L1210/VCR-2 resistant cell subl ines, respectively Significant decrease in ability to accumulate [H-3] -vincristine from cultivation medium was observed for both resistant c ell sublines in comparison to sensitive cells. Accumulation of [H-3]-v incristine by sensitive cells is secured only by passive diffusion of the drug across the plasma membrane. Contrary to that, active efflux o f drug operating against its diffusion across the plasma membrane shou ld be assumed as a factor influencing the [H-3]-vincristine accumulati on by resistant cells. Indeed, the time dependence of [H-3]-vincristin e accumulation by sensitive cells could be fitted using simple monoexp onential kinetic dependence in contrast to biexponential kinetic depen dences that are necessary for fitting [H-3]-vincristine accumulation b y both resistant cell sublines. Kinetic analysis of the experimental d ata indicates that accumulation of [H-3]-vincristine by sensitive cell s grows to a plateau reflecting probably the equilibrium of drug conce ntration is the intracellular and extracellular space. On the contrary , accumulation of [H-3]-vincristine by both resistant cell sublines wa s stabilized after an initial growth on a considerably lower level tha n it was observed for the sensitive cells in the equilibrium.