Differential in vitro interactions of a series of clinically useful topoisomerase-interacting compounds with the cleavage/religation activity of the human topoisomerase IIalpha and IIbeta isoforms.

The topoisomerase II (TOP2)-associated DNA cleavage activity and the DNA sequence preference of 20 antitumor drugs, including 15 TOP2-interacting compounds, have been defined. Four major classes of drugs have been identified: (i) those which enhanced the stabilization of cleavable complexes at a single major site (e.g. amsacrine, doxorubicin), or (ii) at many sites (e.g. etoposide, azatoxin), with chemically related compounds having very similar, although not identical, cleavage patterns (e.g. etoposide, GL331 and Top-53); (iii) those which inhibited DNA breakage (e.g. aclarubicin, actinomycin D); and (iv) those which did not visibly interfere with TOP2-mediated cleavable complexes (e.g. ICRF-187, camptothecin). All drugs tested induced similar overall patterns of sites of preferred DNA cleavage, in the presence either of the two known isoforms, TOP2alpha or TOP2beta, although relative intensities of signals at each position varied. It has been further shown that etoposide and its derivatives blocked the religation step downstream of the DNA cleavage step, whereas amsacrine, ellipticine, azatoxin and genistein acted upstream through enhancement of DNA cleavage. The information provided by this mechanistically based comparison can now be exploited in designing or synthesizing novel TOP2-interacting agents.