Increased gene specific repair of cisplatin induced interstrand crosslinks in cisplatin resistant cell lines, and studies on carrier ligand specificity

Development of resistance to cisplatin in previously treatment-responsivemalignancies is a major obstacle to successful treatment. EnhancedDNA repair as well as enhanced replicative bypass of DNA adductshave been suggested to play a role in the development of resistanceto cisplatin. However, the relative contribution of these mechanismsis unknown. Second generation platinum compounds containingthe 1, 2-diaminocyclohexane (dach) carrier ligand have beenof particular interest in the studies of resistance mechanismssince they have been effective in treatment of cells resistantto cisplatin. We have investigated the formation and repairof interstrand crosslinks (ICL) in the mouse leukemia cell lineLI 210/0 and its carrier ligand specific resistant derivativesL1210/DDP and L1210/ DACH after treatment with ethylenediamine(en)-Pt and diaminocyclohexane (dach)-Pt compounds. ICL in theoverall genome were examined using a modification of the alkalineelution assay. A Southern blot technique was employed for thestudy of ICL in specific regions of the genome. In the overallgenome we found decreased formation of ICL with either -en or-dach carrier ligands in the two resistant cell lines withoutcarrier ligand specificity. Some carrier ligand specificityof ICL formation was observed in the dihydrofolate reductase(DHFR) gene, but it did not correlate with the carrier ligandspecificity of resistance. At the level of the overall genomethere was no difference in repair of ICL between the sensitiveand the two resistant cell lines. When measured in the DHFRgene, however, there was enhanced repair of ICL in the two resistantcell lines compared with the sensitive cell line. The enhancedrepair at the level of the gene did not display any carrierligand specificity.