Background: Topoisomerase I and II (topo I and II) are enzymes which alter the topological state of DNA through DNA strand cleavage, strand passage and religation. They participate in most aspects of DNA metabolism and are therefore vital to the cell undergoing division. Only one form of topo I has been identified whereas two isoenzymes of topo II have been described: the a form (170 kDa protein) and beta form (180 kDa protein). Both topo II isoenzymes have distinct nuclear localisation, are regulated independently, differ in their responsiveness to inhibitors and are differentially expressed in drug resistant cell lines. Results: Several clinically active anticancer drugs (e.g., doxorubicin, m-AMSA, VP-16 and camptothecins) poison these enzymes by stabilizing a putative reaction intermediate called the cleavable complex (cc) where the topoisomerase remains covalently attached to either one strand of DNA (topo I) or both strands of double helix (topo II) after strand cleavage. DNA cleavage sites appear unique for different classes of inhibitor, and are probably critical for defining cytotoxicity. Formation of the cc may cause cell death either by colliding with replication forks, by promoting illegitimate genomic-DNA recombination, by arresting cells in the G2-phase of the cell cycle or by inducing apoptosis. Conclusion: New classes of inhibitor have recently been described with novel mechanisms of action including compounds which do not stabilize cleavable complexes or bind significantly to DNA. These may prove to be more selective and less toxic. They may also avoid the possible problem of therapy-related leukemias associated with topo inhibitors which induce DNA cleavage and chromosomal aberrations.
