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Vitilevuamide, a bicyclic 13 amino acid peptide, was isolated from two marine ascidians, Didemnum cuculiferum and Polysyncranton lithostrotum. Vitilevuamide was cytotoxic in several human tumor cell lines, with LC(50) values ranging from 6 to 311nM, and analysis in a 25-cell line panel revealed a weak correlation with several taxol analogs. Vitilevuamide was strongly positive in a cell-based screen for inhibitors of tubulin polymerization. Vitilevuamide at 9 microg/mL (5.6 microM) had an effect equivalent to the maximal effect of colchicine at 25 microg/mL (62.5 microM). Vitilevuamide was active in vivo against P388 lymphocytic leukemia, increasing the lifespan of leukemic mice 70% at 30 microg/kg. We hypothesized that at least part of the cytotoxic mechanism of vitilevuamide was due to its inhibition of tubulin polymerization. Vitilevuamide was found to inhibit polymerization of purified tubulin in vitro, with an IC(50) value of approximately 2 microM. Cell cycle analysis showed that vitilevuamide arrested cells in the G(2)/M phase with 78% of treated cells tetraploid after 16hr. Therefore, vitilevuamide was tested for its ability to inhibit binding of known tubulin ligands. Vitilevuamide exhibited non-competitive inhibition of vinblastine binding to tubulin. Colchicine binding to tubulin was stabilized in the presence of vitilevuamide in a fashion similar to vinblastine. Dolastatin 10 binding was unaffected by vitilevuamide at low concentrations, but inhibited at higher ones. GTP binding was also found to be weakly affected by the presence of vitilevuamide. These results suggest the possibility that vitilevuamide inhibits tubulin polymerization via an interaction at a unique site.