Ixabepilone and other epothilones: microtubule-targeting agents for metastatic breast cancer.

Taxanes, derived from the bark of the Pacific yew tree, were the last major group of cytotoxic agents to be developed. Their proven efficacy in a variety of malignancies has constituted a real breakthrough in the treatment of cancer. Wider clinical use of taxanes has several important limitations, including acquired and intrinsic tumor resistance, hypersensitivity reactions, and cumulative neurotoxicity and hematopoietic toxicity. Epothilones, naturally occurring macrolide antibiotics that also act on the microtubule, are a novel class of compounds that may circumvent some of these problems. Many synthetic and semisynthetic epothilone analogs have been formulated and have undergone varying degrees of testing. These compounds have demonstrated activity in a variety of tumors, including in tumors and cell lines resistant to taxanes. So far only ixabepilone has been tested in phase II and III trials and licensed by the US Food and Drug Administration for the treatment of metastatic breast cancer. The main dose-limiting toxicity appears to be a sensory peripheral neuropathy, as commonly seen with drugs that act on the microtubule. Further clinical studies assessing the role of ixabepilone in other settings, as well as the clinical investigation of other epothilones, are eagerly awaited.