The marine natural product (+)-spongistatin 1 is an extremely potent growth inhibitory agent having activity against a wide variety of cancer cell lines, while exhibiting low cytotoxicity against quiescent human fibroblasts. Consistent with a microtubule-targeting mechanism of action, (+)-spongistatin 1 causes mitotic arrest in DU145 human prostate cancer cells. More importantly, (+)-spongistatin 1 exhibits significant in vivo antitumor activity in the LOX-IMVI human melanoma xenograft model. (+)-Spongistatin 1 is, thus, an important class of microtubule targeting anticancer agent that warrants further investigation.

AbstractWe report here that des-methyl, des-amino pateamine A (DMDA-PatA), a structurally simplified analogue of the marine natural product pateamine A, has potent antiproliferative activity against a wide variety of human cancer cell lines while showing relatively low cytotoxicity against nonproliferating, quiescent human fibroblasts. DMDA-PatA retains almost full in vitro potency in P-glycoprotein-overexpressing MES-SA/Dx5-Rx1 human uterine sarcoma cells that are significantly resistant to paclitaxel, suggesting that DMDA-PatA is not a substrate for P-glycoprotein-mediated drug efflux. Treatment of proliferating cells with DMDA-PatA leads to rapid shutdown of DNA synthesis in the S phase of the cell cycle. Cell-free studies show that DMDA-PatA directly inhibits DNA polymerases α and γ in vitro albeit at concentrations considerably higher than those that inhibit cell proliferation. DMDA-PatA shows potent anticancer activity in several human cancer xenograft models in nude mice, including significant regressions observed in the LOX and MDA-MB-435 melanoma models. DMDA-PatA thus represents a promising natural product-based anticancer agent that warrants further investigation.[Mol Cancer Ther 2009;8(5):1250–60]

E7974 is a synthetic analogue of the marine sponge natural product hemiasterlin. Here, we show that E7974, such as parental hemiasterlin, acts via a tubulin-based antimitotic mechanism. E7974 inhibits polymerization of purified tubulin in vitro with IC(50) values similar to those of vinblastine. In cultured human cancer cells, E7974 induces G(2)-M arrest and marked disruption of mitotic spindle formation characteristic of tubulin-targeted anticancer drugs. Extensive hypodiploid cell populations are seen in E7974-treated cells, indicating initiation of apoptosis after prolonged G(2)-M blockage. Consistent with this observation, E7974 induces caspase-3 activation and poly ADP ribose polymerase cleavage, typical biochemical markers of apoptosis. Only a short cellular exposure to E7974 is sufficient to induce maximum mitotic arrest, suggesting that E7974's antitumor effects in vivo may persist even after blood levels of the drug decrease after drug administration. Interactions of E7974 with purified tubulin were investigated using two synthetic tritiated photoaffinity analogues incorporating a benzophenone photoaffinity moiety at two different positions of the E7974 scaffold. Both analogues preferentially photolabeled alpha-tubulin, although minor binding to beta-tubulin was also detected. E7974 thus seems to share a unique, predominantly alpha-tubulin-targeted mechanism with other hemiasterlin-based compounds, suggesting that, unlike many tubulin-targeted natural products and related drugs, the hemiasterlins evolved to mainly target alpha-tubulin, not beta-tubulin subunits.

2012 Background: Resistance to tubulin-targeted agents has been associated with differential tubulin isotype expression in cancer cells. In this study we investigated whether β-tubulin isotype composition of tumor cells affects their sensitivity to 2 natural product-based antitubulin agents: halichondrin analog E7389 and hemiasterlin analog E7974. Methods: Antiproliferative effects of E7389, E7974, paclitaxel and vinblastine against a panel of human breast cancer cell lines were determined by methylene blue-based or MTT-based cell growth inhibition assays. Cell lines were tested for evidence of PgP expression by monitoring effects of the PgP blocker verapamil on paclitaxel sensitivity. Using quantitative PCR with specific primers, expression levels of the seven human β-tubulin isotypes were determined (I, II, III, IVa, IVb, V, and VI). Results: No evidence for PgP expression was found in any of the cell lines tested. In growth inhibition studies, interdrug correlations were seen between sensitivities to the 3 microtubule polymerization inhibitors E7974, E7389 and vinblastine, but not between the microtubule stabilizer paclitaxel and the other agents. All 7 β-tubulin isotypes were detected at different levels in each cell line, with isotypes I and IVb showing the least variation while isotypes II, III, and IVa showed the greatest variability between the cell lines. A significant correlation was demonstrated between β III tubulin expression levels and sensitivity to both E7389 and E7974. Conclusions: These findings in cancer cell lines suggest that tumors expressing higher levels of βIII tubulin isotype may be more responsive to treatment with E7389 and E7974. If confirmed clinically, such preclinical insights may lay the foundation for selection of patient populations with enhanced sensitivities to these 2 natural product-based antitubulin agents. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Eisai Medical Research Institute