Non-competitive inhibition of P-glycoprotein-associated efflux of THP-adrimycin by verapamil in living K562 leukemia cells

The decrease of the intracellular concentration of drug in resistant cell as compared to sensitive cells is, in most of cases, correlated with the presence, in the membrane of resistant cells, of a 170-kDA P-glycoprotein (P-gp) responsible for an active efflux of the drug. The fluorescence emission spectra from anthracycline-treated cells suspended in buffer have been used to follow the P-gp-associated efflux of these drugs in the absence or presence of verapamil. In the present study, 4′-o-tetrahydro-pyranyladriamycin (THP-adriamycin) was used. Two different methods were used to determined the kinetics of active efflux of THP-adriamycin: (1) at the steady-state, (2) directly, after the addition of glucose to cells first incubated with THP-adriamycin in the presence of N₃^? and in the absence of glucose. Kinetic analysis indicates: (1) a saturation of the active efflux when the cytosolic free drug concentration increased (the Michaelis constant K_m = 0.5 ± 0.3 μM) and (2) that the inhibitory effect of verapamil on P-gp associated efflux of THP-adriamycin in living cells in living cells is non-competitive.