TAN-1518A

TAN-1518 A is a cytotoxic agent with suppressive activity against Meth A fibrosarcoma in vivo. This compound inhibits calf thymus DNA topoisomerase I (Topo I) but does not stimulate cleavable complex formation in the nuclei of Chinese hamster ovary (CHO)-K1 cells, suggesting that it inhibits Topo I in a manner different from that of camptothecin (CPT). To clarify the mode of action of TAN-1518 A, we examined its effects on the eukaryotic microorganism Schizosaccharomyces pombe (S. pombe), which does not require Topo I as an essential factor for growth. TAN-1518 A inhibited purified S. pombe Topo I as potently as did CPT. TAN-1518 A, unlike CPT, did not stimulate Topo I-induced DNA cleavage; instead, it inhibited CPT-induced cleavable complex formation. We constructed a S. pombe strain, IR9, that produced excess Topo I. IR9 was hypersensitive to CPT, but its growth was not affected by TAN-1518 A. The CPT-mediated death of IR9 cells was reduced dramatically in the presence of TAN-1518 A. These findings clearly demonstrate that TAN-1518 A is a specific inhibitor of Topo I in eukaryotic cells and also suggest that this agent inhibits some earlier step(s) that occurs before the formation of cleavable complex on DNA strands in the catalytic cycle of this enzyme.

New naphthacenecarboxamide antibiotics, TAN-1518 A and B, were isolated from a culture broth of Streptomyces sp. AL-16012. Their structures were elucidated from their reactions and from spectroscopic analyses. The relaxation of supercoiled pBR322 DNA by calf thymus DNA topoisomerase I was inhibited by these metabolites as potently as by camptothecin. However, the decatenation of kinetoplast DNA by calf thymus DNA topoisomerase II was little affected by these agents. The major metabolite, TAN-1518 A, strongly suppressed the growth of various murine and human tumor cells, inducing apoptosis. Unlike camptothecin, TAN-1518 A did not stimulate cleavable complex formation in the nuclei of CHO-K1 cells and had weak activity in intercalating into DNA strands. This metabolite arrested the growth of human tumor cell lines in G1 phase of the cell cycle. These results suggest that TAN-1518 A and B are novel antitumor agents targeting topoisomerase I.