Bleomycin congeners exhibiting altered DNA cleavage specificity.

Sir: The bleomycins (BLM’s) are a family of structurally complex glycoprotein antibiotics possessing significant anticancer activity ( I ) . They have been demonstrated to effect DNA strand scission in a reaction that is both metal ion and oxygen dependent (2); this transformation is believed to form the basis for the therapeutic efficacy of the bleomycins. A number of studies have demonstrated that BLM-mediated DNA cleavage is sequence specific (3-5), but the basis for this phenomenon is not understood. Previous studies have demonstrated that for certain bleomycin congeners the sequence specificity of DNA strand scission can be altered by the choice of metal cofactor (64, and by methylation (9) or platination ( I O ) of the DNA substrate. By the use of a palindromic dodecanucleotide, it has been shown that the ratio of DNA modification at individual sites can vary from one BLM congener to another (11). However, with the exception of tallysomycin (5, 12, 13), which differs substantially in structure from the other bleomycin group antibiotics (14), there is no report of the alteration of sequence selectivity of DNA cleavage among BLM congeners. Reported herein is an analysis of DNA cleavage by seven BLM congeners that establishes the existence of significant differences in the patterns of DNA cleavage. This finding provides important insights concerning the structural factors that control sequence selectivity, further supports the representation of activated FeBLM as a high valent metal-oxo complex, and may bear relevance to the design of a BLM congener having an altered spectrum of antitumor activity. The sequence selectivity of DNA strand scission was studied using three DNA restriction fragments1 and seven Fe(II).BLM derivatives.2 The pattern of DNA strand scission observed3 is illustrated for Fe(II).BLM A2 and Fe(II).epi-BLM A2 in Figure 1 and analyzed for four of the congeners in Table I as a function of cleavage efficiency observed at each of the 16 possible constituent dinucleotides.* The overall pattern of DNA strand scission was the same as that reported previously (3-5) with the exception that 5‘-GA-3‘ was also found to be a prominent cleavage site in this study. Consistent with earlier reports, the sequence selectivity of DNA cleavage mediated by BLM B2 (4,13) or deglyco-BLM A2 ( 1 7,19,20) were found to be essentially identical with that of BLM A> Also quite similar was the pattern of DNA cleavage found here for iso-BLM A2 (supplementary material, Table I). On the other hand, the remaining three congeners ex-