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¹ UMR-CNRS 6032, Université de la Méditerranée, Marseille, France and ² Department of Molecular, Cellular and Developmental Biology and Neuroscience Research Institute, University of California, Santa Barbara, CA

Requests for Reprints: Mary Ann Jordan, Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, CA 93106. Phone: (805) 893-5317; Fax: (805) 893-4724. E-mail: jordan@lifesci.ucsb.edu

Discodermolide is a new microtubule-targeted drug in Phase I clinical trials that inhibits tumor growth and induces G₂-M cell cycle arrest. It is effective against paclitaxel-resistant cell lines and acts synergistically in combination with paclitaxel. Suppression of microtubule dynamics by microtubule-targeted drugs has been hypothesized to be responsible for their ability to inhibit mitotic progression and cell proliferation. To determine whether discodermolide blocks mitosis by an effect on microtubule dynamics, we analyzed the effects of discodermolide on microtubule dynamics in living A549 human lung cancer cells during interphase at concentrations that block mitosis and inhibit cell proliferation. We found that discodermolide (7–166 nM) significantly suppressed microtubule dynamic instability. At the IC₅₀ for proliferation (7 nM discodermolide, 72 h), overall dynamicity was reduced by 23%. The principal parameters of dynamic instability suppressed by discodermolide were the microtubule shortening rate and length shortened. In addition, discodermolide markedly increased the frequency of rescued catastrophes. At the discodermolide concentration that resulted in 50% of maximal mitotic block (83 nM, 20 h), most microtubules were completely non-dynamic, no anaphases occurred, and all spindles were abnormal. The dynamicity of the remaining dynamic microtubules was reduced by 62%. The results indicate that a principal mechanism of inhibition of cell proliferation and mitotic block by discodermolide is suppression of microtubule dynamics. Importantly, the results indicate significant additional stabilizing effects of discodermolide on microtubule dynamics as compared with those of paclitaxel that may in turn reflect differences in their binding sites and their effects on tubulin conformation.

Grant support: GEFLUC Marseille Provence, by the Association pour la Recherche sur le Cancer (ARC), from whom S. H. received a fellowship, and by NIH CA57291 and NS13560.

¹ S. Honore, K. Kamath, D. Braguer, L. Wilson, and M. A. Jordan, unpublished data.

² M. A. Jordan, K. Kamath, B. A. Littlefield, and L. Wilson, unpublished data.

³ For review, see M. A. Jordan and L. Wilson. Microtubules as a target for anti-cancer drugs, manuscript in preparation.

Received 8/ 5/03; revised 9/26/03; accepted 10/ 7/03.

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