The significance of DNA cross-linking to cis-diamminedichloroplatinum(II)-induced cytotoxicity in sensitive and resistant lines of murine leukemia L1210 cells.

The cross-linking of DNA by cis-diamminedichloroplatinum(II) (cis-DDP) has been studied in a murine leukemia L1210 line (L1210/0) and a 30-fold resistant subline (L1210/DDP2) utilizing the technique of alkaline elution. The kinetics of cross-link formation were similar in both cell lines. After 1-h treatment with cis-DDP, cross-linking continued to increase to a maximum at 12 h posttreatment. Although cross-linking decreased by 24 h some lesions were seen to persist. After 72 h interstrand cross-links were completely removed by both cell lines, however 50% of the DNA-protein cross-links still remained in the sensitive cells even though they were undetectable in the resistant subline. To correlate cross-linking with cytotoxicity, a range of drug concentrations were used for concomitant growth inhibition studies and alkaline elution analyses. In addition to the 30-fold resistant subline, two other sublines developed in our laboratory which show 20- and 50-fold resistance to cis-DDP were included in these determinations. All the resistant sublines exhibited the same degree of cross-linking at the same applied drug doses although these doses elicited markedly different cytotoxicities; cross-linking varied only with applied drug concentrations. The degree of cross-linking in L1210/0 was also directly related to the dose of cis-DDP. However, the sensitive cell line displayed equivalent total cross-linking at a 5-fold lower applied dose. This may implicate an uptake phenomenon as a contributor to resistance. However, such a phenomenon can account for only part of the resistance observed as there existed 6-fold more total cross-links and 9-fold more interstrand cross-links in L1210/DDP2 at doses that were equitoxic to L1210. This suggests the existence of a more critical cytotoxic lesion that was not detectable by alkaline elution, probably interstrand cross-links. Resistance could be due to a differential removal of these lesions.