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 it s analogue [H-3]dichloro(ethylenediamine)platinum[II] ([H-3]-DEP) ente r cells relatively slowly. We found that the uptake of [H-3]DEP into 2 008 cells could be increased by treating the cells briefly with the pl asma membrane-selective detergent digitonin. A similar effect was obse rved in an 11-fold DDP-resistant subline of 2008 cells, designated 200 8/C135.25. 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/ C135.25 cells, respectively. The effect was rapid, occurring within 1 min. Increased [H-3]DEP uptake was accompanied by increased platinati on of DNA (8.5-fold in 2008 cells and 18.5-fold in 2008/Cl35.25 cells ), and by enhanced killing of both the DDP-sensitive and -resistant ce lls that was shown to be synergistic by median effect analysis. The co mbination 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 accu mulation associated with acquired DDP resistance. DDP and digitonin in teract synergistically to increase tumor cell kill in vitro.