Formation of blocking lesions at identical DNA sequences by the nitrosourea and platinum classes of anticancer drugs

cis-Diamminedichloroplatinum (II) (cisplatin) compounds and the chloroethylnitrosoureas are two different classes of anticancer drugs that work by modifying DNA covalently. We have compared the platinating drug cisplatin with the alkylating drug bischloroethylnitrosourea and other chloroethylnitrosoureas by modifying double stranded DNA in vitro and identifying blocking lesions that impede the progress of Escherichia coli DNA polymerase. Despite their very different structures and reactivities, cisplatin and the chloroethylnitrosoureas from primary blocking lesions at identical sequences, those containing adjacent guanosines on the same DNA strand. In tumor virus SV 40 DNA, a very strong target for both types of drugs is the regulatory sequence GGGCGG, which is repeated six times and is an important sequence for viral replication and an essential sequence for expression of the viral transforming gene. Sequences related to these GC box elements are known to be present in the flanking regions of many retroviruses and oncogenes, thus raising the possibility that the targeting of these sequences in tumor cells contributes to drug activity.