Characterization of Acquired Resistance to cis‐Diamminedichloroplatinum(II) in Mouse Leukemia Cell Lines

We have established in vivo cisplatin‐resistant mouse leukemia cell lines, L‐1210/DDP and P388/ DDP, in order to elucidate the mechanism of acquired resistance to cisplatin. Resistance indices were 22 and 14, respectively, when the cells were exposed to cisplatin for 48 h. Uptake of cisplatin by both resistant lines was significantly reduced, compared with values for the respective parent lines (17% for L‐1210/DDP and 27% for P388/DDP, at 100 μM for 1 h). While glutathione contents in the resistant cells were 1.7‐1.9 times higher than those in the sensitive ones, their reduction by preincubation with buthionine sulfoximine did not influence the sensitivity of the cells to cisplatin. In addition, the resistant lines dill not show lower sensitivity to CdCl(2), than the respective sensitive ones, suggesting that intracellular SH groups might contribute little to the mechanism of cisplatin resistance in these cells. Postincubation with DNA repair inhibitors, caffeine and aphidicolon, did not selectively enhance the sensitivity of the resistant cells to cisplatin. These results suggested that reduced drug uptake would be a primary mechanism of cisplatin resistance in L‐1210/DDP and P388/DDP. Cross‐resistance patterns to platinum complexes were quite different between L‐1210/DDP and P388/DDP. Colon 26/DDP, another cisplatin‐resistant mouse tumor showed a different pattern from those observed with L‐1210/DDP and P388/DDP. In the development of new platinum complexes we should use plural resistant lines for examining cross‐resistance patterns to candidate platinum complexes.