EFFECT OF S9788, CYCLOSPORINE-A AND VERAPAMIL ON INTRACELLULAR-DISTRIBUTION OF THP-DOXORUBICIN IN MULTIDRUG-RESISTANT K562 TUMOR-CELLS, AS STUDIED BY LASER CONFOCAL MICROSPECTROFLUOROMETRY

S9788 modulating resistance effect has been investigated on the activi ty of THP-DOX against multidrug-resistant K562R cells and compared to that of cyclosporin A and verapamil. Intracellular THP-DOX distributio n and particulary its intranuclear concentration, with or without modu lators, has been measured using confocal laser microspectrofluorometry . The kinetics of intranuclear accumulation of THP-DOX (1 mu M in the medium), as a function of time, were rapid in K562S and K562R cells. M aximum accumulation of THP-DOX is reached in a few minutes (K562S, 400 mu M; K562R, 40 mu M). The addition of S9788, cyclosporin A and verap amil (5 mu M) after one hour THP-DOX incubation, led to respectively 2 90, 250 and 114 mu M. Uptake of THP-DOX was increased in K562R cells b y a factor of 7 when S9788 was added. Results obtained on THP-DOX effl ux from nuclei of K562S and K562R cells, after 3 hours of incubation w ithout drug, showed a very short T-1/2 (time corresponding to 50% decr ease of intranuclear concentration of THP-DOX) in K562R cells (12 min) compared to that in K562S cells (150 min). The addition of S9788, cyc losporin A and verapamil (5 mu M) led to a T-1/2 of 90, 30 and 20 min respectively. The T-1/2 of THP-DOX was increased in K562R cells by a f actor of 7.5 when S9788 was added We tried to correlate these results with those obtained in growth inhibition study. The IC50 (concentratio n which induces 50% growth inhibition) of THP-DOX corresponding to one hour THP-DOX treatment and 3 days culture of IC562S and