MODULATION OF CISPLATIN CYTOTOXICITY BY PERMEABILIZATION OF THE PLASMA-MEMBRANE BY DIGITONIN INVITRO

Killing of human ovarian carcinoma 2008 cells by cisplatin (DDP) is in direct proportion to the amount of drug entering the cell. DDP and its analogue [H-3]dichloro(ethylenediamine)platinum[II] ([H-3]-DEP) enter cells relatively slowly. We found that the uptake of [H-3]DEP into 2008 cells could be increased by treating the cells briefly with the plasma membrane-selective detergent digitonin. A similar effect was observed in an 11-fold DDP-resistant subline of 2008 cells, designated 2008/C13*5.25. A measurable effect was produced by concentrations as low as 5 muM, and 40 muM digitonin increased [H-3]DEP accumulation at 1 hr by 4.4 +/- 0.2- and 6.5 +/- 0.7-fold (means +/- SD) in 2008 and 2008/C13*5.25 cells, respectively. The effect was rapid, occurring within 1 min. Increased [H-3]DEP uptake was accompanied by increased platination of DNA (8.5-fold in 2008 cells and 18.5-fold in 2008/Cl3*5.25 cells), and by enhanced killing of both the DDP-sensitive and -resistant cells that was shown to be synergistic by median effect analysis. The combination index at 50% cell kill was 0.64 +/- 0.14 (values <1 indicate synergy). We conclude that a brief exposure to digitonin can increase [H-3]DEP uptake in vitro, and can overcome the impaired [H-3]DEP accumulation associated with acquired DDP resistance. DDP and digitonin interact synergistically to increase tumor cell kill in vitro.